Question

An electric motor works on the principle:
A.) of electromagnetic induction
B.) when a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously
C.) both A and B
D.) none of the above

Answer 1

Hint: Electric motor converts electrical energy into mechanical energy. This is archived by using magnetic properties of current. Electromagnetic induction is the production of induced potential in a changing magnetic field. This principle can only be used to convert mechanical energy into electrical energy not vice-versa (because it gives potential i.e. electrical energy not mechanical energy).

Complete Step By Step Solution:
An electric motor is a device which converts electrical energy to mechanical energy. Most of the electric motors consist of a rectangular coil placed in a magnetic field. Electric current passes through the coil. Electric motor makes use of the principle that when a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously.

We know that when a current carrying wire is placed perpendicular to a magnetic field, a force is exerted on wire which is given by

$i\left( l\times B \right)$ , where I is current flowing through wire, l is length of wire and B is strength of magnetic field.

Same force is applied on the rectangular coil as well; this produces a torque which rotates the coil. This is how electrical energy gets converted into mechanical energy.
Hence the correct option is B.

Note: It is also true that when the coil rotates electric flux through it changes and an opposing potential is developed in the coil (due to electromagnetic induction), but this is not the principle of working of an electric motor. It only results in reducing the potential.