Electric current



We have two definitions of electricity depending on whether ns refer to physical phenomena on a macroscopic scale or on a microscopic scale.

With the term electricity we refer generically to all physical phenomena in a macroscopic scale that involves one of the fundamental interactions, the electromagnetic force, with particular reference to electrostatics. At the microscopic level, these phenomena are due to the interaction between charged particles on a molecular scale: the protons in the nucleus of atoms or ionized molecules and the electrons. The typical macroscopic effects of such interactions are electric currents and the attraction or repulsion of electrical bodies or charges.

Electricity is the form of energy that produces light effects, mechanical, calorific, chemical, etc., and that is due to the separation or movement of the electrons that make up the atoms.

Electricity is responsible for well-known physical phenomena, such as lightening or electrification, and is the essential element of some industrial applications, such as electronics and electrical engineering through electrical signals. At the same time, electricity has become the most widespread means of transport for energy in electricity networks and one of the most widespread means of transport for information in telecommunications (electrical communications). Electricity has become the symbol of the modern world: housing lighting, makes factories work and makes distant people shut down.

The ultimate objective of photovoltaic solar energy is to generate electricity through photovoltaic solar panels. These panels are composed of photovoltaic cells that, through the photovoltaic effect, generate a small electrical current and, therefore, electricity.

Electric charge

The electric charge is one of those entities that can be measured and used, but it can not be defined in easily understandable terms, since, like space, time and mass, it is not easy to give an exhaustive definition. Perhaps the best way to define it is to observe its effects.

An object equipped with an electric charge exerts a force at a distance on another object that has an electric charge. Unlike gravity, which causes one object to attract another, objects with an electric charge can attract and repel each other. In addition, gravity is directly related to the mass of the objects in question, while the electric charge and the mass are not related when the objects are immobile.

Experiments show that there are two different types of electric charge. The first of these is called positive charge or charge +, and is associated with the nuclei of atoms of all chemical elements. The second is the negative charge or -, and is typical of all the electrons that surround the nucleus of the atom. In general, the positive charge of the nucleus is exactly equal to the sum of the negative charges of the electrons that surround it.

The direction of the forces, which act between objects that have an electric charge, depends on the type of charge on these objects. For example, if two objects have the same type of charge, both are positive or both negative, they repel each other. When the two objects have opposite charge, they attract each other. This force of electric attraction, between the positive nuclei and the negative electrons, joins the latter to the nucleus.

The total amount of electric charges remains virtually constant in the world. Since the two types of charge have opposite effects, the general normal result is electrical neutrality or the apparent lack of charge. Therefore, in order to observe the effects of loading on fairly large quantities of material, it will be necessary to disturb the normal equilibrium and produce an excess load on the object in a desired manner.

Electric charge in matter

Numerous solid substances have a crystalline structure, that is, their atoms are arranged in a regular three-dimensional grid. However, in some substances, the electrons surrounding these nuclei are not tightly bound.

Under certain conditions, it is possible to add or remove a good amount of electrons without seriously disturbing the crystalline structure. In other words, atomic nuclei tend to remain fixed in their position, but electrons can often move. To give a negative charge, only the excess of electrons must be added. However, in relation to the positive and negative charge, it should be remembered that the plus and minus are indicative of an electrical state, not indicators of mathematical operations, as in arithmetic or algebra. When we see a negative sign applied to a charge, we must remember that it only indicates an excess of electrons and has nothing to do with subtraction.

From an electrical point of view, it is possible to classify approximately all the substances that make up the matter in two large groups. The types of substances that contain a relatively large amount of free electrons, which can move from one atom to another, are called electrical conductors. Substances in which electrons are not free to move under moderate stress are called electrical insulators.

Most metals are conductors of electricity, although differently from conductors used by the chemical sector, such as aqueous solutions of acids, bases or salts. On the other hand, most non-metallic substances are electrically insulating. There is neither a perfect conductor nor a perfect insulator, but in practice, a certain number of substances serve very well for this purpose. For example, silver, copper, aluminum and even steel are often suitable as conductors, while glass, porcelain, most plastics, dry air and wood are good insulators. In recent decades, the study of matter has led to the creation of materials that, in extreme conditions, manage to be superconducting.

Definition of electricity and magnetism

The space around an electron or any other object that has an electric charge seems to be in a state of tension, called an electric field. This is what interferes with the electric fields of other electrically charged objects and causes the mutual forces typical of such objects. But if a movement is made to the electrons, their path is surrounded by another new field, called the magnetic field. The intensity of this field is directly proportional both to the number of electrons in motion and to the speed at which they move, that is, to the current.

Therefore, if a current is passed through a coil, that is, a set of coils conveniently arranged, of copper wire, this coil of wire will behave like a steel magnet, attract or repel other similar spools of thread. By winding such a coil in an iron or core structure, it will reinforce the magnetic field produced. If you have several coils of wire around an iron core, free to rotate, placing them in the high intensity field of a series of fixed coils, traversed by the current, they will provide substantial mechanical forces. These will rotate the mobile reels, which will perform mechanical work.

This device is called an electric motor. Currently, electric motors operate all kinds of machinery, from the delicate exercises of the dentist to the gigantic machines of modern factories. There can be many electric motors in a modern house, from the oil boiler to the refrigerator, etc.

Electricity by alternating and direct current

Up to this point, it has been mentioned that, in any given circuit, electrons always move in the same direction within it. A system or circuit of the type mentioned above is referred to as a direct current or continuous system. An example of this circuit is given by any circuit powered by a battery, for example, a magnesium flash or an electric system in automobiles. Sometimes, however, the current does not remain constant, both in terms of force and meaning. Numerous electrical circuits are used in which the current regularly reverses the direction of its flow in the circuit.

This type of circuit is called alternating current. The most common and most used electrical circuits are AC. In an AC circuit, the frequency must also be specified, in addition to specifying the intensity of the current and the voltage of the circuit, as is sufficient for the DC circuit. The frequency measures half the number of times the current changes direction in a second.

Where the current and the voltage change, as it happens continuously in the AC circuits, it is necessary to consider the effect of the reactance. As already mentioned, the current always generates a magnetic field. When the current changes, the magnetic field caused by it changes and this causes a back electromotive force. Therefore, in an AC circuit, the applied voltage must overcome the opposition of the varying magnetic field, in addition to the common resistance of the circuit.

The opposition found by alternating current is called inductive reactance, and it is due to the change of its magnetic field. As we have seen, electrons always repel each other, following the reciprocal action of their electric fields. Therefore, an electron moving in one conductor may force those in another to move, even if the two conductors are isolated from each other.

Therefore, it can happen that an alternating current can flow even through a perfect insulator, whereas a continuous one can not do it (of course, no electron actually moves through the insulator, but it is its interacting electric fields that produce the displacements mentioned above.). This interesting effect is exploited in devices called capacitors, often used for AC circuits. Therefore, an alternating current can apparently flow through a capacitor but not without encountering some opposition.

Opposition to the flow of alternating current due to the action of the capacitor is called capacitive reactance. The inductive reactance, the capacitive reactance and the resistance of a circuit are called, as a whole, the impedance of a circuit. By controlling the amount of inductive and capacitive reactance in a circuit, some interesting effects can be observed. One of the most important effects is resonance. Thanks to this effect, the circuit can be made to resonate, that is, crossed by an alternating current of a particular frequency, ignoring absolutely other frequencies that may also be present. It is thanks to the use of resonance that the radio or TV can be adjusted at a particular broadcast station, excluding others.

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Last review: May 19, 2019