Electricity

Electricity in the most general sense represents one of the forms of motion of matter. Usually, however, this word is understood to mean either the electric charge as such or the very doctrine of electric charges, their motion, and interaction. The word E. originates from the Greek electron—amber, on which the electrification by friction was first discovered. The doctrine of E. consists of the following main sections: electrostatics, which considers the interaction of stationary charges; electric current, which studies moving electric charges; electromagnetism, which studies magnetic phenomena accompanying the motion of electric charges; and the section of electromagnetic oscillations, which considers the propagation of rapidly changing electromagnetic fields. The concept of E. and the view of its essence underwent very strong changes in the course of development. The boundaries of the doctrine of E. were very widely expanded with the appearance of the electromagnetic theory of light and especially after the establishment of the electric nature of all ponderable matter. In the light of modern views, electric charge appears as an excess or deficiency of atoms of negative E., electrons (see). The atoms of all substances represent a collection of equal quantities of positive and negative E. (protons and electrons); if an electron is removed from an atom, it becomes charged positively, if added, then negatively. Electric charges interact with each other, repelling (like charges) or attracting (opposite charges) with a force proportional to their magnitudes and inversely proportional to the square of the distance between them. This dependence is expressed by the so-called Coulomb's law. From this law directly follows the definition of the unit of quantity of E., namely: the quantity of E., interacting with an equal quantity at a distance of one centimeter with a force of 1 dyne, is the unit of quantity of E. in the absolute electrostatic system of units (CGSE). The other units of this system can be derived, starting from the indicated definition of this basic unit. Besides its magnitude (quantity of E.), electric charge is characterized by another quantity—potential (see). The latter shows as it were the degree of chargedness of a given body and determines the direction of the transition of E. from one charged body to another (such a transition always occurs from a higher potential to a lower one). More strictly, the potential of a given point in space is defined as the work necessary for transferring a unit quantity of electricity from a potential equal to zero (from infinity) to that point. The potential of a charged conductor (V) is proportional to its charge (Q) and depends on its size and shape: Q = CV. The quantity C determines the amount of electricity necessary to give a given body a potential equal to 1, and is called capacitance. (In the CGSE system, the unit of capacitance is 1 cm, i.e., the capacitance of a sphere with a radius of 1 cm.) To accumulate large quantities of E., it is advantageous to use bodies of large capacitance. Special instruments serving for this purpose are called capacitors and have in their simplest form the shape of two conducting plates separated by a layer of dielectric (flat capacitor). One variety of capacitor is the Leyden jar (see). Capacitors in which the capacitance can be changed by changing the area of the plates or the distance between them are called variable (widely used, for example, in radio technology).—In relation to electric charges, all bodies can, roughly speaking, be divided into conductors of E. and non-conductors, or insulators (dielectrics). The former transmit electric charge well; because of this, the whole conductor has the same potential. The latter do not transmit it or transmit it very poorly, and therefore the potentials of their individual parts may be different. When two charged bodies are connected by any conductor, electric charges transfer from the body with the higher potential to the body with the lower one, and an electric current arises in the conductor (laws of electric current—see Electric Current). However, the change in the electric charge of a body can occur not only by transferring E. in the form of electric current, but also through an intermediate medium—the “world ether,” filling all space. The fact is that any electric charge creates an electric field in the space around itself, thanks to which, for example, the interaction of electric charges occurs according to Coulomb's law. A charged body, when placed in such a field, itself becomes electrified. This phenomenon is called electrostatic induction (see Induction). Moving electric charges create, in addition to an electric field, a magnetic field in the space around themselves. For example, passing an electric current through a coil of wire—a solenoid—we obtain around it the same magnetic field as from a straight magnet. This magnetic field increases very strongly if an iron core is placed inside the solenoid. Such a device will already represent an electromagnet. The direction of the magnetic field is determined by the so-called right-hand screw rule and depends on the direction of the current. Thanks to the presence of its magnetic field, electric current will interact with an external magnetic field. The rule of the left hand serves to determine the direction of motion of a conductor with current in a magnetic field. On this principle is based the construction of the majority of electric motors. Conversely, if we move a conductor in a magnetic field, then a current arises in the conductor. The direction of the current, depending on the direction of the field and the motion of the conductor, is given by the rule of the right hand. This phenomenon, called electromagnetic induction (see Induction), is applied, for example, for the construction of dynamo machines, inductors, etc. It is necessary to note that the same phenomenon occurs with a stationary conductor, but with a changing force of the external magnetic field. On the electromagnetic action of current is based the second absolute system of electrical units, namely—the electromagnetic (CGSM). The basic unit here is the strength of current, which, encircling an arc of length 1 cm of a circle with a radius of 1 cm, excites at the center of the circle a magnetic field strength equal to 1, i.e., acts on a unit of magnetism with a force of 1 dyne. From this basic unit are obtained all the other derived units of the CGSM system. For practical measurements, however, both these systems (electromagnetic and electrostatic) are equally inconvenient, since their units are too small or too large for the quantities we measure in practice. Therefore, in technology they apply the so-called practical system of units. The basic units of this system are the coulomb, volt, and ohm (see). Below is a brief table giving the relations between the units of these three systems. In many cases, for example, during the spark discharge of a Leyden jar, the electric current arising in a conductor acquires a periodic oscillatory character. This occurs always in the presence of a discharge of the so-called oscillatory circuit, composed of capacitance and self-induction (see Induction). One type of electric oscillation (low frequency) we have in ordinary alternating current. This current is characterized by the fact that voltage and current change with time according to the sine law (along a sine wave). Electric oscillations of high frequency we have, for example, in a receiving or transmitting radio installation. Alternating electric current creates in the space around itself a variable electric and magnetic field of the same frequency. Thus, electromagnetic oscillations arise in the ether. The laws of ordinary oscillations apply to electric oscillations, i.e., for them the phenomenon of resonance, etc., also takes place. An electromagnetic oscillation is characterized by its frequency n or wavelength λ, which are connected by the equality nλ = v, where v is the speed of propagation of the oscillation in a given medium. Radio waves, light waves, X-rays, γ-rays—all of them represent electromagnetic oscillations of different wavelengths. The source of the first is the electric current arising in oscillatory circuits consisting of capacitors and coils of self-induction; the sources of the others are moving elementary electric charges entering into the composition of individual atoms. The speed of propagation of electromagnetic oscillations in a vacuum is 300,000 km/sec.