It was first discovered that current in a wire produces a magnetic field and
then Michael Faraday discovered that the opposite is also true in that a
magnetic field can produce a current inside a conductor.
Electromagnetic Induction is the production of a potential difference (voltage)
across a conductor when it is exposed to a changing magnetic field. When a
conductor is moved across a magnetic field so as to cut through the lines of
magnetic flux, an electromotive force (emf) is produced in the conductor. Magnetic
flux is how any material is affected by a magnetic field. The moving magnetic
field caused by the changing magnetic flux induces emf however the measure of
magnetic flux is not important, it is whether or not there’s change in flux
If the conductor forms part of a closed circuit then the emf produced causes electrons
to flow round the circuit thus a current is produced.
When a magnetic field associated with a magnet, moves towards a coil of wire,
the magnetic flux of the magnet moves across or cuts the coil. It is the
relative movement of the magnetic flux and the coil that causes an emf and thus
current to be induced in the coil.
When the magnet is moved at a constant speed
towards the coil, a deflection is noted on the galvanometer showing that
current has been produced in the coil.
When the magnet is moved at the same speed as
before but away from the coil the same deflection is noted but in the opposite
When the magnet is held stationary, even within
the coil, no deflection is recorded.
When the coil is moved at a constant speed and
the magnet is held stationary, the same galvanometer deflection is noted.
When either the speed of the magnet or coil is
doubled whilst the other is stationary, the galvanometer deflection is doubled.
When a stronger magnet is used, a greater
deflection is noted.
When the number of turns of wire of the coil is
increased, a greater deflection is noted.
Faraday’s First Law: Any change
in the magnetic field of a coil of wire will cause an emf to be induced in the
coil. With this induced emf if the conductor circuit is closed, current will
also circulate through the circuit and this current is called induced current.
Faraday’s Second Law: The magnitude
of emf induced in the coil is equal to the rate of change of flux that links
with the coil. The flux linkage of the coil is the product of number of turns
in the coil and flux associated with the coil.
Lenz’s Law: An electric current,
induced by a source such as a changing magnetic field, always creates a
counterforce opposing the force inducing it.
Fleming’s Hand Rules: Used to aid in
the understanding of magnetic field, motion and induced current directions. Fleming’s
left-hand rule is used for electric motors, while Fleming’s right-hand rule is
used for electric generators. Different hands need to be used for motors and
generators because of the differences between cause and effect. In an electric
motor, the electric current and magnetic field exist (which are the causes),
and they lead to the force that creates the motion (which is the effect).
Electromagnetic induction is the fundamental operating principles of
transformers, inductors and many types of electrical motors, generators and
In an electric motor, the motor has coils turning inside magnetic fields, and a
coil turning inside a magnetic field induces an emf. This emf, known as the
back emf, acts against the applied voltage that’s causing the motor to spin in
the first place, and reduces the current flowing through the coils.
In AC generators, conductors forming an electric circuit are made to move
through a magnetic field. By Faraday’s law an emf is induced in the conductors
and thus a source of emf is created. A generator convert mechanical energy into
In a transformer, alternating current from primary coil moves quickly back and
forth across the secondary coil. The moving magnetic field caused by the
changing flux induces a current in the secondary coil, stepping the voltage
either up or down.