Namkul Ashq

when current in a coil changes the magnetic flux linked with the coil also changes and hence emf is induced in the coil . this phenomenon is known as self induction.

According to lenz law the induced emf opposes the change in magnetic flux
Experimental representation in fig a conducting coil C is connected with a battery and key in series. when current is passed through the key hence magnetic flux is linked with the coil changes. when key is pressed , on increasing current, the change in magnetic flux also increase which produces emf and the induced current oppose the current that flows through the circuit . Similarly when key is opened the current in the circuit is zero . Then , magnetic flux is linked with the coil decreases and hence the induced current flows in the direction of the current supplied. this is shown in figure .

Experiment to demonstrate self induction :
take a solenoid having a large number of insulated wire wound over over a soft iron core . such a solenoid is called a choke coil. connect the solenoid in series with battery . a eheostat and a tapping key . connect a 6 v bulb in parallel with the solenoid . press the tapping key and adjust the current with the help pf rheostat so that the bulb just glows faintly . As the tapping key is released , the bulb glows brightly for a moment and then goes out . this is because as the circuit is broken suddenly . vanishes is the rate of charge of magnetic flux linked with the coil is very large. hence large self induced emf and current are produced in the coil which make the bulb glow brightly for a moment.