How to find the "mass-energy" in a certain field I saw somewhere that for a charged particle of radius R, the method of finding the "mass-energy" in such an electrostatic field (caused by the charged particle is) M = ∫E^2 dV, where E is the electric field of the particle, and the bounds of the integral are from R to infinity. Can someone justify why this integral is correct? Thanks! P.S. What exactly does "mass-energy" mean?