The phenomenon in which an electric
field is generated in an electric
circuit when the number of magnetic
field lines passing through the circuit changes. Electromagnetic induction
was discovered independently by Michael Faraday and Joseph Henry. The voltage induced
is proportional to the rate of change of the field, and large voltages can
be produced by switching off quite small magnetic fields suddenly. Frequently,
the magnetic field is itself generated by an electric
current in a coil, in which case the voltage induced is proportional
to the rate of change of the current (see inductance).
|Simple experiments in electromagnetic induction.
(A) When a permanent magnet is being moved in and out of a small coil,
and "induced" current flows in the coil. But it is only while the
magnetic field in the coil is changing that any induced current flows.
(B) Self-induction occurs when a circuit contains an inductive component
such as a coil. When a circuit is completed through a coil (by closing
a switch), the increasing current in the coil gives rise to a changing
magnetic field which interacts with the coil itself, setting up an
electromotive force which opposes the original current. A sensitive
ammeter in series with such a coil might show a momentary current
(red) in the direction opposite to the subsequent steady current (green).
On breaking the circuit, a similar forward current surge (broken red)
occurs. (C) Mutual induction is exhibited when there is a magnetic
linkage between two circuits containing inductive elements. Closing
the switch in the primary circuit creates a magnetic field in the
secondary circuit, giving rise to a momentary current as in (A). Opening
the switch in the primary causes an equivalent current in the reverse
direction in the secondary
The principle finds numerous applications in electric generators and motors, transformers,
microphones, and engine ignition systems. In the less familiar technique
of induction heating, widely used in metal working, an object is heated
by currents created in it by the voltage induced by a high-frequency current
in a nearby coil; as the coil field will pass through insulators without
heating them, the principle can be applied to produce "cold hob" electric
This law of electromagnetic induction,
deduced in 1834 by the Russian physicist Heinrich Lenz (1804–1865), states that the electromotive
force (emf) induced in a circuit is such as to opoose the flux change
giving rise to it.