Question

The expression p = E/c defines the momentum of a photon. Can it be used for the momentum of an electron or proton?

Answer 1

No. The relation p = E/c applies to photons, which have zero rest mass and travel at the speed of light. For an electron or proton, momentum is p = mv at ordinary speeds; the relativistic relation for total energy is \[E^{2} = p^{2}c^{2} + m^{2}c^{4}\].