Let the flux linking with the coil of turns N be changed by an amount dθ  in short time dt.

E.M.F. induced, e = Rate of change of flux linkage
                             = Number of turns x rate of change of flux = fiux = N dθ
                                                                                                         dt

A minus sign is required to be placed before the right hand side quantity of above expression just to indicate the phenomenon explained by Lenx’s law, therefore expression for induced e.m.f. may be written as

e =-N dθ  volts.
         dt

Example. A coil of 200 turns is linked by a flux of 8 m wb. Determine the e.m.f. induced in the coil if the flux is reversed in 0.02 second

Solution: Number of turns on the coil N = 200
               
             Rate of change of flux, dθ =   0.008 - ( 0.008 )  = 0.8 Wb/S
                                              dt             0.02

             Induced voltage,  e = N dθ = 200 x 0.8 = 160 V
                                                dt

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