ELECTRO MAGNETIC INDUCTION
A
large number of closely wound turns of insulated copper wire. Connect its ends
to the terminals of a galvanometer. Bring the coil swiftly to one pole of a
magnet. Observe the deflection in the galvanometer.
Keeping
the coil at rest bring one end of a magnet swiftly to the coil and then take it
back at the same speed. The needle deflects to opposite directions in the two
different situations and suddenly comes back to the initial position.
When a solenoid swiftly moves in a strong magnetic field, the galvanometer needle deflects indicating the flow of a current.
The process of inducing an emf in a conductor
whenever the magnetic flux linked with it changes, is known as electromagnetic
induction.
The emf so developed is called induced emf and the
current so obtained is called induced current.
The induced emf developed in a conductor is equal to the rate of change of flux linked with it. This is Faraday's Law of electro magnetic induction.
Alternating current (AC);
Direct current (DC)
Connect the terminals of a weak cell to a galvanometer. Observe the
deflections of the needle. Remove the cell and connect a solenoid to a
galvanometer. Observe the deflections of the needle by continuosly moving a
magnet into and out of the solenoid. The needle deflects only in one direction
when the galvanometer is connected to a cell.
Thus a current flowing only in one direction is called direct
current (DC).
The direction of the electric current produced by the movement of
the magnet in opposite directions continously changes. Thus the current which
changes its direction at regular intervals is called alternating current (AC).
AC generator (Alternator)
The device which
produces electricity on the basis of electromagnetic induction by the
continuous motion of either the solenoid or the magnet is called a generator.
An armature is an arrangement
of an insulated conducting wire wound around a soft iron piece. The armature is
continuously rotated by means of mechanical energy.
The induced current developed in one
direction during the first half rotation of the armature of the AC generator
and the induced current developed in the opposite direction during the next
half of the rotation constitute one cycle of an AC.
DC generator
The electric current from a DC generator flows in one direction in the external circuit.
AC induced in the armature is converted into DC in the
external circuit by an arrangement called split ring commutator. The flux cut
in the opposite direction during the second half rotation. Hence the direction
and in the opposite direction during the second half rotation. Hence the
direction of flow of current in the armature changes. When the direction of the
current in the armature changes during the successive half rotations, the
contact of one half of the split ring shifts from one brush to the other. So
the direction of the current in the external circuit does not change.
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