In the energy–momentum density expressions for a relativistic perfect fluid with a bulk motion, one comes across a couple of pressure-dependent terms, which though well known, are to an extent, lacking in their conceptual basis and the ensuing physical interpretation. In the expression for the energy density, the rest mass density along with the kinetic energy density of the fluid constituents due to their random motion, which contributes to the pressure as well, are already included. However, in a fluid with (...) a bulk motion, there are, in addition, a couple of explicit, pressure-dependent terms in the energy–momentum density, whose presence to an extent, is shrouded in mystery, especially from a physical perspective. We show here that one such pressure-dependent term appearing in the energy density, represents the work done by the fluid pressure against the Lorentz contraction during transition from the rest frame of the fluid to another frame in which the fluid has a bulk motion. This applies equally to the electromagnetic energy density of electrically charged systems in motion and explains in a natural manner an apparently paradoxical result that the field energy of a charged capacitor system decreases with an increase in the system velocity. The momentum density includes another pressure-dependent term, that represents an energy flow across the system, due to the opposite signs of work being done by pressure on two opposite sides of the moving fluid. From Maxwell’s stress tensor we demonstrate that in the expression for electromagnetic momentum of an electric charged particle, it is the presence of a similar pressure term, arising from electrical self-repulsion forces in the charged sphere, that yields a natural solution for the notorious, more than a century old but thought by many as still unresolved, 4/3 problem in the electromagnetic mass. (shrink)

It is shown that a newly derived “exact expression” for radiation of an accelerated charge in the recent literature is simply incorrect, having arisen because of a wrong relativistic transformation of the distance parameter. The ensuing claim that the newly derived expression alone satisfies the energy conservation for the electromagnetic radiation, is based on a wrong reasoning where a proper distinction between the time during which the radiation is received and the time for emission (retarded time of the charge) was (...) not maintained. (shrink)

It is shown that the well-known disparity in classical electrodynamics between the power losses calculated from the radiation reaction and that from Larmor’s formula, is succinctly understood when a proper distinction is made between quantities expressed in terms of a “real time” and those expressed in terms of a retarded time. It is explicitly shown that an accelerated charge, taken to be a sphere of vanishingly small radius \, experiences at any time a self-force proportional to the acceleration it had (...) at a time \ earlier, while the rate of work done on the charge is obtained by a scalar product of the self-force with the instantaneous value of its velocity. Now if the retarded value of acceleration is expressed in terms of the present values of acceleration, then we get the rate of work done according to the radiation reaction equation, however if we instead express the present value of velocity in terms of its time-retarded value, then we get back the familiar Larmor’s radiation formula. From this simple relation between the two we show that they differ because Larmor’s formula, in contrast with the radiation reaction, is written not in terms of the real-time values of quantities specifying the charge motion but is instead expressed in terms of the time-retarded values. Moreover, it is explicitly shown that the difference in the two formulas for radiative power loss exactly matches the difference in the temporal rate of the change of energy in the self-fields between the retarded and real times. From this it becomes obvious that the ad hoc introduction of an acceleration-dependent energy term, usually referred to in the prevalent literature as Schott-term, in order to make the two formulas comply with each other, is redundant. (shrink)

We investigate in detail the electromagnetic fields of a uniformly accelerated charge, in order to ascertain whether such a charge does ‘emit’ radiation, especially in view of the Poynting flow computed at large distances and taken as an evidence of radiation emitted by the charge. In this context, certain important aspects of the fields need to be taken into account. First and foremost is the fact that in the case of a uniformly accelerated charge, one cannot ignore the velocity fields. (...) This then leads to other equally vital points. The net field energy turns out to be exactly the same as that of a non-accelerated charge having a uniform velocity equal to the instantaneous velocity of the uniformly accelerated charge. Further, the Poynting vector, seen with respect to the ’present’ location of the uniformly accelerated charge, during the deceleration phase, possesses everywhere a radial component pointing inward toward the charge, becoming nil when the charge becomes momentarily stationary, and during the acceleration phase, points away from the charge position. Last, but not least, when the leading spherical front of the relativistically beamed Poynting flux, advances forward at a large time t to a far-off distance \, the charge too is not lagging far behind. In fact, these relativistically beamed fields, increasingly resemble fields of a charge moving in an inertial frame with a uniform velocity \, with a convective flow of fields in that frame along with the movement of the charge. There is no other Poynting flow in the far-zones that could be termed as radiation emitted by the charge which, in turn, is fully consistent with the absence of radiation reaction and is also fully conversant with the strong principle of equivalence. (shrink)