question_answer 37)

  A (current vs time) graph of the current passing through a solenoid is shown in Fig. 4(EP).10. For which time is the back electromotive force a maximum? If the back emf at t = 3 s is e, find the back emf at t = 7 s, 15 s and 40 s. OA, AB and BC are straight line segments.    

Answer:

                  As back e.m.f., ; e will be maximum when  = max. The graph in Fig. 4(EP).10 shows that  is maximum for 5 s < t < 10 s. For 0 s < t < 5 s,     At  = 3 sec ; For 5 s < t < 10 s,    At  = 7 s,    For 10 s < t < 30 s,   At  = 15 s,  For t > 30 s,      At  = 40 s, e = 0.