(4) The number of turns in the conductor. Each turn of the wire cuts the
lines of force; a 100-turn coil cuts the lines of force 100 times as often as a single wire
moving at the same speed in the same magnetic field.
c. There are three ways in which an electric current can be induced in a
If a wire is moved across a stationary magnetic field.
If a magnetic field moves across a stationary wire.
(3) If the magnetic field varies in strength or direction while a stationary
conductor lies in it.
a. Self-induction is defined as the process by which the magnetic field of a coil
induces a counter EMF in the coil itself. This self-induced voltage will oppose the
(1) Consider a circuit consisting of a coil of wire in which uniform DC flows.
The instant the switch is closed, a magnetic field builds up. This cuts across the coil,
producing a voltage (counter EMF) which will always be opposite in polarity to the
voltage causing the original current to flow. The field surrounds the conductor in
concentric circles, increasing in intensity to a maximum, where it remains as long as
(2) The moment the switch is opened, the current stops, the field collapses,
and the lines of force move across the coil in the opposite direction. This causes a
short-lived induced voltage that tends to keep the current flowing in its original direction.
When the switch is closed, a short-lived voltage tends to oppose the flow of current.
When the switch is opened, the self-induced voltage reverses and tends to keep the
current flowing in its original direction for a short time. In uniform DC, the self-induced
voltage is present only when current flow is started or stopped; but with AC, this process
goes on all the time because of the constantly reversing current.
b. The addition of a soft iron core inside a coil concentrates the magnetic field,
thereby increasing the magnitude of the self-induced voltage (counter EMF) that
opposes the original current. The magnitude of the induced voltage also depends on
how far the iron core is pushed into the coil. The choke coil is designed on this
principle. Changing the placement of the iron core may vary the current delivered
through a choke coil.