Department of Natural Sciences

Clayton College & State University

March 19, 2003

Physics 1112 – Quiz 7b

Name ____SOLUTION________________________________

Consider a 4.00-mH coil carrying a 2.00-A current clockwise. If the current drops to zero and rises in the opposite direction (counterclockwise) to 1.00 A in 500 ms, what average emf will appear across the coil?

e = - L (DI/Dt)

I_i = 2.00 A

I_f = -1.00 A

DI = -1.00 A –2.00 A = -3.00 A

e = - (2.00 x 10^-3 H) (-3.00 A/500 x 10^-6 s) = 12.0 V

How much energy was stored in the inductor before the drop?

ES = ½ LI²

ES = ½ (2.00 x 10^-3 H) (2.00 A)² = 0.004 J

How much energy is stored in the inductor after the drop?

ES = ½ LI²

ES = ½ (2.00 x 10^-3 H) (-1.00 A)² = 0.001 J