A huge number of physical phenomena, both microscopic and macroscopic, are of electromagnetic nature. These include the forces of friction and elasticity, all chemical processes, electricity, magnetism, optics.
One such manifestation of electromagnetic interaction is the ordered motion of charged particles. It is an absolutely necessary element of almost all modern technologies that are used in a wide variety of fields - from the organization of our everyday life to space flights.
The 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 quasiparticles.
A prerequisite for the current is precisely ordered, directed movement. Charged particles are objects that (as well as neutral ones) possess thermal chaotic motion. However, current arises only when, against the background of this continuous random process, a general movement of charges in a certain direction occurs.
When moving a body that is generally electrically neutral, the particles in its atoms and molecules, of course, move directionally, but since the opposite charges in the neutral object cancel each other out, there is no charge transfer, and talk about the current in this case also doesn't make sense.
How does the current arise
Consider the simplest version of direct current excitation. If an electric field is applied to a medium where, in the general case, charge carriers are present, an ordered movement of charged particles will begin in it. The phenomenon is called charge drift.
Briefly, it can be described as follows. At different points of the field, a potential difference (voltage) occurs, that is, the interaction energy of the electric charges located at these points with the field, related to the magnitude of these charges, will be different. Since every 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 electric field intensity vanishes - it disappears. At the same time, the ordered movement of charged particles — the current — also ceases. In order to obtain a stationary field, that is, independent of time, it is necessary to use a current source in which, thanks to the release of energy in certain processes (e.g. chemical), the charges are continuously separated and fed to the poles, supporting 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 conductive circuit introduced into it and causes their directional movement. This current is called induction.
Quantitative characteristics of current
The main parameter with which the current is quantified is the current strength (sometimes they say “magnitude” or simply “current”). It is defined as the amount of electricity (the magnitude of the charge or the number of elementary charges) passing per unit time through a surface, usually through the cross section of the conductor: I = Q / t. Measured current in amperes: 1 A = 1 C / s (pendant per second). In the section of the electric circuit, the current strength is directly related to the potential difference and the reverse to the conductor resistance: I = 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 perpendicularly occurs is called the current density: j = I / S = Q / St. This value characterizes the amount of electricity that flows per unit 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 vectorial and has a direction in the motion of particles carrying 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 toward the opposite pole of the charge of the current source. Particles charged positively drift towards the negative pole ("minus") and, conversely, free negative charges are attracted to the "plus" of the source. Particles can move in two opposite directions at once if charge carriers of both signs are present in the conducting medium.
For historical reasons, it is generally accepted that the current is directed in 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 actual 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 moves. Two different concepts should not be confused: the velocity of propagation of the current (electrical signal) 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 thing - 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 ordered motion 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 speed of thermal random motion of particles (10 5 –10 6 m / s). It is important to understand that under the influence of the field, the simultaneous drift of all free charges begins, so the current occurs immediately in the entire conductor.
Types of current
First of all, currents are distinguished by the behavior of charge carriers in time.
- Constant is a current that does not change either the magnitude (force) or the direction of movement of particles. This is the easiest way to move charged particles, and they always start to study the electric current.
- For alternating current, these parameters change over time. The basis of its generation is the phenomenon of electromagnetic induction arising in a closed loop due to a change (rotation) of the magnetic field. In this case, the electric field periodically changes the tension vector to the opposite. Accordingly, the signs of the potentials change, and their magnitude passes from the "plus" to the "minus" all intermediate values, including zero. As a result of this phenomenon, the ordered motion of charged particles changes direction all the time. The magnitude of such a current fluctuates (usually sinusoidally, that is, harmoniously) from maximum to 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 be made according to such a criterion as the nature of the motion of charge carriers with respect to the medium in which the current propagates.
Conduction currents
The most famous example of current is the ordered, directed movement of charged particles under the influence of an electric field inside a body (medium). It is called the conduction current.
In solids (metals, graphite, many complex materials) and some liquids (mercury and other metal melts), electrons are mobile charged particles. The ordered movement in a conductor is their drift relative to atoms or molecules of a substance. Conductivity of this kind is called electronic. In semiconductors, charge transfer also occurs due to the motion of electrons, but for a number of reasons it is convenient to use the concept of a hole, a positive quasiparticle, which is a moving electron vacancy, to describe the current.
In electrolytic solutions, the passage of current is due to the negative and positive ions that make up the solution moving to different poles - the anode and cathode.
Transfer Currents
A gas - under ordinary conditions a dielectric - can also become a conductor if it is subjected to sufficiently strong ionization. Gas conductivity is mixed. An ionized gas is already a plasma in which electrons and ions move, that is, all charged particles. Their ordered movement forms a plasma channel and is called a gas discharge.
The directed movement of charges can occur not only inside the medium. Suppose, in a vacuum, a beam of electrons or ions is emitted from a positive or negative electrode. This phenomenon is called electron emission and is widely used, for example, in vacuum devices. Of course, such a movement is a current.
Another case is the movement of an electrically charged macroscopic body. This is also a current, since a similar situation satisfies the condition of directed charge transfer.
All the above examples should be considered as the ordered motion of charged particles. Such a current is called convection or transfer current. Its properties, for example, magnetic, are completely analogous to those of conduction currents.
Bias current
There is a phenomenon that is not related to charge transfer and arises where there is a time-varying electric field, which has the property inherent in “real” conduction or transfer currents: it excites an alternating magnetic field. This happens, for example, in AC circuits between the capacitor plates. The phenomenon is accompanied by the transfer of energy and is called the bias current.
In fact, this value shows how quickly the induction of the electric field on a surface perpendicular to the direction of its vector changes. The concept of electric induction includes vectors of field strength and polarization. In a vacuum, only tension is taken into account. As for the electromagnetic processes in matter, the polarization of molecules or atoms, in which the action of the field causes the movement of bound (not free!) Charges, makes some contribution to the bias current in the dielectric or conductor.
The name arose in the 19th century and is conditional in nature, since the actual electric current is the ordered movement of charged particles. The bias current is not related to the charge drift. Therefore, strictly speaking, it is not a current.
Manifestations (actions) of current
The ordered movement of charged particles is always accompanied by certain physical phenomena, by which, in fact, one can judge whether this process is taking place or not. Such phenomena (current effects) can be divided into three main groups:
- Magnetic action. A moving electric charge necessarily creates a magnetic field. If you place the compass next to the conductor through which the current flows, the arrow will rotate perpendicular to the direction of this current. Based on this phenomenon, electromagnetic devices operate, which allow, for example, to convert electrical energy into mechanical energy.
- Thermal effect. The current does the work to overcome the resistance of the conductor, resulting in the release of thermal energy. This is because when drifting, charged particles experience scattering on the elements of the crystal lattice or molecules of the conductor and give them kinetic energy. If the lattice of, say, metal, were perfectly correct, the electrons would hardly 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 correctness, and secondly, lattice defects — impurity atoms, dislocations, vacancies — also affect the motion of electrons.
- Chemical action is observed in electrolytes. Opposite-charged ions into which the electrolytic solution is dissociated, when an electric field is applied, are diluted on opposite electrodes, which leads to chemical decomposition of the electrolyte.
Except when the ordered movement of charged particles is the subject of scientific research, it interests a person in his macroscopic manifestations. What is important to us is not the current in itself, but the above phenomena that it causes due to the conversion of electric energy into other types.
All currents play a dual role in our lives. In some cases, it is necessary to protect people and equipment from them, in others - the receipt of a particular effect caused by the directed transfer of electric charges is the direct purpose of a wide variety of technical devices.