Generalities on the Theory of the Electromagnetic Field and on the Structure of Substance
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The positive electrification of a piece of glass can also be obtained by friction with a piece of cloth or texture. It is worth noting that the non-polished glass will not be electrified. Details are given in the work [12, tome III, p. 362]. The electrification by friction appears on the contact surfaces of two bodies, at least one of them being a dielectric (insulating material), as a result of friction when the two bodies are in relative motion. In the case of non-metallic bodies, electric charges, i.e., electrons, are delivered on a superficial film (pellicle) as a result of friction abrasion. From a microscopic point of view, the electrification state of a body means that it is in the situation of having an excess or lack of electrons. It was experimentally established that there is a great number of possibilities for electrifying a system of bodies: By contact (temporary or permanent) with electrified bodies, by deformation, by thermal action, by irradiation. The electrification can be communicated from an electrified body to a non-electrified one, by contact or by influence. It is useful to mention that the electrification by influence means in fact a modification of the repartition of the electric charge, i.e., of the electrons in a conducting body under the influence of the charge of another body. According to the transmission duration of the electrification state, the materials can be divided into three categories: a. Electric conductors or simpler conductors that transmit the electrification state in a very short time, of the order of 10 12 s, thus nearly instantaneously. The bodies made of such material are also called conductors. b. Electric insulators or simpler insulators that transmit the electrification state in a very long time of the order of hours or days. c. Semiconductors that transmit the electrification state in an intermediary time, of the order of second fractions. The ponderomotive actions that are exerted on the bodies situated in the neighbourhood of electrified bodies make evident the existence of the electrified system, in the space surrounding the electrified field. In accordance with the general definition of Section 1.1, electric field is called the physical system that exists in the space regions in which ponderomotive actions (forces and torques) of electric nature can act on bodies. The interactions between electrified bodies are produced by means of electric field produced by the electrified bodies. This fact was established by Faraday and Maxwell. The preceding statement represents, in fact, a general rule accepted in Physics and called principle of continuity: Two bodies can interact only by means of an intermediary medium (i.e., throughout the intermediary medium between the bodies). Hence all laws have to be expressed in a differential form concerning points infinitely close to each other.
1.6.2. True Electric Charge (Free Electric Charge) The mechanical and thermal quantities do not suffice for the study of the electrification state and of the electric field. Therefore, it is necessary to introduce experimentally new physical quantities that will be primitive (fundamental) quantities in the framework of the theory of the electromagnetic field.
