104 The Poetry of Physics and The Physics of Poetry
induction can be made by recalling that it is the relative motion of the
magnet and the loop of wire, which causes a current to flow.
Let us consider a loop of wire moving with respect to the magnet. The
motion of the loop creates an effective current out of every electron in
the wire. The direction of this current of each electron in the wire is not
along the wire but perpendicular to it i.e. in the direction of the wire’s
motion. Each of these individual currents is acted upon by the magnetic
field of the magnet, which exerts a force perpendicular to the direction of
motion of the wire and hence creates an effective electric field. It is this
force, which causes the induced current to flow in the looped wire. The
same effect occurs when the magnet is moving and the wire is at rest
since it is only the relative motion of the two, which matters.
Electrical induction also operates with two loops of wire facing each
other. If an electric current flowing in the first loop changes in any way,
it causes a momentary current to flow in the second loop. The principle is
the same as that of electric induction caused by a moving magnet. When
the current in the first loop of wire changes, the magnetic field generated
by this current changes at the position of the second loop. In the case of
electric induction involving the relative motion of the loop of wire and
the magnet that we just discussed, the magnetic field at the loop of wire
is also changing when a magnet is thrust into the loop of wire. It is clear
from these two cases that the induced electrical current is caused by a
changing magnetic field. Since the induced electrical current is caused by
an induced electric field, the phenomenon of induction can be expressed
totally in terms of fields. A changing magnetic field induces an electric
field, which in turn causes the induced electric current to flow. Faraday’s
law states that the strength of the induced electrical field produced is
proportional to the rate of change of the magnetic field and acts
perpendicular to the direction in which the magnetic field is changing.