## Finding direction of induced emf

(1) When the current in the primary circuit is decreased, there is a change in the magnetic flux linking the secondary coil and an emf is induced in it. What is the direction of the induced emf?

(2) When the current in the primary circuit is increased, what is the direction of induced emf in the secondary?

See the figure.

The primary circuit has a battery and rheostat. The magnetic flux due to current in this circuits links the secondary circuit. If the current in primary is changed through rheostat, the flux linking secondary also changes and hence an emf is produced in it.

(1) If the current in the primary circuit is decreased, the flux linking the secondary also decreases. Now, the emf produced in secondary will push its current in a direction to maintain the flux constant. So, it should attract more and more flux from the primary. For this purpose, the face of the secondary towards the primary circuit should be an S-face. Therefore, the direction of the current in the secondary circuit should be clockwise (use RHR). This results in the direction of the induced emf to be the same as that of the battery in the primary.

See the figure.

(2) If the current in the primary circuit is increased, the flux linking the secondary also increases. The induced emf in the secondary pushes its current is such a direction to cancel the increase in the current. For this purpose, the face of the secondary towards the primary one should be an N-face to repel its flux. Therefore, the direction of the current in the secondary should be anticlockwise (use RHR). This means the direction of the induced emf is opposite to the direction of the battery in the primary.