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Electrical Permittivity / Relative Dielectic Permittivity

Definitions

Electrical permittivity is the degree to which a material can resist the flow of electric charge & is related to its capacitance, units Farads per metre. The relative permittivity is the ratio of the permittivity of a material to the permittivity of a vacuum & is also called the dielectric constant or the Relative Dielectric Permittivity. It has a value of a fraction over 1 for air, approximately 80 for water & over a thousand for metals. The permittivity of a vacuum is equal to
10⁷ / 4 pi c²
where c is the speed of light in a vacuum.

It is therefore a measure of how easily a material can become polarised by an applied electromagnetic field & to what extent it responds to & attenuates the field.

Affect on electromagnetic radiation

As an electromagnetic wave passes through a material of a certain RDP the radiation is dissipated; the higher the electrical conductivity the greater the spreading of the energy. For an electromagnetic wave to be propagated through a material the electrical & magnetic parts of the wave must remain coupled & if the electrical part is lost through being conducted away the wave will cease to exist. The magnetic part can also be lost if the material has a high magnetic permeability due to magnetisation of the material. For studies of soil this is usually less of a problem than electrical conductivity because the permeability (i.e., susceptibility) of soils is generally relatively low.

Any transition between materials of different RDP causes a change in velocity (& sometimes direction) of an electromagnetic wave with an associated reflection of part of the energy. It is this reflection that is detected by radar & used to determine the boundaries between different buried materials.