THE ELECTRIC MOTOR
Since a current carrying wire experiences a force in a magnetic field, a rectangular loop or coil of wire
carrying a current experiences equal and opposite forces causing a resultant moment(couple) about its
axis of symmetry. The moment is maximum when the coil is horizontal and decreases to zero when the
coil is vertical. The coil is able to overshoots the vertical because of inertia. The coil is mounted on an
axle so that it can rotate freely. To enable it to rotate continuously, the direction of the current in the
coil is reversed every half revolution, and this is achieved by a commutator which rotates with the coil.
Current is passed from the power supply to the commutator through carbon brushes.
Note: (a) The direction of the coil may be reversed by;
(i) Interchanging the magnetic poles.
(ii) Changing the connections to the power supply.
(b) The speed of rotation may be increased by;
(i) Increasing the current in the coil.
(ii) Using a coil with more turn of wire.
(iii) Using stronger magnetic poles.
A much smoother rotation is obtained by the following alterations.
(i) Curved poles are used to form a radial field.
(ii) Several coils are used, at angles to each other, and the commutator consists of several
sections.
(iii) The coils are wound on a cylindrical soft iron armature to concentrate the magnetic field.
Note: Many practical motors are electromagnets instead of permanent magnets.
USES OF ELECTROMAGNETS ON ELECTRICAL APPLIANCES
(a) ELECTRIC BELL
When the switch is closed, the electromagnet is turned on and attracts the soft iron armature, so that
the hammer hits the gong. When the armature moves however it breaks the circuit so that after the
gong is struck, the armature is sprung back by, “springy metal strip”. This completes the circuit again and
so the process is repeated until the switch is opened
(b) ELECTRICAL RELAY
A relay is used to switch on one circuit (often carrying a large current) by switching on another circuit
usually carrying a smaller current.
When switch S is closed above, the armature is attracted to the core, closing the contact C and allowing
current to flow between D and E. If Switch S is opened, the armature springs back, opening the contacts
and disconnecting D from E.
(c) REED SWITCH
When current flows in the coil, the magnetic field produced magnetizes the strips (reeds) of magnetic
material. The ends become opposite poles and attract each other thus completing the circuit connected
to AB. When the current in the coil is switched off, the reeds separate. Reeds can also be operated by
permanent magnets.
THE LOUD SPEAKER
The standard moving coil loudspeaker operates by the same principle as the electric motor. A coil in a
radial magnetic field experiences a force on it depending on the direction of the current flowing in it.
Thus if the current is alternating (ac) with a certain frequency, the force on the coil changes direction
with the same frequency. The coil is wound on a cardboard tube which is connected to a large paper
cone. The oscillation of the cone thus causes a sound wave in surrounding air to be formed.
THE MOVING COIL MICROPHONE
A moving coil microphone generates a varying e.m.f. When you speak into a moving coil microphone the
sound waves sets the diaphragm vibrating. This moves a small coil backwards and forwards through the
magnetic field from a cylindrical magnet and a small alternating current is induced in the coil. When
amplified (made larger), the current can be used to drive a loudspeaker.
Microphones are used in telephones at the speaking end.
Loudspeakers are used in radios or tape recorders.
