Re: Magnetic Field of Finite Wire

Thomas Smid wrote:
> In standard textbooks (e.g. Berkeley Physics Course, Feynman) the
> magnetic field of a wire is derived from the principles of relativity
> by considering the length contraction for the charge distribution in
> different reference frames. This yields a result exactly identical to
> the one predicted by electrodynamics for the field. However, the
> derivation is always only done for an infinite wire. For a finite wire,
> one finds in fact that the results differ:

Such a calculation would make no sense for a finite wire. A finite
length wire cannot carry a uniform current, which is essential for the
application of a Lorentz transformation. The current distribution in a
wire cannot be arbitrary, it must satisfy the charge continuity
equation.

[...]
> So I wonder what the correct
> behaviour in the far field of a current system actually is. Does
> anybody know some observational or experimental evidence in this
> respect?

There are things in our every day lives whose functionality depends on
how well Maxwell's equations model reality. Look at a TV, a radio, or a
cell phone, all have finite length wires that either emit or absorb EM
radiation. The reception and transmission of EM signals by these
devices (that's the experimental part) is modeled by antenna theory
(that's the theoretical part), which is in turn based on Maxwell's
equations. If there were any deviation from expected behavior, we would
have heard of it by now.

Igor

.

Relevant Pages

  • Re: Magnetic Field of Finite Wire
    ... >> In standard textbooks (e.g. Berkeley Physics Course, ... >> derivation is always only done for an infinite wire. ... > Such a calculation would make no sense for a finite wire. ... it must satisfy the charge continuity ...
    (sci.physics.research)
  • Re: Magnetic Field of Finite Wire
    ... >> derivation is always only done for an infinite wire. ... > Such a calculation would make no sense for a finite wire. ... it must satisfy the charge continuity ...
    (sci.physics.research)