For a continuous charge distribution in motion, the Lorentz force equation becomes: d F = d q ( E + v × × B ) where d F is the force on a small piece of the charge distribution with charge d q . If both sides of this equation are divided by the volume of this small piece of the charge distribution d V , the result is: f = ρ ρ ( E + v × × B ) where f is the force density (force per unit volume) and ρ ρ is the charge density (charge per unit volume). Next, the current density corresponding to the motion of the charge continuum is J = ρ ρ v so the continuous analogue to the equation is