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Current in the curcuit from watts and volts

  1. Sep 7, 2009 #1
    1. The problem statement, all variables and given/known data

    A circuit has a 50 V battery, and 3 resistors. they are all 25 Watts, 1 is in series, 2 are parallel. Find the current of the circuit.

    2. Relevant equations

    V=IR
    P=IV

    3. The attempt at a solution

    I know how to do these fine when I'm given the resistances or the amperages, but I'm having problems with this style.

    I've attempted this a few different ways. I tried to find the equalizing resistance using: P=v^2/R
    25=50^2/R
    R=100 ohms.

    then 1/100 + 1/100 = 1/Rparallel = 50. 50+100=150. to find the equalizing resistance, and then using:
    V=IR
    120=I*150
    I= .8 Amps.

    I'm clearly missing something when i'm working with Watts... can someone please walk me through how to actually do this. it's very urgent...
     
    Last edited: Sep 7, 2009
  2. jcsd
  3. Sep 7, 2009 #2

    vk6kro

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    Science Advisor

    The 3 resistors together use 25 + 25 + 25 = 75 watts.

    This must come from the battery, so how much current is being drawn?

    After that it is easy.

    Work out each resistor knowing the power in it and the current (P=I*I*R). The two in parallel have the same resistance but you need to see why this has to be the case.
     
  4. Sep 7, 2009 #3
    thanks
     
  5. Sep 7, 2009 #4

    vk6kro

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    Science Advisor

    Actually, the question doesn't ask for the size of the resistors or the voltage across them. But it would be excellent practice for you to work them out.
    They come out easily.
     
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