example problem about capacitor

Thread Starter

PG1995

Joined Apr 15, 2011
813
Hi

Please help me with the problem in the attachment. It would be kind of you. Thanks.

Regards
PG
 

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jimkeith

Joined Oct 26, 2011
539
Q1: Vo(t) simply means that voltage is a function of elapsed time (after switch is closed)

Q2: calc the Thevenin equivalent resistance (resistors are left to right)

Requiv = r1 * (r2 + r3) / (r1 + r2 + r3)

T = Requiv * C

Hope this is helpful
 
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Thread Starter

PG1995

Joined Apr 15, 2011
813
Thanks, Jim.

But I'm getting wrong answer. I have solved the problem the end. You can have a look on the attachment to see the procedure I have followed. Please help me. Thank you.

Regards
PG
 

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ErnieM

Joined Apr 24, 2011
7,992
Your procedure to calculate tau was correct, you just didn't use uf for the cap:

tau = R*C = (3K * 6K)/9K * 100uF = 2E3 * 100E-6 = 0.2

However I don't track you when you say:

Vc(t) is 8-8e^(-t/0.6)

Where did you get that 0.6 factor? This does not seem to be the correct answer. I think it is:

Vc(t) is 8-8e^(-t/0.2)

That should be calculated using the same R computed as the Thevenin resistance. You find that when you "look" into the circuit where the cap is, short the voltage source and compute the resistance. jim got it right.

I'm not sure what "what is the resistance across the capacitor" means, the question is a bit vague. Does it mean just R2 and R3 or does it also include R1?
 
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