A huge variety of physical phenomena, both microscopic and macroscopic, are electromagnetic in nature. These include forces of friction and elasticity, all chemical processes, electricity, magnetism, optics.
One of such manifestations of electromagnetic interaction is the ordered movement of charged particles. It is an absolutely necessary element of almost all modern technologies that are used in various fields - from the organization of our life to space flights.
General concept of the phenomenon
The ordered movement of charged particles is called electric current. Such movement of charges can be carried out in different media by means of certain particles, sometimes quasi-particles.
A prerequisite for the current isprecisely orderly, directed movement. Charged particles are objects that (as well as neutral ones) have thermal chaotic motion. However, the current occurs only when, against the background of this continuous chaotic process, there is a general movement of charges in some direction.
When a body moves, electrically neutral as a whole, the particles in its atoms and molecules, of course, move in a direction, but since opposite charges in a neutral object compensate each other, there is no charge transfer, and we can talk about the current does not make sense in this case either.
How the current is generated
Consider the simplest version of direct current excitation. If an electric field is applied to a medium where charge carriers are present in the general case, an ordered motion of charged particles will begin in it. The phenomenon is called charge drift.
It can be briefly described as follows. At different points of the field, a potential difference (voltage) arises, that is, the energy of interaction of electric charges located at these points with the field, related to the magnitude of these charges, will be different. Since any physical system, as is known, tends to a minimum of potential energy corresponding to the equilibrium state, charged particles will begin to move towards equalization of potentials. In other words, the field does some work to move these particles.
When the potentials are equalized, the tension vanisheselectric field - it disappears. At the same time, the ordered movement of charged particles, the current, also stops. In order to obtain a stationary, that is, time-independent field, it is necessary to use a current source in which, due to the release of energy in certain processes (for example, chemical), charges are continuously separated and fed to the poles, maintaining the existence of an electric field.
Current can be obtained in various ways. So, a change in the magnetic field affects the charges in the conducting circuit introduced into it and causes their directed movement. Such a current is called inductive.
Quantitative characteristics of current
The main parameter by which the current is described quantitatively is the strength of the current (sometimes they say "value" or simply "current"). It is defined as the amount of electricity (the amount of charge or the number of elementary charges) passing per unit time through a certain surface, usually through the cross section of a conductor: I=Q / t. The current is measured in amperes: 1 A \u003d 1 C / s (coulomb per second). In the section of the electrical circuit, the current strength is directly related to the potential difference and inversely - to the resistance of the conductor: I \u003d U / R. For a complete circuit, this dependence (Ohm's law) is expressed as I=Ԑ/R+r, where Ԑ is the electromotive force of the source and r is its internal resistance.
The ratio of the current strength to the cross section of the conductor through which the ordered movement of charged particles occurs perpendicular to it is called the current density: j=I/S=Q/St. This value characterizes the amount of electricity that flows per unit of time through a unit area. The higher the field strength E and the electrical conductivity of the medium σ, the greater the current density: j=σ∙E. Unlike the current strength, this quantity is vector, and has a direction along the movement of particles that carry a positive charge.
Current direction and drift direction
In an electric field, objects carrying a charge, under the influence of Coulomb forces, will make an ordered movement to the pole of the current source, opposite in sign of charge. Particles charged positively drift towards the negative pole ("minus") and, conversely, free negative charges are attracted to the "plus" of the source. Particles can also move in two opposite directions at once if there are charge carriers of both signs in the conducting medium.
For historical reasons, it is generally accepted that the current is directed the way positive charges move - from "plus" to "minus". To avoid confusion, it should be remembered that although in the most familiar case of current in metal conductors, the real movement of particles - electrons - occurs, of course, in the opposite direction, this conditional rule always applies.
Current propagation and drift speed
Often there are problems with understanding how fast the current is moving. Two different concepts should not be confused: the speed of propagation of current (electricsignal) and the drift velocity of particles - charge carriers. The first is the speed at which the electromagnetic interaction is transmitted or - which is the same - the field propagates. It is close (taking into account the propagation medium) to the speed of light in vacuum and is almost 300,000 km/s.
Particles make their orderly movement very slowly (10-4–10-3 m/s). The drift velocity depends on the intensity with which the applied electric field acts on them, but in all cases it is several orders of magnitude inferior to the velocity of thermal random motion of particles (105–106m/s). It is important to understand that under the action of the field, the simultaneous drift of all free charges begins, so the current appears immediately in the entire conductor.
Types of current
First of all, currents are distinguished by the behavior of charge carriers over time.
- A constant current is a current that does not change either the magnitude (strength) or the direction of particle movement. This is the easiest way to move charged particles, and it is always the beginning of the study of electric current.
- In alternating current, these parameters change with time. Its generation is based on the phenomenon of electromagnetic induction that occurs in a closed circuit due to a change (rotation) of the magnetic field. The electric field in this case periodically reverses the intensity vector. Accordingly, the signs of the potentials change, and their value passes from "plus" to "minus" all intermediate values, including zero. As a resultphenomenon, the ordered movement of charged particles changes direction all the time. The magnitude of such a current fluctuates (usually sinusoidally, that is, harmonically) from a maximum to a minimum. Alternating current has such an important characteristic of the speed of these oscillations as frequency - the number of complete cycles of change per second.
In addition to this most important classification, differences between currents can also be made according to such a criterion as the nature of the movement of charge carriers in relation to the medium in which the current propagates.
Conduction currents
The most famous example of a current is the ordered, directed movement of charged particles under the action of an electric field inside a body (medium). It is called conduction current.
In solids (metals, graphite, many complex materials) and some liquids (mercury and other metal melts), electrons are mobile charged particles. An ordered movement in a conductor is their drift relative to the atoms or molecules of a substance. Conductivity of this kind is called electronic. In semiconductors, charge transfer also occurs due to the movement of electrons, but for a number of reasons it is convenient to use the concept of a hole to describe the current - a positive quasiparticle, which is a moving electron vacancy.
In electrolytic solutions, the passage of current is carried out due to the negative and positive ions moving to different poles - the anode and cathode, which are part of the solution.
Transfer currents
Gas - under normal conditions a dielectric - can also become a conductor if subjected to a sufficiently strong ionization. Gas electrical conductivity is mixed. An ionized gas is already a plasma in which both electrons and ions, that is, all charged particles, move. Their ordered movement forms a plasma channel and is called a gas discharge.
Directed movement of charges can occur not only inside the environment. Suppose a beam of electrons or ions is moving in vacuum, emitted from a positive or negative electrode. This phenomenon is called electron emission and is widely used, for example, in vacuum devices. Of course, this movement is a current.
Another case is the movement of an electrically charged macroscopic body. This is also a current, since such a situation satisfies the condition of directed charge transfer.
All the above examples should be considered as an ordered movement of charged particles. This current is called convection or transfer current. Its properties, for example, magnetic, are completely similar to those of conduction currents.
Bias current
There is a phenomenon that has nothing to do with charge transfer and occurs where there is a time-varying electric field that has the property of "real" conduction or transfer currents: it excites an alternating magnetic field. This isoccurs, for example, in alternating current circuits between the plates of capacitors. The phenomenon is accompanied by the transfer of energy and is called displacement current.
In fact, this value shows how quickly the electric field induction changes on a certain surface perpendicular to the direction of its vector. The concept of electrical induction includes the vectors of field strength and polarization. In a vacuum, only tension is taken into account. As for electromagnetic processes in matter, the polarization of molecules or atoms, in which, when exposed to a field, the movement of bound (not free!) Charges takes place, makes some contribution to the displacement current in a dielectric or conductor.
The name originated in the 19th century and is conditional, since a real electric current is an ordered movement of charged particles. Displacement current has nothing to do with charge drift. Therefore, strictly speaking, it is not a current.
Manifestations (actions) of current
Ordered movement of charged particles is always accompanied by certain physical phenomena, which, in fact, can be used to judge whether this process is taking place or not. It is possible to divide such phenomena (current actions) into three main groups:
- Magnetic action. A moving electric charge necessarily creates a magnetic field. If you place a compass next to a conductor through which current flows, the arrow will turn perpendicular to the direction of this current. Based on this phenomenon, electromagnetic devices operate, allowing, for example, to convert electrical energyinto mechanical.
- Thermal effect. The current does work to overcome the resistance of the conductor, resulting in the release of thermal energy. This is because, during the drift, charged particles experience scattering on the elements of the crystal lattice or conductor molecules and give them kinetic energy. If the lattice of, say, a metal were perfectly regular, the electrons would practically not notice it (this is a consequence of the wave nature of the particles). However, firstly, the atoms in the lattice sites themselves are subject to thermal vibrations that violate its regularity, and secondly, lattice defects - impurity atoms, dislocations, vacancies - also affect the movement of electrons.
- Chemical action is observed in electrolytes. Oppositely charged ions, into which the electrolytic solution is dissociated, when an electric field is applied, are separated to opposite electrodes, which leads to chemical decomposition of the electrolyte.
Except when the ordered motion of charged particles is the subject of scientific research, it interests a person in its macroscopic manifestations. It is not the current itself that is important for us, but the phenomena listed above, which it causes, due to the transformation of electrical energy into other forms.
All current actions play a dual role in our lives. In some cases, it is necessary to protect people and equipment from them, in others, obtaining one or another effect caused by the directed transfer of electric charges is direct.purpose of a wide variety of technical devices.
