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

Shooting stick

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

gas at high

valve open

valve closed

spring

gas under pressure

Aerosol spray

7

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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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If a region of high pressure is connected to a region of ..

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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.

pIece-of

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
I n anycontainer, whatever itsshape, the .liquid.level i~_ tl1esame in all parts. It is often said that 'water finds its

ownlevel'.

Some things to think about I. Why does air come out of a balloon when the mouthpiece is open'? 2. What affects the rate at which a liquid flows out of a hole in a tank'? 3. Why do winds blow'? 4. What happens to the level of water over a very great distance, say several miles'? Does it still find its own level'?

10

PRESSURE DIFFERENCE

MEASUREMENT OF PRESSURE DIFFERENCE Manometers U-tubes can be very useful for measuring pressure differences. You have probably used at school a large utube to measure your lung pressure. Such tubes are given the special name of manometers. You probably also measured lung pressure with a smaller manometer containing mercury in the tube. You get a much smaller difference in that case because mercury is heavier than water: it has a much higher density than water. The difference in height with water is 13·6 times greater than it is with mercury, since mercury is 13·6 times more dense.

II

connected to gas

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.Manometers can be used for measuring gas pressures. If the gas is connected to one side of the tube and the other is left open to the atmosphere, the difference in height measures the difference in pressure between the gas ? and the atmosphere. The diameter of the tube on the right in the diagram does not matter, as you already know. In fact it could even be a large glass bottle as shown in the third diagram; the difference in height would be the same in each case. Problems to think about I. When measuring the pressure of the town-gas supply, water is used in the manometer. When measuring a high pressure, a heavy liquid like mercury is more convenient. Why is this'? 2. In the third diagram above, the supply was connected to a small tube going into the bottom of a large glass bottle. Suppose that the supply was connected instead to the top of the hottle. Where would the liquid level come in the side tube'?

The Bourdon pressure gauge You will be familiar with the toy, shown on the left, often used at Christmas parties. The Bourdon pressure gauge, which is used for measuring the pressure of gases, is based on the same principle. When the pressure in the tube increases, the tube tries to straighten out and the pointer rotates. A scale can be put on the front of the gauge. Any pressure gauge which has a pointer moving over a scale is likely to be this sort of instrument; car oil-pressure gauges usually work in this way. 12

tube

case

pressure to be measured

Bourdon gauge

CONTROL OF PRESSURE DIFFERENCES When a high-pressure liquid or gas is connected to one at low pressure, there is a flow from the high pressure to the low pressure. Many of our everyday appliances depend on pressure difference and we need to be able to regulate this flow. We need to use taps and valves: The tap enables us to stop or reduce the flow when we want to; the valve enables us to control the direction of flow.

Industrial valves

13

PRESSURE DIFFERENCE

gas su pply

The domestic gas tap This consists of a cylinder with a hole through it, which fits into a circular opening in the gas supply tube. When the tap is closed, the hole is not in line with the gas supply. When the tap is open, the hole connects the gas main to the fire or cooker and gas flows.

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closed

open

The domestic water tap The gas tap above would not be very satisfactory as a domestic water tap. Can you suggest why not? When the domestic water tap shown above is opened, the washer is lifted up and water flows. When the tap is closed, the washer is pressed against the bottom and the flow stops. The washer is made of either leather or rubber; it can get worn and has to be replaced occasionally.

14

PRESSURE DIFFERENCE

The ball-cock The ball-cock is a special kind of tap used in the water tanks in the attic and in the lavatory cistern. When water is used from the tank, the level of water goes down. The ball floating on the water descends as the water level falls. The tap then opens to let water flow into the tank again. As the water from the inlet pipe fills the tank, the floating ball rises and closes the inlet. This ingenious device fills the tanks automatically without their overflowing. washer

15

Non-return valves A non-return valve is a device which will allow a flow in only one direction. There are valves for gases, valves for liquids and even valves for electricity, all of which allow flow in one way only. Perhaps the easiest way to understand the idea of a nonreturn valve is to make a very simple one. Take a sheet of paper, moisten your lips, open your mouth wide and press the paper against your lips, all round, as illustrated. Blow air out from your mouth; you will find it comes out quite easily. Now try drawing air into your mouth: it cannot be done because the paper is forced against your lips, blocking the entry. You have made a simple non-return valve for your mouth. It allows air to go out, but not to come in.

A simple liquid non-return valve A simple non-return valve for liquids, like that shown below, can be put within a pipe or at its opening. If the liquid is at a higher pressure on the left, it pushes open the hinge and liquid flows from left to right. If it is at a higher pressure on the right, it presses the rubber washer against the metal and liquid cannot flow. A similar arrangement can be used for gases as well as for liquids.

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liqu.dthrough can flow

16

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PRESSURE DIFFERENCE

The valve on a bicycle tyre This consists of a metal tube with a small hole at one side, closed bya short length of rubber tubing (valve rubber) fitting tightly over the tube. When air is forced in from the bicycle pump, it presses out the rubber tubing and enters the tyre. When the air from the tyre tries to get out, the rubber tubing is forced against the hole and the air cannot escape. Some bicycles now use valves of the motor-car type.

pump

The valve on a motor car tyre When the pressure from the pump exceeds that of the air in the tyre, air is forced past the tyre valve into the tyre. When the pump is withdrawn the air in the tyre forces the valve back against the metal. Air can be released by pressing the metal extension of the valve.

cap for screwing valve onto tyre

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rubber valve

17

PRESSURE DIFFERENCE

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high pressure line

18

The safety valve Many systems contain liquids or gases at a high pressure. If the pressure were allowed to get too high, there would be danger of an explosion. For this reason safety valves are fitted. Many of them are like the liquid valve above, but the hinge is replaced by a firm spring which keeps the valve closed unless a very high pressure is reached, when the spring valve is forced open to allow the pressure to drop. How do you think these valves could be modified so that they would open at different pressures? You may see such a safety valve on a model steam engine in your school laboratory. Pressure cookers and many devices in industry also use safety valves.

Can you spot the safety valve on this industrial boiler?

Pressure cooker

19

PRESSURE DIFFERENCE

Some things to think about I. The heart has two main valves for the blood. What do you think is the function of each? 2. The diagram below shows an attachment for a bicycle pump which is used for blowing up footballs. How does it work? 3. A snorkel is used for swimming face downward, looking at the sea bed. What is the purpose of the table-tennis ball? pulmonary artery (to the lungs)

venae cavae (from the body)

pulmonary vein (from the lungs)

tricuspid valve

mitral (bicuspid valve)

edge of hole bent over to stop ball-bearing

Football adaptor (old type)

20

aorta (to the body)

ball-bearing

smooth circular end

PRESSURE DIFFERENCE

Some things to do at home I. Look at the tanks at home and see the ball-cocks working. Find the main stop-cocks in the water system and decide the use of each. 2. An amusing experiment on pressure difference: shell a small hard-boiled egg (not too hard-boiled) and place it in the neck of a milk bottle. It should be a little too large to go into the bottle. Light a small piece of tissue paper with a match and drop it into the bottle. Replace the egg quickly. The oxygen in the air inside will be used in burning the paper and the gas inside will be at a lower pressure than the air outside, so the egg will be pushed into the bottle. To get the egg out, first wash away the paper, then turn the bottle upside down and blow into it as hard as you can. When you stop, the high pressure inside should blowout the egg.

21

Considerable use is made of the pressure of liquids both in our homes and in industry. Some of these applications will be considered in this section.

APPLICATIONS OF THE PRESSURE CAUSED BY LIQUIDS

THE PUBLIC WATER SUPPLY In every town and city in Britain there is a public water system which supplies water to every house and factory. The water may come from a deep well below the ground. It is pumped from this well into a reservoir, which is often covered. The reservoir is always situated on high ground, above all the places it is going to serve. (Why is this?) Water then flows out from the reservoir to the consumers. The water pressure depends on the height of the reservoir above the height of the consumers.

purnoino house

pump

C.------------_I:: well

Problems to think about I. The pressure in the public supply is sometimes inadequate for water to reach the top of a multi-storied building and a subsidiary pump has to be used. Why is this? 2. Most houses have a water-storage tank in the attic. Does the pressure at the water tap in the kitchen depend on the height of the storage tank or on the height of the reservoir?

22

APPLICATIONS OF THE PRESSURE CAUSED BY LIQUIDS

Water dams Many public water systems depend on artificial reservoirs made by building water dams, usually across a valley in which a river is flowing. The pressure of water produced in this way can be used to generate electricity and the water itself can be used for irrigation or the public supply. The engineer has to calculate the huge pressure that will be exerted on the wall of the dam. It must have deep foundations and very secure connections to the sides of the valley or it would be swept away. Problem to think about

The Kariba dam on the Zambesi river

The walls of a dam are usually very much thicker at the bottom than they are at the top. Why is this?

23

SIPHONS Siphons are widely used to carry water over such barriers as the side of an irrigation ditch. The principle of their operation is quite simple. The siphon will work only if the tube is completely full of water. The water in E starts to fall, creating a partial vacuum at D. There is then a higher pressure on the left at B than at 0, so the water flows, going into the tube at A and out at E. The siphon will continue to work as long as both the ends A and E are below the water level B. c

D

E

t Siphons are most useful for getting liquids out of containers that cannot be tipped easily, for example getting wine out of a barrel. Petrol thieves use siphons for stealing petrol out of cars, which is why it is advisable to have a lock on the petrol tank. Siphons are useful in biology laboratories for emptying or cleaning fish tanks. The siphon can be used as a kind of 'vacuum cleaner' to take away the dirt without completely emptying the tank. In each of these cases, the process is started by sucking the tube until it is full of liquid and then lowering the free end below the level of the liquid in the container to be emptied. Problems to think about I. Is the soda siphon (see page 7) in fact misnamed? Is it a siphon at all?

2. A man fills a swimming pool from a tap as shown on the next page. When the bath is full, he turns off the tap and disconnects the hose from it. What might happen?

24

APPLICATIONS OF THE PRESSURE CAUSED BY LIQUIDS

The water-flushing system Most water-flushing systems use a siphon arrangement. The simplest and most common type is shown below. When the chain is pulled, the disc is raised. Water is lifted up the narrow tube, flows over the bend and a siphoning action starts. The disc has holes in it and there is a rubber diaphragm fixed to the top of the disc at its centre. The rubber acts as a non-return valve, so that the water can flow up into the top of the cylinder, but not down from the top and out of the cylinder. The siphoning continues until the water level falls below the level of the cylinder. There will also be a ball-cock (see page 15) to control the filling of the tank again. (Not all lavatory cisterns have the rubber diaphragm: a loosely fitting piston will act almost as well.)

rubber diaphragm

25

--APPLICATIONS OF THE PRESSURE CAUSED BY LIQUIDS

Some things to do at home I. Obtain two large glass jam jars and fill them both half-full of water. Put a rubber tube into one and suck it so that it is full of water. Then immerse the end in the other jar. Move the jars up and down relative to each other so that water flows one way, then the other: the water always stops flowing when the levels are the same in each jar. Both ends of the tube must always remain under water.

2. Moving water down by steps: obtain four clear squash bottles or milk bottles, and three lengths of rubber tubing. Put the bottles on the stairs as shown. Fill the top bottle half-full of water and the next two a little less than quarterfull. Put the top end of each rubber tube right down to the bottom of the bottle and make sure the other end is lower than the top end. You have to start each siphon by sucking, starting from the top. Watch what happens. Be careful not to use more than one bottle of water altogether!

26

HYDRAULICS When a liquid is confined in a space and a pressure is applied to it, the pressure is transmitted equally to all parts of that liquid. You saw this in your home experiment on p. 8: when the polythene container was pressed hard the water squirted out in all directions. Liquids are practically incompressible. This means that if you have one litre of water, however hard you try to squash it you will still have one litre. If it is pressed down in one place it must come up in another. These two important facts, combined with the fact that {", I" ' pressure = force area' are use d iIn hy drau lilCS. (I :,C{,',cf< ''.'''C:-,:·.·.. '~ ,. A liquid-filled cylinder with a thin tube"' at one end with a piston in it, and a thick tube at the other end with a piston in it, shows the simplest form of hydraulic system. This is how it works. If the area of tube A is I cm-, and the piston is pushed with a force of 5 kgf, a pressure of 5 kgf will be exerted on each square centimetre of liquid. This pressure is transmitted equally to all parts of the liquid. Piston B will experience a pressure all over it of 5 kgf on each square centimetre. Ifpiston B has an area of 10 ern", the total force on it will be 5 X 10 or 50 kgf. This means that pushing the rod at A with a certain force will push out the rod at B with ten times that force. This enables very large forces to be produced using quite small ones. liquid

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50 kgf

large piston B 10 em'

27

APPLICATIONS OF THE PRESSURE CAUSED BY LIQUIDS

It appears that we are getting something for nothing,

but this is not really so. Can you think why? The above experiment shows us that a small area of an enclosed liquid will produce a large force over a large area of the same liquid. This principle is used in all sorts of present-day machinery. Hydraulic presses, car jacks, fork lifts, bulldozers, car brakes, retractable undercarriages, tipping lorries, railway points and dust carts are a few of the many examples.

The hydraulic jack Like all the hydraulic machinery, the jack works on this very simple principle, but a system of valves and reservoirs of liquid (usually oil) is needed to make it practical. When the handle is lifted the small piston is pushed in. This forces the liquid along the tube, closing valve A to the reservoir and opening valve B. Something has to give: the only thing that can move is the large piston. It is pushed up and, as it has a large area, it exerts a large force. large piston

sealing washer handle

small piston

28

reservoir

APPLICATIONS OF THE PRESSURE CAUSED BY LIQUIDS

When the handle is lowered the small piston moves back and liquid must flow into the pipe or a vacuum would be created. Liquid cannot flow back through valve B so the large piston stays up. But valve A opens and liquid flows out of the reservoir into the pipe. When the handle is raised again, the large piston is lifted further and this process can be repeated many times. When you want to lower the large piston, the release valve is opened and the liquid flows straight back into the reservoir. If you have understood this, you have understood the principle on which all hydraulic machinery works.

Motor car brakes Here the actual piston is pressed in by a system of levers from the foot pedal. The master cylinder is connected to a reservoir as in the jack. (This is not shown in the diagram.) The pressure is transmitted through the fluid in the pipes and forces the brake shoes against the brake drums evenly on all four wheels.

off

rear wheel brake drums

front wheel brake drums

fluid pipe lines

fluid pipe lines

29

Hydraulic digger

Hydraulicfork-lift truck

30

Hydraulic press

APPLICATIONS OF THE PRESSURE CAUSED BY LIQUIDS

Problems to think about I. Why is it very dangerous if any of the fluid pipes break in the braking system of a car? If there is a break near the front wheel what happens to the other brakes? 2. Can you adapt the hydraulic jack described to make it into a hydraulic press? 3. To put on the footbrake of a car the pedal is pushed down several centimetres, yet the brake shoes move only a fraction of a centimetre. Can you explain why this is?

Some things to do at home I. For this experiment you need either a rubber hot-water bottle with a cork and rubber tubing, or an air cushion with a blow tube, or a football bladder and tubing, or even a large balloon. Join the tube to the hot-water bottle and pile a large heap of heavy books on the hot-water bottle. Blow down the tube and you will find the huge weight of books rises quite easily. (Get a friend to steady the books.) This shows that even quite a small pressure, if exerted over a large area, can produce a very big force. 2. This is an adaptation of the previous experiment. It is rather more difficult to do. Put some water in the hot-water bottle (not very full), and hold up the tube (transparent tubing is best) as in the diagram. Now stand on a board covering the hot-water bottle and see how high the water comes. If you have a funnel and pour more water into the tube, you will notice that you and not the water level will rise. Why is this? The larger the area of board in contact with the bottle the lower the level of water. Why do you suppose this is?

31

THE PRESSURE OF THE ATMOSPHERE

We have seen that liquids and gases exert pressure on the things around them. The atmosphere is made of gases, chiefly nitrogen and oxygen. Wherever there is an atmosphere it exerts a pressure on everything in contact with it. In the work you have done at school it will have been suggested that gases are made up of tiny particles moving about at great speed. These collide with anything in their path and cause a pressure against them. Something to do at home I. You may have done this experiment at school, but if you have not you can do it now. Get a kitchen balance and a large tin full of marbles or even dried peas. Put a board at an angle on the balance and pour the marbles onto it from a height. You will see that the balance reading is greater than zero even though no marbles actually stay on the board. This shows that the bouncing particles exert a force. Perhaps the most famous experiment of all on the pressure of the atmosphere was that done by a German called von Guerickc. He used two copper hemispheres. The hemispheres fitted together so perfectly that with a greased leather washer between them they made an airtight joint They could be pulled apart quite easily when full of air, but when the air was drawn out by means of an air pump which von Guericke had devised, an enormous force was required to separate them. This was explained by the fact that the pressure of the air on the outside was no longer being balanced by an equal pressure inside. The experiment was done with hemispheres about a foot in diameter before the Emperor Ferdinand 1lI in 1651, when two teams of sixteen horses were required to pull them apart.

32

Problems to think about I. At the top of a mountain the atmospheric pressure is less than at sea level. Why do you think this is? 2. Why may it be difficult to get enough breath to exert oneself on a high mountain? 3. (Difficult) A man walking in space has an oxygen mask over his face so that he can breathe. But he also needs a space suit to cover his whole body. Why?

Some things to do at home I. Place a postcard over a tumbler full of water. Make certain there is no air bubble. When the tumbler is turned over the water will remain in the tumbler. The upward force on the card due to the atmosphere is greater than the downward force due to the water. (It is advisable to try this experiment over the bath I) 2. Lay a thin slat of wood about I m long on a table with one end extending over the edge. Strike this end with your fist and it flies off the table. Now place the slat back on the table and cover the part on the table with a few sheets of newspaper, with perhaps a piece of light cardboard underneath. Now hit the free end again hard. The effect you will find is due to the greater weight of air that presses on the larger surface of the papers.

suck

r

3. This is a spectacular experiment and well worth doing. When the air is taken out of a large tin can (an oil can will do) it crumples up. The pressure of the outside atmosphere is no longer balanced by the pressure of the air inside. Most of the air can be removed from the can by boiling a little water in the bottom, which fills the can with steam and drives out the air. If the can is now sealed and then plunged into cold water the steam condenses, leaving a partial vacuum inside the can. Remember it is very important not to boil the water with the cap on, which could cause a dangerous explosion. A good seal is also important: some Vaseline on the screw cap can help this. 4. A balloon attached to a drinking straw inside a milk bottle will swell out as the bottle is evacuated, since the atmospheric pressure inside the balloon is greater than that outside it. The simplest form of home vacuum pump is yourself. Use two straws, one with a balloon on, and an airtight seal with Plasticine. Suck through the free straw and the balloon expands, blow through it and the balloon collapses. This is probably the first time you will have blown up a balloon by sucking. Our lungs use the same principle, as will be explained below.

33

BREATHING A simple example of how we breathe is provided by the apparatus shown (left). You may have seen this in class. When the marble is pulled the rubber sheet comes down and a partial vacuum is formed inside the jar. No air can get into the jar, but air can come down the tube and blow up the balloon in an attempt to make the pressure the same as the air outside. When the marble is released the balloon is squashed again and the air rushes back into the atmosphere. In your body your ribs and skin take the place of the glass jar used in the model. Instead of the rubber sheet there is a sheet of muscle called the diaphragm across the bottom of the chest cavity. This makes the cavity airtight. When the muscles of the diaphragm, which is shaped like an inverted saucer, contract, it becomes flatter. This enlarges the chest cavity and, together with expansion of the rib cage, causes a partial vacuum into which the air from the atmosphere flows. The reverse happens on exhaling. In order to get the maximum amount of oxygen to the lungs it is important to breathe properly.

34

THE PRESSURE OF THE ATMOSPHERE

MEASURING THE PRESSURE OF THE ATMOSPHERE An instrument for measuring the pressure of the atmosphere is called a barometer. The pressure of the atmosphere is caused by the weight of the air above us, and the simplest form of measurement is to balance this against a column of mercury.

v-

The mercury barometer

vacuum pump

If a large U-tube is filled with mercury and a vacuum

76cm

76cm

pump is attached to one side the difference in levels is 76 ern. This means that the pressure of the air is the same as 76 em of mercury. An easier way to do this is to fill a tube, sealed at one end, with mercury and turn it upside down in a trough of mercury. The level falls until the pressure at the base of the tube is the same as the air pressure. Only the atmosphere is exerting a pressure above the open trough and there is only the pressure of the mercury in the tube, since there is a vacuum above it which can exert no pressure. The thickness of the tube does not matter. A thicker tube would give a bigger weight over a bigger area and hence the same pressure, as we have seen in the manometer on p. 12. By weighing the mercury we discover that the average pressure of the atmosphere is about I kgf on each square centimetre. If the table at which you are sitting is about I m square (10 000 cm 2), the total force on top of the table is about 10 tonnes, like a small steam roller. The only reason why the table does not collapse is that, as we have seen before, the air pressure acts equally in all directions and there is an equal force underneath pressing upward.

The aneroid barometer The most common type of domestic barometer contains no mercury but has a corrugated container with a partial vacuum inside. The can is kept from collapsing by a 35

strong spring. When the atmospheric pressure on the can changes, the spring moves slightly. This movement is magnified by a series of small levers and moves a pointer round a scale marked Stormy, Change, Fair, etc., as a very rough guide to the weather.

THE PRESSURE OF THE ATMOSPHERE

levers for magnifying movement

hmge corrugated metal box partially exhausted of air

The barograph This is exactly the same as an aneroid barometer but it is used for making a continuous record of the pressure. The needle is replaced by a pen, and a drum slowly rotates with a paper on it.

fair

Something to do at home stormy

36

I. To make your own barometer, cut the end from a round toy balloon. Then stretch it smoothly over the mouth of a milk bottle and tie it in place. Fasten one end of a drinking straw to the centre of the rubber cap with a drop of sealing wax. For a scale, mark some cardboard and prop it up beside the straw. (This will give a true result only if readings are always taken at the same temperature.) When the pressure goes up, the cap is pushed in and the straw rises. When the pressure falls the reverse happens.

APPLICATIONS INVOLVING ATMOSPHERIC PRESSURE

Not only do we need the gases in the atmosphere to breathe, but large numbers of things depend on the pressure of the atmosphere.

Sucking through a straw A true scientist will tell you that there is really no such thing as sucking. The reason for this is that the effect of sucking on a straw in a bottle of milk is in fact caused by the pressure of the atmosphere pushing the substance into your mouth. When you suck, you lower the pressure inside the straw and atmospheric pressure does the rest.

Something to try at home I. Get a milk bottle and fill it right to the top with water. Put one straw through some Plasticine and make an airtight seal. Now try and suck the water out of the bottle. You will find you cannot do it. Make a second hole in the Plasticine for another straw to let in air. You can now suck the water out easily. The reason is that you create a lowpressure region in your mouth and the atmosphere is at a greater pressure, so it presses down on the water and presses the water up the tube. If there is no air-hole the atmosphere cannot press down.

37

The syringe This works on just the same principle as the straw: raising the piston produces a low-pressure region and atmospheric pressure pushes the liquid into the syringe.

The lift pump This is used for getting water out of a well or tank. Study the diagram to understand the working of the valves. When the piston is raised a lower pressure is produced in the cylinder. Valve B closes and valve A opens. The atmosphere pushes water up the tube into the cylinder. When the piston is lowered again, valve A is pushed shut and valve B opens and the water passes through. When the piston is raised again, more water comes from the well into the cylinder and the water above B is lifted, coming out of the spout. Thus water comes from the spout on every subsequent upstroke of the pump. Syringe

38

APPLICATIONS INVOLVING ATMOSPHERIC PRESSURE

Problems to think about I. It is not possible to use a lift pump to get water from a well much more than 9 m deep. Why do you suppose this is? 2. This is a diagram of a force pump. There are valves at A and B. Try and work out which way each one opens. The force pump, as its name suggests, sends out water with a greater force than the lift pump; it also sends out a more continuous supply. Can you suggest why?

3. Rubber is a substance that always tends to return to its original shape after being compressed. A pen nib has a small hole from the ink tube rather like the nozzle of the syringe. Can you see how a fountain pen is filled with ink? Why does it work? metal strip

filling lever

rubber tubing

39

APPLICATIONS INVOLVING ATMOSPHERIC PRESSURE

Artificial lung

Artificial lung A patient is put into an artificial lung when his diaphragm muscles have stopped working and he cannot enlarge his chest cavity on his own. His body is enclosed by an airtight metal container. A pump unit alternately increases and decreases the pressure inside the container. When the pressure inside the container is below atmospheric pressure, air is forced through the patient's mouth or nose into his lungs. When above atmospheric pressure the air is forced out. The behaviour is like the glass jar and balloon experiment, described on p. 34.

Altimeter This is essentially the same as an aneroid barometer. The zero is altered according to the prevailing atmospheric conditions communicated to the pilot by radio. As pressure drops with height the needle goes round and is graduated in feet above sea level. The pressure drops approximately the equivalent of I em of mercury for every 120 mr40

Rubber suckers

wall

atmosphere pressing In

A large sheet of plate glass. held by rubber suckers. being moved in afactory

Rubber suckers are used for a variety of purposes: for wall hooks, for picking up sheets of metal and in general for gripping on any smooth surface. Their operation depends on atmospheric pressure. When the rubber cap is pushed against the smooth surface, air is forced out from between the cap and the surface. When the cap is released the rubber goes back to its original shape and a partial vacuum is created inside. The force of the atmospheric pressure on the outside holds the attached object in position against the surface.

Weather maps From large numbers of recording stations over wide areas of the earth the pattern of atmospheric pressure is built up. The lines linking areas of equal pressure are called isobars. The patterns usually form wide loops. If the pressure is high in the centre of the loop it is called an anticyclone, and if it is low, a cyclone or depression. If the isobars are close together the winds are strong and if far apart the winds are weak. 41

You would expect the winds to blow straight from high pressure to low pressure, but because of the effect of the rotation of the earth this is not in fact the case. In the Northern Hemisphere the winds tend to blow clockwise round an anticyclone (high-pressure area) and anticlockwise round a cyclone (low-pressure area). The darker lines that you see are called fronts, either where a mass of warm air is moving over cold air [• • . , a warm front], or where cold air is moving under warm air ~ , .. , a cold front], Rain is often associated with these fronts, particularly with warm ones. With these maps meteorologists are able to give a rough guide to what the weather is likely to be for several hours ahead.

42

Problems to think about I. If a pilot flies into a low-pressure region without having been warned from the meteorological report, it can be very dangerous. Why? 2. This is a diagram of a plunger used for clearing blocked waste pipes. Try to explain how it works. It works better if the pipe and cap are full of water. Why do you think this is? (difficult). This is called 'the plumber's friend'. 3. Why do you think the winds are stronger when the isobars are closer together?

Something to do at home To do this experiment you need some sort of rubber sucker, a tin and some weights. Attach the rubber sucker to the tin with a piece of string. Push the sucker onto a flat surface between two chairs. See how many weights are required to pull it off. This will give you some idea of the force of the atmosphere on the sucker.

smooth surface

tin for weights

43

HIGH PRESSURES

cylinder

We have already seen that a liquid cannot be compressed, but gases are 'squashy' and we can use this squashiness to do useful things. If we enclose a gas and then make the volume smaller the pressure of the gas increases. This certainly makes sense if we consider the gas as made up of lots of fastmoving invisible particles which are causing the pressure. When the volume is made smaller, each particle will hit the side more often and consequently there will be a greater pressure overall. There are many useful applications of gases under pressure. They all depend on the tendency of a compressed gas to expand in volume when the pressure is released and to compress again when the pressure increases. The compressed gas inside the soda siphon (p. 7) expands when the valve is open and pushed the liquid out of the spout. The compressed air inside a car or bicycle tyre causes it to move in and out on rough roads, smoothing the journey over the stones and potholes. We will now consider some of the applications of compressed gas.

The bicycle pump

spring -----.-.,......,

,

~Ieatherwasher

connector to tvre valve

44

The bicycle pump has a piston which moves up and down inside a cylinder. The piston has a cup-shaped leather washer pointing downward and this acts as a pump valve, allowing air to go only downward. When the piston is pushed down, the air between the piston and the tyre valve (p. 17) is compressed and the sides of the washer pressing against the cylinder walls provide an airtight seal. When the pressure between the two valves becomes greater than that in the tyre, air is forced past the tyre valve into the tyre. When the piston is brought back, the washer no longer presses hard against the cylinder and air comes from the handle end filling the space.

The rotary air pump This pump can be used for producing a better and more continuous supply of compressed air. It is driven by a motor and thus the valve effect works very much more quickly. The central disc rotates as illustrated, and has two spring-loaded vanes incorporated in it which maintain contact with the walls of the containing cylinder. This contact is airtight and the whole is immersed in oil. Study the diagrams carefully and see" if you can see how it works. Notice that one part of the central disc is always in contact with the containing cylinder. (The density of dots in the diagram indicates the pressure of the air.) If this pump is connected to a closed vessel at the intake it can be used as a vacuum pump.

Pneumatic drill A pneumatic drill is really an air hammer and it is exceedingly useful for digging and breaking up rocks or concrete. It is really like a pump that works backwards. A tube carries compressed air to the cylinder; inside the cylinder a piston moves back and forth very rapidly as a valve opens and closes.

Riveters A rivet is a type of metal bolt for binding two pieces of metal together. The headless end is passed through two holes and then flattened out. To do this a hammer piston worked on compressed air like the pneumatic drill is used.

The air rifle and the pea-shooter The air rifle is a rather grander automatic version of the simple pea-shooter. Put a small pellet in the end of a drinking straw (or a hollow Biro tube) so that it just fits the hole, and then put that end of the tube into your mouth. When you produce a sudden increase of pressure in your mouth, there is nowhere for the gas to escape except down the tube. To go down the tube it has to press the pellet in front of it and it is shot out at the end. In the air rifle the spring is attached to a cylinder and piston. When the piston is pulled back the spring is caught on a catch. The catch is released by pulling the trigger, the _ . spring compresses the air in the cylinder very suddenly R==r7O"irr and the only way it can escape is by pushing the exactly fitting pellet out of the barrel. The cartridge rifle works on the same principle, but the compressed gas is made even more instantaneously by exploding chemicals with a hammer blow.

pellet

46

washer

piston

mainspring

[HIGH PRESSUREi]

Hovercraft The hovercraft is a comparatively new form of commercial use of compressed air. A powerful air pump forces air downward and the only way that it can escape is by raising the whole craft until the curtains are just clear of the ground. This means that the craft floats on a cushion of air. It is then driven along by propellers on top just like an aeroplane. It can go over land or water and can move much faster than a ship as it does not have the same drag of water to stop it.

47

I HIGH

PRESSURES I

Steam pressure When water boils it turns into steam; if the steam is kept in an enclosed region it will exert a considerable pressure. This pressure can be used to drive pistons or turn turbines. Much of our electricity is made by burning coal or oil, or by using nuclear power to turn water into steam. The steam turns a turbine, which drives a generator, which produces electricity.

Above: Steam-driven piston of a locomotive

Below: A steam turbine at a power station

[~UGH

PRESSURES I

The pressure cooker The pressure cooker is another device which uses gas under pressure. As the gas (steam) above the water is at a high pressure, the temperature at which the water boils is raised and consequently everything cooks more quickly. It is a common misconception that it is the pressure forcing the steam into the food which gives the speed of cooking, but in fact the increase of temperature is the important thing. The more weights that are put on the valve the higher the pressure and therefore the higher the temperature at which the water boils. safety valve

pressure control valve - - - r ' - - - - - - -

Internal combustion engines All internal combustion engines work on the principle that if an explosion of fuel and air takes place in a confined space, considerable pressure is built up. This pressure can be utilised for driving machinery. The petrol engine, diesel engine, jet engine and rockets all work on this principle. These are discussed in detail in other books in this series.

49

[ HIGH PRESSURES I

Problems to think about I. What other devices do you know that use compressed air? 2. This is a diagram of a door check. How do you think it works? /'

/'

//'/'/'/'/'/'\/'/'d oor /'

/

/'

/'

/'

/'

/'

/'

/'

/'

/' ./

/'

./

/'

-:

/'

/'

/'

././

/'

./ /'

./

3. This is a bulb pump for blowing up air mattresses. How does it work?

circular hole~­ intake

--7 r-r-r-r-r--r-r-r-«

to lilo

4. Why is it difficult to hard boil an egg on top of a high mountain?

Some things to do at home I. Take a bicycle pump apart and examine the valve; turn the valve round and see how it becomes a suction pump. 2. Fill a milk bottle almost full of water. Put a block of Plasticine over the top and turn the bottle upside down. Notice that you can change the size of the air bubble by pressing hard on the Plasticine. (Do this over a basin ')

50

PROTECTION AGAINST EXTREMES OF PRESSURE

The human body has evolved in an atmosphere which exerts a pressure equal to I kgf per square centimetre. When this pressure is changed great care has to be taken. The ear is probably the human organ most sensitive to changes of pressure, and you may have noticed your ears 'popping' when going up or down a big hill in a car, the pressure being greater in the valley. You will certainly have felt this pressure on your ears if you have swum to the bottom of a swimming bath at the deep end. The diagram of the ear shows the auditory or Eustachian tube which exists to equalise the pressure on either side of the ear drum. If this is blocked, quite small changes of pressure could burst the ear drum. If it were perfectly free, there would be no discomfort over a range of several atmospheres. The sensation comes from slight blockage and can usually be relieved by swallowing, which opens up the tube. You may have guessed that sound may be some sort of pressure phenomenon as it makes the ear drum move.

51

DEEP SEA DIVING When a diver goes down to examine a sunken ship, he needs a supply of air for breathing. This air is usually supplied through a long tube from a ship on the surface. Air pumps on the ship force air down the tube. As the diver goes deeper, the water pressure becomes greater. To overcome this water pressure, the pressure of the air going down the tube must be increased. If a diver goes down only 20 m the pressure of the air must be three times as great as atmospheric pressure, or about what it is in a car tyre. The deeper he goes, the greater the pressure must be. The diver has to be brought back to the surface very slowly so that the pressure on him lessens gradually; otherwise he is likely to suffer from a very dangerous complaint known as 'the bends'. The bends occur because at high pressure extra nitrogen dissolves in the blood. If the pressure is released quickly bubbles may form in the blood as the nitrogen comes out. If this happens in the brain it is often fatal.

The diving bell This is used for work on the sea bed. The cabin is lowered to the sea bottom and air is forced in from the ship above to clear the water. The divers then work in their bubble.

The bathyscaphe Water depths of up to 10000 m can be examined by deep ocean exploration. At this depth the pressure on any object is some 1000 kgf force on each square centimetre or nearly I tonne per square centimetre. No human being could withstand this pressure without protection, so people are lowered in a spherical steel chamber with walls about 15 ern thick, called a bathyscaphe, to examine the depths. You will understand the necessity of this strength of protection if you have done the experiment of the collapsing can on p. 33. The pressures here are 1000 times greater.

52

-PROTECTION AGAINST EXTREMES OF PRESSURE

PRESSURE IN AEROPLANES Passenger aircraft often travel at heights of 6000 m or more. At these heights the pressure of air is less than half as great as at sea level and the air therefore contains too little oxygen to support life without acclimatisation. Aircraft use compressors to keep air pressure in the craft the same as it is neartheground.Thesecompressors squeeze the thin air together to keep the cabins pressurised.

SPACE SUITS At the altitude of a spacecraft in orbit there is practically noatmospheric pressure, in other words there is nearly a vacuum. Because of this, astronauts must wear a space suit which encloses the air they breathe. The suit must cover their entire body or they would swell and their blood would boil. Problems to think about I. Why does the diver wear very heavy boots? 2. The astronaut is not fat and his clothes are not particularly thick. Why does he look so fat when walking in space? 3. If an astronaut's space suit is punctured, his blood boils. Why? (Hint: consider why the pressure cooker works and think of the reverse.) 4. Does it matter whether or not the diving bell touches the sea bed? 5. Why does an air hostess hand round sweets before take-off?

Astronauts on their way to the launching pad

Walking in space

53

FLOATING AND SINKING

On p. to we discussed how the pressure in a liquid increases with depth. If a block is suspended in a liquid as illustrated , the liquid all round the block will exert a pressure on all its faces. The pressure at the bottom will be greater than it is at the top. As there is more pressure pushing up than pushing down, there will be a net force upwards (an upthrust, as it is often called). Archimedes was the first person to realise that the upthrust must be equal to the weight of the liquid displaced (pushed out of place) by the block. This upthrust leads to apparent loss of weight of the block. This is best shown using a stone hung from a spring balance. When hung in air, the spring balance tells us the weight of the stone. Fill a can to the brim with water and put the can in an empty bowl. Carefully lower the stone into the can. The water that is displaced goes over into the bowl and can be weighed. The spring balance reads less. There appears to be a loss in weight, which will be found to equal the weight of water that overflowed. This is known as Archimedes' principle. Problems to think about I. Why do you think it is easy to keep your legs sticking out horizontally in a bath of water, but it is a great strain to do it if you let the water run out?

I 54

2. Have you noticed that when you walk over pebbles into the sea, the pebbles hurt your feet badly at the edge, but the further you get into the water the less they hurt? Why do you think this is? 3. If a cork is held below the surface ofthe water and released, what happens? How does the upthrust compare with the weight of the cork? 4. When a cork is floating on the surface of the water and is quite still, what can you say about the upthrust? 5. When a model ship floats on a pond, what can you say about the upthrust?

Some things to do at home I. You will need a fairly heavy glass jug, a large saucepan, a plastic washing up bowl and some kitchen scales. Weigh the glass jug on the kitchen scales. Fill the saucepan to the brim with water and put it in the plastic bowl. Float the jug carefully in the saucepan. Some water will be displaced into the bowl. Remove the jug. Remove the saucepan. Weigh the water that overflowed into the bowl. What do you notice? (Hint: to weigh the water that overflowed, weigh the bowl empty and then with the water in it.) 2. Repeat the first experiment several times, but instead of starting with an empty jug put objects of differing weights in the jug (for example, a tin of beans) before weighing it. What do you notice about the depth of floating? What do you notice about the weight of water overflowing?

55

FLOATING AND SINKING

FLOATING We have seen above that when a body is put in a liquid there is an upthrust and Archimedes suggested that the upthrust equals the weight of liquid displaced. If the upthrust is greater than the weight, the body rises - you experienced that when you held a cork below the surface and let go. If the upthrust is less than the weight, the body moves downward; putting an iron block in water shows this. If the upthrust equals the weight, the body floats. Cork is less dense than water: cork weighs less than the same volume of water. It therefore floats in water. Iron is more dense than water: iron weighs more than the same

Right: Queen Elizabeth 2

-

~~

pure water level

volume of water. Therefore a solid block of iron sinks. For this reason, many people said at first that it would be impossible to build ships of iron. Can you explain why an iron ship floats?

-~~puremilklevel

DEPTH OF FLOATING air

mercury

56

An object, put on a liquid surface, will sink until it has displaced its own weight of liquid (then the upthrust will equal the weight). The denser the liquid, the less far it has to sink. Use is made of this principle with an instrument called a hydrometer, which is used for several purposes, such as testing the purity of milk. The hydrometer does not sink as far in pure milk as in water. Another common use is in

checking a car battery. The tester has a small hydrometer inside a glass container. When the rubber bulb at the top is squeezed and let go, the acid is drawn into the glass container. The hydrometer floats in the acid. From the depth at which it floats, the density of the acid can be checked. When a ship floats, the upthrust equals the weight. If a load is added so that the ship has extra weight, an extra upthrust is necessary if it is still to float. It therefore floats deeper in the water. If the load becomes too great, the ship is not able to displace enough water to produce enough upthrust - so it sinks. Special lines, called Plimsoll lines, are marked on the sides of all ships to show to what depth they can be loaded with safety. Below: Empty and loaded

FLOATING AND SINKING

FRESH WATER AND SEA WATER There are two inland seas, the Dead Sea in the Middle East and the Great Salt Lake in the United States of America, where the concentration of salt is so great that bodies float quite high in the water. The photograph shows a bather floating in the Great Salt Lake, in which it is impossible to sink. The bather still weighs the same amount and his volume remains the same. Why does he float higher? A ship will also float higher in sea water than it will in fresh water. Why is this? A person can be made to float more easily in water by using an inflated ring or a life jacket. These do not add much to the weight of the bather, but they increase his volume considerably. The displacement is therefore greater.

SUBMARINES A submarine must be able to float on the surface or sink below it as desired. In the hull of a submarine there are large buoyancy tanks. When the submarine is to submerge, these tanks are opened to allow water to come in. The water adds to the total weight of the submarine, without increasing its volume, and the submarine begins to sink. When it is to return to the surface, the water is forced out of the buoyancy tanks by compressed air, the weight is reduced and the upthrust makes the submarine nse, 58

THE FLOATING DOCK The same principle is used in the floating dock. Large water-tight compartments are filled with water so that the dock sinks deep enough for the ship to sail in over the top. Compressed air then blows the water out of the compartments and the ship is raised out of the water. 59

.... MARINE ANIMALS Most fish are able to remain at a particular depth in the sea. Many fish have an air sac, called a swim-bladder, which occupies about 5% of the total body volume. The air sac can be adjusted so that the upthrust at the depth at which it usually lives and feeds is exactly equal to its weight.

FLOATING IN AIR A balloon full of hydrogen displaces an equal volume of air. Air is more dense than hydrogen and the upthrust is greater than the weight of the balloon of hydrogen; therefore the balloon rises. Archimedes' principle applies in gases just as it does in liquids (though the upthrusts are much smaller since gases are so much less dense than liquids). Something to do at home The obedient diver: take the hollow tapering bottom half of a ball-point pen. Block the hole at the wider end with a little Plasticine. Add more Plasticine until the pen just floats in water. Now put it in a milk bottle full of water. Place a big block of Plasticine on top. (A screw-top squash or lemonade bottle also works very well.jThe screw top can be used in place of the Plasticine.) When you press it, the pen sinks; when you release it, the pen comes up again. The reason for this is that the air in the pen gets compressed as in experiment 2 on p. 50. The pen thus loses its buoyancy, becomes heavier than water, and sinks. When the pressure is released the air expands again and the pen rises. Warning: don't press too hard or the bottle might break.

Problems to think about I. When a swimmer is lifted from the sea into a rowing boat, he appears to get heavier as he leaves the water. Why is this? 2. If an egg is placed in a tumbler and water is added, the egg stays on the bottom. If a large quantity of salt is added to the water and stirred, the egg floats. Why is this? You can try this for yourself. 3. (Difficult) If you are in a rowing boat on a pond and you throw a large anchor from the boat into the pond, does the water level in the pond rise, fall or remain the same?

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APPENDIX 1

THE BERNOULLI

EFFECT

There are several aspects of pressure that are really beyond the scope of this book, but one of them will be mentioned briefly here. The pressure in fast-moving gases or liquids is lower than in slow-moving or stationary ones. If you hold a piece of paper beneath your mouth, so that it hangs down, and then blow over the top of it, the paper moves up because the pressure underneath is higher than that on top. Many important things depend on this Bernoulli effect for their working. The lift of a plane wing, a scent spray, the carburettor of a car, the air hole in a bunsen burner and the swinging of a ball in the air when it is spun are just a few.

'--

I_ow_P"ce:::~

rubber bulb atmospheric pressure

Scent spray

Problem to think about If two ships sail very close side by side they tend to drift together and collide. Why'?

Something to do at home Get a ball-point pen tube, point the narrow end upward, blow hard through it and put a table-tennis ball in the jet. Can you explain what happens'?

APPENDIX 2 SOME FACTS ABOUT PRESSURE

force I. Pressure = - area The international (SI) unit of force is I newton and pressure is measured in newtons per square metre (Njm 1 ) .

The kilogramme force is, however, a useful practical unit of force and the corresponding pressure units are kilogrammes force per square centimetre (kgf/cm-). (On earth I kgf is approximately 9·8 N.) In Great Britain it is still common for many pressures (e.g. motor-car tyres) to be measured in pounds force per square inch (Ibf/in"). 2. An unbalanced pressure on a body will cause it to move. 3. Any pressure will cause a body to deform, in other words to change its size and shape, and in some cases possibly to break. This applies in particular to: (a) Solids - which will break or flow if the pressure exceeds a critical value (hence the stiletto-heel effect) and will go on breaking down until the pressure falls below the critical value. (b) Liquids - which will not change their volume and which will change their shape only if their container changes shape or if they are not fully contained by a solid. (c) Gases - which will be compressed by pressure until their internal pressure matches the applied pressure. 4. An unbalanced pressure will tend to squeeze matter (gaseous, liquid or solid) through an opening. This is really a combination of effects 2 and 3 above. 5. Fluids (liquids and gases) always flow from regions of high pressure to regions of low pressure if they are connected together. The rate of flow depends on the difference in pressure and on how easy it is to get from one to the other (for example, thickness of tube, etc.). 6. A device that controls the flow of a fluid through it is called a valve. If the valve allows the fluid to flow in only one direction, it is called a non-return valve.

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I APPENDIX

2

7. The pressure caused by a liquid depends only on the depth and density of that liquid. If h is the depth of the liquid . _./.,,', p is the density of the liquid ..•. ' g is the strength of the gravitational field then P». hpg, 8. Liquids are (almost) incompressible. 9. The mechanical advantage of a hydraulic press - that is, the load that can be lifted over the force needed to lift it - is given by

.' . mechanical advantage = area of liquid surface lifting the load area of liquid surface being pressed by die force;' ,. 10. The normal pressure of the atmosphere at sea level is about 1 kgf'/cm- (lO"Njm 2) , or the equivalent of a column of mercury 76 em high. 11. An instrument for measuring the pressure of the atmosphere is called a barometer. 12. A perfect vacuum is a space containing no solid, liquid or gas, and thus exerts no pressure. A space containing gas at a lower pressure than the atmosphere around it is called a partial vacuum.

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