Abstract: In order to prove independence of the electrostatic energy of complex system on the process of establishing the system, we apply mathematical method to derive the work done by the electrostatic field according to the Coulomb＇ s law for carrying free charge,conductor and dielectric in any way. The formula of electrostatic energy is obtained. The result shows that the electrostatic energy of linear dielectric only depends on the final distribution of the system. The conclusion provides the important theoretical basis for the imaginary displacement method of solving the electrostatic force.

Key words: Coulomb＇ s law, principle of work and energy, electrostatic energy, dielectric