Answer to Question 2

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The motor in question 1 is turned off improperly by opening the switch.  What is the value of the reverse voltage spike, if the time taken for the current to fall to zero is 20 ms?

Formula:

E = - 150 10-3 H (- 50 A 20 10-3 s) = + 375 V 

(-50 A because the current is going from +50 A to 0 A)