The superposition principle allows for the calculation of the electric field due to a distribution of charge density ρ ρ ( r ) . By considering the charge ρ ρ ( r ′ ) d v in each small volume of space d v at point r ′ as a point charge, the resulting electric field, d E ( r ) , at point r can be calculated as d E ( r ) = ρ ρ ( r ′ ) 4 π π ε ε 0 r ^ ^ ′ | r ′ | 2 d v = ρ ρ ( r ′ ) 4 π π ε ε 0 r ′ | r ′ | 3 d v where