Question

Even though an electric field $E$ exerts a force $qE$ on a charged particle yet the electric field of an EM wave does not contribute to the radiation pressure (but transfers energy). Explain.

Answer 1

The electric field of an electromagnetic wave is an oscillating field. It exerts electric force on a charged particle, but this electric force averaged over an integral number of cycles is zero, since its direction changes every half cycle. Hence, electric field is not responsible for radiation pressure though it transfers the energy. In fact, radiation pressure appears because of the action of the magnetic field of the wave on the electric currents induced by the electric field of the same wave.

Final Answer:

Electric field of an EM wave is an oscillating field and so is the electric force caused by it on a charged particle. This electric force averaged over an integral number of cycles is zero since its direction changes every half cycle. Hence, electric field is not responsible for radiation pressure.