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Jul14-07, 05:00 AM
P: n/a
A month ago I have started a thread called Maxwell versus Compton.
Unfortunately I didn't have time to join the discussion. Nevertheless
it was useful to me to read it and I surely learned something from it
- especially from Daryl McCullough and William R. Frensley whom I am
But it raised new questions which I want to discuss here. I hope
McCullough and Frensley would read this too and take part in a

I was initially very surprised that the action of the E part of the EM
field doesn't show in the movement of an electron. But I realized soon
that surely this can be some explanation since of its mass the
electron can't follow the changes of E. This now gives rise to some
more questions.

1. Daryl McCullough has written:
<Start of citation.
Suppose that initially the electric field is pointing in the y-
direction, and the magnetic field is initially pointing in the z-
direction. The electron is initially at rest, but starts to move in
the -y direction due to the electric field (because it's negatively
charged). But a moving electron is also subject to a magnetic force,
which is perpendicular to both the direction of motion of the electron
(-y direction) and the direction of the magnetic field (+z direction).
So the electron will develop a small velocity in the +x direction.
Later, the electric field rotates slightly so that it is now pointing
with a small component in the +z direction. The magnetic field rotates
so that it is not pointing with a small component in the -y direction.
The effect of the electric field gives the electron a small velocity
component in the -z direction, and the effect of the magnetic field
again gives it a tiny impulse in the +x direction.
End of citation.>

I don't think that the electric field rotates at all. At least I had
in mind a linearly polarized wave. Maybe he had in mind a circularly
polarized light but this unnecessary complicates the case. I supposed
that E just oscillates in y and -y directions. So it can not point
after time in z directions at all. The magnetic field is always
pointing in +z and -z. I suppose this is just an annoying error in the
writing down the letter +z instead -y. But this surely gave me the
clue to the idea about the impulse along the movement of the EM
(plane) wave.
I suppose Daryl McCullough wanted to point out that the E part moves
the electron slightly up and down whereas the B part acts on the
perpendicular velocity always pushing the electron forward. The result
of B is always in +x direction and so it adds up whereas the action of
E is in +y and -y and sums to nil when taking the mean value over a
macroscopic interval.

As William R. Frensley pointed out in Section 34-9 of Volume 1 of the
Feynman Lectures on Physics this exactly was said there.

<Start of citation from Feynman Lectures

Suppose that light is coming from a source and is acting on a
charge and driving that charge up and down. We will suppose that the
electric field is in the x-direction, so the motion of the charge is
also in the x-direction: it has a position x and a velocity v, as
shown in Fig. 34-13. The magnetic field is at
right angles to the electric field. Now as the electric field acts on
the charge and moves it up and down, what does the magnetic field do ?
The magnetic field acts on the charge (say an electron) only when it
is moving; but the electron is moving, it is driven by the electric
field, so the two of them work together: While the thing is going up
and down it has a velocity and there is a force on it, B times v times
q; but in which direction is this force? It is in the direction of the
propagation of light.
Therefore, when light is shining on a charge and it is oscillating in
response to that light ......

End of citation from Feynman Lectures>

Nevertheless I am not convinced it is so.
It would be interesting to solve this problem mathematically. I'd
rather expect a chaotic distribution of up/down directions of E versus
up/down directions of v and hence of B and v - not just that net way
I supposed that Daryl McCullough wanted to point out and that Feynman
described. Than v x B must be effectively near zero - e.g. the
parallel to the direction of the EM wave impulse has to be
approximately zero. So this way of explaining why the electron doesn't
have a perpendicular impulse tends to give a parallel impulse of zero
By the way the electron should hardly move (if not at all) so its not
very approriete to talk about a charge going up and down as Feynman

I wanted to proceed with some other discussion about the
unobservability of v of the electron but the letter became very long
so I would better postpone that for a next discussion Maxwell versus
Compton III.

I wonder if there is at all E perpendicular to the radius vector from
the source to the point of observation (if it can not be verified
directly experimentally).
I. Maybe its possible to see a movement of the electron with a very
very long radio wave
II. Or if one cuts a long radio wave in one half period - than
electron must move up (from point of view of Compton this is not very
probably - though there is some possibility.
III. Another way to check is to take a muon and see whatr changes
occur. It would react much more slowly and hence v x B be less and so
impulse less than for an electron. (if of course v x B is not
effectively zero).

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