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Electromagnetic induction

Induced e.m.f.s

If magnetic flux through a coil is altered then an e.m.f. will be generated in the coil. This effect was first observed and explained by Ampere and Faraday between 1825 and 1831. Faraday discovered that an e.m.f. could be generated by either:
(a) moving the coil or the source of flux relative to each other or by
(b) changing the magnitude of the source of magnetic flux in some way.
Note that the e.m.f. is only produced while the flux is changing.

For example, consider two coils as shown in Figure 1.

Coil A is connected to a galvanometer and coil B is connected to a battery and has direct current flowing through it. Coil A is within the magnetic field produced by B and an e.m.f. can be produced in A by moving the coils relative to each other or by changing the size of the current in B. This can be done by using the rheostat R, switching the current on or off, or (c) using an a.c. supply for B.
(An e.m.f. could also be produced in A by replacing B with a permanent magnet and moving this relative to A.)

Faraday's laws

Faraday summarised the results of his experiments as follows:
(a) An e.m.f. is induced in a coil if the magnetic flux through the coil changes
(b) The magnitude of the induced e.m.f. depends on
(i) the rate of change of flux,
(ii) the number of turns on the coil, and
(iii) the cross-sectional area of the coil.
Points (ii) and (iii) simply refer to the amount of change of flux. The faster the flux is changed the greater is the e.m.f. produced.

Simple observations about induced e.m.fs

A voltage (e.m.f) can be induced in a wire or a coil if the wire is in a region where the magnetic field is changing. This can be done by:
(a) moving the wire through a fixed field
(b) moving a fixed field (a permanent magnet) relative to the wire or
(c) varying the field by using a.c in a coil
(d) the faster the relative motion the greater the e.m.f generated
(e) the direction of motion affects the "direction" of e.m.f

© Keith Gibbs 2011