# RC circuit problem

## Homework Statement

What is the charge on the capacitor?

Kirchoff's rule
change in V =q/c

## The Attempt at a Solution

I want to use Kirchoff rule but I don't know if this is the right way to go.

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rl.bhat
Homework Helper
When the capacitor is completely charges, there is no current in that branch. The voltage across the capacitor is the potential difference across 40 ohm resistor.

When the capacitor is completely charges, there is no current in that branch. The voltage across the capacitor is the potential difference across 40 ohm resistor.

Ya, I was able to figure out that the charge on the resistor would be 80 microCoulombs . The voltage difference between the plates of the capacitor was 40 volts and from there it was plugging in. Now I face another problem where I can't find how long it will take the capacitor to discharge to 25% of 80 microCoulombs. I derived that it should be -RC*ln(1/4)=t. I plug in R= 50, c = 2 and get the wrong answer :/

rl.bhat
Homework Helper
Ya, I was able to figure out that the charge on the resistor would be 80 microCoulombs . The voltage difference between the plates of the capacitor was 40 volts and from there it was plugging in. Now I face another problem where I can't find how long it will take the capacitor to discharge to 25% of 80 microCoulombs. I derived that it should be -RC*ln(1/4)=t. I plug in R= 50, c = 2 and get the wrong answer :/
t = 2 μF.

t = 2 μF.

How does t = 2 MICROFARADS? Don't you mean seconds or milliseconds? I don't quite understand your logic though. Doesn't the capacitor discharge through 2 resistors that become 50 ohms?

rl.bhat
Homework Helper
How does t = 2 MICROFARADS? Don't you mean seconds or milliseconds? I don't quite understand your logic though. Doesn't the capacitor discharge through 2 resistors that become 50 ohms?
Sorry.It is typo. I mean C = 2 μF. Your R is correct. You didn't mention your answer.

Sorry.It is typo. I mean C = 2 μF. Your R is correct. You didn't mention your answer.

Well then it would be -RC*ln(1/4) = -(50)(2)*ln(1/4)= 138.63 ms but that is somehow wrong.. :/

Oh stupid me! I freakin' forgot to convert the units... it should be 138.63 microseconds or .1386 milliseconds.

Thanks for the help! :P