AP Physics C E&M RC circuit FRQ

[erisedk]

Homework Statement

http://apcentral.collegeboard.com/apc/public/repository/_ap06_frq_physicsc_em_51784.pdf

Please please check out Q2 part (d) on this test

Homework Equations

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.

Additionally, please explain how the graphs will be drawn.

 

[Suraj M]

but I don't get how the capacitor which is completely charged, interacts with the circuit right after S2 is closed.

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

 

[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

 

[gneill]

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.

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.

Additionally, please explain how the graphs will be drawn.

Find initial currents at t = 0. Find final currents at t → ∞ (steady state). Connect initial and final states with suitable exponential curves.

 

[SammyS]

Check the following thread. It may help.

Homework Statement

The circuit in the link below contains a capacitor of capacitance C, a power supply of emf E, two resistors of resistances R1 and R2, and two switches S1 and S2. After switch S1 has been closed for a long time, switch S2 gets closed at a time t = 0. Sketch graphs of the current I1 in R1 and I2 in R2 versus time.

https://www.physicsforums.com/threads/rl-parallel-circuit-ap-2006.812704/