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KS4 Physics. Mains Electricity. Contents. Mains Electricity. Alternating current and direct current. Electricity in the home. Fuses and circuit breakers. Buying electricity. Summary activities. What is direct current (d.c.)?.
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KS4 Physics Mains Electricity
Contents Mains Electricity Alternatingcurrentanddirectcurrent Electricity in the home Fuses and circuit breakers Buying electricity Summary activities
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.
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.
Comparing a.c. and d.c. a.c. peak forward voltage d.c. voltage voltage time time peak reverse voltage The voltage of an a.c. supply follows a repeated pattern: it rises to a peak, returns to zero changes direction and so on. The difference between alternating current (a.c.) and direct current (d.c.) can be seen using an oscilloscope. For each current, the oscilloscope trace is a graph showing how the voltage of an electricity supply varies with time. The voltage of a d.c. supply is steady and always in the same direction.
Frequency of alternating current 1 complete cycle a.c. peak forward voltage voltage time peak reverse voltage The frequency of a.c. electricity is the number of complete cycles per second, which is measured in hertz (Hz). 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.
Mains electricity in the UK In the UK, the frequency of mains electricity is 50hertz: 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. ThevoltageofmainselectricitysuppliedtoUKhomesis230V. 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.
Contents Mains Electricity Alternatingcurrentanddirectcurrent Electricity in the home Fuses and circuit breakers Buying electricity Summary activities
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 230V and can easily kill you if you get a shock. NEVER PUT ANYTHING OTHER THAN A PLUG IN AN ELECTRICAL SOCKET!
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.
Inside an electrical cable An electrical cable contains three wires. 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. copper insulating plastic
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.
What does each wire do? The live wire carries a current that alternates between a negative and positive voltage. 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. The neutral wire completes the circuit. It is kept at a zero voltage by the electricity company.
Parts of a plug live terminal earth terminal fuse neutral terminal cable grip cable
Wiring a plug When wiring a plug, always check the following: • Make sure that the wires are connected to the correct terminals: blue to neutral yellow/green to earth brown to live • There are no loose or bare wires. • The grip should firmly hold the cable in place. • A fuse of the correct value is fitted.
Contents Mains Electricity Alternatingcurrentanddirectcurrent Electricity in the home Fuses and circuit breakers Buying electricity Summary activities
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.
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 case made of insulating material thin wire with high resistance When a large current flows the wire becomes hot. If too much current flows, the wire overheats and melts, which breaks the circuit.
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 4.3A, what fuse should it be fitted with? You can choose from fuses of 3A, 5A and 13A. 5A fuse
Calculating which fuse to use A lamp has a potential difference of 230V across it and operates at a power of 60W. a) What current is running through the lamp? b) What fuse should the lamp’s plug be fitted with? P = IV I = P/V = 60W / 230V = 0.26A a) b) The plug should be fitted with a 3A fuse.
Calculating which fuse to use A cooker has a potential difference of 230V across it and operates at a power of 9.2kW. a) What current is running through the cooker? b) Why should the cooker’s plug not be fitted with a 13A fuse? P = IV I = P/V = 9200W / 230V = 40A a) b) A 13A fuse would blow when the cooker was turned on.
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?
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.”
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.
How does a circuit breaker work? switch electromagnet 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.
Domestic wiring system lights 5A 30A ring main power points kWh meter earth live neutral
Contents Mains Electricity Alternatingcurrentanddirectcurrent Electricity in the home Fuses and circuit breakers Buying electricity Summary activities
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 (kWh). The cost of an electricity bill is calculated from the number of units used.
Cost of electricity cost = number of units x cost per unit The cost of electricity is the number of units of electrical energy multiplied by the cost per unit: Example: How much would 10 units of electricity cost at a price of 9p per unit? cost = 10units x 9p/unit = 90p
Calculating the units of electricity electrical energy = power x time 1 unit of electricity = 1 unit of electrical energy = 1 kilowatt hour (kWh) 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: Power is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in? Example: How many units of electricity is 17.6 kWh? 17.6 units
Buying electricity problem A kettle uses 45.2 kWh of energy. If electricity costs 10p 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 = 45.2units x 10p/unit = 452 p or £4.52
Buying electricity problem An iron that operates at a power of 3 kW for 4 hours uses electricity that costs 8p 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 = 3kW x 4h = 12kWh = 12units Cost of electricity: cost = number of unitsx cost per unit = 12 units x 8p/unit = 96p
Contents Mains Electricity Alternatingcurrentanddirectcurrent Electricity in the home Fuses and circuit breakers Buying electricity Summary activities
Glossary • alternating current – A current that is constantly changing direction. • circuit breaker – An electromagnetic device that cuts off the electrical supply if the current is too large. • direct current – A current that is always flowing in one direction. • earth wire – This safety wire connects the metal case of an appliance to earth. • fuse – A length of wire that heats up and ‘blows’ (melts) if the current is too large and so cuts off the electrical supply. • live wire – This wire carries a current that alternates between negative and positive values. • neutral wire – This wire is kept at zero voltage by the electricity company.
