Electrostatics

(Lightning causing a flash of light over Oradea in Romania.)

Electrostatics is the branch of physics that studies the phenomena created by static electric charges for the observer. The obtained laws can be generalized to variable (quasi-electrostatic) systems provided that the distribution of the charges can be considered as being in equilibrium at each moment. Thus the capacitor in an electric circuit is still correctly described by these same laws even if it operates at very high frequencies.

Since Antiquity it is known that some materials, including amber, attract small objects after being rubbed. The Greek word for amber, ήλεκτρον (electron), has given its name to many scientific fields. Electrostatics describes in particular the forces exerted by the electric charges between them: it is about the law of Coulomb. This law states that the force F created by a charge Q on another charge q is proportional to the product of these two charges and is inversely proportional to the square of the distance separating them.

Although they seem, on our scale, relatively weak, the forces of electrostatic origin are extraordinarily powerful. Between elementary electric charges (mainly protons and electrons), they are 40 orders of magnitude greater than the gravitational force. If they seem so weak, it is precisely due to the intensity of these forces, the positive and negative charges are forced to be almost exactly at equilibrium and the forces of attraction and repulsion cancel out at the macroscopic scale. In fact, to understand their real strength, it is necessary to realize that it is they that make solid objects do not interpenetrate and that make the cohesion of the hardest materials. If we succeeded in eliminating even the last layer of electrons from the atoms, the matter would disintegrate only by the repulsive forces that would appear between the nuclei.

The fields of study covered by electrostatics are numerous:

• static electricity;
• explosion of grain silos;
• some photocopier technologies;
• the lightning …

The laws of electrostatics have also proved useful for:

• biophysics;
• the study of proteins;
• nanotechnology (designing a nanoscale-scale engine is more feasible using electrostatic forces than electromagnetic forces.)

Its extensions to moving charges are studied within the framework of electromagnetism, which itself is generalized by quantum electrodynamics.

Overview

There is a simple experience that anyone can do to perceive an electrostatic force: simply rub a plastic ruler with a dry cloth and get close to small pieces of paper: it is the electrification. Papers stick to the rule and stay there until the charges are balanced. The experiment is simple to carry out, however the interpretation is not simple since, if the rule is loaded by friction, the pieces of paper are not a priori. Another experiment of the same style consists in observing that a trickle of water is diverted when approaching a sheet of cellophane.

More simply, a common experience of the effects of electrostatic is the sensation of receiving a discharge by touching a metal object in very dry weather, while going down or up in a car or by removing a garment made of synthetic fabric. These are phenomena in which an accumulation of charges and static electricity has occurred.

From there, we can consider two categories of bodies: the insulators, or dielectric, where the state of electrization is conserved locally, and the conductors where this state is distributed on the surface of the conductor. The electrification of the bodies has been observed thanks to the insulating properties of dry air, which prevents the flow towards the earth of the loads created by friction.

The distinction between insulators and conductors is not absolute; the resistivity is never infinite (but very large) and, for example, a dry insulating paper can become conductive if it is moistened with water.

The free electric charges, practically absent in good insulators, can easily be created by supplying an electron, normally connected to an atomic structure, with a quantity of energy sufficient to release it (by irradiation or heating, for example). At a temperature of 3000 °C, there are no more insulators, but only conductors.

It is also found experimentally that there are two kinds of charges distinguished by their sign, and that matter consists of particles of various charges, all of which are multiples of that of the electron, called “elementary charge”; however, in electrostatics it will suffice to say that when an object is charged in volume, it contains a charge density ρ(x,y,z). This corresponds to a statistical approximation, given the smallness of the elementary charge.

Similarly, a small experiment can demonstrate the importance of static electricity: just load a plastic comb (painting with dry hair) and then approach the comb loaded with a neon tube lamp: in the darkness, by bringing the comb closer to the tube, it lights up locally. The electric field produced by the comb is sufficient to excite the gas inside the tube. Hence the importance of static electricity: if the electric field of a simple comb is sufficient to excite a gas, the discharge of static electricity in a sensitive electronic device can also destroy it.