The time constant is R

1.  The time constant is R*C, so for the time constant to be 2*10^-3 sec, R*C =
2*10^-3, where R is ohms and C is in farads.
C = 2*10^-3 / 181 = 1.1*10^-5 farad = 11µF
You did not specify if this is a series or parallel RC circuit. The time constant is
the same, but the current-time function is different. In a series circuit, the
current is given by
I = (V/R)*e^-t/tc , where tc is the time constant
in a parallel circuit,
I = V/R * (1 - e^-t/tc)
EDIT: this is the current in the resistor only. Your problem probably means a
series circuit, since the overall current in the parallel circuit will depend on the
internal resistance of the battery, which is not given.
Use the values for V (9v) and R (181 ohm) and tc given (2 ms) to find the
current for t = 4 ms
e^-4/2 = e^-2 = 0.135
V/R = 9/181 = 0.05 A
Series circuit I = 0.05*0.135 = 0.0068 A = 6.8 mA
Parallel circuit I = 0.05*(1 - 0.135) = 0.043 A = 43 mA
o 2 years ago