In the circuit shown, the capacitor is initially uncharged and V=9V. At time t =

Question

In the circuit shown, the capacitor is initially uncharged and V=9V. At time t = 0, switch S is closed. If Tau denotes the time constant, the approximate current through the 3 Ohm resistor when t=tau/1000 is: A) 3/8 A B) ½ A C) ¾ A D)1 A E) 3/2 A A charged capacitor is being discharged through a resistor. At the end of one time constant the charge has been reduced by (1-1/e) =63% of its initial value. At the end of two time constants the charge has been reduced by that percent of its initial value? A) 82% B) 86% C) 100% D) between 90% and 100% E) need to know more data to answer the question

Explanation / Answer

originally, the circuit's resistance is

Req = 6 ohm + 3 ohm = 9 ohm

Thus, the initial current there is

I = V/Req = 1 A

At t = T/1000, almost nothing have happened yet to the system. Thus, the answer is OPTION D.

I = 1 A [OPTION D, ANSWER]

*******************

At two time constant, it will eb reduced by

1 -1/e^2

= 0.86   [OPTION B, ANSWER]

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