If we have five balls in a room of volume 1000 m3, we say that the number density (or “concentration”) of balls in the room is c = 0.005m−3. Densities of dimensional quantities are traditionally denoted by the symbol ρ; a subscript will indicate what sort of quantity. Thus mass density is ρm (units kgm−3), while charge density is ρq (units coul m−3).