(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

All capacitors of the open circuit shown below are discharged when, at time t=0, the switch S1 is closed. At some point later, at time t = t1, the switch S2 is also closed. What is the charge Q2(t2) on the capacitor C2 at time t = t2 > t1?

[PLAIN]http://img705.imageshack.us/img705/3282/trashp.png [Broken]

2. Relevant equations

Charging cap: [tex]q = C\epsilon(1 - e^{\frac{-t}{RC}})[/tex]

Discharging cap: [tex]q = {Q_0}e^{\frac{-t}{RC}}[/tex]

3. The attempt at a solution

I thought that when switch S_1 was closed at t=0, the charge on C_2 was zero and that the time constant, RC, was (R_1+R_2)*((1/C_1 + 1/C_2)^-1), abbreviated [itex]\tau[/itex]. Thus, the charge on C_2 the instant before t_1 would be [tex]q = C_2\epsilon(1 - e^{\frac{-t_1}{\tau}})[/tex].

Once S_2 is closed, I said the time constant was given by (C_1+C_2)*(R_2), since it was my understanding that these capacitors would discharge through R2. Given [tex]q = Q_0 = C_2\epsilon(1 - e^{\frac{-t_1}{\tau}})[/tex] (from the previous paragraph), and the fact that [tex]q = {Q_0}e^{\frac{-t}{RC}}[/tex] for a discharging capacitor, I obtained [tex]q_2 = {C_2}\epsilon(1 - e^{\frac{-t_1}{\tau}})e^{\frac{-t_2}{(C_2*R_2)}}[/tex].

I know it's not right, but I'd appreciate it if someone could point me in the right direction.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# RC Circuit capacitors Question

**Physics Forums | Science Articles, Homework Help, Discussion**