High School Physics

# Magnitude and Direction of an induced e.m.f.

To explain electromagnetic induction Faraday suggested that an e.m.f. 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. In this post, let’s see how the magnitude and direction of an induced e.m.f. are determined. [ Read about Faraday’s Law of Electromagnetic Induction here.]

Faraday identified three factors affecting the magnitude of an induced e.m.f.

The induced e.m.f. 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.

These facts led him to state that: The magnitude of the induced e.m.f. is directly proportional to the rate at which the conductor cuts magnetic field lines.

The direction of an induced e.m.f. opposes the change causing it.

In Figure (1a) the magnet approaches the coil, north pole first. The induced e.m.f. and resulting current flow should be in a direction that makes the coil behave like a magnet with its top a north pole.

The downward motion of the magnet will then be opposed since like poles repel.
When the magnet is withdrawn, the top of the coil should become a south pole (Figure 1b) and attract the north pole of the magnet, thus hindering its removal. The induced e.m.f. and current are thus in the opposite direction to that when the magnet approaches.

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