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valarking

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## Homework Statement

All capacitors of the open circuit in fig. 3 are discharged when, at time t=0, the switch S1 is closed. At some point later, at time t = t1, the switchS2 is also closed. What is the charge Q1(t2) on the capacitor C1 at time t = t2 > t1?

http://www.vkgfx.com/misc/fig3.jpg [Broken]

## Homework Equations

Charging capacitor:

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

Discharging capacitor:

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

## The Attempt at a Solution

Ok, I said that when S1 was closed at t1, the resistors arein parallel, giving q at t1 equal to:

[tex]{C_1}\epsilon(1 - e^{\frac{-t}{(R_1+R_2+R_3+R_4)(C_1+C_2)}})[/tex]

Now the thing I need help with is determining the equivalent resistance when S2 is closed. The best I can think of is that only R2 and R4 are acting. That would give:

[tex]{C_1}\epsilon(1 - e^{\frac{-t}{(R_1+R_2+R_3+R_4)(C_1+C_2)}})e^{\frac{-t}{(R_2+R_4)(C_1+C_2)}}[/tex]

Can anyone help me determine the resistance in this last step? I'm out of ideas.

Thanks!

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