Induction
Current
is produced in a conductor when it is moved through a magnetic field
because the magnetic lines of force are applying a force on the free
electrons in the conductor and causing them to move. This process of
generating current in a conductor by placing the conductor in a changing
magnetic field is called induction. This is called induction
because there is no physical connection between the conductor and the
magnet. The current is said to be induced in the conductor by the
magnetic field.
One
requirement for this electromagnetic induction to take place is that the
conductor, which is often a piece of wire, must be perpendicular to the
magnetic lines of force in order to produce the maximum force on the
free electrons. The direction that the induced current flows is
determined by the direction of the lines of force and by the direction
the wire is moving in the field. In the animation above the ammeter
(the instrument used to measure current) indicates when there is current
in the conductor.
If an AC
current is fed through a piece of wire, the electromagnetic field that
is produced is constantly growing and shrinking due to the constantly
changing current in the wire. This growing and shrinking magnetic field
can induce electrical current in another wire that is held close to the
first wire. The current in the second wire will also be AC and in fact
will look very similar to the current flowing in the first wire.
It
is common to wrap the wire into a coil to concentrate the strength of
the magnetic field at the ends of the coil. Wrapping the coil around an
iron bar will further concentrate the magnetic field in the iron bar.
The magnetic field will be strongest inside the bar and at its ends
(poles).
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