(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.

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# Homework Help: RC Circuit capacitors Question

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