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Homework Help: How do you calculate the energy delivered by a capacitor

  1. Jun 1, 2013 #1
    1. The problem statement, all variables and given/known data

    a capacitor of capacitance 10μF is fully charged to form a 20v d.c supply
    1) calculate the charge stored by the capacitor
    2) calculate the energy delivered by the 20v supply

    2. Relevant equations

    1/2 cv2

    3. The attempt at a solution

    1) q =cv
    Q= 2x10-4
    i dont know how to solve the second one pls
  2. jcsd
  3. Jun 1, 2013 #2
    You wrote ## \frac{1}{2}C V^2 ## without an equal sign, check in your notes whats that equal to.
    That equation relates C, V with something that may be interesting for you
  4. Jun 1, 2013 #3
    sorry the equation is
    W=1/2 CV2
    where W=work done
  5. Jun 1, 2013 #4
    Assumineg a stiff power supply with very low source resistance and a very large filter capacitor on the output, the energy that goes to charge the cap is QV, where Q=CV is the charge that rises through the supply through a fixed V=20volts during the near-instantaneous charging.

    This energy expression QV=CV**2 is based on the definition of charge, voltage, and capacitance.

    Of course, the final energy that's stored on the C=10uF capacitor is 0.5CV**2, the remainder of the charging energy having gone to series resistance, radiation, and ?
  6. Jun 2, 2013 #5
    work is energy
  7. Jun 2, 2013 #6
    so is it power im going to solve for??
  8. Jun 2, 2013 #7
    Treat yourself to a nice schematic diagram, with the fixed-voltage supply in series with a switch and the cap.

    Since you are assuming a stiff supply, all the charge is lifted through the supply at a constant voltage, so it's [(joules/coulomb)*coulomb] to get energy. Am an EE here, so most all my charge is positive and flows out of the supply into the cap.

    You do not need to think about power, just energy.

    Not sure if I can invoke the fact that you have a conservative field here so that the work done on a charge moving from the negative supply terminal, through the supply, to the positive supply terminal only depends on the (constant) potential difference.

    Anyway, I believe you can make this a simple problem by saying the energy delivered by the supply is QV.
  9. Jun 3, 2013 #8
    thank you
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