We know that energy is needed to ascertain flux i.e. magnetic flux however it's not needed to keep up it. This is often kind of like the actual fact that the energy is needed to lift the water through an exact height (h) that is 'mgh' joules. However energy isn't needed to keep up the water at height 'h'. This energy 'mgh' gets hold on in it as its potential energy and may be used for several functions.
Consider a solenoid, current through which may be controlled with the assistance of switch S, resistance R shown within the above fig.
Initially switch ‘S' is open, thus current through coil, I is zero. Once switch is closed, current can attempt to build its value capable to I. Neglect the resistance of coil.
It will take time to extend this from 'zero' to 'l' say 'dt’ seconds. within the in the meantime, flux linkages related to the coil can change, because of that there'll be self-induced e.m.f. within the coil whose value is given by,
e = -L (dI/dt)
So at each instant, coil can attempt to oppose the rise within the current. To beat this opposition, supply must give the energy to the circuit. This is often nothing however the energy needed to ascertain this i.e. magnetic flux or flux round the coil.
The minute current reaches its most value '' then change in current stops. Thus there can't be any induced e.m.f. within the coil and no energy is going to be drawn from the source. Thus no energy is needed to keep up the established flux. This is often because; induced e.m.f. lasts as long as there's alteration in flux lines related to the coil, in line with Faraday's law.
When current is once more reduced to zero by opening the switch then current through the coil starts reducing and flux starts reducing. Thus there's induced e.m.f. within the coil in line with Faraday's law. However as per Lenz's law it'll attempt to oppose because creating it that is decrease in current. Thus this induced e.m.f. currently can attempt to maintain current to its original value. So instantly this induced e.m.f. acts as a supply and provides the energy to the supply. This is often nothing however identical energy that is hold on within the magnetic flux that gets recovered whereas field collapses. Thus energy holds on whereas increase within the current is reverted back to the source once current decreases i.e. once field collapses.