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Magnetic field problem

  1. Jun 18, 2013 #1
    1. The problem statement, all variables and given/known data
    This is a problem from Reitz-Milford-Cristy-Problem 9.15(fourth Ed.) .There is a long wire carrying a current "I" above a plane,inside the plane we have to cases a)μ=∞; and b)μ=0. We must find the field above the plane.
    2. Relevant equations

    3. The attempt at a solution
    To solve the problem we split the field sources: 1)wire + 2)Magnetic plane. Part 1) is already known and to find 2) we put H=-grad∅ and solve laplace equation Δ∅=0. To find ∅ we consider an image current in case a) it is a parallel current and in case b) it is an antiparallel one.
    I understand case b), in this case we have B=0 inside the plane so the normal component of H must vanish so the normal derivate of ∅ must cancell that of "I" and the problem is uniquely solved.
    However in case a) we have H=0 and the tangencial component of H must vanish, we can do this with a parallel image however in this case what we know is the tangencial derivate of ∅ and the unicity theorem for laplace equation does not apply.
    Can someone shed some light on this?
  2. jcsd
  3. Jun 18, 2013 #2


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    If the tangential derivative of ∅ vanishes on the boundary plane, what does that tell you about how ∅ varies on the boundary plane?
  4. Jun 18, 2013 #3
    The problem is that the tangencial derivative of ∅ does not vanish but it is equal an opposite to the tangencial component of source 1) (the wire) so the field generated by the to sources combined add up to a null tangencial field
  5. Jun 18, 2013 #4


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    OK. I see. ∅ is the magnetic scalar potential of the image current alone.

    From the known tangential component of H on the boundary plane due to the true current, you know the tangential component of H due to the image current. So, as you said, you know the value of the component of the gradient of ∅ in the direction of the tangential component of H, ∇t∅(x,y), at each point (x,y) on the boundary plane.

    Taking ∅ = 0 at infinity, can you use ∇t∅(x,y) to find (in principle) the value of ∅ at any point of the plane? Does uniqueness follow?
  6. Jun 19, 2013 #5
    Now I see, determining the tangencial component in this case is equivalent to determine the value of ∅ on the boundary surface so uniqueness can be applied. Thank you TSny
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