Art Unwin wrote:
The other particle when in equilibrium is at rest ...

Although it may be possible for an electron to be
"at rest", that concept violates the uncertainty
principle. Free electrons jump from atom to atom
but they are never in a fixed position until they
are measured in that fixed position in which case,
they give up their velocity/momentum as an unknown.

Whoever first said, "One cannot have one's cake
and eat it too." apparently understood the
uncertainty principle. :-)
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com