Work-a-text in physical science (Cambridge work-a-text)

Work-a- Text in

PHYSICAL SCIENCE MILTON GALEMBO

Science Department Cleveland Hill High School Cheektowaga, New York OTHO E. PERKINS

Supervisor of Science Columbus Public Schools, Columbus, Ohio Edited by JAC K ROBBINS

District Supervisor of Science Long Beach City School District, Long Beach, New York BURTON E. NEWMAN

Chairman Science Department Lakeland High School Shrub Oak, New York

CAMBRIDGE BOOK

�

NEW YORK, N.Y. A NEW YORK TIMES COMPANY

COMPANY

©Copyright 1971, 1966, by CAMBRIDGE BOOK COMPANY

A ll rights reserved. No part of this book may be reproduced in any form without the written permission of the publisher.

MANUFACTURED IN THE UNITED STATES OF AMERICA

PREFACE About a hundred years ago, the Brit ish Bureau of Pat ent s announced plans to close its doors because, according to it s director, all wort hwhile discoveries had already been made. Man had nothing more to learn! This was be­ f ore t he first successf ul airplane flight , bef ore t elevision, and bef oret he age of nuclear energy. Today, man realizes f ull well t hat although great discoveries about our environment have been made, much new knowled ge lies just beyond the horizon. In f act, new inf ormation is being accumulated in t he sciences at such a f antastic rate that our amount of knowledge doubles every five to ten years. As more knowledge is gained, science textbooks get big­ ger, bulkier, and more complicated. Teachers are daily f aced wit h t his challenging quest ion : "How can science be taught in an age when new knowledge becomes outdated almost over­ night? " Work-a-Text in Physical Science takes ad­ vantage of new discoveries in teaching and learning to help t eachers and students in this time of rapid change. Alt hough knowledge may change, and new ideas may become out­ moded, basic science principles, concepts, and processes are more stable. While including the most up-to-date inf ormation available, Work-a-Text in Physical Science is designed around an integrated program of these f unda­ ment al aspects of physical science. The ay tivities include various pr ocesses and science concepts, and promot e a logical approach t o problem solvi ng. This Work-a-Text is divided into two major sections, physics ( 1 4 chapters ) and chemistry ( 1 0 chapt ers ) . The teacher can elect to teach either the chemistry or the physics portions first, according to student needs, without dis­ rupting the overall program plan. The new Work-a-Text in Physical Science combines the best f eatures of a comprehensive text, laboratory manual, and activities book. It can be correlat ed with topics covered in all standard text books, and may be used with them or as t he basic classroom text. The reading m aterial of each chapt er is as up-to-date and authentic as research in physi-

cal science it self . The many illustrat ions and diagrams help t o clarif y im port ant concepts. The abundant review tests help to reinf orce processes as well as main ideas, and they em­ ploy a variety of approaches. The Self-Discovery A ctivities help t he st u­ dent s develop a f acilit y wit h t he processes of science by act ive part icipation in t he " doing of science." At t he same t ime main ideas and concepts are supported. The act ivities vary f rom induct ive to deductive in f ormat, and are a depart ure f rom t he old idea of " cook­ book experiment s" in science. Rather than be­ ing placed at the end of t he units, as in pre­ vious editions, t he Self-Discovery A ctivities are integrated wit h t he chapt er m aterial. Int his way, student s are involved in invest igat ions of a concept in a more relevant manner. Act ivities involve simple equipment which is r eadily available or easily improvised, and are de­ signed with t he saf et y of students in mind. Many of t he suggested activities m ay be car­ ried out by the student individually. Ot hers are bett er suited f or work wit h a laborat ory partner, or by larger student groups. The Review Tests are perf orat ed so they can be removed and handed in f or evaluation, if desired. These t est s can also be used t o review t he main ideas of t he chapt ers and can be " graded" by the student s t hemselves. See the teacher's guide m at erial f or f urt her ideas f or using the tests, which can save many hours of the teacher's planning t ime. When all t he unit s of t he Work-a-Text in Physical Science are completed, the student may be encouraged to keep t he book f or his own librar y. On completion of the course, he will have a written record of his work, and a comprehensive, i llust rat ed book t hat he will ref er to many times. Effective use of t he Work-a-Text in Physical Science will give teachers and student s more opportunity f or the exchange of ideas, and a clearer view of science and it s import ance t o man. It is hoped that it will also stimulat e the st udent to f urt her study on his own. -

THE AUTHORS

A C K N OWLEDG M ENTS The Author wishes to thank his many asso­ ciates who offered helpful advice and criti­ cisms and tested the exercises in their classes. My thanks go to Dr. Luis W. Alvarez and J anet L. Alvarez of the U niversity of Califor­ nia, and to This Week Magazine for permis­ sion to reprint the article entitled "Fallout Pro­ tection f or U nder 20 Cents." This article ap­ pears as a Self- Discovery Activity on pages

iv

130-132. Also f or permission to adapt dia­ grams from "Steel in the Making," published b y the Bethle hem Steel C orp oration. My hear tfelt appreciation goes to my wife Louise, who so meticulously typed the manu­ script and to the editori al staff of the C am­ bridge Book C o. , f or their wise counsel and their infinite p atience.

CONTENTS Preface

iii

Safety in the Laboratory

vii

The Scientific Method

viii

Archimedes'

Principle. Why Does an

Object Float? Why Does an Object Sink? Specifi c Gravity.

Determining Specific

Than Water. Specific Gravity of Liquids.

1

Spe cific Gravity. Bernoulli's Self-Discovery A ctivities. Review Tests. Uses

of

Principle.

7. Force s an d Mo tion . . . . . .. . . . . . .. . . . . . . .. . .. . .. . ...

Motion. Speed and Velocity. Uniform

Review Tests. 2. Ph ysic s an d the Me tric Sy ste m . . . . . . . . .. . . ..

Physics - the Science. The Metric Sys-

11

Bodies. Distance Covered by Moving Bodies. Newton's Three Laws of Motion. Conservation of Momentum.

ric

covery Activity. Review Tests.

of

Length,

Weight. Conversion

Volume,

and

Factors . Common

Review Tests.

Static Electricity. Dangers of Static Elec­

3. Forces . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . Gravitational Force. Mass. The Law of Gravitational Attraction . The Force of Friction. The Force of Inertia. Molecular Forces . Cohesion. Surface Tension. Ad­ hesion. Capillary Action. Force Vectors. Concurrent Forces . Self-Discovery A c­ 4. Force san d Work . . . . . . ... . .. . . ... . . . .. . . . . . . . . . ... . . and

Work.

Power?

Horsepower.

Law of Machines.

Measure Electricity.

Is

Work? What Machines.

9. Magne ti sm. . . . . . . . . . . . . . . . . ............................. Magnets. Theory of Magnetism. Electro­ magnets. Generators. Type s of Electrical Currents. Electric Motors. Self-Discov29

Is

How

Simple and Com­

81

ery A ctivities. Review Tests.

10. Sound

. . ................... ...............................

Is

Sound

Transmitted?

Sound

91

Waves. Velocity of Sound. The Pitch of

pound Machines. Levers. The

Law of Moments. Mechanical Advantage. In­ clined Plane. Other Simple Machines.

Sound. The Loudness of Sound. The Re­ flection of Sound (Echoes). Musical Sounds. How We Hear Sounds. Self-Dis­

Self-Discovery A ctivities. Review Tests.

Self-Discovery A ctivities. Review Tests.

Self-Dis­

covery A ctivities. Review Tests.

The

5. Pressure in Fluids . . . .. . . . . ... . . . . .. . . . . . . . . . . . . . . . . Density. Pressure. Liquid Pressure. Total Force. Total Force on a Vertical Sur­ face. Effect of Shape, Size and Volume on Pressure. Liquids Exert Pressure Equally in All Directions at the Same Depth. Pascal's Law and Hydraulics.

Ohm's Law. Electri-

cal Circuits. Power and Energy.

Potential Energy.

Kinetic Energy. What

69

tricity. Current Electricity. Units Used to

19

tivities. Review Tests.

Energy

Self-Dis­

8. Elec tr ci i ty . .............................................

Metric Units. Self-Discovery A ctivity.

61

and Accelerated Motion. Freely Falling

tem. Important Prefixes. Common MetUnits

51

Gravity of Solids Denser and Less Dense

AREA 1. INTRODUCTION TO PHYSICS

1. The Techniques and Tools of Science . . . . Using a Scientific Method III Solving Problems. Theories, Facts, and Laws. Superstitions . Scientific Equipment and Measurement. Self-Discovery A ctivity.

6. Buo yancy an d S peci fic Gra vi ty ..............

covery A ctivities. Review Tests.

41

11. Ligh t .. . .. . . . . . . . . .. .. . . .. . . . . .. . . . . . . . . ... . . . . . . . . . .. ... . .

The Nature of Light. Velocity of Light. Theories of Light. Properties of Electro­ m agnetic Radiations. The Electromag­

netic Spectrum. What Is Refraction? Dis­

persion of White Light. Color of an Ob­ ject. Reflection. Refraction. Convex

97

Lenses. Concave Lenses. Strength of a Source of Light. Photometry. Intensity of Illumination. Optical Instruments. Polar­ ized Light. Self-Discovery A ctivities. Re­ view Tests.

12. Hea t ........ . . . ........ . .................................. 109 Sources of Heat Energy. Heat and Tem­ perature. Measuring Temperature. Heat Transfer. Conduction. Convection. Ra­ diation. Effects of Heat Energy. Self-Dis­ covery A ctivities. Review Tests.

13. Nuclear Energy . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 The Development of Nuclear Energy. Nuclear Reactors. Nuclear Fusion. Uses of Nuclear Energy. Self-Discovery Ac­ tivity. Review Tests.

Self-Discovery A ctivity. Review Tests. .

19. The Periodic Table .......... . . . . . . . . . . . . . . . . . . . . . 171 Reading the Periodic Table. The Ar­ rangement of Elements Into Periods. Ar­ rangement of Elements Into Groups. Iso­ topes and Radioisotopes. Beneficial Uses of Radioisotopes. Self-Discovery A ctivity. Review Tests.

20. Wa ter . ... . . .... ......... ...................... ........... 181 Properties of Water. Types of Solutions. Suspensions. Colloids. Water Purification. Hard and Soft Water. Water Pollution. Self-Discol'ery Activities. Review Tests.

14. Civil Defense . .............................. . .......... 127 Surface, Subsurface, Underwater Bursts. Zones of Destruction. Radiation. Factors Influencing Radioactive Fallout. Protective Measures Against Radioactive Fallout. Basement Fallout Shelter. Self-Dis­ covery Activity. Review Tests.

AREA 2. INTRODUCTION TO CHEMISTRY

15. The Chemis try of Mat ter ........................ 137 Branches of Chemistry. Kinds of Matter. Natural Elements. Synthetic Elements. Transuranium Elements. Compounds. Major Differences Between Compounds and Mixtures. Self-Discovery A ctivity. Review Tests. .

16. Ma tter and Energy ....... . . . . . . . . .... . .......... . 145 The Molecular Theory of Matter. States of Matter. Properties of Matter. Physical and Chemical Changes. The Law of Con­ servation of Matter and Energy. Exo­ thermic and Endothermic Reactions . Self­ Discovery Activities. Review Tests.

17. Chemis try and the A tom ....... . . . .... . ......... 153 Structure of the Atom. Major Atomic Particles. Sub-Nuclear Particles. Atomic Weight. Formula or Molecular Weight. Electron Structure. Self-Discovery Ac­ tivity. Review Tests.

18. Chemical Ac tivi ty ...... . . ........... . ... . . . . . ...... 161 Metals and Non-metals. Activity Series of Metals. The Halogen Activity Series. vi

Formation of Compounds and Ions. Equations. Types of Chemical Reactions.

21. Acids , The Properties of Acids. Common In­ dustrial and Laboratory Acids. The Properties of Bases. Salts. Neutralization. Reaction Between an Active Metal and an Acid. Chemical Reaction Between a Base and a Salt. Self-Discovery A ctivity. Review Tests. .

22. Iron and S teel ................... . ... . ....... . . ... . . . 199 The Occurrence of Iron. The Metallurgy of Iron and the Use of the Blast Furnace. Steel. The Open-Hearth Furnace. The Basic Oxygen Furnace. Steel Alloys . Self-Discovery A ctivity. Review Tests.

23. Organic Chemis try ... . . . . . ... . . .............. . .. . . 207 Crystalline Forms of Carbon. Amor­ phous Carbon. The Compounds of Carbon. Hydrocarbons. Comparison of Or­ ganic and Inorganic Compounds. Some Members of the Methane Series. Self­ Discovery Activities. Review Tests.

24. Chemical Advances ......................... ....... 217 Health. Industry. Petroleum. Plastics. Rubber. Synthetic Fibers. Silicones. Con­ servation of Resources. Self-Discovery A ctivity. Review Tests.

Appen dices ............................................ 225 Distinguished Scientists. Important Scientific Laws and Principles. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . 228 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 240

S O M E I M PO RTANT SAFETY RULES I N T H E LABORATORY Safety in the laboratory is essential to prevent serious accidents to yourself and to others. Heed the following directions and all directions given by your instructor. Most accidents can be prevented if you

MAKE SAFETY A HABIT!

1. Read and follow all directions very carefully.

2. Never mix, touch, taste, heat or inhale chemicals unless you are told it is all right to do so by your instructor.

3. Always wear protective devices such as goggles, gloves, and an apron when these safety devices are needed.

4. Handle all hot objects with clamps or tongs.

S. Take extra precautions in handling dangerous chemicals.

6. Do not perform any experiments unless your instructor is in the room.

7. Turn off the gas when it is not in use.

S. Read the labels on all bottles carefully before using.

9. Never point a test tube you are heating toward anyone.

10. Pay strict attention to your own work. Don't let your interest or at­ tention wander.

FIRST AID FOR ACID AND ALKALI BURNS 1. Flood the tissue with water.

2. Notify your instructor.

3. FOR ACID BURN: Neutralize with a weak alkali (base) such as a sodium bicarbonate solution.

4. FOR ALKALI BURN: Neutralize with a weak acid such as a boric acid solution.

S. When

mixing

acid

and

water - always

add

the

acid

to

the

water, using the greatest caution.

6. When inserting a glass tube into a cork, always lubricate the tube and cork with water.

7. The science laboratory is a place for inteUigent adult behavior.

vii

Gathers evidence

Reads others' works; listens to others

Gathering of evidence, etc., etc.

BUT-a scientist's work never ends-so

to

leads

to

APPLYING THE SCIENTIFIC METHOD

Chapter 1

T H E T EC H N IQU ES A N D TOOLS O F S C I E N C E What i s a "scientist? " I s a scientist a "spe­ cial" kind of per son? Not r eally; however , a scientist does have some special char acter istics. 1. A Scientist Is Curious. His cur iosity leads him to seek knowledge about his envir onment, and plants the seeds of new ideas that taker oot and gr ow.

2. A Scientist Investigates. Cur iosity is of little value if it doesn't lead to investigation. The scientist carr ies out his investigations in an or ganized manner . This appr oach to solving pr oblems is often called the scientific method. 3. A Scientist Is Logical. The scientist uses valid, ver ifiable evidence to suppor t his con­ clusions. He is not swayed by super stition. 4. A Scientist Is Open-minded. He r ealizes that the envir onment is always changing, and that his knowledge is often based upon incom­ plete evidence. He accepts new infor mation and uses it to change and impr ove his ideas. USING A SCI ENTI FIC M ETHOD IN SOLVING PROBLEMS

1. Identify the Problem to be Solved. In or der to identify and under stand a pr oblem, it must be limited to a single, clear ly defined

question. Then the scientist can pr oceed. The failur e of an investigation can fr equently be tr aced to a lack of under standing of what the basic pr oblemr eally is.

2. Gather Evidence. In or der to deter mine whether or not an idea has mer it, a scientist sear ches thr ough all available r efer ence mate­ r ials to see what has been done in the past to solve the pr oblem. When possible, he discusses his ideas with exper ts in the fi eld. This pr events him fr om needlessly r epeating tedious wor k that has alr eady been done by other s, and pr o­ vides clues that help him f or mulate tr ial solu­ tions to his pr oblem. 3. Make a Hypothesis. A tr ial solution to a pr oblem is a hypothesis. It is of ten called an " educated guess," bec ause it is based on what is lear ned as the scientist gather s evidence about the pr oblem. Sever al hypotheses may b e tested and r ejected bef or e the pr oblem is solved. 4. Test the Hypothesis. A scientist estab­ lishes a set of pr ocedur es to test his hypothesis. He keeps accur ate, up-to-date notes so that his r esults can be ver ified if he or other s later r e­ pe at the pr ocedur e under the same conditions. 1

To t est t he hypot hesis, an experiment oft en has t o b e carried out . In sett ing up a scient ific experiment, t he following guidelines are im­ port ant:

( a ) Using a Control. A cont rolled experi­ ment is set up in duplicat e. Procedures, ident ical in every way except one, are carried out . The st ep t hat is carried out in one procedure and omitt ed from t he ot her is known as t he variable. The variab le is t he part of t he invest igat ion t hat is designed t o t est t he hypot hesis. The proceduret hat includest he vari­ ab le is known as t he experiment. The procedure from which t he variab le was omit t ed is calledt he control. As an ex­ ample, suppose t hat a scient ist want ed t o check t he effect of heat energy upon t he product ion of elect rical energy in wires of different kinds ( see Self-Dis­ covery Act ivit y, Invest igat ing a Ther­ mocouple, page 113 ) . He obt ained 100 pieces of copper wire and 100 pieces of iron wire. Then he t wist ed about t wo inches of each piece of copper wire t o t wo inches of each piece of iron wire. He connect edt he loose ends oft he 100 pairs of wires t o a galvanomet er. He heat ed 50 oft he pairs wit h a burn er at t he point where t he wires were t wist ed t oget her, and he ob served and recorded any reading on t he galvanomet er. He connect ed t he remaining 50 pairs of wires t o t he galvanomet er and wit hout heat ing t hem ob served any reading. The scient ist t hen made his conclusions b ased upon his ob servat ions. ( b ) Using a Sufficient Number of Experi­ mental Sets. In t he preceding invest iga­ t ion, t he scient ist used t wo set s of 50 b ecause even carefully mat ched wires may have slight differences. By using many experiment al set s, any variab les t hat could cause invalid result s are less likelyt o affect t he invest igat ion. ( c ) Repetition. A scient ist doesn't rely on t he result s of one experiment t o draw 2

his conclusi ons. A n experiment should b e repeat ed and verified before it can b e considered valid. An experiment t hat cannot produce t he same result s when repeat ed under ident ical condi­ t ions cannot b e relied upon. S. Observe and Record. Accurat e and up­

t o-dat e records should b e kept of all ob serva­ t ions. Careful measurement s and t he accurat e present at ion of dat a prevent t he omission of import ant det ails. 6. Arrive at a Conclusion. Based upon t he evidence he has gat hered, his observat ions, and t he result s of any experiment s, t he scient ist t ries t o arrive at meaningful conclusions. He asks himself some searching quest ions. Was a sufficient amount of informat ion gained as he gat hered evidence t o draw a conclusion wit h­ out experiment at ion? Were his hypot heses properly relat ed t o t he b asic problem? Did t he variable in t he experiment ( or experiment s ) properly t est t he hypot hesis? Finally, were t he result s oft he experiment valid or reliable? Supposet he scient ist who didt he experiment wit h t he wires makes t he st at ement, "When dif­ ferent kinds of wires are t wist ed t oget her and heat ed, elect ricit y is produced." What are some fl aws in such a conclusion? Somet imes, t he conclusion will support t he ideas and hypot heses of t he scient ist , and t he prob lem is solved. Oft en, however, more ques­ t ions arise t o be solved, or experiment s show t he hypot hesis t o be incorrect, andt he scient ist must st art all over again. A negat ive conclu­ sion should not be considered a failure; it is just as valuable as is a posit ive result . Int est ing a hypot hesis, b ot h negat ive and posit ive result s give clues for furt her st udy. TH EORI ES, FACTS, AND LAWS

Scient ist s develop possib le explanat ions for various phenomena which are b ased upon some demonst rat ab le evidence. Such explanat ions are oft en called theories. Theories cannot usu­ ally b e proven "false" or "t rue." However, t hey provide a model from which t o proceed. For example, t here are several t heories relat ed t o

evolution all of which are based upon some observable evidence, but none of t hem has been proven "t rue" or "false." If a theory is proven t o t he satisfact ion of most scient ists, it is often called a science fact. However, even facts are subjectt o change when new information makes t he fact no longer vali d. A scientific law is est ablished whent he s ame results are cons istent ly observed without excep­ tion, although no satisfactory explanation may be available. We speak, for example, aboutt he Law of Gravitation, and aerospace scientists use it to determine precisely the orbits of space­ craft. Yet, we still do not know what gravity really is or what causes it.

t o make accurat e measurements. He must have basic m aterials and equipment wit h which to carry out investigat ions. Proper use of this equipment increases his accuracy in carrying out procedures, observing various phenomena, and making precise quantit at ive measurement s.

SUP ERSTITI ONS

( See also Chapter 2, Physics and the Metric System, pages 1 1 -18 . )

Do you carry a lucky coin or a rabbit 's foot ? Do you knock on wood or avoid walking under a ladder? These are superstitions. They can be traced to primitive peoples who did not under­ stand the laws of nature -t he lightning, rain, thunder, darkness and light. They believedt hat good and bad spirits caused things to happen, so they invented charms to make the evil spirits happy. They were afraid of what they didn' t understand and trusted in magic. Hundreds of years ago, people believed that living things could arise out of non-living things. They believedt hat mice were born from t he mud of t he N ile R iver, that worms and toads came from rain, and that if soiled rags were mixed with wheat grains, young rats would develop. These beliefs last ed u ntil about 1650, when an Italian physician, Francesco Redi, showed that flies and maggots do not come from rott ing meat, as people believed. H is work caused people t o question similar ideas. Superstitions have a strong hold on people' s imaginat ions, but they are not based on fact; t herefore, they are not scientifically valid.

The Metric System of Measurement. The met ric syst em of measurement used by scien­ tists is based on mult iples oft en. There aret wo main advantages t o t he metric system : ( 1 ) Since i t i s based o n mult iples of t en, you can easily convert to a larger or smaller unit by simply moving a decimal point . (2) The metric system is underst ood by all scientist s and is used as the syst em of measurement in most of the countries of the world.

Basic Units of the Metric System Weight - gram ( g ) Volume - liter ( L ) L ength - meter ( m )

Some Important Prefixes

centi

=

milli

=

To test hypotheses, a scientist must be able

centimeter (cm) = 1 / 1 00 of a meter

1/100

1 milliliter (m] ) = 1/1000 of a liter

1/1000

kilo = 1000

kilogram (kg ) 1000 grams

Some Important Conversion Factors 1 meter = 39.37 in . liter = 1.06 g ts. 1 kilogram = 2.2 lbs.

ml. 1 in. lb.

= = =

1 cc. 2 . 54 cm. 454 g.

Temperature Measurement FO

SCI ENTI FIC EQUIPM ENT AND M EASU REM ENT

Example

Prefix

FO

Co=

= =

( CQ X 1 . 8 ) + 32° or 9/5 Co + 32°

FO - 3 2°

_ ___

1.8

or CD

=

5/9 (FO - 32°)

3

COMMON SCIENTIFIC MEASURING INSTRUMENTS

1" ''''''''' I'':' :'':'''' l':' ' 'r'':''i

Measuring length

L

I! I

J

I

\JCC:::>I, " !

!,

0

J

Measuring diameter

Micrometer

I

I

I

!

I

!

I Measuring volume

Vernier caliper.

Measuring angles Measuring air pressure Graduated c yli nder Protractor

Bure.fte

Pipette

Measuring weight. by comparing weights of known and unknown masses

Measuring weight (Mass)

A neroid barometer

Sca re Mercury barometer

Balance

Measuring temperature Measuring nuclear radiation

Measuring electricity

Galvanometer

4

Thermometer

Geiger counter

OTHER COMMON SCIENTIFIC EQUIPMENT

� � 0 �

j

Test

Reagent bottle

tube

. Bunsen burner

Ring stand

Funnel

�

Evaporating dish

Test tube clamp

Gas collecting bottle

Tongs

u

Spatula

Tripod

Crucible

�

Watch glass

Beaker

Erlenm eyer Thistle flask tube

Florence flask

Asbestos screen Forceps

Condenser

Triangle

Mortar and pestle

'N:::;'::;'::;'::;'::;';:";:';;:';:;;"::::::::: ;:::::::::::::::::::::::::::s:· �' """""""" :.� ... ........... . .... . . .

Test tube brush

Bar magnet

Pneumatic trough

Slide rule

Medicine dropper

5

S ELF-DI SCOVERY ACTIVITY

2. Were the maggots visible in or on the jar covered with cheesecloth? ..............

.

Redi's Experiment

Let's try to verify an experiment carried out in 1668 by Redi. His hypothesis was that mag­ gots and flies do not originate from rotting meat.

3. Were the maggots visible in or on the jar covered with plastic? ...................

Materials: Three pieces of raw meat, three jars, a piece of cheesecloth, a piece of plastic, string. Conclusions:

1. What attracted the flies to the jars?

2. Why were there no maggots in the jar covered with cheesecloth? ...............

Procedure:

1. Place a piece of meat in each of the three jars.

2. Leave one jar uncovered. Tightly cover

3. Did new flies come from the raw meat?

the second jar with cheesecloth, and cover the third jar tightly with the plastic.

3. Set all t.hree jars in the open where flies can be attracted by the odor of the meat. After several days inspect the jars closely.

Observations:

1. Were the maggots visible in or on the uncovered jar? ........................

6

4. Was the belief that living things can come from non-living matter proved unscientific? ................................

.

REVIEW TESTS Completion Questions For each of the statements or questions below, write the word or phrase in the space provided that best answers the question or completes the statement. 1. Scientists have a strong

about their environment and seek to determine the

. . .. . . . . .. , answers to questions. 2. What is meant by the term "scientific method?"

3. Who can use a scientific method to obtain answers to questions? . . . .. . . . .. . ... . . .... . ...... .

4. In establishing a scientific experiment, the first step is to . . . . ... .. ... ..... .. ... . . .. . ...

.

5. Why do scientists spend so much time in the library doing research? . . ... ... . .. .. .. .. .. . .

6. What is the meaning of the term "hypothesis?" . . . . . . . . . . . . .. . .. ... .. . .. . .. . ... .. ... . .

.

7. Before carrying out an investigation, it is first essential to gather together all the . . ... .... . .. ... necessary to conduct the experimental procedures.

8. Why is it very important to keep accurate, up-to-date notes of all procedures and observations involved in a scientific experiment? . . . . . .. . . . . . . . . .. . . . . . . . .. . . . . . .. . . . . .. . .. . .. . .. .

9. What is a controlled experiment?

10.

A

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. .

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different factor introduced into an experiment is called a(an) .. .. . . .. . .. .. . .. . . . .. . . . .

11. The part of an experiment in which no new factor is introduced is called the NAME

______

CLASS,

--.JO.JDATE,

__

_______

7

12. A student set up an experiment to determine the effect of a new drug on monkeys. He kept two

monkeys under identical environmental conditions. He injected one monkey with the drug and kept accurate records for one week. Both animals were well after the experiment was finished. The student concluded that the drug was harmless to monkeys and could be safely administered to humans. List five errors in this experiment. (a)

(b)

(c)

(d)

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( e)

13. Careful collection, . . . . . . . . . . . . . . . . . . and presentation of observations are essential in an

experiment. 14. Other than for the information of their colleagues, why should scientists make their findings .

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15. The use of scientific equipment extends man's . . . . . . . . . and increases his . . . . . . . . . . . . . . .

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16. Why is the metric system of measurement used to such a large extent in science? . . . . .. . . .

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available to other authorities?

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17. The temperature of a solution was recorded as 200 C.

(a) Write a formula for converting Centigrade to Fahrenheit.

(b) 200 C =

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18. Body temperature is about 98.6° F.

( a ) Write a formula for converting Fahrenheit to Centigrade.

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( b ) 98.6° F =

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, C.

19. Define the following terms: .

.

( b ) Theory: .... . ... . ... . ... .. . .. ... .... .. . . .. . . .. ..... .. . . .. ... .. . .. . ... .. . .. .

.

(c) Law:

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( a ) Fact:

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. .. . .. . ... . . . .. . . . .. ... .. . ... .. ... .. . . .. . .. . . . .. . . . . .. . . ... .. . . . . . . . .

( a ) Identify the following scientific equipment: A

c

B

G

H

� . ,-

J

A:

... . . . . . ... . . ... ... .

.

D: ... ... . ... ... . ... . .. .

G: . . . . . . . . . . . . . . . . . . . . J:

. . . . . . . . . . ..........

.

=e{

B: .. . . .. . . . . . .. . . .. . .. .

E: . . . . . . . . . . .. . . . . . .. .

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C:

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F:

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I:

H: . . . . . . . . . . ... . . . . . . . .

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(b) Name the scientific instruments used to measure the following: ( 1 ) 100 em: ... . ... ................

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( 2 ) Weight, by comparing unknown and known weights: ...... ..... . .... ......... . . (3) Weight: . . . . . .. .. . . . .. .. . .... . . . . .. .. . . . . . .... .. . .. . .. .... .. . . . ..... . .. .

.

( 4 ) Air pressure: ... .... . ................ ... .... ........... .... .. ......... ...

.

( 5 ) Diameter: .... ... ...... ...... . . .. ........ ...... .... .. . . .. . . . ... ... ... ...

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( 6 ) Volume:

.

. . . . . . . . .. . . . .. .. .. . . .. . . . . . . . . .. . .. . . . . .. . .. . .. . . . .... . .. . . . . .

( 7 ) Angles: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

( 8 ) Nuclear radiation: ..... . ... . . . . .. .. .. . .. . .. .. . .... . .. ... .. . . . .. . .. .. . .. . . .

NAME

___ ____

CLASS,

�DATE,

__

____ _ _ __

.

9

Multiple-Choice Questions In each of the following questions, circle the letter preceding the word or phrase that best completes the statement or answers the question.

1. The basic unit of weight measurement in the metric system is the ( a ) gram, (b) kilogram, (c) centigram, (d) liter. Z. A centimeter is equal to ( a ) 1/1OOth of a meter, (b) 1/100Oth of a meter, (c) 100 meters, (d) 1000 meters. 3. Which of the following scientific instruments is used to measure air pressure? ( a ) micrometer, (b) vernier caliper, (c) burette, (d) aneroid barometer. 4. A protractor is an instrument used to measure ( a ) volume, (b) mass, (c) angles, (d ) diam­ eter. 5. The diagram below depicts a piece of scientific equipment known as a( an) ( a ) Erlenmeyer flask, (b) Florence flask, (c) beaker, (d) reagent bottle.

6. A logical guess which is sometimes used to solve a scientific problem is called (a) the scientific method, ( b ) a hypothesis, (c) a theory, (d) a law. 7. An advantage of the metric system is that ( a ) it is used in other countries such as Great Britain, ( b ) it is easier to measure with metric tools, (c) it is easier to convert to larger and smaller multiples in this system, (d) it is based on mUltiples of 5. 8. It is important in an experiment to use a sufficient number of experimental organisms to ( a ) eliminate variables that could change the results, ( b ) gain practice in the scientific method, (c) eliminate the need for a control, (d) eliminate the need to repeat the experiment. 9. How many grams in the metric system equal one pound in the English system? ( a ) 545, (b) 454, (c) 554, (d) 445. 10. A superstition is ( a ) a belief that Friday the l3th is a lucky day, (b) an unscientific conclusion, (c) a consistent observation for which there is no reasonable explanation, (d) a logical belief in the supernatural.

Matching Questi ons In the space at the left of each it,em Column B that is most closely related to that item.

Column A 1. Second step of scientific method. Z.

Only one difference.

3. Metric system. 4. One liter. S. One cc.

6. Galvanometer 7. 454 grams. 8. 1000 grams. 9. Conclusion.

Column B a. Controlled experiment b. 1000 cc c. One kilogram d. Measures electric current e. Multiples of ten f. Equal to one milliliter g. One pound h. Gather evidence i. Based on facts and evidence j. Procedure k. Variable T. Hypothesis

10. Possible solution. 10

NAME

_____ _______

CLASS

DATE

___

�

__

-----

Chapter 2

P H YSICS AN D T H E M ET R I C SYST E M The physical world, f rom the incredible sm allness of atomic structure to the extreme vastness of outer spac e, is composed of matter and energy.

Physics is the science that deals with matter and energy and their interrelationship. Matter is anything that occupies space and has mass. Mass is the quantity of matter contained in an object. Energy is the capacity or ability to do work; work - the movement of a f orce through a distance - cannot be accomplished without the exertion of energy. This energy is needed to move the f orce through a given dis­ tance. Every day p hysicists learn more about the makeup of our physical world. To organize this knowledge, physicists have devised a set of "laws" related to them. As more inf ormation is gained, these laws are revised or changed. For example, an early law of physics, stated by Aristotle, was that "the rate of f all of an ob­ ject depends upon its weight." Can you detect an error in this law? (You will learn more about it in Chapter 7 . ) We h ave accumulated so much knowledge in science that it is brok en down into many smaller areas. Each of the sciences deals with a special part of nature. Each science contributes knowledge to the others. When you study physics, you are also dealing with other sci­ ences. The th ree main branches of science are ( 1 ) physics, which deals with matter and en­ ergy, (2) chemistry, which deals with changes in matter, and (3) biology, the study of living things. The science of physics is f urther sub­ divided int o such specialized areas as biophys­ ics, geophysics, astrophysics and nuclear phys­ ics. Find out what special aspects of science physicists in these specialized areas investigate. Much scientific work is done as pure or basic science, in which the scientist searches

f or answers to questions without special inter­ est in immediate usef ul applications. Albert Einstein was a brilliant pure physi­ cist. He used mathematics to establish the f or­ mula which defines the relationship between matter and energy - E = mc2• E = energy ; m = mass; c2 = speed of light squared. Ein­ stein helped lay thef oundationf or the develop­ ment of atomic power. It is the work of the applied scientist to de­ velop usef ul applications f or the discoveries of basic research. These applications have led to the development of many new industries. These industries increase man's knowledge, improve his ability to control his environment, and add pleasure to his lif e. Examples of industries based upon the prin­ ciples of applied physics are :

Industry

Function

Machine

Design and construction of machinery

Electronics

Design and construction electronic equipment of such as radio, TV, radar, etc.

Construction

Design and construction of homes, buildings, roads, bridges, and dams

Space and Aeronantics

of

Design and construction airplanes and rockets

Metallurgy

Extraction, purification, testing and application of metals

Optics

Design and construction of instruments using lenses

11

The study of physics is usually divided into specific areas based upon the types of energy studied, such as : ( 1) mechanics, (2) electric­ ity, (3 ) light, (4) sound, (5) heat, and (6 ) nuclear energy. 1 . The Metric System. Since the science of physics uses mathematics to obtain answers to problems involving natural phenomena, it is important that you become f amiliar with the systems of measurement. There are two differ­ ent systems of measurement now used through­ out the world - the English system and the metric system. The English system did not originate in a logical, systematicf ashion; how­ ever, it is used f or everyday measurements in practically all English- speaking countries. In the English system, units of length were based upon the length of an arm or a f oot, the siz e of which, of course, varied f rom person to person. Thus, there were no constant stand­ ards. The metric system, whose origin is scientific, is used in most non-English-speaking countries and is the universal mathematical language of science. Computation in this system is simpler than in the Engl ish system . The metri c system is based upon multiples of ten, making it easy to convert f rom one unit to another. Multipli­ cation and division are done by moving the decimal point of a number. When mU ltiplying by ten or mUltiples of ten ( 100, 1000, 10,000, etc. ) the decimal point is moved to the right a number of spaces equal to the number of zeros in the multiplier. For example, if you are mul­ tiplying by 100, move the decimal point two places to the right, and so on. Examples:

( a) 45.8 X 10 = 458 ( b) 45.8 X 100 = 4580 ( c) 45.8 X 1000 = 45,800 If you are dividing a number by 10 or mul­ tiples of 10, move the decimal point to the lef t a number of spaces equal to the number of zeros in the divisor. If you are dividing by 1000, f or example, move the decimal point three places to the lef t. 12

Examples:

(a) 45.8 ---:- 10 = 4.58 (b) 45.8 ---:- 100 = .458 ( c) 45.8 ---:- 1000 = .0458 A knowledge of important prefixes will help you understand metric terms. I M PORTANT PREFIXES

Meaning

Prefix kilo (k)

1000

hecto (h)

100

deka (dk)

10

deci (d)

.1 (1/10)

centi (c)

.01 ( 1/100)

milli (m)

.001 0/1000)

Measuring in the Metric System. There are three basic quantities f or which units of meas­ urement are required : length (distance), weight (mass) and volume. These quantities are known as the fundamental units of measure­ ment because all other units are either based on them or are combinations of them. 1 . Measuring Length in the Metric System. The meter (m) is the basic unit of length in the metric system. A meter is slightly larger than one yar d and is equal to 39. 37 inches. COMMON M ETRIC UNITS OF LENGTH

kilometer (km)

=

1000 m

decameter (dkm) = 10m decimeter (dm)

= 1m

centimeter (em)

=

.

.01 m

millimeter (mm) = .001 m 2. Measuring Area in the Metric System. Area in the metric system is measured in the same manner as the English system except that metric units are used. Area is always recorded in square units.

Example:

What is the area of a rectangle 6 cm long and 5 cm wide? Area = length X width = 6 cm X 5 cm Area = 30 cm2 ( square centimeters )

SELF-DISCOVERY

ACTIVITY

Measuring Length in the Metric System.

To become familiar with metric units and the techniques of using this system of measure­ ment. Materials: Meter sti ck, graph paper, stiff paper.

Procedure: 1. Exami ne a meter stick. Note that it is divided into 100 units. Each unit, such as 1, 2, 3 and 4, represents the number of centi­ meters. Note that the meter stick is also di­ vided into groups of 10, f rom 0 to 100. You can easily see heavily printed numbers such as 10, 20, 30, etc. T he length f rom 10 to 20 represents one decimeter. T he number 30 rep­ resents 30 centimeters. T he smallest subdivi­ sion on the meter stick is the unit called a millimeter; 1000 of these tiny units are f ound on the meter stick.

2. When usi ng a meter stick you shoul d place it on its edge and avoid using the ex­ treme lef t and right edges of the scale. T hese may be ragged f rom excessive use.

dinary graph paper usually has squares 2 mm w ide and 2 mm long. Mark off the divisions in centimeters and millimeters. In order to mark the millimeters you will have to draw a line caref ully through the center of each small square, bisecting it vertically. The centimeter lines should be 6 mm high ; the half -centimeter lines should be 4 mm high; and th e millimeter lines should be 2 mm high. Cut out the com­ pleted metric ruler and paste it on a piece of stiff paper. Cut away the excess paper. 4. U se a meter stick or a metric ruler to perf orm the procedures that f ol low. Place all your answers in the accompanying chart. ( a) Draw a line 35 mm long. T his is the width of the film used in many cameras. (b) Draw a line 2 inches long and measure it to the nearest tenth of a centimeter. ( c ) Determine the number of centimeters in one inch by dividing the number of centimeters in the two-inch line by 2. ( d) Measure the width of a piece of note­ book paper to the nearest tenth of a centimeter at three different locations. Det ermine the average results to the nearest tenth of a centimeter. ( e) Measure the thickness of a dime to the nearest millimeter.

DATA CHART

( a)

3 5 mm

(b)

2"

(c) (d)

)'''''I''''''''''''''''''''''''I:::?''''''''''''� 30

1

2

3

( e)

4

3. Construct a ruler 15 cm long and 2V2 cm wide by using a sheet of graph paper. Or-

3. Measuring Volume in the Metric System. T he liter (L) is the basic unit of volume i n the metric system. A liter is slightly larger than an English quart. T he liter is equal to 1.06 quarts. 13

COMMON METRIC UNITS OF VOLUM E

kiloliter (kl)

=

dekaliter (did)

= 1 0L

deciliter (dl)

=. 1 L

centiliter (cl)

=

. 0 1L

milliliter (ml)

=

. 00 1 L

1 000L

MENISCUS -

Reading a graduated cylinder.

SELF-DISCOVERY ACTIVITY

cubic centimeter (cc) = .00 1 L Measuring the volume of a solid in the met­ ric system is the same as the method used in the English system . Volume is always recorded in cubic units. Example:

Measuring Volume in the Metric System.

Materials: Water, graduated cylinder, thread, object whose volume is to be determined.

What is the volume of a box 60 cm long, 30 e m wide and 1 0 cm high? Volume = length X width X height = 60 cm X 30 cm X 1 0 cm Volume = 1 8,000 cm3 or = 18,000 cc ( cubic centimeters )

Procedure and Conclusions:

The volume of a liquid is also measured in cubic units. A graduated cylinder is of ten used to obtain accurate volumetric measurements. The surf ace of the liquid is curved slightly since the liquid either f alls or rises where it touches the wall of the container. This curved surf ace is called the meniscus. Always take the reading at eye level, reading the top of the meniscus if the surf ace curves upward and the bottom of the meniscus if the surf ace curves downward.

measure its volume . . . . . . . . . . . . . . . . . . mI.

G R A M 5

�eC/ci/n.j:

Determine the volume of any irregular ob­ ject that will fit into a 1 00 ml graduated cylin­ der by perf orming the f ollowing proced ures: 1. Pour some water into the cylinder and

2. Tie a piece of thread around the object and lower it into the cylinder. 3. Measure the new total volume. . . . . . . ml. 4. Determine the volume of the irregular object by subtracting the initial total volume f rom the final total volume. Volume of the ir-

regular object . . . . . . . . .. . . . . ml.

G R A M 5

T56.8qrams

The scales of a metric balance. 14

4. Measuring Weight in the Metric System.

The gram (g) is the basic unit of weight ( mass) in the metric system. One pound is equal to approximately 454 grams.

SELF-DISCOVERY ACTIVITY Weighing in the Metric System.

Materials: A penny, pencil, 1 50 ml beaker, water, balance.

COMMON M ETRIC UNITS OF WEIGHT

kilogram or kilo (kg)= 1 000 g

Procedure:

dekagram (dkg)

Following the weighing procedure outlined on pages 1 3 , 1 4, weigh each of the obj ects listed in the Data Chart below, using the balance. Record the weight of each in the space provided.

=

10 g

decigram (dg)

= .1 g

centigram (cg)

= .01 g

milligram (mg)

=

.00 1 g

DATA CHART

The balance is a scientific instrument which can be used to determine the metric weight of an object. The accompanying diagram shows the scales of a typical metric balance. The upper scale is divided in to decagrams; the center scale is divided into 1 00-gram sections ; and the lower scale into grams and decigrams. B y moving the sliding weights attached to eac h sc ale, we are able to balance an object that has been placed in the weighing pan. When the object has been balanced, we can determine its weight by totaling the number of grams indicated by the positions of the slides on eac h scale. In the diagram, for exam­ ple, there is a reading of 50 grams on the up­ per sc ale, 1 00 grams on the center scale and 6 . 8 grams on the lower scale. Together they total 1 5 6 . 8 grams and indic ate the wej ght of the object being weighed.

Weight

Object penny penc il 1 50 ml b eaker 1 50 ml beaker

+ 1 00 ml water . . . . . . . . . . . . . . . . . . . . . .

.

1 00 ml water 1 ml water 1 L water 5. Conversion Factors. Many times it is necessary to co nvert from the metric system to the English system and from the English sys­ tem to the metric system. In order to do this, the following conversion factors are used:

CONVERSION FACTORS

Length 1 m

=

39.37 in

2 .54 cm= 1 in 1 km

= .62 mi

Volume 1 L = 1 .06 qts 1 L= 1 000 ml or 1 000 cc 1 ml= 1 cc

Weight 1 kg= 2.2 lbs 454 g =1 lb 28.3 g

= 1 oz

15

C O M M O N METRIC UN ITS

i,""'I� 0 I � . ' ' II il ij i � � 1 il�LIJ J Length

Volume - solids

1

1 cm.

.

�

\ � d ....

C> ...

The equilibrant is equal but opposite to the resultant.

are called the components of the original force. For example, when a boy pulls a wagon he exerts a force on the handle. The two com­ ponents of this single force are ( a) a vertical component which counteracts gravity, and ( b ) a horizontal component that actually performs the useful work of pulling the wagon forward. Finding the components of a single force is called the resolution of a force. F A

=

Horizontal component which

B

=

Vertical component which

=

force. Scale 1

. . .

. .

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.

Scale

2........

.

3. What is the magnitude of the resultant? 4. What would be the magnitude and the direction of the equilibrant force needed to

prevent motion? . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5. Determine the magnitude of the result­ ant by applying two forces at right angles to each other. - ATTACH TO HOOK IN WALL

Single applied force on handle. pulls the wagon forward.

FORCE 1

counteracts gravity.

Concurrent forces acting at right angles to each other.

1. ' " 2. . . . . . . . . . . . . .

6. Record the magnitude of force ·

SELF-D I SCOVERY ACTIVITY

Investigating Resultant and Equilibrant forces. 24

. . .

.

. . . . .

. .

. . Force

7. The resultant is equal to a force of "

.

R EVI EW T ESTS Completion Questions

For each of the statements or questions below, write the word or phrase in the space best answers the question or completes the statement.

provided that

1. What is the meaning of each of the following terms? .

( a) Force :

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( b ) Weight :

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( c ) Mass : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. As an object is moved from the equator to the north pole its weight will . . . . . . . . . . . . . . . . . . . 3. What does the Law of Gravitation state? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

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4. The force of one object upon another is directly proportional to the product of their S. Describe three ways in which the force of friction is useful.

(a) (b)

(c) 6. Attempting to stop a car o n an icy pavement i s difficult because the force o f . . . . . . . . . . . . . is greatly reduced. 7. The efficiency of a machine is greatly reduced by friction which wears its parts and converts much of its energy into undesirable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.

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. . . force tends to draw an orbiting satellite back toward the center of the earth.

9. If it were not for . . . . . . . . . . . . . . . . force an orbiting satellite would be flung into outer space

by . . . . . . . . . . . . . . . . force. 10. The apparent force acting upon a person in a car traveling around a curve is sometimes called . . . . force.

NAME

_____ _____

CLASS.

DATE

___

_ _ _ _ _ _ _ _

25

11. List three devices which make use of a gyroscope. (a) (b) ( c)

12. What is the meaning of each of the following terms? (a) Cohesion: ( b ) Adhesion : (c) Surface tension :

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13. Surface tension is caused by a . . . . . . . . . . . . . . . . . . . . attraction of surface molecules by strong cohesive forces. 14. A meniscus forming an upward curvature in a liquid is caused by . . . . . . . . . . . forces. 15. Three parallel forces are acting simultaneously at the same point on an object. One force is pulling to the left with a force of 25 lbs. and the other two are pulling to the right with forces of 10 lbs. and 5 lbs. respectively. (a) Graphically represent these forces by using the following scale. Scale :

1 inch 10 lbs.

( b ) The magnitude and the direction of the resultant of the applied forces is . . . . . . . . . . . . . .

.

16. If the difference between all forces acting at a point is zero, the forces are in . . . . . . . . . . . . . . . . . . .

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17. The

. . . . . . . . . . . . . . . . is a single force which can balance two opposing forces to prevent

motion. 18. A boy pushes a lawnmower with a force of 25 lbs. at an angle of 30° from the ground. In the space below, draw a vector diagram to determine the force pushing the lawn mower into the ground. Use a scale of 1 inch = 10 lbs. (a) Label the length and force of the horizontal and vertical components, as well as their resultant. ( b ) Label the component providing the for­ ward push and the component providing the downward push. 19. As the angle between two concurrent forces increases, the magnitude of the resultant . . . .

26

NAME ______ CLASS

D � ATE

__ __ __

__ __ __ __ __ __ __ _

20. As a result of a series of experiments involving friction, the following data regarding the applied forces were collected. Use the graph paper to construct a bar graph using this data and your knowledge of frictional forces. There is no specific order to the data listed. Applied forces to overcome the force of friction : 250 grams, 1 60 grams, 230 grams, 1 75 grams, 1 40 grams. OVERCOMING FRICTION

260 250 240 230 220 210

KEY A

=

Starting friction, rough surface

B

=

Starting friction, smooth surface

C

=

Sliding friction, smooth surface

D = Sliding friction, rough surface

+ rollers

E

=

en :::E c

... ...

200

:!: 1 90

... c.:o ... co .... CI ... :::;

aac

Starting friction, rough surface + added weight.

1 80 1 70 1 60 1 50 1 40 1 30 1 20 110 1 00 A

B

C

E

0

Mu ltiple-Choice Questions

In each of the following questions, circle the letter preceding the word or phrase that best completes the statement or answers the question. 1. Which of the following represents a scalar quantity? (d) water pressure.

(a) friction,

( b ) weight,

( c ) length,

2. Roads are banked to prevent vehicles from leaving the road because of (a) gravity, ( b ) friction, (c) centripetal force, (d) inertial force. 3. The magnitude of the resultant of a force pulling on an object toward the west with a force of 20 Ibs. and another force pulling the object east with a force of 60 lbs. is ( a ) 80 lbs. east, ( b ) 40 lbs. west, (c) 40 lbs. east, (d) 80 lbs. west. NAME

______

CLASS,

-L..t DATE

__

_ _ _ _ _ _ _

27

4. As a rocket is hurled into space, which of the following properties will remain constant? (a) mass, (b) weight, ( c ) velocity, (d) temperature. S. The attractive force between similar molecules is an example of

(a) adhesion,

(b) cohesion,

( c ) capillary action, (d) surface tension.

6. An arrow used to illustrate the size and direction of a force is called a ( an ) ( b ) vector, ( c ) equilibrant, (d) component.

(a) resultant,

7. The resultant of two parallel forces of 50 lbs. each acting in opposite directions on a 75 lb. box lying on a fiat surface, would be ( a ) 50 1bs., ( b ) 75 Ibs. , ( c ) 1 25 lbs. , (d) O lbs. 8. A man weighed 1 7 1 lbs. on the surface of the earth. If his distance from the center of the earth were tripled, he would weigh ( a ) 1 7 1 Ibs . , ( b ) 90 Ibs ., (c) 1 9 Ibs ., (d) 43 1bs. 9. Soil water reaches the stem and leaves of a plant with the help of (a) gravity, ( b ) capillary action, ( c ) centrifugal force, (d) surface tension. 10. The force responsible for surface tension is (d) adhesion.

( a ) cohesion,

( b ) gravity,

( c ) water pressure,

Matching Q uestions In the space at the left of each item in Column A, place the letter of the term or expression in Column B that is most closely related to that item.

Column B

Column A a. b. c. d. e. f.

1. Law of Gravitation 2. Point at which all of the weight of a mass seems to be concentrated 3. Causes the destruction of meteors

Gyroscope Atmospheric friction Centripetal force Weightless object Sir Isaac Newton

vF� + F� g. Inertial force

4. Keeps an object moving in a straight line S. Gravitational force = Inertial force

h. Center of gravity Has magnitude and direction j. Gluing objects together k. Concurrent forces I. Cohesive forces m. Scalar quantity i.

6. Object that resists a change in direction 7. Force of a growing root 8. Two or more forces acting on an object at the same time 9. Adhesive forces 10. Attractive forces between iron molecules

28

NAM E_______ CLASS

DATE

____

_ _ _ _ _ _ __

Chapter 4

FORC ES A N D WO R K E N ERGY A N D WORK

What Is Energy? Energy is defined as the ability to do work. Without energy, work cannot be accomplished. Thus, when we say that something has energy, we mean that it is capable of exerting a force on something else and performing work on it. Energy is present in all matter and can be changed from one form into another - but it cannot be destroyed. Until recently, physicists thought that energy also could not be created. How­ ever, in a thermonuclear reaction, such as a hydrogen bomb, energy appears to be created directly from matter. For this reason, most physicists now support the idea that matter and energy are different forms of the same thing. Energy may take on many forms , some of which are mechanical, electrical, chemical, heat, sound, light and nuclear energy. Potential Energy. This kind of energy is in­ active, but is ready to be used. The energy stored in an object or the energy an object possesses because of its position or condition is known as potential energy. For examples of potential energy, just picture a coiled spring, a stick of dynamite, a fully charged, unused battery, water behind a dam, a rock sitting on top of a cliff and the tremendous energy locked up inside an atom. Kinetic Energy. The energy an object pos­ sesses because of its motion is called kinetic energy. The word kinetic means "moving." Kinetic energy i s a form o f energy i n motion; it is active. Let's go back to the examples of potential energy. The coiled spring unwinds, the dyna­ mite explodes, the water behind the dam is released, the rock topples off the cliff, and the atom is split apart. In each of these

examples the object is active - it is now moving. Imagine a rocket standing on its launch pad ready for blast off. Its huge cylinders are filled with fuel which possesses potential en­ ergy. With a tremendous roar, the rocket lifts off the pad and hurtles into outer space. The rocket no longer possesses the potential en­ ergy of its fuel ; it now has kinetic energy due to its motion.

What Is Work? You would probably answer this question in several ways. Mowing the lawn, babysitting, carrying things, digging, pulling weeds, and homework assignments may all be defined as work in the usual sense. How­ ever, scientists have a scientific definition for the term work just as they have for force and energy. Scientists define work as the result of the exertion of a force through a certain dis­ tance to overcome a resistance. Summarizing, we can say that scientifically, in order to do work, ( 1 ) a force must be applied to an object, ( 2 ) a resistance must be overcome, and (3) the applied force must move the object a cer-' tain distance. In the case of the rocket, for example, the chemical energy of the burning fuel produced a force which could overcome the resistance of gravity and lift the rocket off the ground. Four major types of resistance are met in doing work : ( 1 ) gravity, in lifting objects, ( 2 ) friction, in sliding one object over an­ other, ( 3 ) molecular attraction, in cutting, bending or heating objects, and ( 4 ) inertia. Friction is defined as the resistance of one body to the movement of another body along its surface. Inertia is the tendency of an object to re­ main at rest or in motion due to its mass. The greater the mass of an object, the greater its inertia. 29

Inertia must be overcome, for example, in first putting an object in motion or stopping it once it is in motion. The property of inertia explains why the force needed to overcome starting friction is more than sliding friction. It also explains why a person on a bus lurches forward after the bus has stopped. The amount of work done can be found by multiplying the force (F) by the distance (D) it moves an object: w

F

Work

D

X

Force

Distance

The distance moved must be in the direc­ tion of the applied force. Work is usually measured in foot-pounds (ft.-lb. ) when F is expressed in pounds and D in feet. A foot­ pound is the work done by a one-pound force moving an object a distance of one foot.

Since the force applied in lifting the box was upward, opposing the force of gravity, work was done. On the other hand, the force exerted in carrying the box, after it had been lifted, was not in the direction of the applied force but perpendicular to it so that no work was done on the box.

What Is Power? Power is a measure of the rate at which work is done. In determining power, two factors must be taken into con­ sideration : ( 1 ) work, and ( 2 ) time. Suppose a trench can be dug by one man in 1 0 days, or by two men in 5 days, or by a trench-digging machine in one day. In all three cases, the amount of work done is the same. However, the power used in each of the three instances is quite different. What was the power of the machine as compared to that of one man? . . . . . . . . . . . . . . . . . . . . . . . . . . .

Example:

.

If you lift a box weighing 60 pounds through a distance of 5 feet, how much work do you do? Power can be expressed mathematically according to the following formula :

W= F X D = 60 lb. X 5 ft. 300 ft.-lb. =

After lifting the box, you carry the box a distance of 1 5 feet, and place it on a truck. How much work was done on the box? You may be surprised to learn that the answer is zero! Reread the definition of work on page 29 carefully. Do you see that work is accomplished only if the distance the object moves is in the direction of the applied force?

.?Dr"I",:',:." 'J

5 ft.

t

"

:zJ1®i. �

:-:.;.:�-::

"Iff};

� - -----------------------� � Walking with the box. Lifting the box.

W= F X D = 60 lb. X 5 ft. 300 ft.-lb. =

30

I.

W=F = 60

XD X 0

lb. Distance not in direc­ tion of applied force.

Power

=

Work Time

=

Force

X

Distance

Time

=

FXD T Power is expressed in foot-pounds per sec­ ond (ft.-lb. jsec. ) and foot-pounds per minute (ft.-lb. jmin. ) . Example:

in

If a man performed 1 0,000 ft.-lb. of work 5 minutes, how much power did he produce?

P

=

1 0,000 ft.-lb. = 2 , 000 5 min.

ft .-lb. /mm. .

Horsepower. The power developed by rna· chines is usually measured in units called horsepower (HP). In order to produce 1 HP, 33,000 ft.-lb. of work must be done in 1 minute, or 550 ft.-lb. in 1 second. The fol­ lowing formulas are used to calculate the horsepower rating of an engine or machine.

( a ) HP = F

W

_ _ _ __ _

T ( min. ) X

33,000

T ( min. ) X

( b ) HP = F

33,000

D

X

W

_ _ _ _

T ( sec. ) X

550

X D

T ( sec. ) X

550

Example: A mass weighing 660 pounds was lifted a distance of 600 feet in 3 minutes. How much horsepower was developed by the machine which did this job?

( a ) HP =

660 lb. X 600 ft. 3 min. X 3 3,000 396,000 99,000

= 4 HP or ( b ) HP

=

660 lb. X 600 ft. 1 80 sec. X 550 396,000 99,000

= 4 HP

cannot produce more energy or work than what is put into it. The Law of Machines states that under ideal conditions, the work output of any ma­ chine must theoretically equal the work input. A machine, therefore, can never multiply work. Not only is it impossible for a machine to multiply work, it is also impossible for the work output to equal the work input. The Law of Machines is theoretical because it does not take into account the force of friction which is always present" In fact, the efficiency of a machine is always less than 1 00 % , indicating that the amount of work input is always greater than the work output. Efficiency, therefore, is the ratio of the output of useful work to the total work input ex­ pressed as a per cent. Efficiency in %

=

Work Output Work Input

X

1 00 %

Example: What is the efficiency of a machine in which in and

2500 ft.-lb. of work has been put 500 ft.-lb. of work put out? . 500 ft.-lb. Efficlency = X 1 00% 2500 ft.-lb. 0

Efficiency

=

20 %

Machines. A machine enables us to do work more easily or more quickly. Using a ma­ chine, we need to apply only a small force to overcome a large resistance or reduce the time necessary to do work. Some machines can also change one form of energy into another. Electric motors, for example, transform electrical energy into me­ chanical energy. Generators, on the other hand, convert mechanical energy into electri­ cal energy. The chemical energy locked up in fuels such as gasoline or oil may be changed into mechanical or heat energy by automotive engines.

Machines are either simple or compound, depending upon how complex they are. There are six simple machines having only one or few parts. These simple machines include the ( l ) lever, ( 2 ) inclined plane, ( 3 ) wedge, ( 4 ) screw, ( 5 ) pulley, and the ( 6 ) wheel and axle. Compound machines are made up of two or more simple machines which together do a specific job.

The Law of Machines. According to the Law of Conservation of Energy, energy may be changed from one form to another, but it cannot be created or destroyed by usual means. Therefore, under normal conditions, a machine

The Lever. A lever is a rigid bar supported at a point around which the lever can turn. The lever was probably discovered by early man when he learned to use a long pole or bar to move a rock or log. The two parts of the

SIMPL E AND COMPOUND MAC H I NES

31

lever were easy to obtain, and probably made the first machine known to man. To understand better how a lever works sci­ entifically, we must first study its parts : ( 1 ) The fulcrum is the pivot point on which the lever bar is supported. This may be a rock, a brick, or a specially designed part. (2) The effort arm of the lever bar is the part on the side of the fulcrum where an effort or force is being applied. ( 3 ) The resistance arm of the lever bar is the part of the bar that resists the applied force. In doing work with a lever, force is ap­ plied . to the effort arm to overcome the resis­ tance at the end of the resistance arm so that there is movement. Force or effort is abbrevi­ ated ( E ) ; effort arm ( EA ) ; resistance arm (RA) ; resistance (R) ; and fulcrum ( F ) .

�

$

These factors are illustrated in the following diagram of the lever :

E "_-� i�

i �

RA �

m t

F

First-class lever used to increase force.

Since it multiplies the force put in, a first­ class lever allows us to exert a smaller force to overcome a resistance. However, to obtain this increased force, we must increase the distance through which the effort is applied.

E I'�

G D

�/

/�/�;� / F / 1"-?.f.

Applications of the first-class lever used to increase force.

Thus, in a first-class lever, the distance through which the effort must be applied is always larger than the distance through which the resistance is moved.

1. First-Class Levers. In the first-class lever the resistance is at one end, the effort at the other. The fulcrum is located somewhere be­ tween the effort and the resistance.

E ,,,,,(�-- EA tJf======= RA :;:::R :J = �= Ql == == ==ir= =;; == �l

I

-----

• F

----'��I

'f'

First-class lever.

A first-class lever always changes the direc­ tion of the applied force. It may also increase 32

Increasing force by sacrificing distance.

To increase distance and speed, using a first-class lever, the resistance arm must al­ ways be longer than the effort arm.

E

RA ----� �

I I I

t Barber's or tailor's shears used to increase distance or speed.

2. Second·Class Levers. In the second-class lever, the fulcrum and effort are on opposite sides. The resistance is located between the fulcrum and the effort. Since the effort arm is always longer than the resistance arm, a second-class lever can only be used to mUltiply

3. Third·Class Levers. In third-class levers, the fulcrum and resistance are at opposite ends of the lever. The effort is located somewhere between the fulcrum and the resistance. A shovel, your forearm . a fishing pole are ex­ amples of a third-class lever.

I":

1
- 1

I

� t

Third-class lever.

Since the resistance arm is always longer than the effort arm, a third-class lever always increases distance and speed at the exp�nse of force.

E

I���.--- EA ----l----�>�

I

�,,===I ="= = = = ======Til lifb = jJ! i

A .-L� =R=

i-F

:

I

GJ t

Second-class lever .

force but not speed. Some examples of second­ class levers are the wheelbarrow, nutcracker and paper cutter.

E

Examples of the second-class lever used to increase force.

Applications of the third-class lever used to gain speed and distance.

R

The Law of Moments. This law states that when an object is in equilibrium the sum of the counterclockwise moments is equal to the sum of the clockwise moments. The moment of a force is the product of the force and the perpendicular distance from its point of ap­ plication to the fulcrum. A moment produces rotation when the object is not in a state of equilibrium ( balance ) . When the sum of the counterclockwise moments equals the sum of the clockwise moments, the object is in a state of balance and no rotation occurs. The Law of Moments may be illustrated by the following problem of two boys on a seesaw : 33

Problem 1 :

�I


(�r(N> -. �

R/ t E

A C

··· A

C T

I �

��'I �ttS �

0

N

Effects of Newton's Third Law of Motion.

63

For example, when the air in an inflated balloon is suddenly released, the force of the moving air provides the action which propels the balloon forward. The reaction is the equal but opposite force of the escaping air pushing against the air molecules. Jet airplanes , rock­ ets, the recoil of a gun, and rotary lawn sprinklers are examples of machines based on Newton's Third Law of Motion.

Conservation of Momentum. When two ob­ jects are involved in an action-reaction situa­ tion and no other forces are present, the total momentum of these two objects remains un­ changed. For example, if an object exerts a force on a second object, the second body ex­ erts a force on the first object which is equal and opposite to the force exerted on it. This may be expressed mathematically : ml Vl = m2 V 2 In this equation m and v represent the mass and velocity of the two objects. If, for example, a 6-gram mass has a veloc­ ity of 2 cm. jsec. and it collides with a 4-gram mass, the velocity imparted to the 4-gram mass will be 3 cm./sec. and the resulting momen­ tums of the two objects will remain the same. m1 X V 1 = m2 X V i 6 g X 2 cm. /sec. = 4 g X V i 1 2 g-cm.jsec. = V 2 4g 3 cm. jsec. = V 2 The momentum of both masses immediately following the collision will be 1 2 g-cm./sec. The velocity imparted to the smaller mass ( 4 g ) was such as to make its momentum equal to the momentum of the larger mass

Materials: A textbook and a piece of notebook paper.

Procedure: 1. Stand the textbook across the extreme edge of the paper as seen in the illustration, and very slowly pull the paper out from under the book. 2. Moderately in=> 7 crease the force on / the paper. 3. Re-do the experiment but this time very quickly snap the paper from under the book. Observations: Record your observations for each of the procedures above.

1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Conclusions: Explain how Newton's Laws of Motion were involved in the above observations.

1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

( 6 g ) . All action-reaction situations thus in­ volve a transfer of momentums in such a way that the total momentum of the bodies re­ mains constant.

SELF-DISCOVERY ACTIVITY

Investigating an Example of Newton's Laws of Motion. 64

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R EVIEW TESTS Completion Questions

For each of the statements or questions below, write the word or phrase in the space provided that best answers the question or completes the statement. 1. Speed is a . . . . . . . . . . quantity since it has magnitude only. 2. An object traveling through space would travel in a . . . . . . . . . . . . . . . . . . . . . unless some . . . . . . . . . . . . . . . . . . . . . . . acted upon it. 3. If an object covers 1 0 feet every 5 seconds for 1 5 seconds, it would be undergoing

motion. 4. Why would a huge ship moving slowly have more momentum than a speeding bullet? . . . . . . .

5. Velocity is a vector quantity since it indicates . . . . . . . . . . . . as well as magnitude.

6. To avoid an accident, a man applies his brakes and decelerates from a speed of 50 miles per hour to 1 5 miles per hour in 7 seconds. What is his rate of deceleration? . . . . . . . . . . . . . . . . . . . 7. The laws of motion were developed by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. The pull of gravity causes all objects to fall with a constant acceleration of . . . . . . . . . . . . . .

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. . . . . . . . . . . . if air resistance is neglected. 9. An object dropped from the top of the Empire State Building falls for 9 seconds. How far will

it fall, neglecting air friction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

10. An object is moving at a velocity of 50 ft./sec. How far will it travel in 20 seconds? .

travel in 20 seconds? . . . . . . . . . . . . .

11. Why could a person be thrown against the windshield of an automobile after a collision, if seat .

belts were not used? .

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65

12. The product of the mass and velocity is called . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13. According to Newton's Second Law of Motion, why would it be incorrect to state that a rocket is propelled forward by the force of escaping gases? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14. According to Newton's Third Law of Motion, why doesn't a wall collapse when a small force is exerted upon it? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15. How does Newton's Third Law of Motion explain how a rocket can be accelerated in space where there is no atmosphere? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16. According to Newton's Third Law of Motion, forces always exist in . . . . . . . . . . . . . . . . . . . . . . 17. If a 25 g mass having a velocity of 6 em/sec collides with a 1 0 g mass, what is the velocity of .

the 1 0 g mass immediately following the collision? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

18. All action-reaction situations result in a transfer of forces in such a way as to conserve . . . . . . .

19. For every action there is an equal and opposite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.

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. . . . . . . . . . . . . . . helped to discover that all bodies fall at the same rate regardless of their weight, if the resistance of air is neglected.

21. In order to change velocity a ( an ) . . . . . . . . . . . . . . force must be applied. 22. Why can a tiny meteor penetrate the metal hull of a space ship? . . . . . . . . . . . . . . . . . . . . . . .

.

23. What is inertia? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

24. An unbalanced force acting on an object will produce a (an ) . . . . . . . . . . . . . . . of the object

proportional to the force and in the direction of the applied force. 66

�A�E

_______

CLASS

J)ATE

______

__ __ __ __ __�__ __

25. Why will objects of different weIghts or shapes, dropped from a building, fall at different rates?

26. What is the acceleration of an airplane that takes 1 5 seconds to reach a velocity of 360 miles/hour after starting in a motionless position? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,

27. All freely falling objects accelerate at a rate of 32 ft./sec. 2 because of . . . . . . . . . . . . . . . . . . . . .

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28. If a 65-gram mass, traveling at a velocity of 1 0 centimeters/second, collides with an object and imparts a velocity of 8 em/sec to that object, what must the mass of the second object be? . . . .

29. According to the work done by Sir Isaac Newton, as the mass of an object increases, the accele.

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property of matter due to its . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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ration of the object will

30. Inertia is

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Mu ltiple-Choice Questions In each .of the following questions, circle . t he letter preceding the word or phrase that best completes the statement or answers the question.

1. The rate at which an object moves in a given direction is called (c) magnitude, (d) velocity. 2. Which of the following units represents a measure of acceleration? ( c ) ft/sec2 , (d) ft/lb.

(a) speed,

( b ) motion,

(a) mi/hr,

3. All objects will fall at the same rate if they (a) are dropped from a great height, a vacuum, ( c ) have the same weight, (d) are equal in volume.

( b ) ft/sec

( b ) are in

4. In the first second of free fall an object will fall (a) 1 6 ft., ( b ) 32 ft., (c ) 64 ft., ( d) 8 ft.

5. Any object at rest possesses (a) momentum, ( b ) kinetic energy, (c) inertia, (d) acceleration. 6. An arrow released from a bow has great momentum because of its ( c) shape, (d) velocity.

(a) mass, ( b ) weight,

7. If a speedometer shows an increase of 5 mph every 5 seconds . for 5 seconds the automobile would be (a) accelerating, ( b ) in uniform motion, (c) changing velocity, (d) none of these answers.

8. In order to detect motion you need (a) a change in speed, ( b ) to be within the vehicle, ( c) to be isolated from all other objects, (d) a point of reference.

9. If a car traveling 30 miles per hour accelerated to 60 miles per hour in 2 seconds, its rate of acceleration is (a) 30 mi/hr, ( b ) 45 mi/hr/sec, (c) 35 mph, (d) 1 5 mi/hr/sec .

10. The recoil of a gun is an example of (a) Newton's First Law of Motion, ( b ) Newton's Second Law of Motion, ( c ) Newton's Third Law of Motion, Cd) Newton's Law of Gravity. NAME

_______

CLASS

.LJ DATE

_ _ _

_ _ _ _ _ _ _ _

67

M atching Q uestions In the space at the left of each item in Column A, place the letter of the term or expression in Column B that is most closely related to that item.

Column B

Column A

Newton's Second Law of Motion h. 32 ft/sec c. Galileo d. Newton's Third Law of Motion e. 32 ft/sec/sec f. Change of mass g. Equilibrium h. Change of velocity i. Motion j. Archimedes k. Uniform motion I. Acceleration m. Law of Inertia

1. Dense objects will fall to the earth in approxi-

Q.

mately the same time

2. Rotary lawn sprinkler 3. Usually causes a change In momentum 4. No unbalanced forces 5. Double the force, double the acceleration

6. The same distance traveled in the same time 7. Straight-line motion of an object in space. S. Acceleration of free falling objects, neglecting friction 9. Rate of change of velocity

10. Change in the position of an object

68

NAME

______

CLASS,

D �ATE

__ __ __

_ __ __ __ __ __ __ __

Chapter

8

ELECT R I C I TY STATIC E LECTRICITY

All matter is electrical in nature. You may have found this out when you walked across a dry wool rug and then touched a doorknob. Do you know why this happened? First let us discuss the basic nature of matter before we answer the question. The electrical nature of matter comes from its atomic structure. All matter is composed of atoms, the smallest particles of an element. Inside the nucleus of the atom are protons positively charged particles ; neutrons ­ neutral particles. Electrons are negatively charged particles that are found around the core: or nucleus of the atom. Since atoms normally have the same num­ ber of protons and electrons, matter is usually electrically neutral ; that is, it has no electrical charge. -

e_-_ ELECTRON

(-1

�If')IIMt:'Ii{---::-- NEUTRON (0)

The atom.

When two unlike materials rub together an electrical charge, either negative or posi­ tive, results. When you pass a hard rubber comb through your hair, some electrons move from your hair to the comb. The comb now has an excess of electrons and it becomes negatively charged. When a glass rod is stroked against a piece of silk, some of the electrons pass from the rod into the silk. The glass rod now has fewer electrons than protons and is positively charged. The production of an electric charge on an object is called electrification. According to the Law of Charges, unlike

_

_

_

rnllllillII 1I111111l1111iiillili

�

Z'-��, �� 'V '::-- � ,\\\ 1/ ,

-I!1 ..... .. :

;

.

'

.

\

EXCESS ELECTRONS ElECTRONS

GLASS ROD

�hr-�

Negative charge ­ comb.

,

- ,P

r, -- I

, - I

2-

II"", . ,

\

'"

DEFICIENCY OF

.

Positive charge glass rod.

charges attract each other and like charges repel each other. You felt the electric shock when you touched the doorknob because a large number of charges collected on the surface of your body, due to friction. These charges were opposite to those on the knob and caused a momentary discharge of electricity. Static electricity is the momentary transfer of electrons between unlike materials resulting from friction or contact. We can detect static electrical charges with an electroscope. This instrument consists of two very thin leaves of aluminum, tin foil or gold foil. The leaves are attached to one end of a metal rod which is insulated from the glass jar by a rubber stopper (see diagram ) . At the top of the rod is a metal knob.

Negatively charged electroscope. 69

When we bring an object having a static charge near the knob of the electroscope, the leaves spread apart (diverge ) . Why? Because each leaf, receiving the same charge, repels the other. If a negatively charged object is brought near the knob of a negatively charged electro­ scope, the leaves will spread even more. If the charge is positive, the leaves will come together. Why? Because the electrons will move from the leaves toward the object having fewer electrons.

Dangers of Static Electricity. The friction be­ tween air and water molecules against clouds causes a static charge. The bottom of a cloud is usually negatively charged ; the top is posi­ tively charged. As these charges build up, they finally become great enough to cause a huge discharge which we know as lightning. This giant spark is produced between a cloud and some other positively charged object such as the earth, a tree or another cloud. Never stand near trees during thunder­ storms - lightning is greatly attracted to them. Metal lightning rods on buildings pre­ vent lightning from striking them. The metal rods attract lightning and conduct the elec­ tricity harmlessly into the ground. Fires and explosions have often been caused by static electricity. The chain you see drag­ ging along the ground under gasoline trucks prevents sparks by carrying off excess static charges into the ground. Conveyor belts and the areas around toll booths are grounded to avoid electrical shocks to the attendant. Useful Applications of Static Electricity. Cott­ rell precipitators (electrostati c precipitators), are used in industry to remove harmful - and in some cases useful - particles escaping in smoke and dust. These particles are given an electrical charge and are then collected at an electrode having an opposite charge. The use of such devices in the manufacture of iron and steel and certain other industries help re­ duce air pollution. Van de Graaf generators produce large static charges which are used in atomic re70

search. They are also used to produce the high voltages needed to operate huge X-ray machines. SELF-DI SCOVERY ACTIVITY

Investigating the Law of Electric Charges. Materials: Two balloons, thread.

Procedure: 1. Inflate two balloons and suspend each by a piece of thread. 2. Rub each balloon briskly against some wool clothing. 3. Bring the two balloons close together and observe the effect. 4. Repeat step 2 with one of the balloons and place it against a wall. Observe the effect. Observations: 1. What did you observe in step

3?

2. What did you observe in step 4 ? . . . . .

Conclusions: Explain the above observations.

1. Step 3 : . . . . . . . . . . . . . . . . . . . . . . . .

.

2. Step 4 : . . . . . . . . . . . . . . . . . . . . . . . .

.

CU RRENT ELECTRICITY

\Vhen you use a toaster, vacuum cleaner or other electrical appliance in your home you are using current electricity. Current elec­ tricity, also called an "electric current," is a flow of electrons through a conductor. Any material that permits electrons to flow readily through it is a conductor. Some materials permit electrons to flow through them better than others. Silver is the best conductor; aluminum and copper are also excellent conductors. Most metals and carbon are good conductors of electric current. Non-metals, such as glass, cotton, rubber, ' mica and many plastics, do not permit elec­ trons to flow through them. These are called non-conductors or insulators. Fluids that con­ tain charged atoms or groups of atoms called ions can also conduct electricity. These are called electrolytic solutions or electrolytes. Static electricity is "electricity at rest" static means "stationary." Static electricity consists of electrical charges that have col­ lected on the surface of an object. Static electricity, therefore, contains potential or stored energy. Current electricity is "electricity in motion." It contains kinetic energy since the electrons are continuously moving through a conductor. In order to produce an electric current, elec­ trons must be made to move from one point in a conductor to another. In other words, work must be done to keep the electrons moving. In order to perform work, a force must be applied. Where does the force needed to produce and keep an electric current flowing come from? It usually comes from falling wa1ter, steam, gasoline, or diesel oil driving generators which convert mechanical energy into electrical energy. ­ .

U N ITS USED TO M EASURE ELECTRICITY

The Volt (V). The volt is a unit used to measure the force or pressure which pushes

electrons through the conductor. This force is often called the electromotive force (EMF) or the unit of potential difference. The term "potential difference" is used because a difference in potential energy must be maintained between the two points in a conductor in order for electrons to flow be­ tween these two points. The potential differ­ ence is built up and maintained by having fewer electrons at one point than at the other point. A voltmeter measures voltage.

The Ampere (A). This unit is used to measure the rate of current flow; that is, the number of electrons flowing past a given point in a conductor per second. An ammeter measures the number of amperes or amperage. When the electric current is weak a galvanometer is used to measure its strength. The Ohm (0). This unit measures the amount of resistance to the flow of electricity. The resistance of a conductor depends upon the following factors : ( a ) Length. The resistance varies directly as the length of the conductor ; that is, the longer the conductor, the greater the resistance. For example, if a 3 -ft. wire had a resistance of 1 0 ohms, a 6-ft. wire would have a resistance of 20 ohms. ( b ) Temperature. The resistance of most metals increases as the temperature increases. The resistance of carbon, many electrolytes and semiconductors decreases with an increase in tempera­ ture. Semiconductors are substances that behave as conductors or as insu­ lators depending upon conditions such as temperature. Transistors contain semiconductors such as germanium and silicon crystals and are used to replace vacuum tubes in electronic equipment. They are used because they are very small and create little heat. At cryogenic temperatures ( ex­ tremely low temperatures beginning at - 1 50° F ) , some poor conductors will become superconductors and some 71

nonconductors will become conduc­ tors . Superconductors are certain met­ als which become excellent conduc­ tors, that is, offer no resistance to the flow of current, at very low tempera­ tures. For example, lead is a supercon­ ductor at low temperatures. ( c ) Type of Material. Every substance has its own specific electrical resistance. Nichrome wire, for example, has a resistance that is 66 times greater than that of copper. Since high resistance results in the production of a large amount of heat, nichrome wire is often used in devices such as hot plates and toasters. ( d ) The Thickness (Diameter) of Wires. The resistance of a wire is affected by its diameter. A thick wire will have less resistance than thin wire of the same type. The resistance of a wire varies inversely as the cross-sectional area of the conductor. If a wire of 1 inch in diameter had a resistance of 20 ohms, the same type of wire with a 2-inch diameter would have a resis­ tance of 5 ohms.

Ohm's Law. This law states the relationship between current, electromotive force (poten­ tial difference) and resistance in an electric circuit. Ohm's Law states that the electrical current is directly proportional to the electro­ motive force in volts, and inversely propor­ tional to the resistance in ohms. In other words, the larger the voltage, the larger the current. The l arger the resistance, the smaller the cur­ rent. This may be expressed mathematically by the following formulas : I

= !!.... R

I

=

Rate of current in amperes.

E

=

Electromotive force in volts.

R

=

Resistance in ohms .

The following diagram will help you remem­ ber these formulas. Cover the letter of the factor you are trying to find. This letter 72

will represent one side of the formula you are trying to de­ termine. The two remaining letters, in the same positions they occupy in the pyramid, 4-_--1.__� will represent the other side of the formula.

Example: ( a ) What is the rate of current flow through a conductor having a resistance of 20 ohms if it is connected to a 1 20volt line? I

=E

1 20 V

= 6 amps. 20 n ( b ) How much voltage is needed to pro­ duce a current flow of 3 amperes through a resistance of 200 ohms?

E

-

R

=

lR

=

=

--

3A X 200 n

=

600 volts

( c ) How much resistance is produced if a current of 4 amperes is sent through a line having a voltage of 1 20?

R=

�= I

1 20V

=

4A

30 n

E LECTRICAL CIRCUITS

Series Circuits. In a series circuit the electrons follow only one path. They leave one terminal and flow through all the electrical devices one after the other before they go back to the second terminal of the source. There are no other paths through which these electrons can flow. RESISTER (BULB) \

1 /

A series circuit.

The main characteristics of a series circuit: 1. Current. The current is the same in

every part of the series circuit. If one bulb in a series circuit containing more than one bulb burns out or is removed, the circuit is broken and all the bulbs go out.

1 . What is the total resistance?

2.. Resistance. What happens when you add more bulbs to a series circuit? You are adding more resistances, causing the flow of cument to decrease. This will cause the bulbs to dim. We say that the total resistance of a series circuit is equal to the sum of the indi­ vidual resistances and is, therefore, always greater than any single individual resistance. As an equation :

2. What is the voltage drop at r1?

R

= r1

+

r2

+

r3

4. What is the total voltage drop?

. . .

3" Voltage Drop. The voltage in a series circuit is divided among all electrical resist­ ances in the circuit. As the voltage travels across the resistors, the voltage drops. The voltage drop is greatest across the highest resistance. Ohm's Law is used to calculate the voltage drop for each resistance :

IR

E Voltage drop

3. What is the voltage drop at r2?

The total voltage drop of a series circuit is equal to the sum of the individual voltage drops.

Parallel Circuits. In a parallel circuit the electrons may follow separate and complete paths. The devices in the circuit are connected side by side, permitting the current to divide and flow through the resistances at the same time. This diagram of a parallel circuit shows three resistances connected in parallel. I

e,

==

VOLTAGE DROP AT

r,

e.

=

VOLTAGE DROP AT ,

I

/

/

r.

A parallel circuit.

TO POWER SOURCE

A series circuit.

In this drawing of a series circuit the current is 2 amps in all parts of the circuit.

The main characteristics of a parallel cir­ cuit are : 1. Current. The current in the parallel circuit varies and is divided among the re­ sistances. The greatest current is in the smallest 73

resistance. The total current is the sum of the current in the individual branches : I

=

i1

+ i2 + is

. . .

3. Voltage Drop. The voltage across each branch in a parallel circuit is the same. There­ fore, the voltage of the circuit remains constant.

E

= e1 = e2 = es

•

.

•

If one bulb in a parallel circuit burns out, the rest will remain lighted since there are al­ ternate pathways for the flow of electrons and, thus, the circuit remains completed. 2. Resistance. The total resistance of a parallel circuit is always less than that of the smallest resistance. Each new resistance rep­ resents a new pathway for electrons and reduces the total resistance of the circuit. This happens because each resistance acts also as a conductor. If all the resistances have the same value, the total resistance of the circuit may be determined by the following formula :

R = !...

Study the above drawing of a parallel cir­ cuit. Solve the following : 1 . What is the current through r1?

2. What is the current through r2?

n (number of resistances )

Example :

If six 1 2-ohm resistances are connected to­ gether in a parallel circuit, the total resistance would be 2 ohms.

R=

12 6

n=2n

3 . What is the total current?

4 . What is the total resistance?

If the resistances have different values, the following formula is used to calculate the total resistance of the circuit.

If a 3-ohm resistance and a 6-ohm resist­ ance are connected in parallel, the total re­ sistance will be 2 ohms.

1

1

1

R - 3Q + 6"n 1

3

1 or 6 2

R R=2n -

74

=

-

Note that the total resistance of a number of resistances in a parallel circuit is always less than that of the smallest resistance. This helps you check your answer.

3" Voltage Drop. The voltage in a parallel circuit remains constant. In this circuit it is 1 2 volts. TO DRY CELL

POWER AND E N ERGY

The movement of an electric current rep­ resents a form of kinetic energy. This energy is often used as a force to do work. Electrical power is the rate at which electrical energy is converted to work. The unit used to express electrical power is the watt (W). Power in Watts = Volts X Amperes X A W= V One watt of electrical power is equivalent to the work done when one ampere flows under a pressure of one volt. Electrical power is usually measured in kilowatts. As you have learned, kilo means 1 000; therefore, 1 kilo­ watt ( l kw. ) is equal to 1 000 watts. For example, if an electric light were plugged into a 1 20V circuit and the rate of current flow was .5 amps, the lamp would produce 60 watts of electrical power.

W = V X A W = 1 20 volts X . 5 amp W = 60 watts

TO DRY CELL

Series and parallel circuits.

3. Connect to the dry cell and loosen a bulb in the parallel circuit. Observe the results. 4. Replace one of the bulbs in the series circuit with a l . 5v bulb and observe the results. 5. Replace one of the bulbs in the parallel circuit with a 1 . 5v bulb and observe the results.

Observations: Record observations below.

1. Step 2 : . . . . . . . . . . . . . . . . . . . . . . . . .

2. Step 3 : . . . . . . . . . . . . . . . . . . . . . . . .

.

3. Step 4: . . . . . . . . . . . . . . . . . . . . . . . .

.

4. Step 5 : . . . . . . . . . . . . . . . . . . . . . . . .

.

S ELF-DI SCOVERY ACTIVITY InvEistigating Parallel and Series Circuits.

Ma1:erials: Four screw-base miniature lamp sockets, four 2.S-volt bulbs, one 1 . S -volt bulb, insulated wire, two switches, a board 1 � , X 1 � ', large 1 . S-volt dry cell.

Prol�edure: 1 . Connect two 2 . 5 volt bulbs in series and two 2 .5 volt bulbs in parallel as shown in the diagram. 2. After connecting the series circuit to the dry cell, throw the switch. Then loosen a bulb in the series circuit and observe the results.

Conclusions: 1. Explain the results observed when one bulb was loosened in a series circuit. . . . . .

. . . . . . . . . . . .. . .. .

.

.

.

.

.

.

.

.

.

.

. .

.

. .

�

.

.

.

.

75

2. Explain the results observed when one

bulb was loosened in a parallel circuit. . . . .

3. Explain the results observed when a

4. Explain the results observed when a

60W bulb was inserted into the series circuit.

60W bulb was inserted into a parallel circuit.

76

R EVIEW TESTS Completion Questions For each of the statements or questions below, write the word or phrase in the space provided that best answers the question or completes the statement. 1. Static electricity is pruduced by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

2. Matter is usually electrically . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

3. Objects which have a deficiency of electrons are said to possess a . . . . . . . . . . . . . . . . . charge. 4. Why would a rubber comb rubbed through your hair attract small pieces of paper? . . . . . . . .

.

.

6. The production of an electric charge on an object is called . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

5. What is static electricity?

.

.

.

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. .

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7. If an electroscope were negatively charged and a positively charged object were brought near the knob, what would happen to the leaves? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

8. As the diameter of a wire increases, its resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

9. Explain how clouds become electrically charged . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

10. Toll booths and conveyor belts are . . . . . . . . . . . . . . . . . . . . . . . . . . . . . to prevent electrical shocks. 11. In the space at the right, complete the diagram illustrating how you would connect the two dry cell batteries in series and parallel circuits.

NAME

rl

rl

rl

Parallel circuit.

Series circuit. CLASS

DATE

rl

77

12. Fill

III

the following chart with the correct information:

Pa thways

Parallel Circui t

Series Circui t

Current

Total Resistance

Voltage Drop

i3. Why are houses wired in parallel instead of series?

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.

.

14. Answer the following questions concerning the diagram at the right :

(a) What type of circuit is it? . . . . . . . . . . . . . . . . . . ( b ) What is the voltage of the following : r1 . . . . . . . . r2 . . . . . . .

.

(c) What is the current flowing through the 30

n

resistor?

(d) What is the total resistance of the circuit?

( e ) What is the total current in the circuit? . . . . .

78

.

NAME._______CLASS

D ...... ATE

_ _ _

_ _ _ _ _ _ _ _

15. Answer the following questions concerning this diagram : ( a ) What type of circuit is it? . . . . . . . . . . . . . . . .

( b ) What is the current flow at A2? . . . . . . . .

.

.

50

.

( c ) What is the total resistance of the circuit?

(

,

(d) What is the voltage drop across the 5 ohm resistor?

. . . . . . . . . . .. . .. . .. . . . ......... . . .

.

( e ) What is the voltage drop across the 1 0 ohm re. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .:

sistor?

(f) What is the total voltage drop of the circuit?

16. The rate of converting electrical energy into work is called . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 200 kw equals . . . . . . . . . . . . . . . . watts. 18. How many watts are produced in a 1 20V circuit which has 3 amps flowing through it? 19. A . . . . . . . . . . . . . . . . . . can be used to detect the strength and direction of a small electric current. 20. Some poor conductors of electricity may become superconductors at temperatures below - 1 50 ° F. Such temperatures are called . . . . . . . . . . . . . . . temperatures.

21. A Van de Graaf generator produces huge . . . . . . . . . charges useful in atomic research. 22. Lightning rods on buildings attract lightning and carry the electricity harmlessly into the . . . . . . . . . 23. Why is copper used in electrical wire? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(a)

24. List three examples of electrical insulators. (c)

.

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" ,

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(b)

. . . . . . . . . . . . . . . . .

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25. An electrically charged atom or group of atoms is called a ( an ) . . . . . . .

.

.

. .. ............. .

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.

.

Mu ltiple-Choice Questions In each of the following questions, circle the letter preceding the word or phrase that best completes the statement or answers the question.

1. The negatively charged particles spinning around the nucleus of an atom are the (a) protons, ( b ) electrons, (c) neutrons, (d) shells. 2. A device which can be used to detect the type of a small static charge is a ( an ) (a) galvanometer, ( b ) ammeter, ( c ) - voltmeter, (d) electroscope. NAME

_______

CLASS

...... ATE, D

_ _ _

_ _ _ _ _ _ _ _

79

3. The unit used to measure the electromotive force or potential difference is the (a) volt, (b) ampere, (c ) ohm, ( d ) watt. 4. Five 50-ohm lamps connected in parallel would have a combined resistance of (a) 50, ( b ) 55, (c) 250, ( d ) 1 0 ohms. 5. A non-metal which is carbon .

a

good conductor of electricity is ( a ) rubber, ( b ) aluminum, (c) plastic, (d)

6. The best electrical conductor under normal conditions is (a) copper, ( b ) glass, (c ) silver, (d) aluminum.

7. Static electricity might best be compared to ( c ) kinetic energy, ( d ) atomic energy.

(a) potential energy,

( b ) chemical energy,

8. Substances that may behave as a conductor or an insulator under certain conditions are called (a) crystals, ( b ) semiconductors, (c) electrolytes, (d) resistors. 9. If an appliance having a resistance of 20 ohms were plugged into your household current, the rate of current flow would be ( a ) 3, ( b ) 2400, (c) 20, ( d ) 6 amperes. 10. The electrical charge of the nucleus of an atom is ( d ) varies.

(a) positive,

( b ) negative,

(c) zero,

Matching Questions In the space at the left of each item in Column A, place the letter of the term or expression in Column B that is most closely related to that item.

Column B

Column A

a. Path traveled by electrons b . Aluminum c. Ammeter

1. Create little heat, small, replacing vacuum tubes 2. High resistance to electrical current

3. Solution capable of conducting an electrical current 4. Static electricity

5. A device to measure the rate of current flow 6. Electrical power 7. Energy level, rings, shell or orbit 8. Prevents a spark due to static electricity 9. Removes annoying and, in some cases, useful ma­

d. Nichrome e . Friction f· Trees g. Voltmeter h. Transistors i . Cottrell precipitator j. Electrolyte k. Carbon I. Chains on a gasoline truck m. Watt

terial from smoke, thus helping to reduce air pollution

. . . . . . . . 10. Avoid during thunder and electrical storms

80

NAME,

_______

CLASS

D ....... ATE

___

_ _ _ _ _ _ _ _

Chapter

9

M AG N ET I S M MAGN ETS

For thousands of years man has known that certain substances had the power to attract certain other substances. Early man discov­ ered that certain black "rocks" had magnetic properties. These were natural magnets, called lodestones, which contain an iron ore, mag­ netite.

Typ1es of Magnets. Magnets found in nature are called natural magnets. Magnets made by man from iron or steel are called artificial magnets. There are two types of artificial mag­ nets" temporary and permanent. Temporary magnets, commonly made of soft iron, keep their magnetic properties for a short time. Permanent magnets, made of iron, steel or steel alloys such as alnico, keep their magnetism for a long time. Magnetic Fields. We can't see, smell, hear, feel, or even touch magnetism because it is in­ visible; but it does exist. Around every magnet is a region in which the magnetic effects exist. This area, known as the magnetic field, con­ tains invisible magnetic lines of force. You can see these lines of force by -placing a bar magnet under a piece of glass and sprink­ ling iron filings on the glass above the magnet. The pattern that forms shows that the mag­ netk lines of force are concentrated at the op­ posite ends of the magnet called the poles of the magnet. Poles of a Magnet. One end of the magnet is the positive or north pole ( N-pole ) . If a bar magnet is suspended so that it rotates freely, it will finally stop with the N-pole always point­ ing to the earth's magnetic pole (near the earth's geographic north pole). The other end of the magnet, the negative or south pole (S-pole) always points toward the earth's south magnetic pole.

f�Y �c% .. . I#:+.. . . . . . . ::+. . .� . . . '" if ,

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Law of Magnetic Poles. This law states that like magnetic poles repel each other and unlike magnetic poles attract each other. If you bring the N-poles of two magnets together, you will see that they repel each other. Bring the N­ and S-po1es of two magnets together and they will attract each other. The Law of Magnetic Poles is similar to the Law of Electrical Charges. Theory of Magnetism. No one has definitely explained the cause of magnetism. Many sci­ entists believe that magnetic properties are re­ lated to the fact that electrons in an atom spin on their own axis at the same time that they orbit the nucleus (similar to our earth's move­ ment around the sun ) . These spinning motions produce magnetic fields, If these magnetic

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You can magnetize a piece of iron or steel, such as a knitting needle, by stroking it in one direction a number of times with one end of a bar magnet. This causes the dipoles to line up into domains. This is the contact method of making a magnet . In the induction method you don't have to touch the object to the magnet. If you place a nail in the magnetic field of a bar magnet the nail will become magnetized. When you with­ draw the nail it will lose its magnetism. If you heat or hammer a magnet you will disarrange the domains and destroy its mag­ netic properties.

Electromagnets. The electromagnet is a tem­ porary magnet consisting of a current-carrying wire wound around a core. A Danish physicist, Hans Oersted, discovered that when an electric current is passed through a wire it will produce a magnetic field around it. Joseph Henry, an American scientist, produced the first electro­ magnet. Powerful electromagnets are used to lift heavy iron and steel objects. Electrical de­ vices such as doorbells, telephones, telegraphs, radios, phonographs, motors and generators use electromagnets. The betatron, an atom­ smashing machine, uses a giant electromagnet. SELF-DI SCOVERY ACTIVITY Making an Electromagnet

Materials: A dry cell, some insulated bell wire, scissors, 82

a large iron nail, and some small items of iron or steel to test.

Procedure: 1. Wrap about 50 turns of the insulated bell wire around the nail. 2. Remove the insulation from the ends of the wire.

3. Connect the ends to the dry cell as shown in the illustration.

BATTERY WIRE COilS A simple electromagnet.

Observations: 1. Test your electromagnet with some of the small iron and steel items you collected. What do you observe? . . . . . . . . . . . . . . . . . . . . . .

.

2. Disconnect the wire from the dry cell. Now try to pick up the items. What do you

observe? . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

3. Can you think of some ways you might make your electromagnet more powerful? Try them and find out if you are right. What are your results? . . . . . . . . . . . . . . . . . . . . . . . . . .

G E N E RATORS

The generator is a machine that changes mechanical energy into electrical energy. Mi­ cha.�1 Faraday, an English scientist, discovered the principle upon which the generator oper­ ates A n electric current is produced in a con­ ductor forming part of a closed circuit when the conductor moves so that it cuts the lines of force of a magnetic field. A current that is produced in this way is said to be an induced (caused) current. ..

Production of an induced current. The direction of the current alternates as the movement of the magnet is reversed.

The magnet provides the force necessary to do the work in moving the electrons through t e conductor. In order to cut the magnetic lmes of force, either the magnet or the con­ ductor may be moved. If, for example, the con­ ductor is a coil of wire, a current may be in­ duced in the wire by moving a magnet through the coil. The circuit may be completed by con­ necting it to a galvanometer (an instrument for detecting small electric currents) to measure the strength and direction of the current. When the magnet is pushed into the coil (arrow B) the needle on the galvanometer �wings in one direction (A) . When the magnet IS wllthdrawn (arrow A), the needle swings in the opposite direction (B). This shows that the direction of the flow of current has been re­ versed.

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Typ.�s of Electrical Currents. There are two types of electrical currents: ( 1 ) alternating (AC), and (2) direct (DC).

In alternating currents (AC) the electrons constantly reverse their direction of flow. The advantages of AC current are that ( 1) its volt­ age can easily be raised or lowered as needed by a transformer, and (2) it can be sent long distances at high voltage with little loss of elec­ tricity due to heat caused by friction. In direct current (DC) the electrons flow in one direction only. Direct current is produced by the generator of an automobile when the motor is running. It supplies the current for the lights, horn and other equipment and also charges the storage battery. Electro­ plating, a process in which metals are coated with other metals, uses direct current.

The AC Generator. The AC generator con­ sists of ( 1 ) an armature, (2) field magnets, (3) slip rings, and (4) brushes. The armature is a large coil of wire wound around a core of soft iron. The core is mounted on an axle which permits it to rotate in the magnetic field. The field magnets have opposite poles which provide the magnetic field. The slip rings, two brass rings connected to the armature, conduct the current from the armature to the brushes. The carbon or metal brushes conduct the cur­ rent from the slip rings to the wires. The wires carry the electricity to the outside circuit. To simplify our discussion we will consider the armature as having only one loop of wire. As the armature turns on its axle, the coil of wire cuts the lines of force of the magnetic field between the field magnets. This causes a cur­ rent to be induced in the coil.

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During one-half of the rotation of the loop, the voltage ( electromotive force, or EMF) in­ duced in the armature rises until the greatest number of magnetic lines of force are cut by the loop. During the other half of the rotation, the induced voltage builds up to a maximum, but the direction of the current is reversed. Why? Because the loop cuts the lines of force in the opposite direction. The greatest voltage is induced when the loop is in a horizontal position because at this time the most lines of force are cut. When the loop is in a vertical position no lines of mag­ netic force are cut. Thus, the induced voltage at that moment is zero.

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Careful examination of the graph shows that most solids become more soluble in water as the temperature increases. At 0° C, the freezing point of water, 1 00 m!. of water will dissolve only about 1 3 grams of potassium nitrate ( KNO:l ) , whereas the same quantity of water can dissolve 1 00 grams of this compound at about 5 5 ° C. We can increase the rate at which a solid ( solute ) dissolves in a liquid (solvent ) by ( 1 ) pulverizing or grinding the solid, ( 2 ) stirring or mixing the solution, and ( 3 ) increasing the temperature of the solvent. An increase in tem­ perature thus increases both the solubility of

most solutes and the rate at which the solute will dissolve. Solutes affect the freezing and boiling point of the solvent. They raise the boiling point and lower the freezing point. In winter, antifreeze is added to water in an automobile radiator to prevent freezing by lowering the freezing point. In summer, antifreeze may be added to raise the boiling point. SELF-DISCOVERY ACTIVITY Exploring the Effect of Particle Size, Stirring and Heating on the Rate at Which a Given Solute Will Dissolve.

Materials: Five test tubes, copper sulfate ( euSO 4 ) crystals, mortar and pestle, bunsen burner.

Procedure: Fill each of the 5 test tubes one-half full of water. Select 5 crystals of euso 4 of about the same size. ( a ) Drop one crystal into a test tube and allow the tube to stand. ( b ) Drop one crystal into a test tube and heat the tube over the flame of a bun­ sen burner.

Observation: How did heat affect the rate of solubility?

( c ) Drop one crystal of euso 4 into a test tube and shake the tube vigorously while holding your thumb over the mouth of the tube.

Observation: How did pulverizing affect the rate of solubility?

. . .. . . . .. . . . . . . . . . . . .. . . . . . . .

.

( e ) Pulverize one crystal of euso 4 and drop the material into a test tube. Shake the tube vigorously.

Observation: How did shaking the pulverized euso 4 affect the rate of solubility? . . . . . . . . . . . . .

.

Conclusions: Did your investigation support the statement of the authors about the effects of particle size, stirring, or heating on a solid solute?

Types of Solutions. Solutions may be classified by comparing the amount of solute with re­ spect to a given amount of solvent present. 1. Dilute Solution. A dilute solution con­ tains a large amount of solvent compared to a relatively small amount of solute.

How did shaking affect the rate of solubility?

2. Concentrated Solution. A concentrated solution contains a comparatively large amount of solute dissolved in a small amount of sol­ vent. A concentrated hydrochloric acid solu­ tion ( Hel ) contains a relatively large amount of hydrochloric acid in a comparatively small amount of water.

( d) Drop one crystal of pulverized euso 4 into a test tube and allow the tube to stand.

3. Saturated Solution. A saturated solution is one in which the solvent has dissolved all the solute it can dissolve at a given temperature and pressure. 4. Supersaturated Solution. A supersatu­ rated solution contains more dissolved solute

Observation:

183

than it could normally hold at a given tempera­ ture and pressure. Since more solute can be dissolved at higher temperatures, lowering the temperature of the solution will cause some of the dissolved ma­ terial to settle out. Some substances, such as photographic hypo ( sodium thiosulfate ) , how­ ever, will not settle out if the solution is cooled slowly, thus making a supersaturated solution. Supersaturated solutions are not very stable. Adding a single crystal of solute or shaking the container will cause the excess dissolved solute to settle out, leaving a saturated solution behind. This intended agitation or disturbance is called seeding.

Suspensions. A suspension is a non-uniform mixture of tiny, insoluble solid particles dis­ tributed in a liquid or a gas. The particles eventually settle out on standing. The sus­ pended particles are visible and can be re­ moved by ordinary filtration. Muddy water, oil base paints, and dust in the air are some com­ mon examples of suspensions . Colloids. Colloids ( colloidal dispersions ) are special kinds of suspensions. They are com­ posed of particles intermediate in size between those of a solution and a suspension. Colloidal particles are thus larger than the molecules found in a solution but smaller than the tiny visible particles found in a true suspension. The particles in a colloidal suspension are so small they never settle out. This results from the constant bombardment of the particles by the vibrating molecules of the suspending medium. The erratic, zigzag motion of the particles is called Brownian movement. The haphazard movement of dust particles visible in a strong beam of light also illustrates Brownian move­ ment. Some examples of colloids include : milk, smoke, jello, and protoplasm - the liv­ ing material of a cell . S ELF-DI SCOVERY ACTIVITY I nvestigating the Differences Between a Solu­ tion and a Suspension. 1 84

Materials: Three 1 00 ml. beakers, graduated cylinder, stirring rod, tablespoon, water, sugar, soil, alcohol, paper toweling or filter paper, funnel.

Procedures and Observations: ( a ) Place 7 5 ml. of water in beakers A, B, and C. Place a tablespoonful of sugar in A, a tablespoonful of soil in B, and a tablespoonful of alcohol in C and thor­ oughly stir each. A

B

c

( b ) Describe the appearance of the ma­ terial in each of the beakers, including presence or absence of visible particles. H 2 0 + sugar: . . . . . . . . . . . . . . . . .

.

H 2 0 + soil : . . . . . . . . . . . . . . . . . . . H 2 0 + alcohol : . . . . . . . . . . . . . . . . ( c ) Allow the samples to stand for about 20 minutes. In which sample or samples did the solute settle out? . . . . . . .

( d ) After thoroughly remixing the samples, try to separate the solutes by filtration. Which solutes were removed by this process? . . . . . . . . . . . . . . . . . . . . . .

.

Conclusions: ( a ) Tell whether the following samples were solutions or suspensions :

H 2 0 + sugar: . . . . . . . . . . . . . . . . . . H 2 0 + soil : . . . . . . . . . . . . . . . . . . . H 2 ° + alcohol : . . . . . . . . . . . . . . .

.

( b ) What properties were illustrated in this experiment that enabled you to differ­ entiate between solutions and suspen­ . Slons ?. . . . . . . . . . . . . . . . . . . . . . . . .

5. Filtration. Water is allowed to pass through a filter composed of sand and gravel to remove suspended solids. Charcoal is also used to remove bad odors and colors. -

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Water Purification. Water found in nature usually contains impurities. Some impurities, when present in large quantities, may give water a disagreeable taste or odor, or make it unsafe for drinking. Water may be made fit to drink (potable) by the following methods : 1. Boiling. If water is boiled for a sufficient length of time, most harmful germs will be destroyed.

2. Distillation. This method consists of boil­ ing and evaporating a fluid, in this case, water, and condensing the water vapor to remove im­ purities. Distilled water is often used to prepare water solutions of medicines. The preceding two methods are only suit­ able in purifying small quantities of water. Water purification on a large scale uses the following processes to bring s afe drinking water to communities : 3. Sedimentation. In this process the water is collected in reservoirs or settling basins and the large solid particles settle to the bottom. 4. Coagulation. The addition of chemicals, such as alum or lime, will cause fine particles to j oin together and clump or coagulate in a jellylike mass which settles to the bottom.

6. Aeration. Filtered water is usually sprayed into the air (aeration) to help remove odors, kill bacteria and eliminate gases that give water a bad taste. 7. Chlorination. Chlorine compounds are added to water to kill harmful bacteria. Chlo­ rine is added to the water in swimming pools as well as to drinking water.

Hard and Soft Water. Water for home use may be classified as hard water or soft water, de­ pending on the type and amount of minerals it contains. 1. Hard Water. Hard water contains the ions of calcium, magnesium or iron. These ions interfere with the ability of soap to form suds. As a result scum forms, often visible as a ring in the bathtub. Boilers and water pipes are often clogged by deposits of calcium carbonate. Temporary hard water contains calcium or magnesium salts in solution which can be re­ moved by boiling. Permanent hard water contains calcium or magnesium salts ( calcium or magnesium sul­ fates or chlorides ) which cannot be removed by boiling. 2. Soft Water. Soft water contains very little dissolved minerals. Water softeners are often added to water used for washing clothes. Some commercial water softeners contain a sub1 85

stance called "zeolite," which removes many dissolved minerals. Certain resins, called "ion­ exchange resins" are also used to remove the minerals which cause hard water.

A New Kind of Water? In the mid- 1 9 60's, Russian scientists announced that they had dis­ covered a new and different form of water. Found in tiny capillary tubes, the new "water" had the consistency of grease, and did not boil until a temperature of 3 00 0 C. was reached. Its freezing point was below - 1 0 0 C. After preparing some of the mysterious m aterial, U.S. scientists proposed that the greasy substance is a form of water in which the molecules form a chain or polymer. Thus, the name "polywater" was given the unusual substance. Even today, scientists are still studying polywater to find out more about its unique properties. Water Pollution. Approximately three-quarters of the earth's surface is covered with water, and about 97 % of this water is salt water, un­ suitable for human consumption. Much of the remaining 3 % of the water is unavailable since it is deep in the ground or locked up in polar ice caps. An increasing population and tremen­ dous industrial growth are steadily demanding more of the limited fresh water supply. Indus­ trial and community wastes are dumped into streams, lakes and rivers, and are greatly re­ ducing the available supply of fresh water and destroying much of the aquatic life. Because of these factors and the general scarcity of water in many areas of the world, the lack of water is a major concern. The prob­ lem is so widespread that in 1 965 the United Nations sponsored an international organiza­ tion to help solve this problem. A huge step forward is the erection of desalinization plants to make fresh water from sea water on a large economical, commercial basis. S ELF- D I SCOVERY ACTIVITY Discovering Some Materials Which Can Be Used to Soften Hard Water.

1 86

Materials: Hard water ( containing calcium sulfate ) , soap solution, 3 test tubes, 3 corks, graduated cylinder, washing soda, borax.

Procedure: To 3 test tubes add 1 0 m!. or about one-third of a test tube of hard water. To each test tube add the following : ( a ) Test tube 1 . Three drops of soap solu­ tion. Shake the tube. ( b ) Test tube 2. A small piece of washing soda, 3 drops of soap solution. Shake the tube. ( c ) Test tube 3 . A pinch of borax, 3 drops of soap solution. Shake the tube.

Observations: Describe the amount of suds formed in each tube after shaking. Test tube 1 : Test tube 2 : Test tube 3 :

Conclusions: What conclusions would you draw from the evidence resulting from this experiment? . . . .

R EVIEW TESTS Completion Questions For each of the statements or questions below, write the word or phrase in the space provided that best answers the question or completes the statement. .

1. About

.

.

% of an adult's weight is due to water and about 90% of his . . . . . . . . . is com-

posed of water. 2. Name five life functions common to all living things which are in part dependent upon water.

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(a) What is the meaning of the term "hydrate?"

( b ) What does the dot in the formula C US04 ' 5H20 mean? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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187

9. Complete the following chart:

Suspension

Solution

Colloid

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1 88

NAME

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18. How could you differentiate between a true suspension and a colloidal suspension?

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19. What is the meaning of the term "distillation?" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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20. Despite the fact that a large portion of the earth's surface is covered with water, why is the shortage of water a world problem?

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Multiple-Choice Questions In each of the following questions, circle the letter preceding the word or phrase that best completes the statement or answers the question.

( b ) 90% , (e) 1 5 % ,

(a) 75 % ,

1. The earth's surface is covered by approximately water.

(d) 60%

2. Water takes a long time to heat and to cool because of its (a) molecular shape, ( b ) impurities, ( c ) density, (d) high specific heat. 3. Water, upon freezing, (a) contracts, ( b ) expands, (e) chemically changes, (d) remains the same.

4. The freezing point of water is (a ) 1 00 0 C, ( b ) 00 P,

( c ) 00 C,

(d) 320 C.

( a) excretion,

5. Water acts as a solvent to eliminate wastes in the process of (c) reproduction, (d) secretion.

( b ) digestion,

6. When water is driven off from copper sulfate ( CUS04 . 5H20) , the remaining product is called

(a) hydrated, ( b ) dry, (c) carbonated, (d) anhydrous 7.-11. The following questions relate to the graph : 7. Which letter represents the compound with the highest solubility at 30° C? ( a ) B, ( b ) C, (c) A, (d) E. 8. As the temperature increased, the solubility of compound C ( a ) decreased, ( b ) in­ creased, ( e ) remained the same, (d) im­ possible to determine from the graph. 9. How many grams of compound C will be needed to saturate 1 00 ml. of water at 500 C? ( a ) 20, ( b ) 80, ( c ) 40, ( d) 10 . 10. Which letter represents the compound least soluble at 7 3 0 C? ( a ) B, ( b ) D, (c) E, (d) C. 11. Which letter represents the compound whose solubility increases most slightly as the temperature increases? ( a ) B, ( b ) E, (e) A, (d) C. NAME

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copper sulfate. EFFECT OF TEMPERATURE ON SOLUBILITY

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