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Homework Help: Total energy

  1. Mar 3, 2007 #1
    1. The problem statement, all variables and given/known data
    A 12 V battery is rated at 200 Ah (Amps hours). How much total energy can it supply?

    2. Relevant equations

    3. The attempt at a solution
    First I did E = 1/2 C V^2. (Got 4,320,000 J, although I'm not sure if that is correct). But now I need to convert it to kwh so my question is can I use P = IV in this manner:
    P = 200 Ah * 12 V
    P = 2400 Wh = 2.4 kWh
    or do I need to manipulate my original energy finding?

    4,320,000 J != 2.4 kWh so one must be wrong
    Last edited: Mar 3, 2007
  2. jcsd
  3. Mar 3, 2007 #2
    Huh? amp hours? That's a new one.. You plugged in 200 Ah for I..meaning that Ah is a measurement of current? According to Wikipedia, Ah is a measurement of electrical charge..
  4. Mar 3, 2007 #3
    Where did 1/2 C V^2 come from? What did you use for C, and why?

    P has units of power, not energy - to get energy you have to multiply by a time, which is what you did, though this might have been accidental since amp-hours are units of charge, not current.

    Also, 2400 Wh is not equal to 24 kWh.
  5. Mar 3, 2007 #4
    Alright, using P=IV was the wrong approach

    Heres how I calculated Capacitance:
    First 200 Ah * 60 min/h * 60 s/min = 720,000 Coulombs.
    CV = q
    C*12 = 720,000 C
    C = 60,000 F (which seems really high)

    1/2 * 60,000 F * 12^2 = 4,320,000 J which is equal to 1.2 kWh ( i think )

    Am I calculating the correct amount of charge?
    Last edited: Mar 3, 2007
  6. Mar 3, 2007 #5
    CV = q ??
    the C refers to capacitance. do you have a capacitor in the circuit?
    Hint: what do you have to do to the equation P = IV to get units of energy on the left hand side?
  7. Mar 3, 2007 #6
    Heh, whoops, wasn't thinking about actually needing a capacitor in it.

    Anyway, now I got it, thanks all.
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