dielectric constant (relative permittivity, symbol ∈r) Measure of the polarity of a medium. The force (F) between two electric charges (e) at a distance (d) apart in a vacuum is expressed as: F = e2/d2. In any other medium: F = e2/∈rd2 where ∈r is the dielectric constant. Typical values are: 1.0 for air; 1.013 for steam; 15.5 for liquid ammonia; 80.36 for water at 20°C; usually 3.18 for ice, although at low frequencies it can be up to two orders of magnitude higher; 5–19 for granite; and 3–105 for dry to moist sand. The dielectric constant is temperature dependent, increasing as temperature increases, and strongly frequency dependent with low-frequency (less than 100 Hz) values being up to 30% higher than those of high frequency (more than 100 Hz). It is analogous to the magnetic permeability. The complex relative permittivity, ∈× is given by ∈×= ∈′ − j∈″ where ∈′ and ∈″ are the real component (relative permittivity) and the imaginary component (dielectric loss factor) respectively, and j = √ −1.
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