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Generators
Electromagnetic induction Faraday’s law Lenz’s law Simple a.c. generator (alternator)
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The effect of producing electricity from magnetism was discovered in 1831 by Faraday and is called electromagnetic induction. It led to the construction of generators for producing electrical energy in power stations.
●● Electromagnetic induction
Simple d.c. generator (dynamo) Practical generators Applications of electromagnetic induction
b) Bar magnet and coil The magnet is pushed into the coil, one pole first (Figure 43.2), then held still inside it. It is then withdrawn. The meter shows that current is induced in the coil in one direction as the magnet is moved in and in the opposite direction as it is moved out. There is no deflection when the magnet is at rest. The results are the same if the coil is moved instead of the magnet, i.e. only relative motion is needed.
Two ways of investigating the effect follow.
a) Straight wire and U-shaped magnet First the wire is held at rest between the poles of the magnet. It is then moved in each of the six directions shown in Figure 43.1 and the meter observed. Only when it is moving upwards (direction 1) or downwards (direction 2) is there a deflection on the meter, indicating an induced current in the wire. The deflection is in opposite directions in these two cases and only lasts while the wire is in motion.
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wire
coil (600 turns) bar magnet
Figure 43.2 A current is induced in the coil when the magnet is moved in or out.
●● Faraday’s law
magnet
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sensitive centre-zero meter
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6
4 2
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To ‘explain’ electromagnetic induction Faraday suggested that a voltage is induced in a conductor whenever it ‘cuts’ magnetic field lines, i.e. moves across them, but not when it moves along them or is at rest. If the conductor forms part of a complete circuit, an induced current is also produced. Faraday found, and it can be shown with apparatus like that in Figure 43.2, that the induced p.d. or voltage increases with increases of (i) the speed of motion of the magnet or coil, (ii) the number of turns on the coil, (iii) the strength of the magnet.
sensitive centre-zero meter Figure 43.1 A current is induced in the wire when it is moved up or down between the magnet poles.
These facts led him to state a law: The size of the induced p.d. is directly proportional to the rate at which the conductor cuts magnetic field lines. 199
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