A straight copper wire is held between the poles of a horseshoe magnet. The wire is connected to a sensitive voltmeter. Explain what happens to the voltmeter reading when the wire is moved quickly upwards, then held still, and then moved slowly downwards.

When the wire is pulled up, the magnetic flux through the wire decreases, so an e.m.f. is induced in the wire (Lenz’s law). The faster the upward motion, the larger the rate of change of flux and therefore the larger the induced e.m.f., so the voltmeter shows a higher positive voltage. When the wire is held still, the flux is constant, no e.m.f. is induced and the voltmeter reads zero. When the wire is pulled down, the flux increases, giving an induced e.m.f. of opposite polarity; a slower downward motion gives a smaller rate of change of flux and therefore a smaller (negative) voltage on the voltmeter.