Unknown capacitance

blackhar

18th February 2007 - 02:26 AM

I'm having the hardest time with this problem.

An isolated capacitor of unknown capacitance has been charged to a potential difference of 113 V. When the charged capacitor is disconnected from the battery and then connected in parallel to an uncharged 12.3 uF capacitor, the voltage across the combination is measured to be 31.2 V. Calculate the unknown capacitance.

I setup up these two equations:
q=CV1
q=(C+X)V2
I have C as the unknown capacitance and X is the capacitance given to me.
V1 is the first potential and V2 is the second potential
I set them equal to each other and then solved for C
CV1=(C+X)V2
CV1=CV2+XV2
C(V1-V2)=XV2
C=XV2/(V1-V2)
so C=5 uF

I enter this in on my homework site and it says it's wrong.

mr_homm

18th February 2007 - 02:33 AM

Your method is correct, but you have rounded off the answer too far. Try entering more digits. I get 4.69uF. Also, be sure that the homework site is expecting uF not F. You may need to enter 4.69E-6 instead.

Hope this helps!

--Stuart Anderson

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