In the realm of electricity, dissipation factor is a measurement of the parasitic loss (as heat) that results from subjecting a dielectric to an alternating electric field. This parasitic loss occurs because the polarization of the dielectric that results from the increasing electric field requires energy that is not recovered when the field collapses. This effect is analogous to hysteresis in magnetic materials.

Usually, dissipation factors are given as the resulting power factor when an alternating current is run through a purely capacitive circuit (i.e. no inductance or resistance) in which the capacitor is using the dielectric in question. An ideal capacitive circuit with no dissipation factor would have a power factor of zero, but the parasitic losses cause the power factor to be very slightly over zero.

To find the power lost as heat because of dielectric losses, the following formula should be used:

Power = Dissipation Factor * Voltage * Current

Dissipation factor does increase somewhat with thefrequency of the alternating field, so it is usually quoted at a certain frequency. Below are some sample dielectrics and their dissipation factors:

  • Vacuum: 0 @ all frequencies
  • Air: 0* @ all frequencies
  • Rexolite: 0.00012 @ 1 MHz
  • Polyester: 0.002 @ 1 kHz
  • *: Air technically does have a dissipation factor higher than zero, but it is so close it can be ignored.

    Cooking food using a microwave oven utilizes this principle. Most foods contain water, a dielectric with a relatively high dissipation factor. Microwaves contain rapidly alternating electric fields, subjecting the water molecules to repeated polarizations and de-polarizations. The high dissipation factor of water leads to a high parasitic loss, heating the food.

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