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

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SUMMARY

The discussion centers on proving that the magnetic field in a current-carrying wire possesses only a theta component. Utilizing Maxwell's equations, specifically the divergence and curl equations, the magnetic field is expressed in cylindrical coordinates as B = Br(r)rhat + Bθ(r)θhat + Bz(r)zhat. The divergence of Ampere's Law leads to a system of differential equations that must be solved to demonstrate the dependency of the magnetic field on the radial distance from the z-axis. The conversation highlights the importance of proper mathematical notation, particularly in LaTeX, for clarity in presenting the equations.

PREREQUISITES
  • Understanding of Maxwell's equations, specifically ∇ ⋅ B = 0 and ∇ x B = μ J.
  • Familiarity with cylindrical coordinate systems in electromagnetism.
  • Knowledge of differential equations and their applications in physics.
  • Proficiency in LaTeX for formatting mathematical expressions.
NEXT STEPS
  • Study the derivation of Ampere's Law and its implications in cylindrical coordinates.
  • Explore the solution techniques for systems of differential equations in electromagnetism.
  • Learn how to effectively use LaTeX for writing complex mathematical equations.
  • Investigate the physical significance of magnetic field components in various geometries.
USEFUL FOR

Students in physics or electrical engineering, particularly those focusing on electromagnetism, as well as educators seeking to clarify concepts related to magnetic fields in current-carrying conductors.

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Homework Statement



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

Homework 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

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 =/
 
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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:

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