Before we consider the mathematical treatment of Faraday's laws we will
look at the phenomenon of eddy currents. These are induced currents in metal objects larger than
pieces of wire; the e.m.f.s induced may not be very great but because the resistance of a lump of
metal is low the induced currents can be large.
Since the induced currents always act so as to
oppose the motion (Lenz's law) eddy currents can be used as a very effective electromagnetic brake.
A simple example of this is shown in Figure 1.
A piece of metal is swung between the poles of
magnet and currents are induced in the metal which quickly damp the oscillation as is shown in Figure
1 (a). In figure 1 (b) slots have been cut in the metal so increasing its resistance, the induced currents
are reduced and the damping is very much less – the metal swings freely.
Eddy currents become a problem in the cores of transformers
where they can cause large energy losses. For this reason the cores are made of thin leaves of metal
called laminations. The laminations are coated with an insulating paint, thus increasing the resistance
of the core and limiting the eddy current flow. The energy loss is proportional to the square of the
lamination thickness and the square of the frequency of the current.
Eddy currents can be
used as electromagnetic damping, to melt metals in a vacuum, so giving metals of a high purity free
from atmospheric contamination, and to heat metal parts of valves.
