Invariance of charge refers to the independence of electrostatic potential of a particle from its speed. That is, the charge of a particle does not vary with its speed. In other words charge is not relativistic as opposed to relativistic quantities such as mass, energy etc.

A non-relativistic particle is any particle not traveling at a speed close to the speed of light. This is not a property of particular type of particle; any particle may in general travel at any speed (below the speed of light).

An exception are particles which are massless such as photons and gluons, these MUST travel at the speed of light.

Yes. A particle of zero rest mass has ONLY its relativistic mass when in motion. There are actually no photons just sitting around.

F. Rohrlich has written:

'Classical charged particles' -- subject(s): Particles (Nuclear physics)

'Relativistic particle electrodynamics' -- subject(s): Electrodynamics

what are the impacts of relativistic gravity on falling object on ligh?