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Homework Help: Finding amount of charges

  1. Jul 6, 2011 #1
    1. The problem statement, all variables and given/known data
    The e.m.f of a battery is 12V and provides an electrical energy of 500J. What is the amount of charges flowing in the battery for 5 seconds?

    2. The attempt at a solution

    Since V=W/Q, rearranging the equation gives you Q=W/V:

    Q= 500/12 = 41.6666667 C

    But, I don't know what is the 5 seconds for. So, I just put 41.6666667 C as the answer and I got it wrong.
  2. jcsd
  3. Jul 6, 2011 #2


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    Hi Kyoma! :smile:
    nooo :redface:, V = W/I

    (watts = volts times amps :wink:)​
  4. Jul 6, 2011 #3
    My 'W' is work done and 'Q' is amount of charge?
  5. Jul 6, 2011 #4


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    are you sure the question says that?

    the question would normally give the power of a battery, not the (total stored?) energy :confused:
  6. Jul 7, 2011 #5
    Yeah. :(
  7. Jul 7, 2011 #6
    Kyoma,did you post the actual question or did you change the wording?
  8. Jul 7, 2011 #7
    Hello tiny-tim. I'm assuming that the energy is provided in a time period of 5 seconds but the question as presented above does not make that clear.
  9. Jul 12, 2011 #8
    I posted the actual question.

    My answer is correct. My professor marked it wrong.
  10. Jul 12, 2011 #9
    Kyomo,a 12V battery transfers 12J of energy to every coulomb of charge that passes through it so 500 J would be transferred to a charge of 500/12=41.7C.This is the answer you got but the question asks for something different, it asks how much charge flows in 5 seconds.
    The question does not make sense and needs clarifying.
  11. Jul 12, 2011 #10

    You are correct , Kyoma .
    But Tiny Tim is also not wrong .
    He conceptually refers to the work done by a unit charge in t seconds .


    V=12 V
    W=500 J
    T= 5 s


    W = VxI
    500 = 12 x Q/t ( since I=Q/t)
    500 = 12 x q/5
    2500 = 12Q
    Q=208.33 C

    Is this the answer ? Your professor hasn't marked it wrong .:wink:


    W work done on Q charge in T sec .
    So On 1 unit of charge work done will be w/q

    so it will do more work in t sec so final formula will be :



    Hey By sheer logic , Kyoma what you do is you multiply your answer by 5 , ie
    41.6666667x5 = 208.5 (approximately).

    Last edited by a moderator: Apr 26, 2017
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