Electrets

Electret (formed of elektr- from "electricity" and -et from "magnet") is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet. Although Oliver Heaviside coined this term in 1885, materials with electret properties were already known to science and had been studied since the early 1700s. One particular example is the electrophorus, a device consisting of a slab with electret properties and a separate metal plate. The electrophorus was originally invented by Johan Carl Wilcke in Sweden and again by Alessandro Volta in Italy.

Electrets, like magnets, are dipoles. Another similarity is the radiant fields: They produce an electrostatic field (as opposed to a magnetic field) around their perimeter. When a magnet and an electret are near one another, a rather unusual phenomenon occurs: while stationary, neither has any effect on one another. However, when an electret is moved with respect to a magnetic pole, a force is felt which acts perpendicular to the magnetic field, pushing the electret along a path 90 degrees to the expected direction of 'push' as would be felt with another magnet.

There is a similarity between an electret and the dielectric layer used in capacitors; the difference is that dielectrics in capacitors have an induced polarization that is only transient, dependent on the potential applied on the dielectric, while dielectrics with electret properties exhibit quasi-permanent charge storage or dipole polarization in addition. Some materials also display ferroelectricity; i.e. they react to the external fields with a hysteresis of the polarization; ferroelectrics can retain the polarization permanently because they are in thermodynamic equilibrium, and are used in ferroelectric capacitors. Although electrets are only in a metastable state, those fashioned from very low leakage materials can retain excess charge or polarization for many years. An electret microphone is a type of condenser microphone that eliminates the need for a power supply by using a permanently charged material.

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