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A-hrs calculation help

  1. Oct 2, 2008 #1
    My teacher posted a practice exam with no answers so I want to make sure I am doing my problems right.

    A car battery is rated at 70 A-hrs. How many minutes can this battery last if it operates at a current of 15A?

    I made an equation 70A/60min = Xmin/15A

    So, X=17.5 min

    Am I right?
     
  2. jcsd
  3. Oct 2, 2008 #2

    LowlyPion

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    Re: A-hrs

    An amp is one coulombs per sec. That means that an Amp hour is 3600 coulombs.

    70 amp hours is 252,000 coulombs.

    15 amps is 15 coulombs per sec = 54,000 coulombs in an hour.
     
  4. Oct 2, 2008 #3
    Re: A-hrs

    so 900 minutes?
     
  5. Oct 2, 2008 #4

    LowlyPion

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    Re: A-hrs

    How did you get 900 minutes?

    You have 252,000 coulombs to spend and you spend 54,000 an hour? How many hours is that again? Times how many minutes an hour?
     
  6. Dec 7, 2008 #5
    Re: A-hrs

    Just divide 70 A-h by the 15 A, the A cross themselves out and your left with just the hours not necessary to switch to coulombs.
     
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