Currents from magnetism

A further connection between electricity and magnetism was discovered by Faraday, who found that changing magnetic fields though loops of wire will cause currents to be induced. For example, consider the wire loop below, and imagine a bar magnet is brought into the vicinity.

 

Figure 9.16: Wire loop

If a magnetic field is pushed into the plane of this loop, a counterclockwise current will be induced, as indicated. Alternatively, if the magnetic field is pulled out of the loop, a clockwise current will be induced. These induced currents only exist as long as the magnet is moving, and will die off when the magnet becomes stationary.

These induced currents have an interesting aspect as far as there magnetic properties are concerned. Recall from the last section that currents induce magnetic fields. Thus, by either pushing or pulling the magnet into or out of the wire loop, one is inducing magnetic fields within this loop. The direction of these induced magnetic fields are such that if one is externally increasing the magnetic field through the loop by pushing a magnet in, then the induced field will be such as to decrease the magnetic field through the loop; this is indicated in the previous figure. Alternatively, if one is externally decreasing the magnetic field through the loop by pulling a magnet out, then the induced field will be such as to increase the magnetic field through the loop. This feature that the magnetic effects of the induced current are such as to oppose the external change is known as Lenz's law.

The induction of currents from changing magentic fields has a number of important applications, some of which we now discuss.