Finding kilowatt hours from parallel capacitors

  1. Please Help! Finding kilowatt hours from parallel capacitors

    1. The problem statement, all variables and given/known data

    A parallel-connected bank of 5.00 µF capacitors is used to store electric energy. What does it cost to charge the 2900 capacitors of the bank to 45,000 V, assuming a unit cost of $0.03 per kW·h?


    2. Relevant equations

    CV = Q
    1 Joule = 2.77E-7 kWh
    1 Volt = 1 Joule per Coulomb

    3. The attempt at a solution
    I'm not sure how to convert from Volts to Joules and then to kilowatt hours. I tried CV=Q to find the Coulombs, from where I wanted to find the number of Joules. If my thinking is correct, the 45,000 Volts means there are 45,000 Joules per Coulomb. When i multiplied 5E-6 times 45,000 to find Q my result was that the charge was only .225 C for one capacitor, which seems low. I'm not sure where to go from here, and I'm very unsure of how to convert from Volts to Joules to Kilowatt hours. I know at the end I need to remember to multiply my final result by 2900 for the number of capacitors. I also don't know how the fact they're connected by parallel plays into this.

    Please help!
     
  2. jcsd
  3. ideasrule

    ideasrule 2,323
    Homework Helper

    Re: Please Help! Finding kilowatt hours from parallel capacitors

    Are you familiar with the formula E=1/2CV^2 or E=Q^2/2C? Those are the formulas for the energy stored in a capacitor.
     
  4. Re: Please Help! Finding kilowatt hours from parallel capacitors

    Okay, i'll try that and see if i can work out the right answer
     
    Last edited: Mar 24, 2010
  5. Re: Please Help! Finding kilowatt hours from parallel capacitors

    The units for E is in joules correct?
     
  6. ideasrule

    ideasrule 2,323
    Homework Helper

    Re: Please Help! Finding kilowatt hours from parallel capacitors

    That depends on what you use for C, V, and Q. If you use standard SI units, E will come out in joules.
     
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