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Homework Help: Voltage question

  1. Mar 14, 2010 #1
    I have a homework problem I am stuck on can some one help?

    A bird stands on an electric transmission line carrying 3100 A. of current. The line has 3.0E-05 ohm resistance per meter, and the bird's feet are 2.0 cm. apart. What voltage does the bird feel?

    Thanks!
     
  2. jcsd
  3. Mar 14, 2010 #2

    Andrew Mason

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    What is the resistance of the wire between the bird's feet? The voltage is just IR.

    AM
     
  4. Mar 14, 2010 #3
    That is all the info that was given for the problem
     
  5. Mar 14, 2010 #4

    cepheid

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    No, you misunderstand. Andrew Mason was hinting at the first step in answering this question. It is possible to answer Andrew Mason's question using the information you have been given. You know the resistance per unit length, and you know what length of cable is between the bird's feet.

    By the way, please post homework problems in the Homework Help subforum in the future.
     
  6. Mar 14, 2010 #5
    yes but I am looking for resistance and dont have the cross sectional area of the wire. the formula I have was R= (resistivity of wire) Length/Area
     
  7. Mar 14, 2010 #6

    cepheid

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    You don't need any of that information, because the problem already tells you how much resistance a one-metre section of this type of wire has. All you need to do is figure out how long a section spans the distance between the bird's feet.
     
  8. Mar 14, 2010 #7
    So R=(3e-5 ohm).02m/A ??? Still have 2 unknowns
     
  9. Mar 14, 2010 #8

    cepheid

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    I don't understand what you are doing. Where does 'A' come from? What you have been given in the problem is:

    [tex] \frac{\textrm{resistance}}{\textrm{length}} [/tex]​

    in units of ohms/metre. To figure out the total resistance in the section of wire of interest, all you have to do is multiply this number by "length" to end up with just "resistance."
     
  10. Mar 14, 2010 #9
    it is the formula we were given for the resistivity of material
     
  11. Mar 14, 2010 #10
    so 3e-5/.02m = .0015ohms

    Is that right? If so how do I translate that to the voltage?
     
  12. Mar 14, 2010 #11

    cepheid

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    I realize that. What i'm saying (and what I said in post #6) is that it is not relevant here. You don't need to know the resistivity or the cross-sectional area. They would only be useful if you were trying to calculate the total resistance of a piece of material from scratch. But this problem tells you how much resistance per metre this wire has. Therefore, to figure out the resistance of a section of it, all you need to know is the length of that section.
     
  13. Mar 14, 2010 #12

    cepheid

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    No, it's not. Think about it. You have resistance OVER length. How do you combine that with 'length' in order to get just 'resistance.' Hint 1: the answer is not division. Hint 2: the result should obviously end up with units of ohms.

    Assuming you eventually find the right answer for the resistance of that section of wire, Andrew Mason already told you how to use that to find the voltage across that section.
     
  14. Mar 14, 2010 #13
    sorry I read the one post to quick and didnt understand right. The bird feels .00186V. Thanks!
     
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