67. Interaction of stationary electric charges inside a uniform dielectric

The force of interaction between charged bodies depends on the properties of the environment in which they are. Let the charged small metal balls be placed in a uniform substance that does not conduct electricity - in an insulator (dielectric). The dielectric must be liquid (kerosene, oil, etc.), as it is difficult to measure the interaction force of the charged bodies inside the solid dielectric.

The interaction force between charges in a uniform dielectric is always smaller than in a vacuum, as experience shows. And the relation of force of interaction of charges in vacuum to force of interaction of the same charges in the given environment does not depend neither on magnitude of charges, nor on distance between them. It is defined only by properties of the given environment. If to designate this relation of forces through \(\varepsilon\), the law of Coulomb for interaction of charges in a dielectric is written down as

\( F ~= ~\frac{1}{4 \pi \,\varepsilon{_0} \,\varepsilon} \cdot \frac{q_1 \,q_2}{r^2} \) (8-5)

Then the ratio of the interaction force in vacuum \((8-2)\) to the interaction force in the environment \((8-5)\) will be equal to a constant value \(\varepsilon\).

This value \(\varepsilon\) is called dielectric permittivity of the given environment. It is the value of unnamed. For vacuum \(\varepsilon ~= ~1\). The dielectric permittivity characterizes the electrical properties of the substance. Later we will find out the reason why the interaction force of charges in the dielectric is less than in vacuum.

In system CGSE (8-5) we will have the form

\( F ~= ~\frac{q_1 \,q_2}{\varepsilon \,r^2} \)

The table gives examples of dielectric permittivity values of some substances.

Substances	dielectric permittivity
Air	1.000594
Kerosene	2.1
Ebonite	2.7-2.9
Quartz	4.5
Glass	5-10
Ethyl alcohol	27
Water	81