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Homework Help: Current Voltage Power

  1. Jan 3, 2008 #1
    1. The problem statement, all variables and given/known data
    A 100 kW generator supplies power at a p.d of 10kV through cables of total resistance 5 Ohms. Calculate the supply current and the power dissipated in the cables.

    2. Relevant equations
    P = IV
    3. The attempt at a solution
    Supply current:
    100 kW = (I)(10kV)
    Supply current 10 A.

    But I get different answers when I try to find the power dissipated in the cable.
    By using this formulae , P = (V*V)/R I get a huge number which is wrong because it is more than the power generated but when I use P = ( I * I ) / R , I get the correct answer.

    Think I have a misconception about this [ P = (V*V)/R ] . Can someone tell me where I am wrong? and why so? Thanks in advance. =)
  2. jcsd
  3. Jan 3, 2008 #2
    The power equation is P = I*I*R (sub in ohm's law into the power equation)
  4. Jan 3, 2008 #3
    Yah I think he typo'd there. What he's missing is that power generator is FORCING a certain power output, and thus FORCING a certain current through that potential difference, which you found correctly

    However if that power generator wasn't limiting you, you would have 10kV across 5 ohms, which corresponds to a current flowing of 2000amps, which when put into P=IV, doing 2000*10kV gives you the incorrect huge answer you got

    by using P=V^2/R you're finding the power dissipated in 5ohm wires with a potential difference of 10kV, ignoring the fact that you're limited to 100kW from the power generator. By finding the actual current through the wires, you incorporate that
  5. Jan 4, 2008 #4
    Ohh.Thanks blochwave! Understood why I was wrong. =)
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