Re: [Phys-L] feeler-dealer, third law, et cetera

[John Denker <jsd@av8n.com>]

On 12/11/2013 05:37 PM, Bruce Sherwood wrote:
> There's also the well-known case of lack of reciprocity for magnetic
> forces, which makes the answer to the second question be false even without
> retardation. A proton at the origin moves to the right, and an electron
> somewhere along the x axis is headed in the y direction. The proton
> contributes no magnetic field at the location of the electron, but the
> electron does contribute a magnetic field at the location of the proton.
> Hence the magnetic portion of the force on the proton is not equal and
> opposite to the magnetic portion of the force on the electron (whereas the
> electric portion of the force doesn't have this behavior).

An excellent point.  I just now added a section addressing this:
  http://www.av8n.com/physics/force-intro.htm#sec-field-momentum

FWIW, I don't think of it as "lack of reciprocity for magnetic forces" 
but rather as
   _momentum in the fields_ 
which seems like a more general concept.