# AP Physics C E&M RC circuit FRQ

1. May 10, 2015

### erisedk

1. The problem statement, all variables and given/known data
http://apcentral.collegeboard.com/apc/public/repository/_ap06_frq_physicsc_em_51784.pdf

2. Relevant equations

3. The attempt at a solution
I understand that before S2 is closed, current through the circuit is zero.
As soon as S2 is closed, I don't really understand what happens? I understand that both the currents will be ε/R1+R2 after some time, but I don't get how the capacitor which is completely charged, interacts with the circuit right after S2 is closed.

2. May 10, 2015

### Suraj M

The current doesn't flow through the capacitor, hence take into consideration of only the resistors,

3. May 10, 2015

### erisedk

By your argument, the current after S2 is closed should instantaneously become ε/R1+R2, not gradually (which can only happen if the capacitor interacts with the circuit in some way after the switch is closed) as given in the answer.
Check out the scoring guideline and a sample solution here---
http://apcentral.collegeboard.com/apc/members/repository/ap06_physicscem_samples_q2.pdf

4. May 10, 2015

### Staff: Mentor

For purposes of analyzing a circuit for the instant after a switch has changed it, you may replace a capacitor with a voltage source having a voltage equal to that of the capacitor immediately before the switching. Perhaps that will help you to sort out the conditions.
Find initial currents at t = 0. Find final currents at t → ∞ (steady state). Connect initial and final states with suitable exponential curves.

Last edited: May 10, 2015
5. May 10, 2015

### SammyS

Staff Emeritus
Check the following thread. It may help.