Pressures (Longman Physics Topics)

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LONGMANS' PHYSICS TOPICS

General Editor: John L Lewis

[PRESSURES A. R. Duff M.A. (Oxon), Assistant Master, Malvern College

Illustrated by T. H. McArthur

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LONGMANS

LONGMANS, GREEN AND CO LTD

London and Harlow Associated companies, branches and representatives throughout the world

© Longmans, Green and Co Ltd 1969 SBN 582321794 First published 1969 Printed in Great Britain by Butler and Tanner Ltd, Frome and London

I

ACKNOWLEDGEMENTS I

I CONTENTS]

The author and publisher are grateful to the British Oxygen Company Ltd, page 7 (above), and P. B. Cow Ltd, Page 50 (middle), for help with the diagrams, to Mr H. J. P. Keighley, Mr F. R. McKim and Pergamon Press Ltd for permission to base the diagrams at the foot of page 17 and on page 28 on those in The Physical World 1, and to the following for permission to use photographs: page 6 Canada House Information Service; page 7 Dunlop Co Ltd; page 13 (above) Smiths Industries Ltd, (below) Esso Petroleum Co Ltd; page 19 Prestige Group Ltd, Samuel Birkett Ltd, Hartley & Sugden Ltd; page 23 Camera Press; page 30 (above) Barnaby's, (below, left) Fox Photos, (below, right) Coventry Climax and George Cohen 600 Group Ltd; page 32 Science Museum, London (Crown Copyright reserved); page 36 Meteorological Office (reproduced by permission of the Controller, Her Majesty's Stationery Office, Crown Copyright reserved); page 40 Cape Engineering Co Ltd; page 41 Pilkington Bros; page 45 Barnaby's; page 47 Camera Press; page 48 (above) British Rail, (below) The English Electric Co Ltd; page 52 French Embassy; page 53 (left) United States Information Service, (right) National Aeronautics and State Administration; page 56 Cunard Steam-Ship Co Ltd; page 57 (above) Ford Motor Co, (below) Esso Petroleum Co Ltd; pages 59 and 60 U.S.l.S. The front cover photograph is reproduced by permission of Paris-Match and the Daily Express, and the back cover photograph was taken by Michael Spincer.

Pressure 5 Pressure difference 9 Applications of the pressure caused by liquids 22 The pressure of the atmosphere 32 Applications involving atmospheric pressure 37 High pressures 44 Protection against extremes of pressure 51 Floating and sinking 54 Appendix I 61 Appendix 2 62

NOTE TO THE TEACHER

This book is one in the series of physics background books intended primarily for use with the Nuffield O-Level Physics Project. The team of writers who have contributed to the series were all associated with the Project. It was always intended that the Nuffield teachers' material should be accompanied by background books for pupils to read, and a number of such books have been produced under the Foundation's auspices. This series of books is intended as a supplement to the Nuffield pupils' material: not books giving the answers to all the investigations pupils will be doing in the laboratory, certainly not textbooks in the conventional sense, but books that are easy to read and copiously illustrated, and which show how the principles studied in school are applied in the outside world. The books are such that they can be used with a conventional as well as a modern physics programme. Whatever course pupils are following, they often need straightforward books to help clarify their knowledge, sometimes to help them catch up on a topic they have missed in their school course. It is hoped that this series will meet that need. This background series will provide suitable material for reading in homework. This volume is divided into sections, and a teacher may feel that one chapter at a time is suitable for each homework session for which he wishes to use the book. This particular book is written as a background book for the Forces and Pressures sections in Years I and II. It will also be relevant in Years III, IV and V, but has been kept simple throughout. It is hoped that the examples. given, which range rather beyond the Nuffield course, will help pupils to appreciate the importance of pressure in everyday life. Emphasis is placed on the practical applications of pressure and pressure differences. This is essentially a book for pupils to browse in, taking up points which capture their interest and possibly pursuing them further.

3

INTRODUCING THIS BOOK

4

In your work at school, you will have realised that it is not always the force that is significant, but also the force that acts on a particular area, to which scientists give the word pressure. This book attempts to show the significance of the pressure caused by liquids and gases in the world around us, and how much use is made of pressure in devices that we often take for granted. The first section considers various facts about pressure with which you are probably already familiar. In the next section we discuss the significance of pressure differences, how they are measured and also various devices for controlling pressure differences. The next section considers a series of applications in which the pressures caused by liquids are put to practical use. In later sections we turn to the pressure caused by the atmosphere, reminding you of how it is measured and how atmospheric pressure is used. Gases at high pressure are also useful and there are various devices that use compressed air. There are also special problems these days related to extremes of pressure, for example in deep-sea diving and ocean exploration, in high-altitude flying and of course in space exploration, and reference is made to these problems later in the book. It is hoped that when you have read this book, you will realise how significant pressure is in the world in which we live today.

I PRESSURE

FORCE AND PRESSURE I

When you pull a piece of elastic you exert a force on it; when you push on a wall you exert a force on it; when you stand on the floor you exert a force on it. A fo_~~e, iHLPush or a pull. When'your mother weighs sugar for cooking, she puts it on a kitchen balance and the needle goes round to show how much sugar she has. The balance works by stretching or compressing a spring and most forces can be measured in this way. It is the same when you stand on the scales: if you weigh 50 kg it means that you always exert a force of 50 kgf on the ground. (A weight of 50 kg is denoted by 50 kgf. The f shows that it is a force; for units see Appendix 2.) This force will always be the same, provided you are not carrying anything. If you stand on the grass with both feet on the ground you make very little impression on it. But if you sit on a stick with a spike on the end - a shooting stick-it sinks into the ground until the plate is reached. In each case the force is about the same. But standing on the ground, the soles of your shoes cover a much larger area than does the point of the shooting stick. The area over which a force acts is very important, so much so that we use the special word pressure, defining it as force divided by area. The word pressure has many meanings in everyday life. We talk about the pressure of work, the pressure of exams, political pressure from the Government or even high-pressure selling by travelling salesmen. In science we have only the one meaning for it: force divided by area. I[ you weighed 50 kgf and the area of the soles of your . 50 kgf shoes was 100 cm-, you would exert a pressure of 100 cm 2

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or 0·5 kgf'/cm", But the area of the end of the shooting 50 kgf stick might be 0·5 em", So the pressure would be . 5 cm 2 O which is the same as 100 kgf'/cm--a very much greater pressure. It is the pressure on a surface, as well as what the surface is made of, that decides how much the surface dents. 5

Problems to think about I. Why does the shooting stick stop sinking into the ground when it reaches the plate? 2. Why does it depend on which way up a drawing pin is when someone sits on it? 3. Can you think of one reason why a footballer has studs on his boots? 4. Why does a girl weighing 50 kg and wearing high-heeled shoes do more harm to a wooden floor than a man weighing 100 kg? 5. Why do Eskimos and other people in snowy regions wear snow shoes? 6. Why does a grocer use a fine wire for cutting cheese? 7. Can you explain why a sharp knife cuts better than a blunt one?

PRESSURE OF L1aUIDS Snow shoes

Because of the force downward caused by its weight, a solid exerts a pressure on the ground or any surface on which it rests. Do liquids also exert a pressure? Liquids also have a weight: if they are in a container they must therefore exert a pressure on the bottom of that container. But does the liquid pressure only act downward? If you have not already done this at school, take an old tin (the larger the better), hammer a hole in the side with a round nail and fill the tin with water. What happens? To see if water pressure acts upward as wellas downward and sideways, batter a tin as shown and make a hole abovesome of the water but belo"\\.; the-highest point when the tin is filled. In what direction does the water start to come out? Take another tin can and make three holes in the side with a nail at different depths. Fill the can with water and watch the water run out. Can you suggest what happens to the pressure as one gets deeper in the water? (The best place to try this is in the bath !) You may have noticed that if you swim down to the bottom at the deep end of a swimming bath, it is painful 1'1-," on the ears. This is because the ears are very sensitive to the increase in pressure as you go lower toward the ~bottom of the bath.

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I. Why does a bubble of gas coming up from the bed of a lake get bigger and bigger as it gets nearer the surface? 2. Why is it necessary for men investigating the depths of the ocean to go down in steel containers with walls several centimetres thick? 3. Would you expect the hot-water tap in the kitchen downstairs to run faster than the hot-water tap in the bathroom upstairs if the hot tank was in the attic and the taps were exactly similar?

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PRESSURE OF GASES Like liquids, gases also exert a pressure on their containers. If you blow up a balloon with a small hole in it, you will find that the air comes out of it whichever direction you point the hole. A soda siphon is a good example of a gas exerting a pressure. The space above the liquid is filled with gas at high pressure. When the valve is raised by the depression of the lever, the gas pushes down on the liquid and forces it up the tube and through the outlet. Another example is the aerosol spray, which also contains gas under pressure that forces liquid out when the valve is opened. Air pressure plays a most important part in the tyres of

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your bicycle and the tyres of a motor car. It manages to support the bicycle and to prevent the road rubbing on the rim of the bicycle wheel. Problems to think about I. When a steam train blew its whistle, clouds of steam came out through the whistle. What made the steam come out? 2. A car has its tyres at a pressure of 1·5 kgf/cm- (22Ibfjin'). What will happen to the tyres if four heavy people get in as passengers? 3. If you put a cork loosely in a bottle of fizzy lemonade and then shake the bottle, why will the cork fly out? 4. A bicycle pump can force air into a tyre, even when the air inside is at quite a high pressure. If you make a hole in the tyre, air would come out and not go in. How do you think the pump works? 5. The pressure of the air in our atmosphere has a considerable effect on the weather; can you think how pressure variations might be responsible for winds?

Some experiments to try at home I. Find a flat piece of firm soil in the garden and stand on it in your ordinary flat shoes. Borrow an old pair of your mother's high-heeled shoes and try again. Notice the difference in the depth of the imprints. 2. Take a heavy pile of books and tic them up with thin string. Make a loop at the top and hold the books up for a full minute with your fingers through the loop. Then slip a rolling. pin through the loop and hold the books up with it. Why does it hurt in one case and not in the other? 3. Obtain an old plastic detergent bottle, preferably one about 25 cm high. First make a hole in it near the bottom, using a drawing pin. Then fill the bottle with water and watch the water running out of the hole with the cap screwed back on and also with the cap off. Can you explain the different effects? Secondly, make many holes with the drawing pin all round the container and at different depths. Fill the bottle with water, screw the cap back on and squeeze the bottle. Does this tell you anything about the direction of the liquid pressure? 4. Obtain a milk bottle, a drinking straw and some Plasticine.';. Fill the bottle one-third full of water and put the straw through the Plasticine, which should be pressed down on top of the bottle as illustrated. Make sure that you have an air-tight join between the Plasticine, the bottle and the straw. Throughout the experiment you must push down firmly with your hands on the Plasticine. See how many bubbles of air you can blow into the bottle through the water. When you have done this, and are still keeping your hands pressed down, take your mouth away from the straw. Can you explain what happens? (This experiment is better done in the open air, or over a sink or in the bathroom.) 5. Take a bicycle pump, put your finger over the end and feel the 'springiness' of the air when you compress it. Can you explain this?

* Many of the experiments in this book are done with milk bottles, drinking straws and Plasticine. A /I these will give satisfactory results, but if rubber corks and glass tubes are available they are sometimes easier to use. A summary of the effects of pressure can be seen in Appendix 2 (Nos 2 to 5).

8

PRESSURE DIFFERENCE

We have discussed how liquids and gases exert pressure.

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pressure, the liquid or gas will flow from the region of higher pressure to the lower-pressure region until the .pressures balance. Whenever two pressures are unbalanced, movement occurs trying to restore the balance. You can show this with a very simple example. Blow out your cheeks keeping your mouth shut: this gives a high-pressure region in your mouth. Then put your hand in front of your mouth and open your mouth. You will feel the air rushing out of the high-pressure region until it reaches the same pressure as the atmosphere outside.

VACUUM AND PARTIAL VACUUM Some standard has to be taken for absolute pressure, and the pressure of the air in the atmosphere at ground level is the normal standard. We shall be considering this in a later chapter. _A space containing gas which exerts a pressure less than that of the atmosphere outside is saidto contain a partial vacuum. The extreme case of this is if all the gas is removed from a space and thus no pressure is exerted in it. In this space we say that there is a vacuum. You know already that in liquids the pressure increases . with depth below the surface of the liquid. Think about a tubing, half-full of water, held in the shape of aU.

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9

PRESSURE DIFFERENCE

The water level on each side will be the same. Suppose one side of the tube is raised quickly as shown. The pressure at the bottom of the water on the right is greater than that at the bottom of the water on the left because of the greater depth. The water will then flow from the right to the left until the pressures balance and the levels are again the same. (If you can get a piece of transparent tubing, try it at home for yourself.) In the laboratory you will have seen another most important experiment. What happened if water was put in a U-tube in which the two arms were different sizes? This experiment suggests that the pressure ina liqyid