If there are free charge carriers in any medium (for example, electrons in a metal), then they are not at rest, but move randomly. But you can make the electrons move in an orderly fashion in a given direction. This directed movement of charged particles is called electric current.
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How electric current is generated
If we take two conductors, and one of them is charged negatively (adding electrons to it), and the other is charged positively (taking away some of the electrons from it), an electric field will arise. If you connect both electrodes with a conductor, the field will force the electrons to move in the opposite direction of the electric field vector, in accordance with the direction of the electric force vector. Negatively charged particles will move from the electrode where they are in excess to the electrode where they are deficient.
For the occurrence of electron movement, it is not necessary to impart a positive charge to the second electrode. The main thing is that the negative charge of the first one is higher. It is even possible to charge both conductors negatively, but one conductor must have a charge greater than the other. In this case, one speaks of a potential difference that causes an electric current.
By analogy with water, if you connect two vessels filled with water to different levels, a stream of water will appear. Its pressure will depend on the difference in levels.
It is interesting that the chaotic motion of electrons under the action of an electric field is generally preserved, but the general vector of motion of the mass of charge carriers acquires a directed character. If the "chaotic" component of motion has a speed of several tens or even hundreds of kilometers per second, then the directional component is several millimeters per minute. But the impact (when the electrons move along the length of the conductor) propagates at the speed of light, so they say that the electric current moves at a speed of 3 * 108 m/sec.
In the framework of the above experiment, the current in the conductor will not exist for long - until the excess electrons in the negatively charged conductor run out, and their number at both poles is not balanced. This time is small - insignificant fractions of a second.
Going back to the initially negatively charged electrode and creating an excess charge on the carriers does not give the same electric field that moved the electrons from minus to plus. Therefore, there must be an external force acting against the strength of the electric field and surpassing it.Similar to water, there must be a pump that pumps water back to the upper level to create a continuous flow of water.
Current direction
The direction from plus to minus is taken as the direction of the current, that is, the direction of movement of positively charged particles is opposite to the movement of electrons. This is due to the fact that the very phenomenon of electric current was discovered much earlier than an explanation of its nature was received, and it was believed that the current goes in this direction. By that time, a large number of articles and other literature on this topic had accumulated, concepts, definitions and laws appeared. In order not to revise a huge amount of already published material, we simply took the direction of the current against the flow of electrons.
If the current flows all the time in one direction (even changing in strength), it is called direct current. If its direction changes, then we are talking about alternating current. In practical application, the direction changes according to some law, for example, according to a sinusoidal one. If the direction of the current flow remains unchanged, but it periodically drops to zero and increases to a maximum value, then we are talking about a pulsed current (of various shapes).
Necessary conditions for maintaining electric current in the circuit
Three conditions for the existence of an electric current in a closed circuit are derived above. They need to be considered in more detail.
Free charge carriers
The first necessary condition for the existence of an electric current is the presence of free charge carriers. Charges do not exist separately from their carriers, so it is necessary to consider particles that can carry a charge.
In metals and other substances with a similar type of conductivity (graphite, etc.), these are free electrons. They weakly interact with the nucleus, and can leave the atom and move relatively unhindered inside the conductor.
Free electrons also serve as charge carriers in semiconductors, but in some cases they speak of "hole" conductivity of this class of solids (as opposed to "electronic"). This concept is needed only to describe physical processes, in fact, the current in semiconductors is the same movement of electrons. Materials in which electrons cannot leave an atom are dielectrics. There is no current in them.
In liquids, positive and negative ions carry charge. This refers to liquids - electrolytes. For example, water in which salt is dissolved. By itself, water is electrically fairly neutral, but when solid and liquid substances enter it, they dissolve and dissociate (decompose) to form positive and negative ions. And in molten metals (for example, in mercury), the charge carriers are the same electrons.
Gases are mostly dielectrics. There are no free electrons in them - gases consist of neutral atoms and molecules. But if the gas is ionized, they speak of the fourth state of aggregation of matter - plasma. An electric current can also flow in it, it occurs with the directed movement of electrons and ions.
Also, current can flow in a vacuum (the action of, for example, vacuum tubes is based on this principle). This will require electrons or ions.
Electric field
Despite the presence of free charge carriers, most media are electrically neutral. This is explained by the fact that negative (electrons) and positive (protons) particles are located evenly, and their fields compensate each other. For a field to arise, the charges must be concentrated in some area. If electrons have accumulated in the region of one (negative) electrode, then there will be a shortage of them on the opposite (positive) electrode, and a field will arise that creates a force acting on charge carriers and forcing them to move.
Third party force to carry charges
And the third condition - there must be a force that carries charges in the direction opposite to the direction of the electrostatic field, otherwise the charges inside the closed system will quickly balance. This extraneous force is called the electromotive force. Its origin may be different.
Electrochemical nature
In this case, the EMF arises as a result of the occurrence of electrochemical reactions. Reactions may be irreversible. An example is a galvanic cell - a well-known battery. After the reagents are exhausted, the EMF drops to zero, and the battery "sits down".
In other cases, reactions may be reversible. So, in a battery, EMF also occurs as a result of electrochemical reactions. But upon completion, the process can be resumed - under the influence of an external electric current, the reactions will take place in the reverse order, and the battery will again be ready to give current.
photovoltaic nature
In this case, the EMF is caused by the action of visible, ultraviolet or infrared radiation on processes in semiconductor structures. Such forces arise in photocells (“solar batteries”).Under the action of light, an electric current is generated in the external circuit.
thermoelectric nature
If you take two dissimilar conductors, solder them and heat the junction, then an EMF will appear in the circuit due to the temperature difference between the hot junction (the junction of the conductors) and the cold junction - the opposite ends of the conductors. In this way, it is possible not only to generate current, but also measure the temperature by measuring the emerging emf.
Piezoelectric nature
Occurs when some solids are compressed or deformed. An electric lighter works on this principle.
Electromagnetic nature
The most common way to generate electricity industrially is with a DC or AC generator. In a DC machine, a frame-shaped armature rotates in a magnetic field, crossing its lines of force. In this case, an EMF arises, depending on the speed of rotation of the rotor and the magnetic flux. In practice, an anchor is used from a large number of turns, forming a plurality of series-connected frames. EMF arising in them add up.
AT alternator the same principle applies, but a magnet (electric or permanent) rotates inside the fixed frame. As a result of the same processes in the stator, EMF, which has a sinusoidal shape. On an industrial scale, AC generation is almost always used - it is easier to convert it for transportation and practical use.
An interesting property of a generator is reversibility.It consists in the fact that if voltage is applied to the generator terminals from an external source, its rotor will begin to rotate. This means that, depending on the connection scheme, the electric machine can be either a generator or an electric motor.
These are just the basic concepts of such a phenomenon as electric current. In fact, the processes that occur during the directed movement of electrons are much more complicated. To understand them, a deeper study of electrodynamics is required.
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