Electric Currents (Longman Physics Topics)

LONGMAN PHYSICS TOPICS

General Editor: John L. Lewis

ELECTRIC CURRENTS John L. Lewis Senior Science Master, Malvern College formerly Associate Organiser NufJield a-level Physics Project

and Philip E. Heafford Senior Science Tutor and Lecturer University of Oxford Department of Education

Illustrated by T. H. McArthur

LONGMAN

LONGMAN GROUP LIMITED

[

London Associated branches, companies and representatives throughout the world

© Longman

Group Ltd iformerly Longmans, Green & Co Ltd) 1969

All rights reserved. No part of this publication may be reproduced. stored in a retrieval system, or transmitted in any form or by any means, electronic. mechanical. photocopying. recording. or otherwise. without the prior permission of the Copyright owner First published 1969 Reprinted with corrections 1970

SBN 582 32175 1 Printed in Great Britain by Butler & Tanner Ltd. Frome and London

I

ACKNOWLEDGEMENTS I

The author and publisher are grateful to the following for permission to use photographs: Belling and Co Ltd, page 8; Bill Switchgear, page 50; British Lighting Council, page 19 (left); British Railways (Western Region), page 24; Copper Development Association, page 30; Electricity Council, Belling and Co Ltd and Heatstore, page 6; English Electric Co Ltd, page 9; Fox Photos Ltd, page 4; General Electricity Co Ltd, pages 19 (right) and 20; Institute of Welding, page 14 (right); Keystone Press Agency Ltd, page 36 (right); Morphy Richards Ltd, page 7; Paul Popper Ltd, page 36 (left); Radio Times Hulton Picture Library, pages 13 and 14 (left); Venner Ltd, page 55; Wilmot Breeden, page 28; Hamblin and Co Ltd, page 2 I. The photographs on pages 29 and 32 are by Philip Heafford.

1

NOTE TO THE TEACHER

This book is one in a 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 that project. It was always intended that the Nuffield teachers' materials should be accompanied by background books for pupils to read, and a number of such books is being produced under the Foundation's auspices. This series is intended as a supplement to the Nuffield pupils' materials: 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, and sometimes to help them catch up on any 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 the teacher may feel that one section at a time is suitable for each homework session for which he wishes to use the book. This particular book was written as a background book for the electricity section in Year II of the Nuffield course but it can also be used with other courses. The chapter on the magnetic effects of a current stops short before the consideration of motors, the moving-coil meter and electromagnetic induction. These are topics which are in Year III of the Nuffield course and for which there is a separate background book, Electromagnetism. Though there is brief reference to the transmission of electricity, no detail is given as this topic is studied in Year IV.

3

[

INTRODUCING THIS BOOK

!CONTENTS

In your work in school, you investigated electric currents. Electricity plays a very large part in the lives of all of us. So important is it that we tend to take it for granted, but it would make an immense difference to our lives if we had to do without all the things that depend on electricity. You might try a game listing all the things that use electricity - first in your home - then in your town or village - finally throughout the country. The list grows very large. In this book we will discuss how the things you investigated in your laboratory are applied in everyday life. We will discuss some of the electric appliances used in our homes, as well as some industrial applications of electric currents. Electricity can be very dangerous, especially for those people who do not know how it should be used correctly, and safety precautions are very important. We have therefore included a chapter on these in this book. There is also a chapter on the wiring system in a house. We hope that by the time you have read this book you will agree how important electricity is in the lives of all of us and that you will understand some of the ways in which it is applied.

Heating effects of an electric current Electric light Magnetic effects of an electric current Chemical effects of an electric current Safety Electric wiring in a house Switches and automatic control Appendix

7 12 20

26 31 39

47 56 5

Living room showing storage fan heater in the window, radiator and convector heater in the fireplace, television set, tape recorder and strip lighting.

6

HEATING EFFECTS OF AN I ~~ECTRIC L£URRENT

ELECTRIC FIRES You have already discovered in the laboratory that copper wire is a very good conductor of electricity. Certain wires, however, are not such good conductors: these are known as resistance wires. You found that they got hot when an electric current passed through them. This heating effect, due to an electric current, is used in a large number of electrical appliances in your home. The most obvious one is the electric fire. In this, a long length of resistance wire is usually wound into a long coil, to save space, and then connected to the electric socket (the electrical supply) in the wall. When switched on, the wire glows red hot and gives out energy as heat. Problems to think about" I. The resistance wire in an electric fire is usually wound on fireclay, an insulating material. Why is an insulating material used? What would happen if the bare resistance wire were wound on a piece of iron? 2. You have discovered in the laboratory that some materials conduct electricity: for example pieces of iron or copper. Others, like paper, do not appear to conduct, and we call them insulators. Wood is an insulator. It might be a good material on which to wind the resistance wire of the fire element, but it is not. Why not? 3. Why does the resistance wire get hot and not the copper wire of the cable connecting the fire to the plug')

Single-bar electricfire. ':' Throughout this book, there will be a number of these problems for you to think about yourself. They will usually be easy, but if you have difficulty thinking out an answer, try discussing them at home or ask your teacher what he thinks.

7

HEATI NG EFFEC TS OF AN ELECT RIC CURR ENT

Refle ctors in an electr ic fire In the electri c fires illustra ted there is a shining metal

surfac e behind each elemen t. Why is it there? Somet hing to do at home

Investigate the heat given out by any electric fires you have at home find in the homes of your friends. See how the direction of the heator you can radiation depends on the shape of the reflector. Moving the back of your hand in a large circle around the fire is a good way of investigating this.

The numb er of elem ents Some electri c fires have only one elemen t or bar. Some have two or even three bars. The electri c fire below has three bars, all of them alike, but each has its own switch, so that one, two or three bars can be used as require d. How must they be connec ted to the supply ? In series or in paralle l? (Hint: How did you conne ct two lamps to one cell in your circuit board in order to get each lamp equally bright at norma l brightn ess?)

Three-bar electricfire.

8

HEATING EFFECTS OF AN ELECTRIC CURRENT

Problems to think about 1. When using the circuit board, you used a short length of resistance wire with one or two cells and the wire got hot, but not red hot. If you shortened the wire, it got hotter but still not red hot. In the electric fire you have a much longer length of resistance wire, but it glows red hot. What is the difference? 2. (This is a more difficult problem.) An electric fire is designed so that it can be used on the 240-volt mains supply in England. It is taken to America where the supply voltage is only 110 volts. What will happen when the fire is plugged into the mains in America and switched on? Someone suggests that to get the wire to its usual brightness two such fires should be connected in series. Someone else says that the resistance wire should be shortened to half its length. Which of these suggestions would be more effective?

OTHER HEATING DEVICES Not only is electricity used for electric fires, but it is also used for heating purposes in electric kettles, immersion heaters, electric toasters, electric blankets, electric cookers and electric irons. In each of these it is the heating effect of an electric current that is used.

Electric cooker with pre-set thermostats and automatic timer. Flat hot-plate with heating element. Tubular. spiral hot-plate.

*

There are two types of hot-plates used on electric cookers, One is a perfectly fiat hot-plate which makes contact with an equally fiat-bottomed saucepan. Its heating elements are set inside the body of the hot-plate itself. The other type has its heating elements wound inside tubular metal spirals. These can heat any kind of saucepan by radiation and not by conduction; they can even be made red hot. In an electric iron, the heat must again be spread evenly over a large surface, so the heating element is set inside the metal baseplate and insulated from it. The heating 9

HEATI NG EFFECTS OF AN ELECTRIC CURR ENT

water inlet

Heating element in an electric iron and (right) a steam iron.

elemen t is wound on a flat sheet of mica and sandw iched betwee n two thin insulat ing sheets. To iron clothe s flat, it is necess ary to have them slightly damp: that is why water is often sprink led by hand on to dry clothes when ironing with an ordina ry iron. This is not necessary if a steam iron is used. Some water is poured into the special contai ner in this type of iron and the electri c curren t not only heats the basepl ate, but it also boils the water so that steam comes out undern eath, betwee n the iron and the clothes , autom aticall y dampi ng the clothes. Proble m to think about The makers of steam irons usually suggest that they should be filled with distilled water. It does work. however, if ordinary tap water is used and one does not get a shock from it. This suggests the reason for distilled water is not because of electrical safety. Why do you think distilled water is suggested? (Hint: Have you tried boiling away both tap water and distilled water in your Chemistr y course? What was the difference in each case")

In each of the following applian ces, the heatin g elements have very differe nt shapes. Why do you think the particu lar shapes were chosen in each case'? The small immer sion heater can be used for heatin g liquids. Larger ones are used in big tanks for provid ing domes tic hot water. 10

Electrical appliances, showing the heating element in each.

Problems to think about I. You found in the laboratory that pure water (distilled water) did not conduct electricity, but tap water did conduct. If the heating element in an electric kettle merely consisted of a length of bare resistance wire coiled in the bottom of the kettle, what would happen when the kettle was filled with tap water and it was switched on? 2. (A little more difficult.) An electric kettle is used to boil water. If it is left on, the water eventually boils away. There is a special 'cut-out' incorporated in the kettle, which switches off the current. Why do you think it is necessary to have such a safety cut-out" What might happen if there was not one" 3. (More difficult.) Some electric kettles have another type of cut-out which switches off the electric current when the water actually boils. In what way is this different from the cut-out in problem 2 above? How do you think it works') 4. If you look down into an electric toaster as it is toasting bread, you will see the wires of the heating element glowing red. The manufacturers are always very insistent that no one should poke any object inside such a toaster. Why do you think this is so serious? Suppose a small child did push a knife into the toaster so that some of the wires are pushed together. What do you think would happen when the toaster is switched on? 5. Electric hair driers can blow either hot air or cold air. How is this possible"

11

,.------------,

ELECTRIC LIGHT

Early Greek lamp. Roman oil lamp. Paraffin lamp.

12

A SHORT HISTORY OF LIGHTING The mastery of fire and the control of light were among the first advantages man gained over the wild animals. He learned to use fire to give warmth and protection. He found that the fire provided him with light which lengthened his day and thus changed many of his habits of living. It was no longer necessary to count his day from sunrise to sunset. The first lamp was probably a wick floating in a crude bowl of oil, in much the same way that Eskimos today often use saucerlike vessels filled with blubber, and some moss as a wick. At the beginning of the Christian era, candles had begun to be used. The Greeks and Romans used threads of flax coated with wax, often beeswax, and until the nineteenth century candles provided the only means of artificial illumination available to common people. Candlemaking was an important industry in the Middle Ages. Oil lamps, using vegetable oil, were used by the early Jews, Greeks and Romans. Many types of wick and different kinds of oil were used. In 1490 Leonardo da Vinci devised an oil lamp in which there was a glass chimney fitting into a glass globe filled with water, an improvement which gave a much steadier light suitable for study at night. The discovery of paraffin in 1859 provided an inexpensive and highly efficient substitute for the crude oil. Circular wicks were made and air was admitted from a pipe leading through the centre of the lamp. This produced a hot blue flame which was then used to heat a mantle hot enough to give off light itself. This is called incandescence and these were the first incandescent lamps. In 1664 natural gas was discovered near a coal mine in Lancashire. Coal gas was first used for lighting when, in 1798, it was installed in a Birmingham factory. Because of the explosive properties of the gas and the danger of fire, gas was only slowly introduced for lighting purposes. Westminster Bridge was illuminated in 1813. Paris adopted gas for street lighting in 1818. By the end of the century gas lighting was firmly established.

eleCTRIC LIGHT

Gas lighting on Westminster Bridge in the 1870s.

None of the above methods of providing artificial light is very convenient. The use of electricity for lighting purposes has considerable advantages and it has transformed lighting throughout the world. In 1809 Sir Humphry Davy used a battery of 2000 cells to pass a current through two sticks of charcoal a few centimetres apart. This produced a brilliant arch-shaped flame, which from then onwards was called an arc light. Development of arc lamps continued throughout the century. They produce an intense white flame, but also a great deal of heat, which shows that it is not a very efficient form of lighting when so much of the electrical energy that goes into it is transformed into heat and not light. Use is made of the concentrated and intense heat of the electric arc to weld two metals together in the process known as electric welding. or to heat substances to a very 13

Arc lighting in the City ofLondon in 188/. A rc we/ding ofan oil pipeline in Hampshire.

14

high temperature in electric arc furnaces. Electric arc welders wear eye-shields to protect their eyes from the intense light and other radiation. It was the inventions of Sir Joseph Swan in England and Thomas Edison in America in the years before 1880 that led to the first practical carbon filament incandescent lamps that started the modern electric lamp industry. They were more efficient and more convenient to use than the electric arc lamp. Finally, this century has seen the development of fluorescent tubes for lighting. Their efficiency is much greater than that of incandescent lamps: each is discussed below.

ELECTRIC LIGHT

ELECTRIC LIGHT

In the laboratory you investigated how a single cell lights a lamp with a lamp's worth of current. You found how the lamp glows with much greater brightness (it becomes a miniature 'photo-flood') when two cells are put in series with it. Lamps, however, should not normally be overloaded like this, for they will not last long if they are. The figures given by one manufacturer for the kind of lamps used in your laboratory experiments show this: CURRENT

Normal use Overloaded Overloaded Overloaded

0.25 0.50 0.75 1.00

A A A A

LIFE

almost indefinite I hour 5 minutes nil

Of course the length of life quoted above is only an average value. Sometimes a bulb will last longer than the time given, sometimes it burns out straight away when overloaded, as you may have found in your own laboratory experiments. Electric lamps used on the mains react in the same way to excess voltages, and this is one reason why the mains voltage is kept very steady. You have done another experiment in the laboratory in which you put a length of fine steel wire (a strand of steel wool) in a circuit. You probably put a variable resistor (a 'dimmer') in series with it. At first the current was small. As the current was made larger, the steel wool got warmer. It got red hot, then briefly white hot and then it broke. But if you look inside one of the small .amps, you will see it has inside a very much finer wire than the strand of steel wool. It too glows white hot when switched on, but yet it does not break. This does seem surprising. Problems to think about

copper wire

clip

steel wool

clip

I. If you were making an electric bulb. what would be the properties needed for the wire you chose to put inside it 0 2. In the experiment with a strand of steel wool. what would have happened if you had used fine copper wire instead') What would have happened if you had used an insulator. like a strand of wool? What would have happened if you had used a strand or two of steel wool in series with some copper wire" 3. Collect as many different kinds of clear lamps as you can. including oldfashioned mains lamps. and see how fine are the filaments inside.

15

carbon lamp

tungsten filament lamp

coiled tungsten, gas filled lamp

coiled filament

coiled coil filament

16

Throughout the nineteenth century, many attempts were made to produce an incandescent electric lamp. In 1860 Sir Joseph Swan used a paper filament coated with carbon to make it conduct electricity. The resistance was such that the carbon heated and glowed, giving off light as well as heat. Carbonised bamboo (charred bamboo) was used in America until 1894. Metallised carbon-filament lamps were widely adopted and in 1907 the first use was made of tungsten filaments. Tungsten had the advantage of a very high melting point (why was this an advantage?) and proved particularly suitable for filaments. But there is a further problem in the construction of lamps. The filament could burn up in air much as the steel wool strands burnt up in your own laboratory experiment. How could this be overcome? To avoid burning - combining chemically with the oxygen in the air - the filament was en.c1osed in a glass bulb, from which the air was pumped out. In 1910 the lamps widely used were all tungsten filament vacuum lamps. There was, however, a tendency for the tungsten filaments to evaporate in the vacuum and this limited the life of the lamp. This was therefore the next technical problem to be solved. How could this be overcome? In 1913, it was discovered that if the lamps contained some gas such as nitrogen or argon at low pressure, it prevented the metal from vaporising. These gases however are 'inert', unlike air or oxygen, in that they will not promote burning. So successful were these tungstenfilament gas-filled lamps that they rapidly replaced all other types. There were further technical advances. A long length of tungsten wire inside a bulb was not very robust and easily led to 'shorting'. So they started using a coiled filament which also 'concentrated' the heat. In the 1930s, the single coil was replaced by a coiled coil. Another significant development began in 1925, when lamps frosted inside ('pearl' lamps) were used instead of clear glass and most domestic lamps these days are frosted ones. (What is the advantage of this?) A recent development which provides a very intense

ELECTRIC LIGHT

source of light is the quartz iodine lamp. This is compact and has a small quartz envelope. Inside it includes iodine vapour, which enables the filament to operate at an even higher temperature and thus give out more light. Something to do at home I. Obtain an old electric-light bulb. preferablv a clear one so that the inside can be seen. Examine how the wires are sealed in through the glass and how the coiled filament is supported. You might also try wrapping the bulb in an old cloth (to prevent the glass flying about) and then break it carefully. so that you can examine the inside. Try taking a broken 'bayonet holder' to pieces to see how the pins at the side of the base are connected to the filament. 2. Make a collection of old electric-light bulbs. trying to include each of the types mentioned above. Electric-light shops and radio shops often have old ones which they are pleased to give away.

The scientist and the engineer The development of the electric-light bulb shows very well how scientists work. The first discovery of the heating effect of a current also showed that light could be produced by a current. The scientist experimented to try to make it more effective, trying new materials. At each stage there were faults, and the next experiments were directed at eliminating them. Carbon filaments had to be run at too low a temperature. Tungsten filaments with their high melting point were found more suitable, but there was the difficulty of their burning out. This was overcome by using tungsten filament vacuum lamps. But then the metal vaporised. So the scientist suggested next the introduction of inert gas to prevent the vaporisation. At each stage, a new idea was incorporated and the lamp became more effective. At the same time, there were technical problems to be solved by the engineer if the lamps were to be reliable or even made at all. Seals were necessary in the base for the electrical connections to the filament. They had to withstand the temperature changes and the pressure difference between the inside and the outside of the bulb. As the scientist strove to suggest how the various faults could be overcome, so the engineer had to ensure the production and reliability of each new development: a partnership that enables us to have electric light in common use in our homes today. 17

**

ELECTRIC LIGHT

Efficiency Although the tungsten-filament coiled-coil gas-filled light bulb is effective, unfortunately it is not very efficient. A high proportion of the electrical energy that goes into it is transformed into heat. You have only to place your hand close to it after it has been on for a little while to convince yourself of this. Then place your hand cl.ose to a fluorescent tube which has been switched on for the same time and judge for yourself how little heat is being wasted. The difference between the incandescent lamp and the fluorescent tube will then be obvious. The fluorescent tube has a greater efficiency - about three times as great. The relative efficiency of the different types can be seen in the table below. TYPE OF LAMP

Carbon arc Carbon filament Tungsten, vacuum Tungsten, gas filled Fluorescent tube

UNITS OF LIGHT PER WATT

7 2.6

10 19

63

Fluorescent tubes, therefore, give better value for money on running costs, but they are more expensive to buy and install. Furthermore, the expected life of a fluorescent tube is about five times that of a tungsten lamp.

Fluorescent tubes It would be out of place here to describe the fluorescent tube in great detail, but essentially it consists of a long glass tube coated inside with a fluorescent material that glows, giving out visible light, when ultra-violet light falls on it. The tube is filled with mercury vapour and a small amount of the gas argon which helps the electric current to start to flow through the mercury vapour along the length of the tube. The conducting mercury vapour then gives out the ultra-violet light and this in turn causes the fluorescent coating to glow brilliantly with visible light. As a light source it gives a soft light, free from glare. 18

fluorescent coating inside tube

A.C. mains supply

Fluorescent tube.

The usual colour is blue-white, but it can give a redder light by mixing together suitable fluorescent materials on the tube. An advantage of fluorescent tubes for lighting is that they cast less shadow than other lamps. Why do you think this is the case? Of the input electrical energy, over 20 per cent is transformed into visible light, whereas it was only 7 per cent with the best incandescent lamp.

Modern lighting in a home. Fluorescent lighting in a factory.

19

MAGNETIC EFFECTS OF AN ELECTRIC CURRENT

Large electromagnet moving iron.

20

ELECTROMAGNETS In your laboratory investigations you found the effect that magnets have on pieces of iron. Using your circuit board you found that a current through a coil of wire had a magnetic effect. It had a much stronger effect if you put an iron nail inside the coil. A great deal of use can be made of this magnetic effect due to a current.

An electromagnet can be made by wrapping a coil of wire around an iron core. It has the advantage that it can be switched off in a way that is not possible with a permanent magnet. If soft iron is used for the core (and not steel), almost all the magnetism will have gone when the current is switched off. (If steel is used, a lot of magnetism remains - and this is therefore a way of magnetising a steel rod so that it becomes a weak permanent magnet.) A small electromagnet can provide a very simple way of releasing a steel ball, and an arrangement like this will be used later in your course. Large electromagnets are used for sorting iron and steel from other scrap metal. A specially designed electromagnet is often used in hospitals for taking bits of iron or steel out of a patient's eye.

The electric bell Use is made of this effect in the electric bell. The second illustration below shows the two coils of the electromagnet. When a current passes they pull on the soft iron armature, on the end of which is an iron clapper which strikes the bell. This, however, would merely give a single clang. The mechanism is made so that the movement of the armature breaks the circuit to the electromagnet; the two coils no longer attract the armature and the spring causes it to go back to its original position. Once again Electromagnetfor use in an eye hospital.

21

MAGNETIC EFFECTS OF AN ELECTRIC CURRENT

the circuit is closed, the electromagnet attracts the armature, and the cycle is repeated. This 'make and break' arrangement enables the bell to go on ringing. Something to do at home Examine an electric bell and identify all the working parts mentioned above.

The electric relay The relay is a type of eiectrical switch. It contains a coil that becomes an electromagnet when a current is passed through it. The electromagnet pulls on a metal armature, which is pivoted. In moving, the armature closes other contacts. Look at the illustrations below, which show the type of relay used by the Post Office. (An automatic

relay contacts pivot

r"""l

~" " :

r"""l

r"'"1

\~ ~

0 relay coil

Post Office relay with single contacts. Post Office relay with multiple contacts. When activated. the top contacts close, the next open, the next close. the bottom open.

22

leads to coil

telephone exchange might use as many as 100000 relays.) The electromagnet will pull the armature to the right, and this movement closes the two contacts on the top. The relay on the right is of a more elaborate type, for it has several sets of contacts. When the current passes, the top contacts are closed, but the bottom ones open. With such a relay a number of different circuits can be switched on and others off, all at the same time. The disadvantage of the above type of relay is the corrosion at the contacts. A recent development has been the introduction of the dry-reed switch. This has diffused gold contacts on the two metal 'reeds' or strips which are enclosed in a glass envelope. This magnetic switch is put inside an operating coil, and otherwise it works like the

(((('( 'II '((////////////(((/////'('(// ~

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A reed switch. A reed switch inside an operating coil.

conventional relay already described. It is likely that this type of relay will eventually replace the older type. How is a relay used? One simple use is inside a car. When the starter switch is pressed, it sends a small current through the relay coil, which closes the relay contacts, thereby switching on a large current to the self-starter motor. But why not have a simple switch at the dashboard, which just switches on the motor without bothering with a relay? The self-starter motor needs a very large current and it would be very inconvenient to have long heavy thick leads, with low resistance, trailing to the dashboard area. The relay enables a small current to control a much larger one.

Starting system in a car.

long thin wires to dashboard

starter switch relay

car battery

self-starter motor

short thick cables

23

MAGNETIC EFFECTS OF AN ELECTRIC CURRENT

The picture below shows a signal-box control panel. The whole of this can be operated by one man pushing buttons at the control panel, which operate relays that control signals and points. The electromagnets in the relays are all operated by small currents, but control heavy motors requiring considerable power and large currents.

Railway control panel on Western Region, British Railways,

Relays are extensively used in telephone exchanges; they are used for automatic traffic lights and a large variety of other purposes. You can have a lot of fun with relays if you have some electric trains at home, as explained in the next section. 24

c

A

* L

D

B

leads to relay coil

A

c

B

D

leads to relay coil

Relays with model electric trains Relays for some model electric trains':' consist of a coil, through which a metal strip is pulled when a current flows through the coil. In the first position, shown on the left, the strip makes contact between A and B, and C and D are open. When a current is passed through the coil, the metal strip is pulled through with such force that it is now in the second position. C and 0 are now connected, A and B are open. To reverse the process, a current is again passed through the coil, the metal strip is again pulled through the coil, so that A and B close, while C and 0 open. In other words, the first electric pulse opens one switch (AB) and closes the other (CD); the second pulse reverses the process. With such devices, points can be operated, trains can be stopped and started, light signals can be switched on and out. You can have great fun arranging relays, and relays are used in very similar way when operating real railways. To operate the relay, manufacturers supply an attachment to the railway track, such that there is no contact between the copper strip and the rail when in position. However, as soon as a train goes over it, the copper strip touches the rail and it closes the circuit. Then a current flows through the coil and the relay operates. Something to do at home I. If you have electric trains at home, set up a relay so that when a train passes the contact strip, the relay switches out a green light and switches on a red one. Then insert a second contact strip further along the line, so that the train again operates the relay and changes the lights back again. 2. Set up a relay to change a rail point. A first strip can switch it one way, a second strip somewhere else on the track can switch it back. 3. If you have some isolating tracks, arrange these so that the track immediately behind the train is isolated by a relay. This will prevent a following train catching up with the first and colliding with it. 4. Most difficult of all. Arrange relays at the end of a track, so that when the train arrives it reverses and goes back to the start again . .,. For example, Triang.

* **

25

CHEMICAL EFFECTS OF AN ELECTRIC CURRENT

anode

electrolyte

26

ELECTROLYSIS .>

In the laboratory you investigated how electric currents pass through liquids. Would electricity pass through pure distilled water? Through tap water? What happened when salt was added to the water? What about soapy water or dirty bath water? Why do you think it is dangerous to have electric fires in a bathroom?

You also passed electricity through special liquids like copper sulphate solution and found that copper was deposited. As in all scientific work, it makes it easier to discuss the details if we give names to parts of the apparatus. A solution through which a current passes is called an electrolyte. The two electrical contacts immersed in the liquid are called electrodes; the one connected to the positive terminal of the battery is called the anode, the one to the negative terminal the cathode. The whole process is called electrolysis. Do not worry about these words; they merely make it easier to describe what we are talking about and you will soon get used to them. Your laboratory experiment showed that when the electrolyte was copper sulphate and you had copper electrodes, copper was deposited on the cathode. You may have tried using a penny (or even a sixpence) as the cathode and getting a deposit of copper on it. You will also have seen a demonstration when an electric current was put through an apparatus hke the one illustrated opposite. When filled with distilled water, no

current flowed. But as soon as a little sulphuric acid was added, a current passed. Gas was given off at each electrode and collected in the glass burettes. (What can you remember about the quantity in each case? Was it the same in each burette? How much did they differ by?) One gas, coming from the anode, was identified as oxygen: a glowing wood splint burst into flame when put in the gas. The other was identified with hydrogen: the gas exploded when collected in a tin can and lit. In both cases energy from the battery was causing a chemical change to take place. Details of the chemical processes involved are outside the scope of this book, but you will study them in your chemistry course. However, the process of electrolysis has very important industrial applications. For example, the electrolysis of a solution of bauxite, found in large quantities in certain areas like Ghana, Yugoslavia and the South of France, produces pure aluminium. The process requires large electric currents and is therefore undertaken where electricity is cheap and plentiful. Other metals such as calcium, sodium, potassium and magnesium can all be extracted by the process of electrolysis. Perhaps the most interesting example is the way copper can be obtained by electrolysis in a pure state directly from the earth, using a lake as the electrolytic bath. The water in the lake with a small quantity of copper sulphate in it is the electrolyte, the impure copper in the earth is the anode, and a strip of pure copper in a convenient position for easy removal is the cathode.

copper sulphate lake

27

CHEMICAL EFFECTS OF AN ElECTRIC CURRENT

ELECTROPLATING The process by which a metal is deposited on another is called electroplating. It is done for a variety of reasons. Steel is strong and comparatively cheap, and is a good material for the bumpers of a car, but it rusts very easily. Electroplating the steel bumper protects it from rust and gives it a more attractive finish.

Automatic electroplating ofcar bumpers.

At one time, the steel was electroplated with nickel. This protected the steel from rusting. But nickel goes dull and soon appears rather drab. Furthermore, it is not very hard and scratches easily, and deep scratches. through to the steel, led to rusting, Chromium is a much harder metal, and it stays bright and shiny. Unfortunately 28

CHEMICAL EFFECTS OF AN ELECTRIC "CURRENT

if it is of a certain thickness it is liable to crack, leading to rusting underneath. What is the solution to all this '? (This is yet another example of our discussion in the section on Electric Light: how the engineer tries first one, then another of the discoveries of the scientists, gradually getting over one difficulty after another.) The usual process now is to nickel-chromium-plate the

- - - - s t e e l or iron

steel. Using the correct electrolyte, a layer of nickel is first deposited to protect the steel from rusting. (This may be about 0·0004 em thick, though the actual thickness depends on whether the appliance is to be used outside in severe weather conditions; it can be thinner if the plated appliance is used indoors.) Then, with a different electrolyte, a thin layer of chromium (usually about 0·000004 em thick) is deposited on top of the nickel. This protects the nickel from scratches, as it is so much harder, and also gives a shiny finish. Very few people can afford to eat with solid silver spoons and forks. Furthermore, they are weak to use. A strong spoon or fork can, however, be made from a nickel alloy and then electroplated with silver, giving it the pleasant, shiny appearance of a solid silver spoon. You will often find EPNS stamped on the back of a spoon: this stands for Electro-Plated Nickel Silver. Something to do at home 1. Have a look at your spoons and see if you have any with the EPNS mark. If you find a spoon with a small lion on it, do you know what this means? 2. Look round your mother's kitchen, your bicycle, your father's car, and make a list of all the things that you think might be electroplated.

Large numbers of articles can be electroplated automatically. They are usually hung on metal frames which are carried slowly through an electrolytic tank by a 29

CHEMICAL EFFECTS OF AN ELECTRIC CURRENT

moving belt. They pass right through in about an hour; in successive stages they are cleaned with weak acid, rinsed, nickel-plated, washed and finally dried.

REFINING OF

METALS

In the process of electrolysis, the metal deposited is very pure. It is very important, for example, that the copper used for electric wires should be very pure. Even very small quantities of impurity make copper a moderately poor conductor. Electrolysis is therefore used industrially for producing pure substances. Copper, aluminium, calcium, gold, silver, zinc are all purified in this way. You will learn more about this in your Chemistry course. To refine copper, thin sheets of pure copper are made the cathodes in large electrolytic tanks of copper sulphate solution. The anodes consist of impure copper. The current is passed and pure copper is deposited on the cathode,

Copper refining in A/rica.

30

I

SAFETiJ

Electricity can be very dangerous. It must therefore be treated with great respect, and only by those of us who know how to use it and take every safety precaution. The dangers are real. (I) A short circuit can cause a fire, an explosion or shock damage to a person. (2) Faulty insulation can be responsible for a short circuit and all its consequences. (3) Excessively large currents can seriously damage equipment. Unless adequate safety precautions are taken, electric shocks (which could be fatal) or serious burns are all possible. The heating effect of an electric current is rapid and could be quickly responsible for much damage.

FUSES A wire gets hotter as the current flowing in it increases. The electric wiring in a house will stand a certain current without the wires getting too hot to cause any damage. But consider the experiment illustrated. In the first circuit board, the three lamps will glow equally bright with normal brightness. What will happen if a length of copper wire (copper is a very good conductor) is put across the connecting leads as shown in the second diagram? What happens to the current? (If you are in any doubt, try the experiment yourself.)

31

fuse wire

Now consider what would happen if the leads to an electric fire touched each other or if a short length of copper wire accidentally fell across the points A and B in the illustration. (This is NOT an experiment to try: it could be very dangerous and you might get a shock.) The current would clearly be very great indeed, and the mains wire might get very hot and cause a fire unless there was some automatic way of switching off the current. A small length of wire with a low melting point meets this need. This is called a fuse wire. In the wiring circuit such a fuse is included which will burn out if the current exceeds a stated amount. The fuse wire is normally fixed in a special holder inside a fuse box. When the current gets too great, the fuse burns out and the main electric wiring is protected. It would be difficult to repair the wire in the wall of a room, but it is an easy business to go to the fuse box, take out the fuse holder and replace the fuse wire. Something to think about Spare fuse wire can be obtained on a card as illustrated. Such cards often have three different fuse wires on them, varying in thickness. Which fuse will be thicker, the 5 A or the 15 A?

When the two leads to an electric fire, or another electric appliance, touch or are accidentally joined in some way as illustrated with the electric fire above, we call it a 'short circuit' or we say the circuit is 'shorted'. 32

Reasons for a fuse blowing It is important to remember that there is always a reason why a fuse wire burns out. It is no good just replacing a fuse wire, or the new fuse wire will burn out again. It may be caused by a faulty appliance, in which case the appliance - an electric fire, an electric iron or whatever it is must be disconnected first before repairing the fuse. Another common cause for a fuse blowing is overloading the circuit. What would happen when the current is switched on if an electric kettle, an electric toaster and an electric fire were all plugged into a light socket, using two- or three-way adaptors? It is always wrong to connect electric fires, toasters, or kettles which need quite a lot of current to the electriclight circuit. A common fault is frayed leads or old leads trailing around a room. It is not satisfactory to rely on two lengths of flex tied up together with insulating tape. Such a connection is always a possible source of trouble. Why is it unwise to tack long lengths of flex around the walls of a room? What might happen in the picture illustrated below? What other causes of fuses blowing can you think of? What might happen if one of the leads to an appliance is not securely fixed inside the plug?

33

A fused plug and a cartridgefuse.

Fused plugs It is now very common practice to use electric plugs which have a fuse fixed inside them. (The advantages of this will be discussed in the section on electric wiring in a house.) The fuse in these is usually a little cartridge fuse: the fine wire inside melts as soon as the current exceeds the stated value. What kind of fuse will be used in the plug will, of course, depend on the particular appliance. The following table shows the probable currents and the type of fuse that might be used with a 240 volts mains supply. APPLIANCE

Table lamp Stronger lamp Refrigerator Television set Electric blanket Hair drier Electric iron Small electric fire Electric kettle Electric fire Immersion heater

POWER

60 watt 120 watt 120 watt 120 watt 120 watt 600 watt 720 watt 750 watt 2000 watt 2000 watt 3000 watt

CURRENT

FUSE NEEDED

REQUIRED

FOR SAFETY

A

2A 2A 2A 2A 2A

A 3.0 A 3.1 A

5A SA SA

8.3 A 8.3 A 12.5 A

10 or 13 A 10 or 13 A

0.25 0.5 0.5 0.5 0.5 2.5

A A

A

A

13 A

Fuses in electrical apparatus You will often find cartridge fuses inside some of the electrical equipment in the laboratory. This saves the equipment from being overloaded and burnt out. The 34

illustration shows the back of a transformer unit, which uses such fuses. Something to do at home

Cartridge(use at the back ora power 5Upp/V.

Motor cars also need to be fitted with fuses. These must be the cartridge type, where the wire is sealed in a glass tube. (This avoids any risk of a spark being produced when the wire melts, setting fire to any petrol fumes which may be around.) You might like to look inside your father's car and identify the fuse box. Find out from the car booklet the 'rating' of the fuses, the current at which they melt. (Do not remove any fuse, as this could easily lead to a road accident.)

Cut-outs In some equipment, instead of a fuse, a cut-out device is incorporated. This uses the magnetic effect of a current. The current flows through a small coil. When it becomes too large, the magnetic field due to this current operates a trip switch, which cuts out the current itself. What is the advantage of this kind of cut-out over an ordinary fuse?

EARTHING In the laboratory you have seen sparks produced by a van de Graaff generator. You probably noticed how much easier it was for the spark to pass to a pointed object.

35

I

SAFETY

I

Tree struck bv lightning. Lightning can strike a building.

36

Lightning is caused when an electrical charge builds up in a cloud. A lightning 'spark' is produced when the charge flows from cloud to cloud or from the cloud to earth. As with the van de Graaff sparks, it is easier for the lightning to travel to a pointed object. To prevent this damaging a building, a lightning conductor is normally attached to it. There is usually a spike or several spikes on top of such a conductor. Why does it have this spike? The top is connected to a thick copper conducting wire or strip, the lower end of which is attached to a plate in the earth. If lightning does strike a house with such a conductor, the electricity flows through the copper to earth and avoids damaging the house. This principle of 'earthing' in order to ensure safety is not only used in lightning conductors, but also with electric wiring in houses.

, SAFETY

I

A lightning conductor on a church

Have you noticed that the electric points in your home or in your school laboratory all require three pins? But your laboratory experiments with circuit boards only required two leads from the cells. What is the third pin for? Suppose a live wire to an electric fire should accidentally come into contact with the frame of the fire and you touched it. If you were wearing rubber gloves or happened to be wearing rubber boots, you would probably be all right as you would be well insulated. It is much more likely that you would not be wearing either of these, and then a current would flow through you to earth. You could get a serious electric shock; you might even be killed. The one way out of this difficulty would be to provide an easier path for the current to flow to earth than through your body. This is where the third pin comes in. There are three wires to the electric socket and the third wire, coloured green, ':' is connected directly to earth. This is often done by connecting directly to a metal plate buried in the ground or to a metal (not a plastic) water pipe. As we shall understand later, the neutral wire is connected directly to the substation, where it is there 'earthed'. The earth lead from the socket is shown on the next page connected to the metal of the electric fire. If now the live wire were accidentally to come into contact with the frame, the current would have a much easier path than " From Julv 1970. the earth wire is coloured green and yellow.

37

The earth connection to an electricfire.

through you. The current would be large by this easy path to earth, the fuse in the circuit would melt and no one would be electrocuted. This earth lead is therefore most important when connecting appliances, even though the fire would work without it. The diagram below shows the connection from a threepin plug into an electric kettle. The connecting cable carries three wires. The earth wire connects the earth lead at the wall socket to a strip of metal which makes contact with the metal body of the kettle when the plug is inserted.

Problems to think about I. Why is it particularly dangerous to stand under a tree in a thunderstorm? 2. Why is it relatively safe to be in an aeroplane in a thunderstorm? 3. Why is it considered so dangerous to have an electric fire in a bathroom? (Remember water, especially dirty bath water which is very different from pure distilled water, is a very good conductor.) 4. Even if the wiring is such that it will take the current, why is it most unwise to plug an electric fire or electric kettle into an electric light fitting? 5. When earthed leads are so important to electric appliances for safety reasons, why is it not considered necessary to have more than two leads to an electric light hanging from the ceiling? 6. Why are the old-fashioned brass lampholders no longer recommended? Why should you always switch off the light before changing a bulb in a lampholder? 7. Some electrical devices which operate from the mains voltage are not 'earthed'. An electric shaver is an example. They use the principle of 'double insulation'. What do you think this means?

38

SOURCE OF

THE ELECTRIC WIRING IN A HOUSE

ELECTRIC POWER

You will learn about the generation of electric power later in the course and also about how electricity is transmitted around the country. This will therefore be a brief introduction to it: later books in this series will give you the details. Electricity is generated at power stations at many places throughout the country. The necessary energy may come from fuel in the form of coal, gas, or oil, it may come from nuclear energy within the atom (a nuclear power station), or it may come from falling water (a hydroelectric power station). The power station usually produces electricity at 11 000 volts and it is then transmitted around the country at very high voltages. (The reason for this will be discussed later in the course, as will the transformers which 'step up' the voltages or later step them down.) The National Grid operates at 132 000 volts: there is also a Supergrid at 275000 volts (and even 400000 volts as well). Such high voltages are quite unsuitable for domestic purposes. Various substations, using transformers, bring the voltage down to 240 volts, at which it will reach your home.

Power distribution/rom power station to home.

grid line at132 000 volts

power station

transformer house

power line at 33000 volts

transformer

E====

~

underground cable at 6 600 volts

factory

~. =_j'd '"""'J_.==_=-_(~ ~)~~

=- __

-----l __

0

240 volt underground cable

39

Something to think about

ELECTRIC WIRING IN A HOUSE

Sometimes electricity reaches a factory at 6600 volts. If, instead of supplying electricity to your home at 240 volts, the Electricity Board supplied it at 6600 volts, what would be the consequences') What special precautions would have to be taken?

ELECTRIC WIRING IN A HOUSE In the previous chapter we discussed the importance of fuses in electric circuits in order to safeguard electrical applianc.es and to avoid the danger of fire. Fuses therefore play an important part in the electrical wiring system in a house. In domestic wiring, it is important that the third pin at each socket be connected to earth. For this reason threecored cable is used: the LIVE wire bringing the electric current is usually coloured red, the return wire (often called the NEUTRAL wire) is black and the EARTH wire green.':' In order to avoid too much complication in the drawings below showing the wiring in a house, the earth wire has been omitted, but of course it is an essential part of the installation,

" From July 1970 the following colour code is to be adopted: LIVE brown NEUTRAL blue EARTH green and yellow

The Electricity Board's installations The main cable will pass your house, probably under the road or the pavement outside. A short length of cable joined to the main cable will bring the electricity to your house. This will usually be underground and the cable will enter in the basement or ground floor. The cable will go into the Electricity Board's main fuse box. This fuse box usually has a 60-A fuse inside, through which the live lead passes. The box is sealed by the Electricity Board; it is the Board's property and

~

D-

(

undergound cable from road

Board's fuse box

40

meter

mains switch and fuse

1--------1

Board's fuse box and seal

meter

mains switch box and fuse

cannot be opened by you without breaking the seal. If something went seriously wrong with your electric wiring, the Board's fuse would blow and this would protect the rest of their installations and ensure that the supply to neighbouring houses did not fail as well. If their fuse blows, the Board's electrician has to come and repair it. The leads then pass through the meter, which records the electrical energy used. They then go to the mains switch. When switched off, none of the lights or appliances in the house will work at all and there can be no possible danger. It is often recommended that, when a house is to be left empty for a long period, the main switch should be turned off. Something to do at home Find where the main supply cable enters your home. Then identify the Electricity Board's fuse box and look for the seal closing it. Examine the meter and identify the mains switch.

After the mains switch, the electricity has to be distributed around the house. What are the requirements going to be? There must be a convenient method of distribution. There must be fuses. There must be leads going to all parts of the house. There are two ways in which this is commonly done, and both are described below.

Distribution fuse box The leads from the mains switch enter the distribution box, where each is connected to a bus bar. The two bus bars consist of two thick copper bars. From the bus bars in the distribution box, leads go off to all parts of the house. Immediately following the bus 41

l

,---------- ----, I

fuses

I

I

lamps or other appliances in parallel

I

I neutral

:

bus bars

I

I I

l-7-

m

distribution box with fuses

n

I

-+--, L--

---'

00

bar the live lead passes through a suitable fuse. Then the live and neutral leads, together with the third 'earth' wire, provide electricity in one part of the house. In the above diagram only two sets of leads are shown leaving the distribution box; one might be providing lighting upstairs, the other lighting in the downstairs rooms. Something to think about In older installations there was often a fuse in both the live and neutral sides, but it is in fact much safer only to have a fuse in the live side. What would happen if the neutral fuse blew and left the live fuse intact?

Suppose the wiring to the upstairs rooms is such that it will take 5 A with safety. Then the fuse in those leads will be a 5-A fuse. Remembering that a 120-watt lamp will take about 0.5 A and a 750-watt electric fire will take about 3 A, there is a definite maximum to the number of lamps or appliances one can add in parallel to a particular set of leads without exceeding the 5-A maximum load. As soon as more are wanted, it is necessary to go back to the distribution box and put in another set of leads and another fuse. Something to think about Why do you think that old houses which use this system tend to have different circuits for the "lighting' sockets and the 'power' sockets? (Hint: remember that a two-bar 2000-watt electric fire would use 8.3 A of current, whereas a 60-watt lamp would use only 0.25 A, and that the less wire used for the installation the less it will cost.)

42

to bedroom

ELECTRIC WIRING IN A HOUSE

~

to berJroom ~

)

cl

live

neutral

(

:)

0 0

(

?

(

~

to djnin~! room ~

I

to livinq room

\

to kitchen

bus bars

~

I

~

The complete distribution scheme for the whole of a house might be something like this. The figures in the circles give the maximum current that might be used. bedroom lights

power poin-ts in bedrooms

living room lights

r::=:::==:~~~ garage and hall lights

;-;====:--

power points for living rooms

kitchen lights

power point in kitchen

~======J:15

15 power poi nt for hot water immersion hr;ater

43

ELECTRIC WIRING IN A HOUSE

Something to do at home If your home has this kind of distribution, investigate the wiring system and make a chart of it similar to the above. First list all the lights in the house and check which are on the same circuit. (This can be done with the aid of your father: turn on all the lights and see which go out when your father takes out one of the fuses in the distribution box.) Then list all the power points, noting which are 2-A, which 5-A, and which l5-A type. Check which are on the same and which on independent circuits. When you have these details, you can prepare your own chart of your wiring system.

Are there any disadvantages in the above scheme? It requires the use of plugs of various sizes as shown on the left, and this can be very inconvenient. Another disadvantage is that when a fuse blows, it puts out all the other lights on the same circuit from the distribution box. Furthermore the fuse has to be repaired at the distribution box, which may be some way away or in an inconvenient spot. But there is one greater disadvantage. Suppose you have a new electric appliance, which needs a 15-A socket in an upstairs room. To put in the new socket, it is necessary to put in new leads all the way back to the distribution box. Do you see why this will be an expensive business and what a disadvantage this is?

The ring main system This system is rapidly replacing the system described above and is usually installed in all new houses. As its name implies, it consists of a ring circuit. This starts from the main fuse box, which usually has a 30-A fuse, runs AA/\

AAA \

v

\

v

2-A, 5-A. 15-A plugs and sockets.

L

I

I

N

I

I

I

I

<

S < I-Q.--O--.

fuse box with 30-A fuse

\~

;'J

\;, vvvv

44

LA v

vv

round the main rooms of the house and back to the fuse box again, as shown below. There are two routes that the electricity can travel to anyone appliance, so even though a 30-A fuse is included, it is usually only necessary to have 15-A cable. It is possible to have plugs and sockets all of the same size, and the plugs used have their own fuse of the type discussed on page 34. When a fuse blows, it only affects the single appliance concerned. It is easier to repair than groping around at a distribution box. Furthermore, it is very much easier to install a new power point without having to put in long leads all the way back to the distribution box. Of course, this assumes that the total load on the ring circuit will not exceed 30 A at anyone time, that it will not be used for more than, say, three two-bar electric fires at once. In large houses, it may be necessary to have two or more ring mains running round different parts of the house. As explained in the previous chapter on safety, whatever system is used, it is necessary to have an earth wire as well as the live and neutral leads. The earth lead is connected to the third pin at all the sockets. The earth lead is shown in the drawing below of a ring circuit around a living-room.

ELECTRIC WIRING IN A HOUSE

., ~

0 [Q

[Q

0.

:

i

""

I

»>:

\:~ Fused plug and socket.

--_/

\

-------I !

7· ~~======---

I

~ ~l~

-~==::=:cc:::=--~~c:c~~~--=--__

~

main fuse

--====-~=~

45

ELECTRIC WIRING IN A HOUSE

Something to do at school It is possible to demonstrate the two methods of distribution with the circuit boards you have used in the laboratory. If time allows, ask your teacher to let

you try it out. For the first method, use one cell on the circuit board and get it to light one lamp in the usual way. To light more lamps with equal brightness from the same cell, they are connected in parallel and leads are taken back to the cell as the 'distribution' point.

For the ring main, set up the following ring circuit on your board. In the first diagram, one lamp is connected. To add further lamps - as in the second diagram - it is not necessary to use extra leads back to the cell.

46

SWITCHES AND AUTOMATIC CONTROL

Circuit board switches. Knife switches: single-pole, one-way and two-way.

SWITCHES You have used switches in the circuit-board work in your laboratory. You may have had simple push-button switches like the one illustrated below, or you may have used one of the spring connecting leads and pivoted it about one of the pillars. Switches play an important part in all work with electricity: elaborate switches for switching on or off the electrical power from a power station to the National Grid, for switching on electric motors or

~III!I!i!!i!l!lE:J ~! television sets, for operating your lights in your home, for operating electric bells, There are switches of many different types and in this chapter we can only consider a few of them, The two-switches on the left are both called single-pole switches. The first is a simple on-off switch that connects or disconnects the single wire in the circuit. The second is a single-pole two-way switch, The current comes in along the central wire and can be switched to either of the other two terminals, (Suggest a way in which such a switch might be used.) Single-pole three-, four- or five-way switches are often used in electronic apparatus. The picture below shows a control knob on a voltage supply. In one position the supply is off, in another it gives I 000 volts, in another 2000, in another 3000, and so on.

Multi-wav switch.

47

SWITCHES AND AUTOMATIC CONTROL

In a circuit diagram, these switches can be represented like this:

single pole on-off

single pole 2-way

single pole 5-way

Sometimes it is convenient to use a double-pole switch. This will connect or disconnect two wires simultaneously. A particular use of a double-pole two-way switch is as a reversing switch.

Can you see for yourself how the connection in the second drawing causes the current to be reversed? In circuit diagrams, the double-pole two-way switch might be represented like this: Bell-push.

~

0----

~ 0----

double pole switch interrupting two circuits

double pole switch as a two-way switch

double pore switch as a reversing switch

Another very common type of switch is that used for ringing electric bells. It merely switches on the current when the button is pushed, bringing the copper strip in contact with the rest of the circuit.

Switches for use with mains voltages None of the switches you used on your circuit-board experiments, nor the single-pole and double-pole 'knife' switches illustrated above, would be suitable for use with the 240-volt mains. Why not? 48

SWITCHES AND AUTOMATIC CONTROL '----.-----l

I

A much safer insulated switch is necessary for a light switch or for a power point. Such a switch is shown below. In the off position, there is an air gap between the two metal springs. In the on position, the gap is closed when the good conductor comes between the metal springs. Good switches usually have strong springs, which snap the conductor into place. This explains the sharp click when the switch is operated.

Light switch in the offand then the on position.

Various special switches are used for special purposes. Illustrated below is first a rotary on-off switch, secondly a type of switch often used on reading lamps, and thirdly a cord switch. Why do you think that a cord switch is recommended for bathroom use?

Various types ofswitch.

49

Mains switch box.

There is always a danger of sparking when a circuit is switcheo off, especially when the current is large. The heat generated can damage the switch, and that is one reason why thick metal plates are used for the switch in the mains fuse box where the mains cable enters the house. In large switches, such as those needed to control the flow of electricity to the National Grid, the danger of serious sparking or 'arc-ing' is very great and elaborate circuit breakers are necessary. Sometimes these are oilimmersed; sometimes there is an air blast which effectively blows away the arc; but details of these are outside the scope of this book. Something to do See how many different switches you can find. and examine their working when they are not connected in any electrical circuit. You might make a collection of different kinds of switches and keep them at school. Electricians can often let you have old ones and your father may have some at home.

Circuits with switches When switches are inserted in the wiring system of your house, they are always connected in the live wire and never in the neutral wire. Why do you think this is the case? Would the light, for example, be switched out just as effectively if the switch were in the neutral lead ? What therefore is the reason? Very often in a house one wants to switch a light on or off in two separate places. What would be wrong with this circuit?

N

50

SWITCHES AND AUTOMATIC CONTROL

Instead the way it ment:

IS

usually done is to use this arrange-

/P>-------~O L --------<~A B

N

This enables the light to be switched on or off at either A or B, whereas, in the first circuit opposite, the light could be switched on at A or B, but it could only be switched off at the same switch. If you have not already tried these circuits on your circuit board in your school laboratory, you should persuade your teacher to let you try. Something to think about I. Suppose you wish to install in a house a hall light and a landing light , both of which can be switched on or off independently either upstairs or downstairs. Work out the electric circuit necessary. This is not very difficult, as it is merely a matter of doubling up the circuit given above . What is more difficult is to work out the most economical way of doing it, that is, using the least amount of wire. Can you double up any wires? 2. Try and work out a circuit which would enable you to switch a single light on or off in anyone of three different places. Then try to switch a light on or off from four different places. 3. Electric bells are usually operated at low voltage, say 6 volts. Work out a simple circuit so that an electric bell can be operated using a bell-push in one of two places. 4. One terminal of an electric bell is connected through a bell-push to one terminal of a battery. The other terminal of the bell is connected to an electric radiator, whilst the other terminal of the battery is connected to a water pipe.

radiator

bell

switch

battery

pipe

Pressing the bell-push causes the bell to ring. Why is this possible when the circuit does not appear to be completed? Would it work If the pipes were thickly painted or the water pipes were made of plastic" (Gas pipes are never recommended for earthing, because of the fire danger.)

51

Three-heat control switches Many electric cookers, hot-plates and electric blankets have control switches which give three different heats. For this the appliance has two separate elements connected in series and wired to the switch as shown below. In the first switch position, the supply is off. In the second, the mains is connected across both elements A and B in series: there is a large resistance and the current will not be very great, so the heat produced is 'low'. In the third position, the switch connects the supply across the element A only: the resistance is less, the current is greater and the heat produced is 'medium'. In the fourth and final position, the mains supply is connected across both the elements A and B in parallel: there will be double the effect that there was in the medium position and the heat produced will be 'high'. element A

element B

N

B

N

52

N

L

A

L

B

A

L

B

A

N

L

SWITCHES AND AUTOMATIC CONTROL

AUTOMATIC SWITCHES In your study of heat, you will find that solid materials expand when they are heated. But not all materials will expand the same amount. For example, brass expands further than an equal length of iron for the same rise in temperature. (In the drawings below the amount of expansion is exaggerated.) Suppose a length of brass and the same length of iron are riveted together at 20°e. What must happen to the double strip (it is usually called a bimetallic strip) when heated to lOOOe? I I

brass

iron

...... :.. rr

-. . ·····.··1

~~~1t=== ···c·.·.·.·: ~ ···;,··b

1--..· ···.·,··.· ..

I

I

same length at 20"c

rivets

brass iron

~r":"":""'T""C='--------.--:c-=

;:;;f'F"2"F++EiEEI same lengths riveted together at 20'c

at 100'c

Use is made of the bending of a bimetallic strip when heated, to provide an automatic switch. An automatic fire alarm can be set up using a bimetallic strip, as shown below. At room temperature there is a gap between the strip and the contact point, the circuit is not complete and the bell is silent. If a fire develops and the strip gets hot, it will bend and make contact, so that the alarm bell rings.

53

SWITCH ES AN D AUTOMATIC CONTROL

Automatic switches are included in most modern electric irons. The switch cuts off the current when the iron is at the right temperature and then switches it on again when the iron cools slightly. This keeps the hot iron at a nearly constant temperature. The iron includes a bimetallic strip which bends upwards when the temperature rises, until it breaks contact at a certain temperature. The control knob adjusts the position at which contact is broken, in other words it adjusts the temperature at which the current stops flowing. As the iron cools, the strip straightens and electric contact is again made, so that the current flows to heat up the iron agam. Bimetallic strips are used for many forms of automatic temperature control. They are used, for example, in controls for central heating, kettles and blankets.

knob for setting temperature

A utomatic control switch in an electric iron which uses a bimetallic strip.

iron

brass

Another form of automatic control is that used in thermostats in aquaria. Find out what type of thermostat is used in the aquaria in your biology laboratories. One type contains a sealed tube of liquid - often toluene, which expands considerably on heating - with some mercury in the other end. Electrical leads are set into the side of the glass tube. Look at the two diagrams and 54

SWITCHES AND AUTOMAT!C CONTROL electrical leads

cool

hot

decide for yourself how such an automatic switch works. Remember: mercury is a very good conductor of electricity, toluene is not.

Time switches This account of switches would not be complete without brief reference to time switches. These include a clock mechanism which enables the switch to be set so that the current will flow for a certain interval of time during the day, but will be cut off the rest of the time. To what uses do you think such a switch could be put? In a kitchen? In a home? In an office? In the street?

Venner time switch.

55

I APPENDIX \

Throughout this book we have avoided using mathematics. Our object has been to show how the principles you have studied in the school laboratory are applied in everyday use. In order to work out in detail the size of the resistance wire needed in an electric kettle, it is necessary for the scientist to use mathematics. You will learn more about this as your school course continues, but for the sake of those interested we have included this brief appendix which summarises some of the details.

Charge It is necessary for scientists to measure electrical charge

and this is done in units called

COULOMBS.

Current When electric charge moves, we have a current of electricity. This is measured in AMPERES (or A for short). If the rate at which charge is moving is I coulomb per second, we say that we have a current of I ampere. If there is a current of I ampere in a circuit, and this is such that in t seconds, Q coulombs flow, the rate of flow is Q]t coulombs per second. In other words we have a current of Q/t amperes. This gives us a useful formula: I=Q/t, or Q=/t, or coulombs e amperes x seconds.

Energy In order to do a job of work, whether it is to lift a brick, to accelerate a train or to push an electric charge through a wire, energy is needed. Scientists measure energy in units called JOULES.

Power When energy is transformed from one form to another, it is often the rate at which the energy is transformed that is interesting. The rate at which energy is transformed is called the power. This is measured in units called WATTS. If a joule of energy is transformed per second, we say 56

I APPENDIX I

that the power is I watt. If J joules are transformed in t seconds, the rate is J It joules per second or J It watts. This gives another useful formula: watts = joul~_, seconds

or

joules = watts X seconds.

Voltage In order to get a current to flow there must be an electrical force (electro-motive force) produced by cells or some other source. This electrical force is measured in VOLTS. When a cell causes a current to flow in a circuit, it converts some of its chemical energy. For a cell with an electro-motive force of I volt, I joule of energy is transformed if I coulomb moves round the circuit, 2 joules for 2 coulombs or Q joules for Q coulombs. A volt is defined as a joule per coulomb. This gives us: volts= joules coulomb'

or

joules-e volts xcoulombs.

Resistance You found in the laboratory that if you double the voltage across a resistor, you get double the current. If you treble the voltage you get treble the current. In other words the current is proportional to the voltage. This was appreciated as a result of the work by a German physicist called Georg Ohm and this relationship is now known as Ohm's law. It is a relationship that holds for the resistance wire you used in the laboratory; there are many other substances for which it does not hold under normal conditions. Since the voltage Vand the current I are proportional, then VI I is a constant and this constant is called the RESISTANCE of the conductor concerned. Resistance is measured in units called OHMS. A resistor whose resistance is one ohm is such that one volt causes a current of one ampere to flow in it, two volts causes two amperes, and so on. This gives us:

_\,ol~__ = ohms or amperes

'

f

I

= R or V = RI, or 1= V. ' R 57

APPENDIX I

Electrical power We introduced, above, the watt as the unit of power, the rate at which energy is transformed, and showed that watts

joules seconds

We also showed that joules=volts x coulombs. Combining these two, we have: watts volts X coulombs seconds But So that we have: watts = volts X amperes This is very useful to us. If we know that we have a 60watt lamp and the voltage is 240 volts, we can calculate the current. Since

amperes =watts/volts,

in this example, current= 60/240 A, =0.25 A. Look at the table on page 34 and you will now see how we were able to calculate the current for each electrical appliance and decide on the kind of fuse necessary, since we know the wattage of the appliance and the voltage being used.

Paying for electricity When paying an electricity bill, it is energy we are paying for. Energy is measured in joules and we showed above that this was the same as watts xseconds. A watt-second is a very small unit of energy. Instead the Electricity Board sells its energy in UNITS, which are KILOWATTHOURS. A kilowatt is 1000 watts and an hour is 60 x60 58

I APPENDIX I

seconds, so a unit of I kilowatt-hour is 3600000 wattseconds or 3600000 joules. One unit (or 1 kilowatt-hour) usually costs the average domestic consumer about lp. One unit of electrical energy is needed to operate a 1000-watt heater for I hour or a 100-watt lamp for 10 hours or a 25-watt lamp for 40 hours.

I>,·'

, ,

'.