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Homework Help: Proving the B field in a wire only has theta component?

  1. Jan 22, 2015 #1
    1. The problem statement, all variables and given/known data

    Prove in a current carrying wire the magnetic field only has a theta component.

    2. Relevant equations

    ∇ ⋅ B = 0 (dive of magnetic field zero, 2nd Maxwell Eq)

    ∇ x B = μ J (Ampere's Law, 4th Maxwell Eq)

    Cylindrical symmetry means B field only dependent on r (distance from z axis) so that

    B = Br(r)rhat + Bθ(r)θhat + Bz(r)zhat

    3. The attempt at a solution

    Divergence of Ampere's Law = 0 gives

    - ∂B/∂r θhat + 1/r ∂/∂r(rBθ) zhat = 0

    Not really sure where to go from there =/
  2. jcsd
  3. Jan 22, 2015 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    You'll end up having to solve a system of DEs.

    Note: you can write the equations better in LaTeX... i.e. $$\vec B = B_r\hat r + B_\theta\hat\theta + B_\phi\hat\phi \\ -\frac{\partial B}{\partial r}\hat\theta + \frac{1}{r}\frac{\partial}{\partial r}\left(rB_\theta\right) \hat z = 0$$ ... If I understand you correctly that the last equation is supposed to be ##\nabla\cdot(\nabla\times\vec B) = 0## then that does not look right to me.
    Please show your working.
    Last edited: Jan 22, 2015
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