# RC Circuit, is my solution wrong?

Discussion in 'Homework Help' started by blacktrack8, Jul 15, 2010.

1. ### blacktrack8 Thread Starter New Member

Jul 6, 2010
2
0
Hi,
I'm doing homework to study for my final exam tomorrow. The final will contain RC, RL, and RLC circuits. I have the solution to problem 7.21 in Nilsson Riedel (sounds like they'd write a circuits book, huh?) but it doesn't click with me.

As I understand it, parallel capacitors equivalent capacitance is calculated by adding the capacitances.

In this example there are two capacitors of 2 microFarads and 8 microFarads separated by a 5 kiloOhm resistor.

My professors solution calculates the time constant by calculating the equivalent capacitance, but he does (8*2) / (8+2). Would it not just be 10 microFarads? He's probably correct. Are they in series because of the resistor?

Here's a picture of the solution, the bottom part is what the circuit looks like after the switch is closed with the two capacitors and the resistor.

File size:
47.5 KB
Views:
25
2. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
To calculate the time constant of an RC loop, you must reduce it to a single resistance and capacitance. Isn't it rational to add in series two capacitors that share only one common end? Clearly C1 and C2 are connected in series.

3. ### blacktrack8 Thread Starter New Member

Jul 6, 2010
2
0
Thanks Georacer. I just had a lightbulb go off. I think my circuits were fried when I asked this question.

Last edited: Jul 15, 2010
4. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
You 're wellcome.