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Homework Help: Two Parallel Wires( in 2 hrs!)

  1. Oct 17, 2007 #1
    1. The problem statement, all variables and given/known data
    Two long parallel wires are a center-to-center distance of 1.50 cm apart and carry equal anti-parallel currents of 1.80 A. Find the magnetic field intensity at the point P which is equidistant from the wires. (R = 4.00 cm).

    2. Relevant equations
    [tex] B= \frac{u_{0}I}{2\pi r} [/tex] infinite wire

    3. The attempt at a solution
    Ok I have been workin this problem for a while.
    In terms of vectors, the y's cancel out.
    For the X direction I get:
    [tex] B_{totalx}=\frac{u_{0} I R}{\pi \sqrt( (d/2)^2 +R^2)}[/tex]
    I dont get why I am wrong, I took the sum of the b-fields in terms of vectors.
    And no the answer is not 0 T!

    Attached Files:

  2. jcsd
  3. Oct 17, 2007 #2
    Why do you think that the Magnetic field is not zero?
  4. Oct 17, 2007 #3
    Thats what I thought originally , but it is incorrect.
  5. Oct 17, 2007 #4
    If they have equal, anti-parallel currents, and it's a point that's equidistance from the wires, then the answer has to be zero. Unless you decide to ignore the fact that magnetic fields are vectors.
  6. Oct 17, 2007 #5
  7. Oct 17, 2007 #6
    Thats what I used to think too, but when I entered 0 T it is incorrect.
    there is something else I am missing.
  8. Oct 17, 2007 #7
    got an idea?
  9. Oct 17, 2007 #8
    Go back and ask your instructor what he means by anti-parallel. It has more than one meaning. It shouldn't but it does.
  10. Oct 17, 2007 #9
    I agree, word choice could be better.
    However I don't have contact with my prof. right now, this assignment is due in an hour.
  11. Oct 17, 2007 #10
    If I was you I would give your best guess (ignore that you know it's "not right") and argue for points later. Chances are others having the same problem.
  12. Oct 17, 2007 #11
    perhaps his answer is wrong by mistake? talk to other people in the class
  13. Oct 17, 2007 #12
    he is not, there is an answer I people got. There is some trick though, but my math tells me otherwise,
  14. Oct 17, 2007 #13


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    Science Advisor
    Homework Helper
    Gold Member

    ??? The two B fields add up, they don't cancel!!
    It's when the currents are in the same direction that the total B field is zero at the point midway between them.
    Last edited: Oct 17, 2007
  15. Oct 17, 2007 #14

    Doc Al

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    Staff: Mentor

    Show how you got that answer--it's not dimensionally correct, for one.
  16. Oct 18, 2007 #15
    OK, now that I can see your diagram, I see the instructor used anti-parallel in the correct sense.
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