# dielectric constant

**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*=

*e*

^{2}/

*d*

^{2}. In any other medium:

*F*=

*e*

^{2}/∈

_{r}

*d*

^{2}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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**dielectric constant**