A (current vs time) graph of the current passing

From the graph, we can say that when there is a maximum rate of change of magnetic flux, the electromagnetic force will be maximum which is proportional to the rate

of change of current. The rate of current will be maximum when the time axis makes a maximum angle at AB.

Therefore, when t = 3 seconds, s = slope of OA

Therefore, the rate of change of current at t = 3 = ¼ A/s

Therefore, the electromotive force is L/5.

When the emf is between 5 to 10 seconds, it is -3e

When the emf is between 10 to 30 seconds, it is +1/2e

When the emf is at 40 seconds, dI/dt = 0