r41​ Energy density $U$ is given byU=21​∈0​E2 The electric field created by a point charge at a distance r is given by E=4πϵ0​r2q​ On putting the above form of E in eq. 1 , we getU=21​∈0​(4πϵ0​r2q​)2 Thus, U is directly proportional to r41​ .