There may be, in addition, the  ‘induced dipole
moment’ effect as for non-polar molecules, but generally the alignment
effect is more important for polar molecules Thus in either case, whether polar or non-polar, a dielectric develops
a net dipole moment in the presence of an external field The dipole
moment per unit volume is called polarisation and is denoted by P For
linear isotropic dielectrics,
0
P=ε χ
eE
(2