In an a.c.circuit containing only inductance, for example, it is said that the current is lagging behind emf by 90 degree. We can understand this that when the emf is zero,the current is at its negative maximum and when the emf reaches its positive maximum, the current becomes zero and so on.. my doubts are: 1. How did they find that one is lagging behind the other when both are rapidly changing with time? 2. How is it possible when the emf is zero the current becomes maximum or vice-versa? To produce current itself in a circuit is not the potential difference required? When pd becomes zero, how the current reaches its maximum? What all these things actually mean in practical terms? If any body gives some simple explanation, it will be very useful.