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How long does it take for the batter to lose its charge?

  1. Feb 5, 2015 #1
    1. The problem statement, all variables and given/known data
    The two headlights of a car sonsume a total power of 120 W. A driver parks the car but leaves the lights on. The 12-V battery is rated at 95 A * h.

    2. Relevant equations
    P=IV
    V=IR

    3. The attempt at a solution
    I don't know any equation that associates this with time.
     
  2. jcsd
  3. Feb 5, 2015 #2

    gneill

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    Staff: Mentor

    Hi pandamonium, Welcome to Physics Forums.

    Can you determine the amount of current drawn by the lights?
     
  4. Feb 5, 2015 #3
    Since power and voltage is given, we use P=IV to solve for the current I, correct ?
     
  5. Feb 5, 2015 #4
    Since power and voltage is given, we use P=IV to solve for the current I, correct ? And thanks!
     
  6. Feb 5, 2015 #5

    gneill

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    Staff: Mentor

    That's right. What value do you get for the current?
     
  7. Feb 5, 2015 #6
    I got I=10.
     
  8. Feb 5, 2015 #7

    gneill

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    Staff: Mentor

    Okay, now take a closer look at the units of the battery capacity. How long will you have to draw 10 A to equal that capacity?
     
  9. Feb 5, 2015 #8
    Oh, since 95 is the total charge that the battery can produce before failing, I can divide 95/10 to get the time before it fails, correct ?
     
  10. Feb 5, 2015 #9

    gneill

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    Staff: Mentor

    Yes. You can check your logic by working with the units.
     
  11. Feb 5, 2015 #10
    Thanks for all the help ! I really appreciate it c:
     
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