Circuits: voltages and switches

1. Jul 28, 2008

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 [Broken]

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.

Last edited by a moderator: May 3, 2017
2. Jul 28, 2008

dragonrider

Re: Circuits

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!

3. Jul 28, 2008

Gear300

Re: Circuits

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.