A simple formula for the interaction energy of two non-overlapping but concentric charge distributions can be derived. Let the first charge distribution ρ ρ 1 ( r ′ ) be centered on the origin and lie entirely within the second charge distribution ρ ρ 2 ( r ′ ) . The interaction energy between any two static charge distributions is defined by U = d e f ∫ ∫ d r ρ ρ 2 ( r ) Φ Φ 1 ( r ) .