Circuits: voltages and switches

[dragonrider]

017 (part 1 of 2) 10 points
For a long period of time the switch S is in position "b". At t = 0 s, the switch S is moved from position "b" to position "a". Find the voltage across the 1 MΩ center-left resistor at time t₁=5s. Answer in units of V.

http://img74.imageshack.us/img74/3606/capturear5.th.jpg

018 (part 2 of 2) 10 points
Much later, at some time t'₀ = 0 s, the switch is moved from position "a" to position "b". Find the voltage across the 1 MΩ center-left resistor at time t' = 2.4s. Answer in units of V.

 

[dragonrider]

I did the part 1 and got 3.15 V which is right. I used the equation (V)(2.718)^(-t/RC)

And for part 2 I derived the equation (((V/(R1+R2))8((2.718)^(-t/(R1+R2)C)))*R1 but I am getting the wrong answer. Please Help!

 

[Gear300]

For part 2, the circuit is essentially limited to an RC circuit with no constant emf. So it is essentially a capacitor discharging. Using Kirchoff's method, find the appropriate differential equation for charge on the capacitor as a function of time (you'll have to take into account the initial charge on the capacitor that is gained from the time in which the switch is at position a). The derivative of that equation is current and you can use that to find the voltage across the center-left resistor.