Electrostatics.—If a pair of ebonite rods be electrified by friction with flannel, then by suspending the one rod and presenting the other to it, it is easily demonstrated that a mutual mechanical force of repulsion exists between them. If now a glass rod be electrified by friction with silk, it will be found that it attracts the suspended electrified ebonite rod. These experiments reveal the facts that electric charges may be of two opposite kinds, and that like charges repel one another, while unlike charges attract one another.
The charge produced on glass by friction with silk is called positive; that produced on ebonite by friction with flannel is called negative. The kind of charge produced depends not merely on the material rubbed, but also on the material of the rubber. Thus a warm dry glass rod becomes negatively electrified when rubbed with fur. The rubber always becomes electrified with a charge of the opposite kind to that produced on the material rubbed, and these two charges are equal in amount. All bodies may be electrified by friction, but those which allow a free movement of the charge over them (such bodies are called conductors, to distinguish them from insulators, which do not allow this free movement) must be held by an insulating handle, or else the charge will be removed as quickly as it is produced.
Coulomb proved that the magnitude of the mutual mechanical force exerted between two charged bodies depends on the amounts of the charges and the distance between them. Faraday called attention to the influence of the medium
in which the charges are placed. Thus if two charges of q1 and q2 units respectively are placed d centimetres apart in a given medium, the mechanical force f in dynes exerted between them is given by the equation f = (q1q2)/(Kd2), provided the dimensions of the bodies on which the charges are concentrated are small in comparison with d. The coefficient K is called the dielectric constant of the medium, and its value is taken as unity for air.
In accordance with this relationship, unit charge is defined as that charge which repels an equal and similar charge placed at a distance of 1 centimetre in air, with a force of 1 dyne.
If the medium surrounding a charged body be explored with a unit charge, a mechanical force varying in magnitude and direction from point to point will be found to act on the unit charge. In such a case, an electric field is said to exist in the medium.
The strength of the electric field at any point is defined as numerically equal to the mechanical force which would act on a unit charge placed in air (or more strictly in a vacuum) at that point. The direction of the electric field at any point is defined to be the direction of the mechanical force acting on a unit positive charge placed at that point.
It should be noted that the strength of the electric field and the mechanical force are numerically equal only when the dielectric constant of the medium is unity. Thus if F is the field strength, K the dielectric constant, and U the mechanical force acting on a unit charge, F = KU.
It is very convenient to represent an electric field by means of what are called lines of electric force. If lines are drawn, starting from a positive charge and ending on a negative charge, such that the tangent to the line at any point is the direction of the electric force at that point, these lines are called lines of electric force. They can be drawn in such a way that the strength of the electric field at any point is numerically equal to the number of lines of electric force passing through unit area surrounding that point (and taken at right angles to the direction of the force). The lines of electric force will thus completely represent the electric field.
Further, if the following properties are attributed to the lines of electric force, viz. (a) that a line of electric force tends to shorten itself as far as possible; (b) that lines of electric force mutually repel one another; then all the phenomena due to the presence of an electric field may be interpreted by the behaviour of the lines of electric force.