One of the factors affecting the size of the electrostatic force is the permittivity ( ) of the medium surrounding the charges. The medium through which the force is exerted is called the dielectric.
e
If the permittivity is low, the force is big; permittivity and force are inversely proportional. The force is greatest when the charges are in a vacuum9.
e0 Permittivity of a vacuum (Permittivity of free space) 8.9 1012 F m1
Relative permittivity All media have a greater permittivity than that of a vacuum; they are all some multiple of the permittivity of free space.This factor is called the relative permittivity of the medium.
Material Vacuum Air Oil
PERMITTIVITY OF CERTAIN MEDIA Relative permittivity ( r)E 1 (by definition) 1.0005 2.2
Polythene Perspex Mica
Water
2.35 3.3 7
80 Table 15.2: Permittivity of some common media Forces between collinear charges
• The size of the force is the same on each body. • The direction of the force is always along the line joining the two charges and is an attractive force if the charges are unlike, and repulsive if they are like charges.
• If the charges are not point charges, but the distance between the charges is large compared with the dimensions of the charge, Coulomb’s law still holds to a high degree of accuracy.
SAMPLE PROBLEM 15A The relative permittivity of polystyrene is 2.5. Calculate the permittivity of polystyrene.
SAMPLE ANSWER 15A The permittivity of polystyrene is 2.2 1011 F m1.
e E ErE0 (2.5)(8.9 1012 ) 2.225 1011
Permittivity ( )E 8.9 1012 F m1
8.9 1012 F m1 2.0 1011 F m1 2.1 1011 F m1 2.9 1011 F m1 6.2 1011 F m1 7.1 1010 F m1
RELATIVE
PERMITTIVITY e e
r 0 Permittivity
e e
e 0 Relative of free space e
r Relative Permittivity
9The word ‘permittivity’ is somewhat misleading, as the higher the value of , the less will be the force between the two charges. Perhaps a term like ‘unpermittivity’ would be more descriptive, but unfortunately we have to use the term given to us, which is ‘permittivity’.
