An RC circuit contains a 3.9 kΩ resistor and a 5 µF capacitor. When the switch is first closed in the circuit, a maximum current of 1.5 mA flows in the circuit. Using Equation (1), find the time at which the current flow will have fallen to 1.0 mA.

Answer 1

Hmmm.

I=V/R e^-t/rc is that you equation?

1.0=3.5e^t/rc
take the ln of each side

ln1.0-ln3.5=t/RC
solve for time t.

Answer 2

1.5/e^x = 1.0
e^x = 1.5
X = 0.405 = t/RC.

RC = 3.9*5 = 19.5 milliseconds.
t/19.5 = 0.405
t = 7.91ms.