Question

A resistor and an inductor are connected in series to a battery. The battery is suddenly removed from the circuit. The time constant for of the circuit represents the time required for the current to decrease to . a) 50% of the original value. b) 75% of the original value. c) 37% of the original value. d) 25% of the original value. e) 63% of the original value

Answer 1

The time constant is how long will the current decrease by 1/e. The numerical value of this is approximately equal to 0.37. Since we are looking at the decrease which is equal to 1 less than this value, the answer would be 0.63. Thus, the answer to this item is letter E. 

Answer 2

E, 63% of the value. I forget the rationale behind it but I learnt that in engineering. 90% confident for that answer.