(Power line near a railway track in Poland. )
Electricity is the effect of the movement of charged particles inside a conductive material, under the effect of a potential difference at the ends thereof. This physical phenomenon is present in many contexts: electricity constitutes both the nerve impulse of living beings and the lightning of a thunderstorm. The discoveries of the natural laws of electricity have led to the invention of many techniques grouped together in electrical engineering. The latter is widely used in developed societies, for example to transport large amounts of easily usable energy.
The properties of electricity were discovered during the 18th century. The control of electric current allowed the advent of the second industrial revolution. Today, electrical energy is omnipresent in industrialized countries: produced from different sources of energy, mainly thermal, nuclear and hydraulic, electricity is an energy vector used in many domestic and industrial uses and essential in remote communications.
It is the movement of the electric charges of matter which is at the origin of electricity. Like mass, electric charge helps explain the origin of certain phenomena. While no one has ever directly observed an electrical charge, scientists notice similarities in the behavior of certain particles. They deduce that these particles share common characteristics, the properties of which coincide with their observations.
Unlike mass, two types of electric charges behave as if they were “opposite” to each other: by convention, one is said to be positive and the other negative. An atom has a positive charge when the number of protons is greater than the number of electrons.
(Forces generated by two charged atoms:
Equal charges of opposite natures cancel each other out: a particle which has as many positive as negative charges behaves as if it did not have any. It is said to be electrically neutral.
In nature, electrons are carriers of negative charges and protons are carriers of positive charges. The atoms that make up ordinary matter include electrons that move around a nucleus made up of protons and neutrons, the latter of which are electrically neutral. When the number of electrons is equal to the number of protons, the whole is electrically neutral. It is static electricity when there is no flow of electric charges. Experimentally, this is generally obtained by using materials in which the charges are “trapped”, insulating materials such as plastic, glass, paper… which resist the circulation of charges.
When we rub certain materials together, the surface electrons of the atoms of one are torn off and recovered by the atoms of the other. For example :
- a glass rod rubbed on a silk fabric charges positively, because the atoms of the glass lose electrons for the benefit of the silk;
- a balloon rubbed on dry hair charges negatively, because it picks up electrons from dry hair;
- a plastic ruler rubbed on the fabric of a garment charges negatively, it can then attract small pieces of paper. The rule modifies, by electrostatic influence, the distribution of the charges in the paper: the negative charges of the rule repel the negative charges at the other end of the piece of paper and attract the positive charges of the atoms of the paper.
In industry, the use of sources of 241Am, alpha emitter, in the form of ribbons placed at the end of production machines (of paper, plastics, synthetic textiles) a few millimeters from the material allows, by making the surrounding air conductive , suppress the build-up of static electricity.
Some materials are said to be electrically conductive (metals, salt water, human body, graphite, etc.), when they allow electrical charges to move easily.
When walking on a carpet, the friction of the feet on the ground tears electrons away and the body becomes charged with static electricity. If you then touch a metal door handle, you feel a small electrostatic discharge, potentially accompanied by a spark, caused by the sudden displacement of the electrical charges which flow from the body to the ground through the conductive materials of the door.
This flow, or current, is due to the fact that there is at this moment a difference in electric charges between the body and the ground; this difference in charges is a potential difference; the sensation felt comes from the electric current generated by the potential difference existing between the handle and the human body. We deduce that:
- the carpet is an electric voltage generator and an insulator;
- the human body and the door handle are conductors of electricity.
To create an electric current, therefore, a circuit of conductive materials is needed, which allows electric charges to move, and a system capable of creating a potential difference between the two ends of the circuit. This system is called a generator: it can be, for example, a battery, a dynamo or an alternator.
Direction of current
In an electric circuit, we say that the electric current, noted I, circulates between the electrodes from the positive pole to the negative pole of the generator. This sense is purely conventional, since the current can just as well be caused by positive charges (lack of electrons), which will be attracted by the negative pole of the generator, as by negative charges (electrons) which will move in the opposite direction, towards the positive pole. However, we are mainly interested in the movement of electrons, which are the only ones able to move (except in radioactive materials in the process of decay).
In some cases, positive and negative charges move at the same time and this double displacement is responsible for the overall electric current. This is the case in ionic solutions, where cations and anions move in opposite directions, and in semiconductors like a diode, where electrons and “holes” do the same. The charges cannot all move under the action of the electric field and it is thus that in an electric wire, the positive charges (the nuclei of the atoms) remain fixed in the structure of the metal and cannot constitute any electric current; the electric current in a metal is created only by the displacement of the negative charges (the free electrons) towards the positive pole of the generator: it is an electronic current, however, one uses in all the cases the conventional direction “I” of the current , instituted before the discovery of the negative charge of the electron.
We speak of direct current when the direction remains constant and of alternating current when it changes periodically. The frequency of an alternating current is the number of periods per second. It is expressed in hertz (Hz), for example, the current distributed in electrical installations is at a frequency: 50 Hz in Europe and 60 Hz in the United States.
To understand certain properties of electric current, it is interesting to compare it to water flowing in a circuit of pipes. The generator can then be seen as a pump responsible for pressurizing the liquid in the pipes.
The potential difference, or voltage, then resembles the pressure difference between two points in a water circuit. It is denoted “U”, and is expressed in volts (V).
The intensity of the electric current can be likened to the flow of water in the pipe. It gives an account of the number of charges which pass each second in a point of the circuit; it is often noted “I”, and measured in amperes (A). In other words the electric voltage would be the height of a waterfall and its intensity the diameter of the waterfall.
The resistance of an electrical circuit would then be the analog of the diameter of the pipes. The smaller the pipes, the more pressure is required to obtain the same flow rate; similarly, the higher the resistance of a circuit, the more a high potential difference is required to have the same intensity. Electrical resistance accounts for the ability of a material to more or less oppose the flow of current. It is denoted “R” and it is expressed in ohms (Ω).
It is possible to push this analogy much further but it has its limits and certain properties of electric current deviate significantly from this model based on a fluid, pipes, and pumps.