3while applying kirchoff's law u need to confirm whether the current inside the inductor is growing or decreasing.
the emf induced by the inductor is given by e= -L(di/dt)
so when we apply the kirchhoff's law in the growth of RL circuit then the emf induced by the inductor should come to be +ve i.e. the eqaution should come "e - iR + L(di/dt)=0 but the actuall equation come to be "e - iR - L(di/dt)=0
any explanation for this????
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2 Answers
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1take an example: 
now,from this diagram,by applying kirchoff's law, -e + iR + Ldi/dt=0( i added the losses and
subtracted the gains) here current should be entering the inductor at the point having -ve
sign,as it has emf source property( current flows from -ve to +ve inside the inductor).
thus -(-Ldi/dt) is used. hope, now u cn analyse other cases also