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When an external electric field is applied to a polar dielectric, the dipole moments get partly lined up, In general this induced Polarization disappears when the external electric field is switched off. In some dielectrics, polarization remains even after the external field is removed. The duration for which these induced charges remain inside the dielectric depends on the nature of the material, its polarization condition and its surroundings. In some materials it remains for many years. Such materials are called electrets. Electrets posses a gross permanent electric dipole moment. Materials from which electrets can be constructed are called ‘Ferro-electric’.

Organic waxes and plastic materials are commonly used in the preparation of electrets. When an external electric field is applied to a dielectric in the molten state, the molecular dipoles are partly aligned in the field direction. The dielectric is then allowed to solidify. The molecular dipoles are “frozen” in their new orientations. They do not return to their original orientations when the external field is removed. Electrets prepared by this method are called thermo electrets. Electrets are also prepared by the simultaneous application of light and an electric field. They are called photo electrets.

Electrets are used in instruments such as microphones, high voltage generators, computers, radio and television receivers and other electronic devices. They are also used in xerography.

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