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Calculating Capacitor Variance/Uncertainty

  1. Jul 15, 2006 #1
    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?

    Last edited: Jul 15, 2006
  2. jcsd
  3. Jul 17, 2006 #2
    Hi Blog,

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


  4. Jul 17, 2006 #3
    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

  5. Jul 17, 2006 #4


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    Staff Emeritus
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    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
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