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Two parallel wires in plane. determine B field as f(x)

  1. Jul 30, 2009 #1
    1. The problem statement, all variables and given/known data

    Let two long parallel wires, a distance d apart, carry equal currents I in the same direction. One wire is a x=0 and the other is a x=d. Determine B along the x axis between the wires as a function of X.

    Repeat problem if wire at x=0 is 2I and in the opposed direction.



    2. Relevant equations

    Answer key = u/2pi * I [(d-2x)/(x(d-x))]



    3. The attempt at a solution

    I understand the physics, but I am unsure how to arrive at that final answer. I do not understand determining the point along the x axis with those variables. Can someone please help me with the deriving?
     
  2. jcsd
  3. Jul 30, 2009 #2

    berkeman

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    Staff: Mentor

    Start with Ampere's Law, and do the vector addition. You are familiar with Ampere's Law and the Right-Hand Rule, right?

    http://en.wikipedia.org/wiki/Ampère's_law

    .
     
  4. Jul 30, 2009 #3
    thanks for the reply. I know Amperes and vector, and maybe I'm not thinking this through properly. I know the direction of the magnetic fields. But how do you come up with a formula that is based on a variable "x" that could be any distance between the two currents?
     
  5. Jul 30, 2009 #4

    berkeman

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    Staff: Mentor

    Write the expression for the B-field for a single wire, as a function of the distance away from the wire (and with a vector direction from the RH Rule). Then stick the 2nd wire in there, and combine the two equations. With 2 wires and the currents going the same way, is the B-field halfway between them zero or twice what each wire makes on its own?
     
  6. Jul 30, 2009 #5
    I know the expression is B=u/2pi * I/r ----- It is r that I have the problem with. How do you know r when it can vary from between 0 and d. where d is the max.
     
  7. Jul 30, 2009 #6

    berkeman

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    Staff: Mentor

    Exclude r=0 because of the infinity it introduces. For all r>0, the solution is straightforward, no?
     
  8. Jul 30, 2009 #7
    Unfortunately no.

    For the equation with just one wire i have:

    B=u/2pi*I/x


    for the second wire it would be

    B=u/2pi*I/d-x


    Even if this is right, I am not sure where to go next.
     
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