please answer this :

Question

A certain capacitor, in series with a resistor, is being charged. At the end of 10 ms its charge

is half the final value. The time constant for the process is about:

A) 0.43 ms

B) 2.3 ms

C) 6.9 ms

D) 10 ms

E) 14 ms

Answer 1

For a series RC circuit, the charge, Q, vs. time equation is:

Q(t) = Qmax exp(- t / RC)

Since Q( .01) / Qmax = 1/2

1/2 = exp( -.01 /RC) and RC = .01 / ln(2) = .014 or 14 ms