(a) Compare the conservation laws for energy and linear momentum for a spherical volume. Use this to define an angular distribution of the rate at which electromagnetic waves radiate linear momentum, dP_EM/dt dΩ, by analogy with the definition of the angular distribution of the rate at which electromagnetic waves radiate energy, dP/dΩ.
(b) Express dP/dΩ and dP_EM/dt dΩ entirely in terms of E_rad.
(c) Show that the relation between the two quantities computed in part (b) is consistent with the mechanical properties of electromagnetic plane waves.