Electrical Engineering ⇒ Topic : Magnitude of Self-induced E.M.F.

Magnitude of Self-induced E.M.F.

Consider a coil of N turns carrying a current of I amperes. If current in the coil changes, the flux linkages of the coil will also change. This will set up a self-induced e.m.f. e in the coil given by

Since flux is due to current in the coil, it follows that flux linkages (= NΦ) will be proportional to I.

where L is a constant called self-inductance or inductance of the coil. The unit of inductance is henry (H). If in eq. (i) above, e = 1 volt, dEdt= 1 A/second, then L = 1 H.Hence a coil ( or circuit ) has an inductance of 1 henry ¡fan e.m.f of 1 volt is induced in it when current through it changes at the rate of 1 ampere per second.

Note. The magnitude of self-induced e.m.f. is e = Ldll dt. However, the magnitude and direct on of self-induced e.m.f. should be written as

The minus sign is because the self-induced e.m.f. tends to send current in the co 1 in such a direction so as to produce magnetic flux which opposes the change in flux produced by the change in current in the coil. In fact, minus sign represents Lenz's law mathematically.