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Homework Help: Current and Resistance

  1. Jul 27, 2008 #1
    The headlights of a certain moving car require 11 A from the 12 V alternator, which is driven by the engine. Assume the alternator is 84% efficient (its output electrical power is 84% of its input mechanical power), and calculate the power (in hp) the engine must supply to run the lights.

    I plugged the equation into P = iV and multiplied it by .84 efficiency, then got my answer in watts, which I converted to hp. When I checked in my book, however, the answer was not correct. I'm pretty much stumped on how else to approach it.
     
  2. jcsd
  3. Jul 27, 2008 #2

    G01

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    You should be dividing by .84, not multiplying by it:

    [tex]P_{out}=.84P_{in}[/tex]

    Therefore,

    [tex]P_{in}=\frac{P_{out}}{.84}=\frac{iV}{.84}[/tex]

    See where you went wrong?

    Next time, please post homework type questions in the appropriate homework help forum.
     
  4. Jul 28, 2008 #3
    Ah! Yes!! Thank you so much!!
     
  5. Jul 28, 2008 #4

    G01

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    Anytime.:smile:
     
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