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Homework Help: Power loss in a cable

  1. Jan 4, 2016 #1
    1. The problem statement, all variables and given/known data
    A 400kV line is carrying 1GW of power and experiences a line resistance of 5 ohms. What is the power loss in the cable due to resistive losses?

    2. Relevant equations

    3. The attempt at a solution
    You are given 4 potential answers to this question
    a) 3.2x10^7 W
    b) 78 W
    c) 5000 W
    d) 2x10^8 W

    I have tried using I=P/V and then substituting this value into P=I^2/R but this doesn't seem to give any of the answers. Can anyone help me out?
  2. jcsd
  3. Jan 4, 2016 #2
    You might want to check your equation relating power and current.
  4. Jan 4, 2016 #3


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    Gold Member

    I suspect you are probably using the LINE resistance, when you should be using the line resistance plus the load resistance, to computer the current. Since you did not bother to post any work, I'm just guessing here. Please post your work when you ask such questions. How can we tell where you've gone wrong when we don't see what you did?
  5. Jan 4, 2016 #4
    I used I=P/V to get 1x10^9/400x10^3 = 2500A. I then substituted this into Power loss= I^2/R to get 2500^2/5 = 1.25x10^6 W.
  6. Jan 4, 2016 #5


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    Gold Member

    So you are agreeing with me; you made exactly the mistake I thought you probably had made. Think about it.
  7. Jan 4, 2016 #6
    ah yes, gosh that was stupid P=i^2*R not divided like I was doing
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