Generating Electricity

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2 Generating Electricity The dynamo effect An electric current is produced when a magnet is moved into a coil of wire in a circuit. The direction of the current is reversed when the magnet is moved out of the coil. It can also be reversed if the other pole of the magnet is moved into the coil. Check your understanding of the dynamo effect with this simulation:

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4 Increasing the induced current To increase the induced current: move the magnet more quickly use a stronger magnet increase the number of turns on the coil.

5 DC electricity If the current flows in only one direction it is called direct current, or DC. Batteries and solar cells supply DC electricity. A typical battery may supply 1.5 V. The diagram shows an oscilloscope screen displaying the signal from a DC supply.

6 AC electricity If the current constantly changes direction it is called alternating current, or AC. Mains electricity is an AC supply. The UK mains supply is about 230V. It has a frequency of 50 Hz, which means that it changes direction and back again 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an AC supply.

7 Dynamos One simple example of a generator is the bicycle dynamo. The dynamo has a wheel that touches the back tyre. As the bicycle moves, the wheel turns a magnet inside a coil. This induces enough electricity to run the bicycle's lights. AC generators

8 One side of a coil in an AC generator moves up during one half-turn, and then down during the next half-turn. This means that, as a coil is rotated in a magnetic field, the induced current reverses direction every half-turn. Generators

9 Conventional power stations There are four main stages: fuel is burned to boil water to make steam steam makes a turbine spin spinning turbine turns a generator which produces electricity electricity goes to the transformers to produce the correct voltage.

10 Energy transfers Power stations generate significant heat losses to the environment. Below is an energy transfer diagram for the generation of electricity from a fossil fuel such as coal: Chemical energy is stored in coal. This energy is transferred as heat and stored in water as steam. The energy in steam is transferred to movement in a turbine and to electrical energy in the turbine.

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12 Efficiency The efficiency of a device or process is the proportion of the energy supplied that is transferred in useful ways. You should be able to calculate efficiency as a decimal or as a percentage. The efficiency of a power station can be calculated as follows: efficiency = useful energy out total energy in (for a decimal efficiency) efficiency = (useful energy out total energy in) 100 (for a percentage efficiency)

13 The efficiency of the power station is 3,800 10,000 = 0.38 (or 38 per cent) This means that 62 per cent of the chemical energy supplied is not usefully transferred. For example, it may be transferred to the surroundings as heat energy and sound energy. Note that the efficiency will always be less than 100 per cent. You might be given the power in W instead of the energy in J. The equations are the same - just substitute power for energy.

14 Which one of these changes will reduce the voltage induced in a coil by a magnet? Adding more turns to the coil Using a weaker magnet Moving the magnet more quickly

15 Answer Using a weaker magnet!

16 Which statement about how generators work is correct? A coil of wire is rotated in a magnetic field A coil of wire is rotated in an electric field A magnet is rotated in a magnetic field

17 Answer A coil of wire is rotated in a magnetic field

18 What is the approximate voltage 230 V produced by power stations? 25,000 V 400,000 V

19 25,000V Answer

20 What is the approximate voltage of large overhead electricity cables? 230 V 25,000 V 400,000 V

21 400,000 V Answer

22 Which statement about transformers is correct? They work with dc They work with dc and ac They work with ac

23 A transformer produces 100V from an input of 50V. What does this tell you? It is very efficient It is a step-up transformer It is a step-down transformer

24 Answer It is a step-up transformer

25 How AC Generators Produce Electricity XXhQU

26 AC Generators in the Power Plant Deerhaven Power Station xldqu4 Regulation of AC Generator MozZOk

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