Electricity
Electric current

Electric current

Electric current

Electric current is the transport of electric charge. In an electrical network, this transport is mainly carried out through the movement of electrons through conductors and semiconductors under the influence of a potential difference. In the field of electricity, the movement of ions in an electrolyte or plasma also causes an electric current. In all these cases, cargo transportation is done by moving the cargo carriers. In addition, an electric current is also created as a change in the electrical flow, such as between the plates of a capacitor during charging and discharging, without moving the charge carriers.

The intensity of the electric current is measured in amps (A), such as the amount of charge displaced per unit of time, that is, in coulomb (C) per second (s): 1 A = 1 C / s.

In dilute gases, electrolyte solutions and molten electrolytes, positive and negative ions move in opposite directions; In a metal conductor, negatively charged electrons move from the negative pole (surplus of electrons) to the positive pole (electron deficiency).

Direction and force of the electric current

Traditionally, electric current is expressed as the displacement of the positive charge. When it was learned that the electric current is usually caused by electrons moving in the opposite direction, the electron was assigned, by definition, a negative charge. The old definition of flow direction, therefore, remained in force.

The electric current is usually shown with the letter I (current intensity) and can be described as the displacement of the electric charge per unit of time. For a constant force current:

Electric current intensity

where:

  • I is the electric current is expressed in amps (A).
  • t the considered time, expressed in seconds (s).
  • Q the time t amount of electric charge moved expressed in coulomb (C).

The intensity of the electric current is measured in amps (symbol: A), and informally it is also known as amperage by analogy with the voltage of the electric voltage.

Current measurement

An ammeter is used to measure electrical current. The ammeter is connected in series with the circuit to be measured. In order not to influence the circuit to be measured too much, the instrument must cause the least possible voltage loss.

Another method of measuring electric current is with a current clamp or an ammeter clamp. This is a measuring instrument for alternating current, designed as a clamp that is held around a conductor that carries current. In the clamps, the magnetic field created around the conductor induces a current, which is a measure of the current flowing through the subject conductor.

Different types of electric currents

If the charged particles move within the macroscopic bodies in relation to a particular medium, that current is called a conductive electrical current. If the particles move macroscopic charged bodies (for example, charged raindrops), this current is called convection.

We must distinguish between direct and alternating electric currents, as well as all types of alternating current. In such concepts, the word "electric" is often omitted.

DC

The direct current is a current whose direction and magnitude do not change over time. The electricity provided by the solar panels in a photovoltaic solar energy installation is obtained in direct current.

Alternating current

Alternating current is an electric current that varies over time. Any current that is not constant would fall within this classification. The current inverters in a photovoltaic solar installation are responsible for converting the electricity obtained thanks to the photovoltaic effect into alternating current.

Sinusoidal current

The sine current is a periodic electric current, which is a sinusoidal function of time. Among the alternating currents, the main one is the current, whose value varies according to a sinusoidal law. In this case, the potential of each end of the conductor changes with respect to the potential of the other end of the conductor alternately from positive to negative and vice versa, passing through all intermediate potentials (including zero potential).

The result is a current that changes direction continuously: when it moves in one direction, it increases, reaching a maximum, called the amplitude value, then decreases, at some point it becomes equal to zero, then it increases again, but in the other direction and also reaches its maximum value. Subsequently, it decreases to return to zero, after which the cycle of all changes resumes.

Quasi-stationary current

The quasi-stationary current is "a relatively slow alternating current, for whose instantaneous values ​​the laws of constant currents are met with sufficient precision. These laws are Ohm's law, Kirchhoff's rules and others. The quasi-stationary current, such as the current continuous, it has the same current intensity in all sections of an unbranched circuit.

Induction capacitance and inductance are taken as grouped parameters. Normal industrial currents are almost stationary, with the exception of currents in long-distance transmission lines in which the quasi-stationary condition along the line is not met. Electromagnetic disturbances propagate along the electrical circuit with the speed of light, therefore, for currents that change periodically. For example, an industrial frequency current of 50 Hz is almost stationary for circuits up to 100 km long.

High frequency electric current

A high frequency electric current is an alternating current (from a frequency of approximately tens of kHz), for which phenomena, such as electromagnetic radiation and the effect of the skin, become significant. In addition, if the wavelength of the alternating current radiation becomes comparable with the dimensions of the electrical circuit elements, the quasi-stationary condition is violated, which requires special approaches to the calculation and design of said circuits.

Eddy Currents

Foucault currents (eddy currents) are closed electric currents in a massive conductor that occur when the magnetic flux that penetrates it changes, so eddy currents are induction currents. The faster the magnetic flux changes, the stronger the stray currents will be. Foucault currents do not flow along certain paths in the wires, but when closed in a conductor they form Eddy contours.

The existence of parasitic currents leads to an effect on the skin, that is, the fact that alternating electric current and magnetic flux propagate mainly in the surface layer of the conductor. The eddy current of the conductors leads to energy losses, especially in the coils of the AC coils.

To reduce energy losses by stray currents, the division of AC magnetic circuits into separate plates isolated from each other and located perpendicular to the direction of stray currents is used, which limits the possible contours of their paths and reduces greatly the magnitude of these electric currents. At very high frequencies, instead of ferromagnets, magneto-insulators are used for magnetic circuits, in which, due to the very high resistance, the parasitic currents practically do not occur.

Other types of electric current

The ripple current is a periodic electric current, whose average value over a period is different from zero.

Unidirectional current is an electric current that does not change its direction.

The periodic current is an electric current, whose instantaneous values ​​are repeated at regular intervals in a sequence without changes.

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Last review: October 7, 2019