(a) What is an electric motor ? With the help of a labelled diagram, describe the working of a simple electric motor.

Explanation

Electric motor: It is a device which converts electrical energy into mechanical energy.
Construction: Electric motor consists of following parts:
(a.) Rectangular coil: A large number of turns of insulated copper wire wound on iron core in rectangular shape forms a coil ABCD as shown in figure.
(b.) Strong magnet: The coil is placed in between two pole pieces (N and S) of a strong horse shoe magnet which provides a strong magnetic field. The sides AB and CD are perpendicular to the direction of magnetic field.
(c.) Split ring: It consists of two halves of a metallic ring `(R_(1) and R_(2))`. The ends of the coil are connected to these rings. The outer conducting surfaces of spilt rings `R_(1) and R_(2)` are in contact with the two stationary carbon brushes. Spilt rings reverse the direction of current in the coil.
(d.) Brushes: Two carbon brushes `B_(1) and B_(2)` are used to press the split rings.
(e.) Axle: The two halves of the rings have resistive coating in their inner surfaces and are tightly fitted on the axle. The main function of axle is to rotate with the coil.
(f.) Battery: The battery supplies the current to the coil.

Working:
(a.) When the circuit is completed, the current passes through the brush `B_(1)` from A to B in branch AB.
(b.) As, the magnetic field acts from north pole to south pole, according to Fleming's left hand rule, the force experienced by arm AB is in the downward direction and arm CD is in the upward direction.
(c.) Both these forces are equal in magnitude and opposite in direction and hence, the coil rotates in anticlockwise direction.
(d.) After half rotation, the split rings `R_(2) and R_(1)` come in contact with brushes `B_(1) and B_(2)` respectively.
(e.) Again the current starts passing through the coil in opposite direction.
(f.) The force acting on branch BA is upward and DC is downward, this force again moves the coil in anticlockwise direction.
(g.)This shows that the current in the loop ABCD is reversed after every half rotation, and the loop continues to rotate in same direction.
(h.) Thus, the coil rotates with the help of electrical energy.