Re: [Phys-l] Magnetic force and work - space-time approach?

[John Denker <jsd@av8n.com>]

Rauber, Joel wrote:

> Is it the time-like components of F that are doing mechanical work on
> the charge? 

Yes.

The question makes sense only in a particular frame, because the
decomposition of F into a spacelike part (B) and a timelike part (E)
only makes sense w.r.t a particular frame.

So, then, in any particular frame, we can say that it is the timelike
part (E) that does all the work.  This is mentioned at
  http://www.av8n.com/physics/maxwell-ga.htm#sec-lorentz
near the end of the section.

The point of my note from this morning was that the electron only
responds to F;  it doesn't know how F was produced.  For any static
F such that B^2-E^2 is >0, you can find a frame where F is purely
magnetic.

  Of course the question "did work get done" is frame-dependent in
  a big way.  Consider a ball bouncing off a bat in the rest
  frame of the bat!  So in some sense the frame where F is purely
  magnetic is the "bat frame" of the field.

===========

We should of course keep in mind John M's point that sometimes the
magnetic force is so strongly associated with another force that
it "looks like" the magnetic force is doing work.

This is neither completely the same nor completely different from
my earlier point that you can do magnetic work on the permanent
dipole moment of elementary particles.  An electron is *not* just
a test charge;  it has a magnetic moment, too.