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What is direct current (d. c. )? Direct current (d. c. ) is an electric current that always flows in one direction. Direct current is produced by cells and batteries. Electronic circuits such as those in computers and stereos need direct current electricity in order to work. Direct current cannot be transferred efficiently over large distances. 3 of 30 © Boardworks Ltd 2006

What is alternating current (a. c. )? Alternating current (a. c. ) is an electric current that is constantly changing direction. Alternating current is produced by most generators and is used in mains electricity. Motors often work using alternating current. The voltage of alternating current is easily changed with a transformer. Alternating current can be transferred efficiently over large distances. 4 of 30 © Boardworks Ltd 2006

What is the difference between a. c. and d. c. ? The difference between alternating current (a. c. ) and direct current (d. c. ) can be seen using an oscilloscope. voltage d. c. time voltage For each current, the oscilloscope trace is a graph showing how the voltage of an electricity supply varies with time. peak forward voltage a. c. time peak reverse voltage The voltage of a d. c. supply is steady and always in the same direction. 5 of 30 The voltage of an a. c. supply follows a repeated pattern: it rises to a peak, returns to zero changes direction and so on. © Boardworks Ltd 2006

Frequency of alternating current voltage The frequency of a. c. electricity is the number of complete cycles per second, which is measured in hertz (Hz). 1 complete cycle peak forward voltage a. c. time peak reverse voltage The a. c. frequency can be determined from an oscilloscope by counting the number of complete waves per unit time. If the frequency is increased, the number of complete waves shown on the screen increases. For example, if the frequency is doubled, the number of waves doubles. 6 of 30 © Boardworks Ltd 2006

Mains electricity in the UK In the UK, the frequency of mains electricity is 50 hertz: this alternating current flows backwards and forwards 50 times per second. This frequency is the same at any point in the electricity supply system but the voltage varies in different parts of the national grid. The voltage of mains electricity supplied to UK homes is 230 V. This is an effective voltage which is equal to the voltage of a d. c. supply that would produce the same heating effect. The peak (maximum) voltage is higher than this. 7 of 30 © Boardworks Ltd 2006

Comparing a. c and d. c. 8 of 30 © Boardworks Ltd 2006

Which type of current – a. c. or d. c. ? 9 of 30 © Boardworks Ltd 2006

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What is a fuse? A fuse is a safety device that protects an electric cable from overheating so that the insulation does not catch fire. A fuse also makes appliances with a metal case safer. If the live wire becomes loose and touches the metal case, a very large current flows along the earth wire and blows the fuse. This makes it safe to touch the appliance. 11 of 30 © Boardworks Ltd 2006

How does a fuse work? A fuse is a built-in weak point in a circuit. It contains a thin wire with a lower resistance than normal wire. terminals thin wire with low resistance case made of insulating material When a large current flows the wire becomes hot. If too much current flows, the wire overheats and melts, which breaks the circuit. 12 of 30 © Boardworks Ltd 2006

How do you choose the correct fuse? The circuit symbol for a fuse is: A fuse is labelled with the maximum current that it will allow to flow through it. To choose the correct fuse for an electrical device, always choose the one with the closest rating that is greater than the operating current of the device. Example: If a kettle operates with an electrical current of 4. 3 A, what fuse should it be fitted with? You can choose from fuses of 3 A, 5 A and 13 A. 5 A fuse 13 of 30 © Boardworks Ltd 2006

You choose which fuse to use! 14 of 30 © Boardworks Ltd 2006

Steve the student buys a DVD player Steve bought a second-hand DVD player that did not work. He changed the fuse for one of the same size but the DVD player still did not work. What might be the problem? 15 of 30 © Boardworks Ltd 2006

Steve the student buys a DVD player When Steve changed the fuse on his second-hand DVD player it still did not work. Steve and his flatmates suggested different solutions to the problem: Steve: “I think I should throw it away and buy one from a shop where I know it will work. ” Fatema: “I would put a bigger fuse in it because the fuse in it can’t be large enough if it keeps on blowing. ” Kieran: “It sounds as if something is wrong, I would get an electrician to have a look at it. ” 16 of 30 © Boardworks Ltd 2006

What is a circuit breaker? The electrical wiring in a building must be protected from being overloaded so that it does not overheat. This is the job of a ‘fuse box’, which used to contain fuse wire but now contains circuit breakers to protect the wiring. Circuit breakers do the same job as fuses but they are electromagnetic switches which are easy to reset. A house has several circuits and each one is protected by a separate circuit breaker. Circuit breakers have different ratings as shown by the coloured dots. 17 of 30 © Boardworks Ltd 2006

How does a circuit breaker work? The circuit breakers in a ‘fuse box’ are some of the most important safety mechanisms in your home. Each circuit breaker is an electromagnetic switch which is designed to break the circuit when the current gets too high. Too much current makes the magnetic field produced by the electromagnet strong enough to open the switch. The circuit breaker is said to ‘trip’ and switches off the current. 18 of 30 switch electromagnet © Boardworks Ltd 2006

What is a residual current device (RCD)? A residual current device (RCD), also called a residual current circuit breaker (RCCB), is another type of circuit breaker. It prevents electric shocks when using extension cables to appliances like lawnmowers. This safety device compares the current in the live and neutral wires, which are the same when the appliance is working properly. If the current is leaking, the RCD detects a difference between the live and neutral wires and instantly shuts down the power. The advantages of an RCD are that it is very sensitive and quickly cuts off current before it is too large to cause injury. 19 of 30 © Boardworks Ltd 2006

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What is a transformer? The voltage of an alternating current can be changed using a device called a transformer. A transformer contains two coils that are wound around a soft iron core The alternating current in the primary (input) coil produces an alternating magnetic field. This alternating magnetic field induces an alternating current in the secondary (output) coil. 21 of 30 primary coil secondary coil © Boardworks Ltd 2006

How does a transformer change voltage? The voltage induced in the secondary (output) coil depends on the number of turns on the primary and secondary coils. A step-up transformer has more turns on the secondary coil and so increases voltage. 22 of 30 A step-down transformer has fewer turns on the secondary coil and so decreases voltage. © Boardworks Ltd 2006

Where are transformers used? A television needs a very high voltage to operate. It contains a step-up transformer, which increases the voltage of the electricity supplied to the television. 23 of 30 This outdoor transformer decreases the voltage of the electricity carried by the national grid. It is an example of a step-down transformer. © Boardworks Ltd 2006

How are transformers used to supply electricity? 24 of 30 © Boardworks Ltd 2006

How does electricity get to your home? 25 of 30 © Boardworks Ltd 2006

Glossary (1/2) l alternating current – A current that constantly changes direction. It is produced by most electrical generators. l circuit breaker – An electromagnetic device that cuts off the electrical supply, if the current is too large. l direct current – A current that always flows in the same direction. It is produced by cells and batteries. l earth wire – This safety wire connects the metal case of an appliance to the earth. l fuse – A length of wire that heats up and melts (‘blows’) if the current is too large, and so cuts off the electrical supply. l mains electricity – The domestic electrical power supply with a voltage of 230 V. 27 of 30 © Boardworks Ltd 2006

Glossary (2/2) l national grid – The network of high-voltage power lines that carries electricity from power stations across the country. l step-down transformer – A transformer that decreases the voltage of alternating current. l step-up transformer – A transformer that increases the voltage of alternating current. l substation – A place where the high voltage electricity from the national grid is reduced by transformers so that it can be used in homes. l transformer – A device that increases or decreases the voltage of alternating current. 28 of 30 © Boardworks Ltd 2006