Electricity, all the phenomena due to the electric loads(responsibilities). When they are still, the electric loads(responsibilities) engender fields of force electrostatics which can influence the surrounding bodies!; when loads(responsibilities) are in movement, they generate magnetic fields.

The electric loads(responsibilities) are associated to the constituent particles of atoms. So, the protons of the pit(core) have a positive said load(responsibility), while electrons have a negative charge (of the same absolute value). See Electron!; Proton. The particles of loads(responsibilities) of the same sign grow again (a proton pushes another proton) while the particles of opposite loads(responsibilities) incur (as the proton and the electron). The electric properties of particles and their influence on the environment were gradually updated by the philosophers, the physicists and the chemists throughout the history of civilization.


The electric phenomena of attraction and aversion, obtained by rubbing of the ambre or the other glassy substances, were known from the High Antiquity, cities notably by the Greek philosophers Thalès de Milet (VI-th century BC) and Théophraste (III-th century BC). But it was necessary to wait for the year 1600 so that the English physicist William Gilbert publishes a first methodical analysis, proposing the electric term (from élektron Greek, "! Ambre! ") to describe the force engendered by these bodies. One knows today that the friction of a baguette of ambre or glass on a material tears away(extracts) electrons from the atoms of this last one to transfer them on the baguette: this one is in charge of then negatively and can so exercise a force electrostatics on its environment.

In 1672, the German physicist Otto von Guericke conceived the first machine electrostatics, producing loads(responsibilities) by mechanical effect. She(it) was established(constituted) by a sphere of sulfur which one made turn in the crank: a load(responsibility) was led(inferred) by getting(touching) there the sphere. At the same time, the French researcher Charles de Cisternay Du Fay made(did) an important observation, allowing to distinguish positive charges and negative charges. The first condenser, the device accumulating loads(responsibilities) electrostatics to free(release) them then, was Leyde's bottle, finalized(worked out) in 1745. She(it) was established(constituted) by a glass bottle covered with a sheet(leaf) of tin, establishing(constituting) the armature external!; the internal armature was established(constituted) by sheets(leaves) of beading (gold, money(silver), etc.) connected with a metal stalk, the whole placed in the bottle.

The researcher and the American diplomat Benjamin Franklin realized for his part a famous experience(experiment) at the end of the XVIII-th century: it(he) collected the atmospheric electricity - responsible for the lightning and the flashes of lighting - by means of a kite, proving that it was the same nature that loads(responsibilities) electrostatics of Leyde's bottle. Franklin explained the observed phenomena by proposing that the electricity was one "! Fluid! " Present in any material(subject), the electric demonstrations being due to a lack or to an excess of this one.

Simultaneously developed a mathematical theory of the electricity. The law according to which the forces between electric loads(responsibilities) vary as the opposite of the square of their distance was experimentally demonstrated by the English chemist Joseph Priestley in 1766. For his part, the Frenchman Charles Coulomb invented, at the same time, a balance of twisting to measure exactly the forces exercised by the electric loads(responsibilities), confirming Priestley's observations and demonstrating besides that the electric force practicing between two loads(responsibilities) is proportional in the product of the individual loads(responsibilities).

The movement of the electric loads(responsibilities), producing an electric current, was studied by the Italian physicists Luigi Galvani and Alessandro Volta. Galvani observed the contraction of muscles to the frog, when in the course of autopsy he applied them an electric current: the biologic phenomena showed themselves so, too, of nature électrochimique. Volta, for its part, first source of continuous electric current built in 1800 the chemical pile.

The similarity between electric field and magnetic field was investigated in 1819 by the Danish researcher Hans Orsted: this one observed that an electric current generates in its circle of acquaintances a magnetic field. As for the British Michael Faraday, it(he) demonstrated in 1831 that a current circulating in a reel could lead(infer) a second electric current in a nearby reel, through the magnetic field. The French physicist André - Marie Ampère studied the action of electric currents on magnets(affectionates) and elaborated the first mathematical theory of the electromagnetism.

A new stage in the understanding of the electromagnetic phenomena was brought in 1864 by the British James Maxwell. This last one demonstrated that the light is itself an electromagnetic phenomenon, and that a brilliance is obtained by oscillations of an electric field. This work opened the way to the German physicist Heinrich Hertz, who produced electromagnetic waves in the atmosphere in 1886, and to the Italian engineer Guglielmo Marconi, who finalized(worked out) in 1896 the first system of transmission and reception of these radioelectric waves (to see Radio).

The electronics, foundation of the modern theory of the electricity, was elaborated throughout the XX-th century, to begin with the first precise measure of the load(responsibility) of the electron, made in 1909 by the American physicist Robert Milliken. The electricity was exploited(run) as source of energy and work, the most important inventions being in the asset(active person) of the Americans Thomas Edison, Nikola Tesla and Charles Steinmetz.


In physical appearance(physics), one distinguishes two domains of studies of the electricity: the electrostatics, which is interested in the still loads(responsibilities), and the électrocinétique, which studies loads(responsibilities) in movement. In electrostatics, the electricity is characterized by the forces of aversion and attraction between still loads(responsibilities). These are measured in coulombs (of symbol C), unity named(appointed) in reference to the French researcher Charles de Coulomb (to see Electric, measure). The force which practices between particles carrying(wearing) loads(responsibilities) q1 and q2 can be determined by the law of Coulomb

According to this law, the force electrostatics is proportional in the product of loads(responsibilities) divided by the square of their distance. The constant of proportionality k is called constant diélectrique, function(office) of the environment(middle) surrounding loads(responsibilities).

To describe the influence of a load(responsibility) electrostatics on its environment, one says that she(it) is surrounded by a field of force. In such a field, to move a load(responsibility) electrostatics from a point to the other one requires some energy, called difference of potential between these points, and expressing himself in volts (unit of measure named(appointed) in reference to the Italian researcher Alessandro Volta). The Earth, which can be likened to an uniform electric sphere, serves as reference in these calculations: the potential energy is declared no by definition. In this reference frame, the potential of a body in charge of(loaded with) positively is characterized by a positive value in volts, over the potential of the Earth, and that of the body in charge of(loaded with) negatively, by a negative value in volts, lower in this(they) even potential of reference.

Being Électrocinétique

When an electric current circulates in a driver, one observes two important effects: the temperature of the driver increases and the needle of a compass placed near the driver is diverted in a perpendicular direction(management) in the vital lead.

The first phenomenon explains itself by the collision of electrons with the atoms of the conductor, the freed(released) energy demonstrating itself in the form of heat, expressed in joules ( J ), the clear power expressing himself herself(itself) in joules by second, or watts (of symbol W). The power loosened(kicked away) in a circuit can be calculated according to the equation P! =! E! ×! I or P! =! I2! ×! R, power which does not demonstrate itself only in the form of heat, but also by the supplied mechanical work, or by the magnetic emitted(uttered) brilliance (brilliant waves, radio, infrared, etc.).

Electric properties of solids

The atoms which establish(constitute) solids can each free(release) one or several electrons which move in the atomic network of the material. Electrons are more easily freed(released) in certain bodies, called conductive. Metals, in particular the copper and the money(silver), are excellent drivers (to see Conductor). The materials(subjects) which do not possess free electrons, and so which can not lead(drive) the electric current, are called insulating. One can quote the glass, the rubber and the dry wood.

The other materials let escape a small number of electrons of their atoms by leaving "! Gaps! " Or "! Holes! ". These gaps, corresponding to the absence of negative charge, behave as a positive charge. The application of an electric field can then provoke the movement of electrons (and associated positive cavities) in the material, by producing there an electric current. Such a material, called semiconducting, presents generally a resistance more important for the passage of the current than a driver as the copper, but a resistance weaker than an insulation as a glass. If the majority of the current are due to electrons, the semiconductor is said about type N. If the majority of the current are due, on the contrary , to cavities, loaded(charged) positively, the semiconductor is told about type P (to see Transistor).

A perfect driver would offer no resistance to the passage of the electric loads(responsibilities), as well as a perfect insulation would allow to pass no load(responsibility). However, such materials do not exist in the nature, at least in ambient temperature. However, certain metals and metal compounds lose any resistance and become remarkable drivers in the low temperatures, close to the absolute zero. This phenomenon is the supraconductivité.

Electric loads(responsibilities)

The electric loads(responsibilities) can be measured by means of a called device électroscope. The first device of this type, such as it was used in the XIX-th century by the physicist Michael Faraday, is represented on the figure 1. The électroscope contains two fine metal sheets(leaves) (insulating, generally glass has, has _) suspended from a metal support ( b ), the whole in a bowl ( c ). A metal ball d collects the electric loads(responsibilities) to be measured and passes on them by means of the support to the two sheets(leaves) of metal. As the loads(responsibilities) of the same sign grow again, both sheets(leaves) deviate proportionally in the quantity of loads(responsibilities) in question.

A body A, loaded(charged) negatively, is placed among a neutral driver B, and an insulation C. The free electrons in the driver are pushed away(repelled) towards the side the most remote from the body A, while positive charges are attracted(enticed) in the driver towards the most close side. The body B, in general, is attracted(enticed) towards A, because the attraction of opposite and close loads(responsibilities) is more important than the aversion of loads(responsibilities) of the same sign but more taken away (the forces between electric loads(responsibilities) being proportional contrary to the square of their distance). In the insulation C, electrons are not free to move, but atoms and molecules transfer so that their associated electrons are as remote as possible of A!; the insulation is also attracted(enticed) towards A, but in a degree lesser than the driver.

The movement of electrons in the driver B of the figure 2 and the reorientation of the atoms of the insulation C confer on these bodies of positive charges on sides close of A and to negative charges on remote sides of In. One calls such loads(responsibilities) of the led(inferred) loads(responsibilities).

Electric measures

When electric loads(responsibilities) move in a vital lead, their passage is called common(current). Measured in a point of the circuit, the strength of current corresponds to the quantity of electricity which crosses this point in 1 s One expresses so the intensity in coulombs by second, or amperes (of symbol A), the unity named(appointed) in reference to the French physicist André - Marie Ampère. One ampere corresponds to the passage about 6,25.1018 electrons by second.

When a load(responsibility) of 1 C crosses a difference of potential of 1 V, a made work (joule), unity was named(appointed) according to the English physicist James Joule is worth by definition 1 J. This definition allows to get in touch mechanical sizes(greatnesses) and electric sizes(greatnesses).

Another unity of energy widely used in atomic physical appearance(physics) is the électronvolt ( eV ). This unity corresponds to the quantity of energy acquired by an electron when this one is accelerated by a difference of potential of 1 V (to see Électronvolt).

Electric current

When two bodies of equal and opposite electric loads(responsibilities) are connected by a metal driver, electrons move of the negative body towards the positive body, so as to restore a neutral balance (by agreement, one sometimes identifies the current as the inverse operation, that is the passage of positive charges of the positive body in the negative body). In any conductive circuit, electrons pass by of the point of lower potential to the point of superior potential. Such an electric current is said continuous if it circulates constantly in the same direction(management), or alternate if it circulates alternately in both senses(directions).

Three independent parameters allow to describe a current: the difference of potential - also called force électromotrice ( f.e.m ). - the strength of current, generally expressed in amperes, and the resistance of the circuit. The unity of the resistance is the ohm, defined as the resistance in a circuit crossed by a current of 1 In and subjected to a difference of potential, or a tension, 1 V. Known under the name of law of Ohm, the name of the German physicist Georg Ohm, who discovered her(it) in 1827, the relation can be represented by the algebraic expression E! =! I! ×! R, in whom(which) E is the électromotrice force in volts, I, the intensity in amperes and R, the resistance in ohms. The law of Ohm can also express itself under the shape I! =! E/R.


The abnormality of the needle of a compass placed near a driver crossed(gone through) by a current indicates the presence of a magnetic field (to see Magnetism) around this one. When two currents cross(go through) unofficial circuits, magnetic fields incur when the currents have the same direction(management) and the same sense(direction), and grow again when they have opposite senses(directions). The magnetic field created by the current has an influence on the vital lead: if this one is suspended in the ground magnetic field, it(he) will behave as a magnet(affectionate) or the needle of a compass and will turn so as to be perpendicular in the lines of field connecting the north and south magnetic poles of the Earth.

The magnetic field created by a driver crossed by a current can be represented as a network of concentric circles around the driver. The action of the lines of magnetic forces in such a field is made in opposite sense(direction) of the needles of a watch, when one observes them with regard to the sense(direction) of movement of electrons in the driver. Let us note that a magnetic field around a driver is constant when the current which crosses him(it) has a constant intensity.

When a driver crosses the lines of force of a magnetic field, the field acts on electrons free of this driver to move them, generating a common(current) levy induction current. The same effect obtains as a driver moves in a still magnetic field, or as a magnetic field is brought to vary around a still driver.

When the current of a circuit increases by intensity, for example when the circuit is closed, the magnetic field created around the driver varies proportionally: this variation of the field creates in return in the driver an induction current, of informed opposite sense(direction) of origin. In a rectilinear vital lead, this effect is very weak!; but, when the thread is rolled up in the form of helical reel, the effect is much more important, fields created by every buckle of the reel influencing the nearby buckles to lead(infer) there a current. The sum these induction currents is such as, when the circuit is closed, the passage of the primary current is thwarted by the induction current. Also, when the primary current does not pass, the diminution of the magnetic field around buckles creates an induction current, which has this time the same sense(direction) as the current of origin. The reel tends so to maintain a little the passage of the current. So, a reel opposes to any modification of the current, showing an electric slowness known under the name of inductance. This slowness has notably a big importance in circuits for alternating current. To see below, Alternating currents.

Conduction in liquids and gases

When an electric current crosses(goes through) a metal driver, the stream of the current is made in a single sense(direction), because only electrons are implied. However, in liquids and gases, a double stream is made possible by the phenomenon of ionization (to see Electrochemistry). In a liquid solution, in particular, positive ions go points of positive potential raised(brought up) towards the points of lower potential, while the negative ions move in the opposite sense(direction). Also, in an ionized gas, the ions move in two opposite directions(managements), as their load(responsibility) is positive or negative, and please , generate a current (see Electric arc!; electric Lighting).

Sources of forces électromotrices

In any electric circuit, the production of a current requires a électromotrice force or a difference of potential. The available sources are multiple: devices electrostatics producing loads(responsibilities) by mechanical means, as Van der Graaff's generator, electromechanic devices where the current is generated by drivers moving in magnetic fields, voltaic piles which produce a force électromotrice by électrochimique action, the devices which produce a électromotrice force by thermic action, the cells which produce a électromotrice force by direct action of the light ( photovoltaic effect) and finally, the devices which pull(fire) their electricity of the pressure, as crystals make him(it,her) piezoélectriques (see Crystal).

Alternating currents

When a driver is moved in a magnetic field, current exchange of sense(direction) as often as the driver changes himself physically sense(direction). Several types of electric generators work by using this principle to supply an oscillating current, called alternating current ( AC). The alternating current is favorite informed continuous as electric source of energy, as for the domestic manners as manufacturers. In particular, an alternating current can see its tension adjusted by a device of a big simplicity: the transformer. The principle is the following one: when an alternating current passes in a reel, the intensity of the generated magnetic field is brought to vary constantly. If a second conductive reel is placed in the magnetic field of the first, the variations of the magnetic field lead(infer) there a secondary alternating current. Now, if this second reel contains more buckles than the first, the led(inferred) tension is more important for it, because the field acts on a bigger number of conductive buckles. Conversely, if the number of buckles is lower in the second reel, the tension is there weaker than in the primary reel.

The principle of the transformer allows to forward the electric current on long distances, without excessive losses of the useful power. Indeed, if, originally, 200 000 W are supplied in the network, the current can be transported as well under a strong tension of 200 000 V with an intensity of 1 In, that under a weak tension of 2 000 V with an intensity of 100 In (because, we saw him(it), the power is the product of the tension by the strength of current). Now, if the line resistance is 10 w ( a typical value), the loss of power under 200 000 V is only 10 W, while under 2 000 V, the loss of power reaches(affects) 100 000 W, that is half of the total energy to pass on. It is so the first solution which is held(retained) to forward the electricity in networks, transformers increasing the tension, so that the current can borrow(take) lines to very high tension (to see Electricity, production and distribution of him(her,it)).