If the switch in an RL dc circuit is closed at t=0, the current i in the inductor rises exponentially. But at t=0, i=0 because the inductor generates a back emf equal to the applied voltage at t=0. So in absence of any current,how does the inductor come to know that the switch has been closed? How does it come to know the exact magnitude of applied voltage?? The initial current is 0 due to initial back emf but the initial back emf is due to what??