I am stuck on the problem below. If the capacitor was not in parallel with the resistor I could solve for tau. I hope my make shift circuit looks ok.

For the circuit below, switch is closed at t=0. Determine:
1. the time constant, tau
2. the initial current in the capacitor
3. the final current in the capacitor
4. the capacitor voltage at t = 2 time constants
5. the capacitor voltage at t = 10 ms

see attached

 

Hint: Do you know how to calculate Thevenin equivalent voltages and resistances?

 

Is the time constant troubling you?
In order to find the time constant, you need to reduce the whole circuit to a single resistance and capacitance and then multiply them. Nullify the voltage source and merge the two resistances (they are in parallel). Then the answer will be obvious.

 

I had a feeling that I needed to thevanize the circuit. So is this right Rth=300||600=200 ohms and tau=200ohm x 5uF= 1ms? Then my initial current is V/R = .09A and my final current is zero. Now I can plug my time constant in and solve for the two capacitor voltages.

 

I had a feeling that I needed to thevanize the circuit. So is this right Rth=300||600=200 ohms and tau=200ohm x 5uF= 1ms? Then my initial current is V/R = .09A and my final current is zero. Now I can plug my time constant in and solve for the two capacitor voltages.

No, the current is wrong. You forgot to "thevanize" the voltage.

 

thanks Ron H, so Vth = 18V x (600 / 300 + 600) = 12V and then the current is Vth/Rth = .06A ?

 

thanks Ron H, so Vth = 18V x (600 / 300 + 600) = 12V and then the current is Vth/Rth = .06A ?

That's what I get.