# Felt like I understood circuits until

1. Oct 14, 2009

### Newtime

...I read over a few problems from previous exams. I have a few specific questions about some of the stuff that just is not making sense with my notes and the book.

First of all, here's the page with all the old exams: http://people.physics.tamu.edu/kochar/indexOLD.php?id=ME [Broken]

My first question is about "Fall 2007 MEx 2 Ver. 1 " at the bottom of the left column, and the third and final question on the exam itself:

I. Find the potential at Point a and at point b: I understand that the current through the capacitor is zero in a steady state regime, and I understand why current through b must be zero thus vb=0, but then why is the current through a non-zero?

II. Find the potential at point a and at point b: same question here as above. except now both are non-zero. Is it because now the current flows from the battery in sort of an odd loop through the s ohm resistor, from a to b, through the 4 ohm resistor and then back to the battery, bypassing the capacitors since no current flows through them ina steady state regime?

Find the charge on each capacitor: if my question above is answered I feel like I could get this one, but it's still confusing given the work she has on the exam.

Now look at "Fall 2005 MEx 2 Ver. 1 " in the left column.

For the first part of the third (last) question:

-Why do we not consider teh equivalent resistence of the whole system? In the solution, it appears current was calculated as if it was flowing through the 20 ohm resistor then bypassing the 40 ohm resistor and flowing through the capacitor and back through the battery. Is this correct? At the instant the switch is closed is current only going to flow through a capacitor if a capacitor and resistor are connected in parallel? This part just wasn't making sense...

-If my logic above is correct then for the second part of this question, the current "a long time later" I noticed they did in fact use the equivalent resistence since no current flows through the capacitor. Is this too correct?

I'll leave it as just these questions for the time being since I think if these are cleared up the answers to my other questions will immediately follow. Thanks in advance.

Last edited by a moderator: May 4, 2017
2. Oct 14, 2009

### Mapes

Nowhere does it say that the current through a is non-zero in part I. The potential is non-zero because it is connected to the positive terminal (the voltage drop through the resistors is zero because no current flows).

Yes.

Capacitors act like they have zero resistance at high frequencies (or short time periods after a signal change) and infinite resistance at low frequencies (or long time periods after a signal change). Does this help?

3. Oct 14, 2009

### Newtime

YES! Let it be known that I love this place! Also, chances are I'll be posting again in several hours with another question or two so stay tuned...