Electromagnetic energy is converted to thermal energy, causing wave attenuation and material heating.

The affect of losses on the propagating wave is due to the materials response to either the electric or magnetic field. In most cases it is usually the electric field that produces a response. This response can be attributed to two main dielectric mechanisms: ionic polarisation and dipole rotation. There are other mechanisms which can dominate in certain situations, however we are mainly concerned with ionic polarisation and dipole rotation since industrial microwave systems usually involves heating of liquids and non magnetic solids.

As the wave progresses into a lossy material, its amplitude decreases due to the transfer of energy as heat into the material. The field and power flux density falls exponentially with distance from the surface. The rate of decay is proportional to the materials dielectric properties and the wave frequency. The depth of penetration is defined as the distance into the material at which the power flux has fallen to 1/e(=0.368) of its surface value. The wave still penetrates beyond this point however the power flux density is a value less than 0.368 of that on the surface.
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