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1.6.2. True Electric Charge (Free Electric Charge
Generalities on the Theory of the Electromagnetic Field and on the Structure of Substance 45
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].
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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 thattransmit 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.
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.
46
From Sub-section 1.6.1, it follows that there are two kinds of electrification state: positive and negative. The electrification state of an electrified body, namely the electrified piece of resin of Sub-section 1.6.1, will be considered as reference state. This electrification state is, as shown, negative.
To introduce the physical quantity called true (free) electric charge; the following measuring procedure can be used. All bodies under consideration in this procedure are considered to be very small, i.e., their dimensions are negligible compared with the distances between them.
Several small bodies in identical electrification state will be considered. All these bodies will be termed reference bodies.
As shown in Sub-section 1.6.1, no ponderomotive force acts on the system composed of a piece of glass and a piece of resin if the system is under the action of an electrified body, hence, if it is in an electric field. This system is called neutral system. The operation of bringing together two or several electrified bodies, so that the resulting system will not be subjected to a ponderomotive force, in an electric field, is called neutralization. Therefore, the true electric charge of any body is proportional to the number of reference bodies necessary for the neutralization of the given body.
The unit (of measure) of the true (free) electric charge may be adopted by convention that of one reference body, [8, Vol. I], [23, Vol. I, p. 46].
The proportionality constant is the negative unity. In this case, the true (free) electric charge of the given body is equal to the number of reference bodies and with opposite sign with respect to the last ones.
Hence, the electric charge has been introduced in a non-correlative way, i.e., regardless of the electric field strength. At the same time, it follows that the electric charge introduced in this way is independent of the reference frame (reference system) or of the velocity. The previous terms true and free can be considered synonyms, however certain authors are using the term true in the macroscopic approach and free in the microscopic approach.
It is to be noted that the electric charge of a moving body does not vary like the mass of the body with the velocity. This fact has been established experimentally observing that, in the case in which the number of protons of one atom nucleus is equal to the number of electrons turning about the nucleus, the neutral state of atoms is maintained, although the velocity of electrons and nuclei, respectively, are different. For this reason, hydrogen atoms or molecules are not deflected by an electric field. This fact is referred to as constant charge principle [18, p. 17].
It follows that the true electric charge is a property of bodies, characterized by a scalar quantity proportional to the number of electrons that are not neutralized by the positive charge of the nuclei of the constitution of bodies. The electric charge of a body is negative if it is an excess of electrons and positive if it is a lack of electrons. From the microscopic point of view, the electric charge is distributed among the microscopic particles of the respective body. Each microscopic particle of a certain type has always the same electric charge. From a microscopic point of view, the electric charge is distributed discontinuously in space.
From a macroscopic point of view, it is assumed that the true (free) electric charge, like the substance, is distributed continuously within the whole domain occupied by the considered body. Therefore, the macroscopic representation is an idealized one but
General Theory of the Electromagnetic Field
