In the context of energy harvesting, electrostatic refers to capacitive harvesting . Electrostatic generators are widely used though they are not as popular as piezoelectrics or thermoelectrics. Electrostatic (capacitive) energy harvesting is based on the changing capacitance of vibration-dependent variable capacitors called varactors. Vibrations separate the plates of an initially charged varactor , and mechanical energy is converted into electrical energy. To put it more precisely, with electrostatic generators, mechanical forces are employed to do work against the attraction of differently charged parts, so they are mechanically variable capacitors with plates separated by the movement of the source. They are either switched (switched charge or potential) or continuous. All electrostatic generators require a potential and inorganic or organic electrets are one way of providing this. Electrets have a permanent charge buried in the dielectric. For example, PVDF and certain inorganic oxides act as electrets and some are used as such in microphones. An active charge system could permit tuning to duty cycle.