Inertia is the property of an object to oppose any change in the direction of its motion. Because of inertia, a
body at rest tends to remain at rest; a body in motion tends to remain in motion. Electrical circuits have a
property which is very similar to inertia. If current in a circuit changes, a force in that circuit opposes the
change. This opposition of a change in current is called INDUCTANCE.
In 1819, Hans Christian Oersted, a Danish physicist, discovered that a magnetic field surrounds any
conductor that is carrying current. The strength of this magnetic field depends on the amount of current
flowing in the conductor. The direction of the magnetic field depends on the direction of the current flow
within the conductor. The left hand rule for conductors, illustrated below, is used to determine the direction
of the magnetic field.
When the current is increased, the magnetic lines of force expand; when the current is decreased, the lines
of force tend to collapse. The expansion or collapse of the magnetic field (the magnetic field in motion)
causes a voltage to be induced in the conductor. This voltage is called self-induced emf. The three
requirements for producing an electromagnetic force (EM) by electromagnetic induction are:
(1) a conductor,
(2) a magnetic field,
(3) and a relative motion between the two
The drawing on the next page illustrates how self-induced emf is created in a conductor. Note that the
magnetic field originates in the center of the conductor; hence, the field cuts the conductor, which causes
the same effect as relative motion between the field and the conductor.