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Terminal Voltage Problem

  1. May 2, 2009 #1

    jgens

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    1. The problem statement, all variables and given/known data

    The emf of an automobile storage battery is 12.20 V, and its internal resistance is 0.008 ohms. what is the terminal voltage of the battery when the starter motor is drawing 150. A from the battery?

    2. Relevant equations

    N/A

    3. The attempt at a solution

    I have no idea how to approach this problem and a suggestion on how to get it started would be much appreciated. Thanks!
     
  2. jcsd
  3. May 2, 2009 #2
    What is the voltage drop across this internal resistance? And if there is a voltage drop due to the resistance, then the terminal voltage will be seen as less by this amount.
     
  4. May 2, 2009 #3

    jgens

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    I'm not certain. That's all the information that was given to me.
     
  5. May 2, 2009 #4
    Well don't you know the formula for voltage drop across a resistor?
     
  6. May 3, 2009 #5

    jgens

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    V = ir?

    Edit: Bah, the forum keeps changing all my caps.
     
  7. May 3, 2009 #6
    That is correct. You know the current through the internal resistance, so you can find the voltage drop through this resistor. So the current sucking device will see a lower terminal voltage from the battery.
     
  8. May 3, 2009 #7

    jgens

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    So, since V = ir = (150 A)(0.008 ohms) = 1.2 V, then Terminal Voltage = (12.20 - 1.20)V = 11 V?
     
  9. May 3, 2009 #8
    Correct.
     
  10. May 3, 2009 #9

    jgens

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    Thanks nickjer!
     
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