Poetry of Physics and the Physics of Poetry

Electricity and Magnetism 103

100 The Poetry of Physics and The Physics of Poetry

Fig. 11.3

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.

Fig. 11.3

While the concept of the electric and magnetic field might solve the
mystery of action at a distance, it leaves us with a new mystery; to wit,
how does a charged particle create an electric or magnetic field at some
distance from it without a connecting medium. Faraday and his successor
actually believed in the existence of an invisible medium, which they
called aether, which, according to them, filled up all of space. This
concept survived until the advent of the theory of relativity when
Einstein showed that the existence of an aether was inconsistent with
experimental facts. The electric and magnetic field, as far as I am
concerned, do not really exist in the same sense that charged particles
exist. The field concept is an abstraction, a pictorial description of
the electric and magnetic forces. It is, nevertheless, a conceptual
construction, which is very useful. The concept of electric and magnetic
fields will help us to understand the phenomenon of electrical induction.
It will also help to explain the creation, absorption and propagation of
light as an electromagnetic phenomenon.
The phenomenon of electrical induction was first discovered by
Faraday, who observed that an electric current momentarily exists within
a loop of wire into which a magnet has been thrust as is illustrated in
Fig. 11.3. The current flows only while the magnet is in motion either
being inserted into the loop or being withdrawn from it. It was not just
the presence of the magnet but its motion that induced an electric current.
Faraday discovered that a current could also be induced by moving the
loop of wire with respect to the magnet. In other words, it is the relative
motion of the magnet and the loop, which induces a current. If the
current flows in a clockwise direction when the magnet is inserted, then
it flows counter-clockwise when the magnet is removed. Furthermore, if
the opposite pole of the magnet is inserted into the loop, then the current
also changes direction. An understanding of the mechanism of electric