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Homework Help: Energy Stored in a capacitor

  1. Feb 9, 2009 #1
    1. The problem statement, all variables and given/known data

    1) A 4.7-mu or micro FF capacitor is charged to 290 V. How much energy is stored in the capacitor?

    2) How much additional energy is required to charge the capacitor from 290 V to 780 V?

    2. Relevant equations

    U = 1.2*C*V2

    3. The attempt at a solution

    The first one I got right...

    1) U = 1/2*(4.7 * 10-6)*290 = 1.975E-1

    2) I thought to find this one you could use the change in energy would have to equal the change in potential? So I tried this...but its not right, can anyone tell me where I went wrong? thanks!

    Uf - 1.975E-1 = .5*(4.7E-6)(780)2 - .5*4.7E-6*2902

    Uf = 1.4297 J
  2. jcsd
  3. Feb 9, 2009 #2
    I did Uf - 1.975E-1 = .5*(4.7E-6)(780)2 - .5*4.7E-6*290^2

    but I got 1.232
    Last edited: Feb 9, 2009
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