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Homework Help: How to calculate power provided by a battery

  1. Nov 13, 2012 #1
    1. The problem statement, all variables and given/known data

    The diagram is of a circuit with 2 resistors in series (1 ohm and 2 ohms) connected to 2 batteries (a 9 V battery and a 12 V battery). The question asks, how much energy is provided by the 12-volt battery in 3 seconds?

    2. Relevant equations

    Power = IV = I^2R = V^2/R

    I = current, V = voltage, R = resistance

    3. The attempt at a solution

    I used P = I^2R at first, since I already calculated I (= 1 amp) for a previous part. Thus, P = ((1)^2)*(1+2) = 3 watts. It's over 3 seconds, so it would be 3 watts * 3 seconds = 9 joules. However, when I use P = IV, I get a different answer; P = (1)*(12) = 12 watts. 12 watts * 3 sec = 36 joules.

    How is this possible? I thought the equations were equivalent--if anyone could shed some light on this, that would be great! Thank you so much.
  2. jcsd
  3. Nov 13, 2012 #2
    How much energy is being supplied by the 9v source?
  4. Nov 13, 2012 #3


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    Science Advisor
    Homework Helper
    Gold Member

    That suggests the 12V and 9V batteries are arranged to give 3V across the resistors. Correct?

    That's the equation for the power dissipated in the resistors which might be different to that delivered by the 12V battery. aralbrec has given you a good clue as to why these two are not the same thing. What's happening to the 9V battery?
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