Contents Mains Electricity Alternating current and direct current

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Contents Mains Electricity Alternating current and direct current Electricity in the home Fuses and circuit breakers Buying electricity Summary activities © Boardworks Ltd 2005

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. © Boardworks Ltd 2005

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. © Boardworks Ltd 2005

Comparing a. c. and d. c. The difference between alternating current (a. c. ) and direct current (d. c. ) can be seen using an oscilloscope. 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. 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 2005

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. © Boardworks Ltd 2005

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. © Boardworks Ltd 2005

Alternating current simulation © Boardworks Ltd 2005

Which type of current – a. c. or d. c. ? © Boardworks Ltd 2005

Contents Electricity: Mains electricity Alternating current and direct current Electricity in the home Fuses and circuit breakers Buying electricity Summary activities © Boardworks Ltd 2005

Electrical sockets The mains electricity system in your house is made up of three types of wires: live, neutral and earth. Electrical sockets are a convenient and safe way of making the electricity available for use. Remember the electricity in a plug socket is at 230 V and can easily kill you if you get a shock. NEVER PUT ANYTHING OTHER THAN A PLUG IN AN ELECTRICAL SOCKET! © Boardworks Ltd 2005

Plugs and electrical cable An electrical plug is inserted into a socket and the cable which is attached to the plug carries electricity to the appliance. The plug has three pins which fit the three holes on the socket in one way only, so the plug cannot be connected wrongly. The pins are made of brass, which is a good conductor but is hard and will not bend. © Boardworks Ltd 2005

Inside an electrical cable An electrical cable contains three wires. copper These wires are made of copper because it is a good conductor of electricity. Each wire is made of thin strands of copper to keep the cable flexible. Each wire is encased in plastic to stop the wires touching and causing a short circuit. Plastic is used as it is a good insulator as well as being tough and flexible. The whole cable is encased in another layer of plastic. insulating plastic © Boardworks Ltd 2005

Materials in a plug © Boardworks Ltd 2005

Live, neutral and earth The three wires in electrical cables have different functions. The colour of the plastic casing tells you what the wire does. live earth neutral This colour-coding is a safety feature that makes the wires easy to identify in poor lighting conditions. © Boardworks Ltd 2005

What does each wire do? The live wire carries a current that alternates between a negative and positive voltage. The neutral wire completes the circuit. It is kept at a zero voltage by the electricity company. The earth wire is a safety wire that is needed to earth appliances with a metal case. For example, if the live wire becomes loose and touches the metal case, a very large current flows to earth and blows the fuse. This makes it safe to touch the appliance. © Boardworks Ltd 2005

Parts of a plug earth terminal live terminal fuse neutral terminal cable grip cable © Boardworks Ltd 2005

Wiring a plug When wiring a plug, always check the following: l Make sure that the wires are connected to the correct terminals: blue to neutral yellow/green to earth brown to live l There are no loose or bare wires. l The grip should firmly hold the cable in place. l A fuse of the correct value is fitted. © Boardworks Ltd 2005

Plugs – true or false? © Boardworks Ltd 2005

Contents Electricity: Mains electricity Alternating current and direct current Electricity in the home Fuses and circuit breakers Buying electricity Summary activities © Boardworks Ltd 2005

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 plug contains a fuse to prevent the case of a metal appliance from becoming live if a fault develops. All plugs contain a fuse like the one shown and are simple to change if they ‘blow’ and break the circuit. © Boardworks Ltd 2005

How does a fuse work? A fuse is a built-in weak point in a circuit. It contains a thin wire with a higher resistance than normal wire. terminals thin wire with high 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. © Boardworks Ltd 2005

Choosing the right fuse The circuit symbol for a fuse is: Fuses are labelled with the maximum current that they are designed to allow. To choose the correct fuse for a device, always choose the nearest one rated above 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? 5 A fuse You can choose from fuses of 3 A, 5 A and 13 A. © Boardworks Ltd 2005

Choose which fuse to use © Boardworks Ltd 2005

Circuit breakers 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. © Boardworks Ltd 2005

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. switch 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. electromagnet © Boardworks Ltd 2005

Contents Electricity: Mains electricity Alternating current and direct current Electricity in the home Fuses and circuit breakers Buying electricity Summary activities © Boardworks Ltd 2005

Buying electricity Electricity costs money, which is why every home has an electricity meter. The meter records how much electricity is used in a house in units of electrical energy. The units of electrical energy are called kilowatt hours (k. Wh). The cost of an electricity bill is calculated from the number of units used. © Boardworks Ltd 2005

Cost of electricity The cost of electricity is the number of units of electrical energy multiplied by the cost per unit: cost = number of units x cost per unit Example: How much would 10 units of electricity cost at a price of 9 p per unit? cost = 10 units x 9 p/unit = 90 p © Boardworks Ltd 2005

Calculating the units of electricity The amount of electrical energy (i. e. the amount of electricity) used by an appliance depends on its power and how long the electricity is used for: electrical energy = power x time Power is measured in kilowatts (k. W) and the time is measured in hours (h), so what are the units of electricity measured in? 1 unit of electricity = 1 unit of electrical energy = 1 kilowatt hour (k. Wh) Example: How many units of electricity is 17. 6 k. Wh? 17. 6 units © Boardworks Ltd 2005

Buying electricity problem A kettle uses 45. 2 k. Wh of energy. If electricity costs 10 p per unit, how much does it cost to use the kettle? Number of units: number of units of electricity = number of kilowatt hours = 45. 2 units Cost of electricity: cost = number of units x cost per unit = 45. 2 units x 10 p / unit = 452 p or £ 4. 52 © Boardworks Ltd 2005

Buying electricity problem An iron that operates at a power of 3 k. W for 4 hours uses electricity that costs 8 p per unit. How much does it cost for the electricity used by the iron in that time? Number of units: number of units of electricity = number of kilowatt hours = 3 k. W x 4 h = 12 k. Wh = 12 units Cost of electricity: cost = number of units x cost per unit = 12 units x 8 p / unit = 96 p © Boardworks Ltd 2005

Contents Electricity: Mains electricity Alternating current and direct current Electricity in the home Fuses and circuit breakers Buying electricity Summary activities © Boardworks Ltd 2005

Glossary l alternating current – A current that is constantly changing direction. l circuit breaker – An electromagnetic device that cuts off the electrical supply if the current is too large. l direct current – A current that is always flowing in one direction. l earth wire – This safety wire connects the metal case of an appliance to earth. l fuse – A length of wire that heats up and ‘blows’ (melts) if the current is too large and so cuts off the electrical supply. l live wire – This wire carries a current that alternates between negative and positive values. l neutral wire – This wire is kept at zero voltage by the electricity company. © Boardworks Ltd 2005