71001

Linear lossless dielectrics

In most non-crystalline dielectrics, at least for electric field strengths below that of dielectric

breakdown, the polarisation is proportional to and in the direction of the internal electric field. This is expressed by the equation P = _Xe²oE(5.1)

where _Xeis a dimensionless constant called the dielectric susceptibility. Fromthe above

equation it follows that the electric flux density D is given by D = (l_+Xe) ²o E (5.2)

An alternative expression of this result is D = ²r²oE (5.3)

where ²r= (1+_Xe) is a further dimensionless parameter known as the relative dielectric permittivity, or alternatively as the dielectric constant. Alternatively we may write D = ² E (5.4)

where ² is called the dielectric permittivity and has the same units as ²o, i.e. units of Fm-i. It may be shown that with this type of dielectric behaviour, no losses are involved in changing the State of polarisation of the materiał. Models of dielectric media

suitable for a wide variety of purposes can be obtained by combining the above lossless

D - E relation with the linear conductivity relation, which does involve energy loss, and

isdefined in Section 5.2.11 below