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Homework Help: Calculating capacitance -Got wrong answer

  1. May 20, 2012 #1
    1. The problem statement, all variables and given/known data
    Find the capacitance of a capacitor that stores 320 MJ(MegaJoules) when the potential difference across its plates is 170V.

    Answer must be in nanofarads.

    So basically q: is 320MJ
    V: is 170V
    I need to find C the capacitance.

    2. Relevant equations

    To find the capacitance is q=CV.

    3. The attempt at a solution

    I converted 320MJ to 320* 10 ^ 6 Joules for q. 320* 10 ^6 J is also 320 * 10 ^ 6 Coulombs. Let V the way it is 170V.

    Divide (320 * 10 ^ 6 Coulombs) / (170 V) = 1882352.941 Farads. to convert farad to nanofarad I multiply the answer by 10^ 9 and got 1.882352941 * 10 ^ 15 nanofarads.
    I put 1.88 * 10 ^ 15 nanofarads but apparently it is the wrong answer.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. May 20, 2012 #2


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    Homework Helper

    In your equation Q=CV, Q is the charge stored.

    You are given 320 MJ which is a unit of energy. What is equation for the energy stored by a capacitor?
  4. May 20, 2012 #3
    that equation would be (1/2)C(V^2). but i do not see your point. i should be using that equation.
    Last edited: May 20, 2012
  5. May 20, 2012 #4
    you know the energy and you know the voltage so using this equation will give you the value of C.
    Have you read the question carefully? You have the energy stored as 320 megaJoules.....that sounds totally unreasonable !!! it is about 100kWhr!!!!!
  6. May 20, 2012 #5
    Oh! I see. Thanks for the help. Answer is 22 nanofarads.
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