試題詳解

15. A circuit contains a resistance and a charged capacitance. The resistance is 20 ohms, and the capacitance is 5 F. If the circuit is switched off, how long will it take when the current decreases to a half of initial value? (ln2 = 0.69)
(A) 34 s
(B) 50 s
(C) 69 s
(D) 100 s
(E) 127 s