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Re: [Phys-L] Visualizing the Liénard-Wiechert Potentials



On 3/29/2013 12:46 PM, John Denker wrote:
The Liénard-Wiechert potentials are discussed in Feynman volume II section 21-5. If
you have not re-read that section recently, I recommend you do so, but first let me
offer a few words (and diagrams) that might clarify a couple of concepts.

Issue #1 has to do with "retarded potentials". That's simple enough for a
particle that is not moving, or not moving very much ... but if the particle
is moving there are different positions and therefore different amounts of
retardation and it gets kinda confusing.

Issue #2 has to do with the mysterious factor that Feynman sticks in, to obtain
equation 21.29 from the previous unnumbered equation. Feynman devotes a couple
of complicated figures and an entire page of text to explaining this factor, but
not everybody considers the explanation easy to follow.

We can resolve both of these issues in grand style with the help of a spacetime
diagram. In particular, the mysterious factor just comes from geometry. It is
the density of source-points near the light-cone.

This is discussed in more detail, with diagrams, at
http://www.av8n.com/physics/lienard-wiechert.htm

I'm not even sure this counts as a relativity problem, but it reinforces (again!)
the lesson that it really pays to draw the spacetime diagram.
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*

What are we talking about again? :-) I found it soothing to read that....

Liénard-Wiechert potentials* describe the classical electromagnetic </wiki/Electromagnetism> effect of a moving electric point charge </wiki/Electric_charge> in terms of a vector potential </wiki/Vector_potential> and a scalar potential </wiki/Scalar_potential>. Built directly from Maxwell's equations </wiki/Maxwell%27s_equations>, these potentials </wiki/Potential_%28physics%29> describe the complete, relativistically </wiki/Special_relativity> correct, time-varying electromagnetic field </wiki/Electromagnetic_field> for a point charge </wiki/Point_charge> in arbitrary motion, but are not corrected for quantum-mechanical </wiki/Quantum_mechanics> effects. Electromagnetic radiation </wiki/Electromagnetic_radiation> in the form of waves </wiki/Wave_%28physics%29> can be obtained from these potentials.

...even more so to be assured that...
The Liénard--Wiechert formulation is an important launchpad into more complex analysis of relativistic moving particles.

Both from <http://en.wikipedia.org/wiki/Li%C3%A9nard%E2%80%93Wiechert_potential>


Brian W