(i) Principle of an electric motor: It works on the principle of the magnetic effect of current. A current carrying coil rotates in a magnetic field. An electric motor is a rotating device that converts electrical energy to mechanical energy.
Working: When a current is allowed to flow through the coil ABCD by closing the switch, the coil starts rotating anti-clockwise. This happens because a downward force acts on length AB and at the same time, an upward force acts on CD. As a result, the coil rotates anticlockwise. Direction of force is given by Fleming’s left hand rule. After half a rotation, the position of AB and CD interchange. The split ring P now comes in contact with Y and split ring Q comes in contact of X. Hence, the direction of current in the coil ABCD gets reversed. Now, current flows in the direction DCBA. The reversal of current through the coil repeats after each half rotation. As a result, the coil rotates unidirectionally.
- The split rings help to reverse the direction of current in the circuit. These are called the commutator.
(ii)
Definition: Electromagnetic induction is the production of an electromotive force across a conductor when it is exposed to a varying magnetic field.
Experiment: The coil 1 having larger numbers of turns is connected in series with a battery and a plug key. Coil 2 is connected with a galvanometer as shown. Now, the key is closed. We will observe that galvanometer shows deflection and then quickly returns to zero, indicating a momentary current in coil 2.
Now, open the key. Again, there is deflection in the galvanometer but to the opposite side. It means that, now the current flows in the opposite direction in coil 2.
Observations: From these observations, we conclude that a potential difference is induced in the coil 2, whenever the electric current through the coil 1 is changing (Starting or Stopping). As the current in coil 1 changes, the magnetic field associated with it also changes. Hence, magnetic flux linked to coil 2 also changes, which is the cause of induced current in it. In practice, we can induce current in a coil either by moving it in a magnetic field or by changing the magnetic field around it. It is convenient in most situations to move the coil in a magnetic field.
