Permittivity describes the effect of material in determining the electric field in response to electric charge. For example, one can observe from laboratory experiments that a particle having charge
gives rise to the electric field
is distance from the charge,
is a unit vector pointing away from the charge, and
is a constant that depends on the material. Note that
, which makes sense since electric charge is the source of
. Also note that
is inversely proportional to
, indicating that
decreases in proportion to the area of a sphere surrounding the charge – a principle commonly known as the inverse square law. The remaining factor 1/
is the constant of proportionality, which captures the effect of material. Given units of
, we find that
must have units of farads per meter (
). (To see this, note that
) describes the effect of material in determining the electric field intensity in response to charge.
In free space (that is, a perfect vacuum), we find that
The permittivity of air is only slightly greater, and usually can be assumed to be equal to that of free space. In most other materials, the permittivity is significantly greater; that is, the same charge results in a weaker electric field intensity.
It is common practice to describe the permittivity of materials relative to the permittivity of free space. This relative permittivity is given by:
for a few representative materials is given in Appendix 10.1 . Note that
ranges from 1 (corresponding to a perfect vacuum) to about 60 or so in common engineering applications. Also note that relative permittivity is sometimes referred to as dielectric constant. This term is a bit misleading, however, since permittivity is a meaningful concept for many materials that are not dielectrics.
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