# Calculating Capacitor Variance/Uncertainty

#### blog

I can't seem to find out how to calculate a charge's uncertainty when given capacitance & voltage across a series & parallel circuit. Simple circuit setup w/voltage and the 2 capacitors.

Any suggestions?

Thanks!

Last edited:
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#### tommyers

Hi Blog,

Could you give somemore details - and what you mean by 'charge uncertainty'?

Thanks

Tom

#### blog

Hey Tom,

I had to adjust the question, because it was formatted as a "homework question" so I posted the entire question here: https://www.physicsforums.com/showthread.php?t=126165

Basically, I'm given Capacitance uncertainty(5%) & Voltage uncertainty(1%), as well as the Capacitance & voltage across a simple DC circuit(one Series & one Parallel circuit).

The question was a 2 part, find Q(charge) as well as its uncertainty based on the information above.

I finally came across this equation:

(Delta.Z / Avg.Z) = sqrt( (delta.X/avg X)^2 + (delta.Y/avg.Y)^2 )

I was able to find each capacitor's charge uncertainty, except for Q.total's Charge Uncertainty, when using C.eq

Thanks.

#### Office_Shredder

Staff Emeritus
Gold Member
Hmmm... well, we know c=q/v, so q=cv (obviously not straight out of the problem, but you can calculate the nominal values). I would guess the uncertainty is just the range of uncertainties possible for c and q after that. So you know the nominal value easily, and you can calculate the minimum and maximum values, giving you your uncertainty range

"Calculating Capacitor Variance/Uncertainty"

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