Circuit Elements: Battery, Resistors & Capacitor Charge 195μs Post Switch Open

[deerhunt713]

Three resistors and a capacitor are in a circuit with a battery:
R1 = 23 Ω
R2 = 13 Ω
R3 = 15 Ω
C = 6.2 µF
V = 17 V
The switch has been closed for a long time. What will the charge be on the capacitor 195 μs after the switch is opened?

picture
https://online-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys212/fall09/homework/06/11_paired/image.gif

 

[kuruman]

WHat are your thoughts on the matter? Is this a charging or discharging capacitor 195 μs after the switch is opened? What are the relevant equations to describe this?

 

[deerhunt713]

ok well the capacitor is fully charged, so the voltage of the capacitor is the same as the battery, from there i can find the max charge. then i used the equation Qmax(1-e^(t/CR))
where i used 28 for the resistance. but that's not giving me the right answer

 

[kuruman]

Try again. The voltage across the capacitor is the same as across the series combination R₂+R₃. That's not the voltage across the battery. What is it?

 

[willem2]

Try again. The voltage across the capacitor is the same as across the series combination R₂+R₃. That's not the voltage across the battery. What is it?

That's not the only problem. The current through C is not the current through either R1 or R2+R3 so you can't use those in Qmax(1-e^(t/CR) either.

I recommend replacing V, R1, R2 and R3 with their Thevenin equivalent.