if a point charge Q is vibrating in some mechanical way along the X axis with respect to a certain point xQ, then the value of electromagnetic energy density(which is a point function like E) on the same axis will be also oscillating. Immediately the question arises: how does the test charge q at the point of observation, lying at some fixed distance from the point xQ along the continuation of the X-axis, “know” about thecharge Q vibration?
In other words, we have a rather strange situation: the Poynting vector S = c/4π [E × H] is zero along this axis(because H is zero along this line) but the energy and the momentum, obviously “pass” from point to point along this axis.
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