The current strength is a physical quantity that characterizes the speed of a charge in a conductor. The process in the conductor is accompanied by the release of a certain amount of energy according to the Joule-Lenz law. The current was discovered by Alessandro Volt on the basis of the experiments of Galvani, and the theory for experimental phenomena was laid down in 1794.
How the electric current is formed
For 2,5 thousand years BC in Ancient Egypt they knew about electric fish and considered them as defenders of aquatic inhabitants. The Greeks and Romans had an idea on this subject, sometimes they tried to use this feature for the treatment of headaches or gout. It is noticed that the charge is perfectly transmitted by metal objects. The first tried to study static electricity for 600 years BC.e. Thales of Miletus. Then they already recognized the property of amber, worn with wool, to attract dissimilar dielectric materials. But the pundit quickly reached an impasse.
The concept of electricity began to develop in the 1600s by William Gilbert, who experimented with magnetic iron ore and rubbed amber. The term originates from the Greek language. Translated electricity means "like amber", showing similar properties. Apparently, the first print publication covering the subject is Thomas Brown’s Pseudodoxia Epidemica, published in 1646.
Further research is going on separately. For example, in 1752, Benjamin Franklin tied a metal key to a kite and threw it into a stormy sky. He saw sparks jumping off his hand and suggested the electric nature of lightning. In Arabic, by the way, the natural phenomenon has long been called the same word with the name of electric rays. Benjamin Franklin believed that any matter contains a fluid, the lack of which is manifested in a negative charge, and the excess - in a positive one. For unknown reasons, glass was attributed to the first type of materials, and rubber to the second. Fluid movement produces electrical current.
As a result of the described assumption, it turned out that the direction of flow of the fluid is opposite to the direction of movement of the electrons. And today in physics, the current is indicated by an arrow pointing in the opposite direction. This movement is not fast and is formed not exclusively by electrons. The speed of the elementary particles is units of centimeters per second. And the electric wave moves much faster. Therefore, the current occurs in the environment and propagates approximately at the speed of light and quickly disappears.
Returning to the experiments with rubber and glass. It is noticed that worn, they attract, but repel a piece of identical material. Thus arose the idea of two types of fluid. Bodies capable of exhibiting properties similar to rubber or glass are called electrically charged. Some materials contain a positive, and some negative fluid, repel glass, but are attracted by rubber, and vice versa.
The current can be transported by electrons( negative particles) or protons( positive particles).Often in the theory of semiconductors the term "hole" is used. This is the place where at a certain moment there is a shortage of electrons. The charge of this carrier is positive. Often there is no difference in which particle the current is formed.
Unit of measurement of electric current
Electric current is represented as the volume of charge transferred per unit of time through a unit of cross-sectional area of a material. Ampere is recognized as the unit of measurement, and the Latin letter I, derived from the French phrase intensite de courant, is used as the designation. This symbol was used by Ampere, whose name is called the unit, although until 1896 private journals continued to use C. There is another definition of ampere in physics:the force of interaction in the area of 1 meter length of 0.2 μN ".
Interpretation is due to the fact that the flowing current creates a magnetic field around the conductor, successfully interacting with others. The process is normalized by Ampere's law, derived in 1820.Initially, the formula included magnetic induction, but then it turned out to be an optional value. It depends on the magnitude of the current, the distance to the point under study and the magnetic constant( physical constant).
Before they did not focus attention, but in everyday life it is much more convenient to use alternating current. It is easier to transfer along the circuits, thanks to the possibility of using transformers that carry out the isolation of individual segments and the transformation of parameters. The frequencies of the industrial network are usually within the range of 50 - 60 Hz, and most people are interested in the causes of the indicators. For example, Nikola Tesla showed that the current frequency of more than 700 Hz practically does not harm the human body, moving along the surface( skin).
The specified effect is widely known in electrical engineering. It is called - superficial( in English skin - skin).The phenomenon is reduced to the fact that the current with increasing frequency penetrates less and less into the thickness of materials. For copper conductors at a frequency of 60 Hz, the depth reaches 8.57 mm. For the reason mentioned, high current conductors are often hollow. Due to the large diameter, the current will never penetrate into the core. Hollow conductors allow you to save on materials and reduce the mass of wires.
Here lies the reason why the industry has not yet moved to a new level. After all, the use of a current of 700 Hz will significantly secure the network for ordinary citizens. Such a step will require a radical review of the design of multiphase engines, significantly increasing their efficiency( to reduce the amount of transmitted power).What is often impossible at the present stage of development of technology.
Alternating current is usually formed in a conductor by changing the direction of the external magnetic field. This happens at the power station. The massive turbine shaft makes a couple of revolutions per second on the force, and the high frequency is generated by switching the stator winding. So changing industry standards is relatively simple. It is rumored that with increasing frequency losses in ferromagnetic materials to eddy currents grow. Moreover, the dependence is quadratic. This is readily believed, the power of induction cookers is often increased by increasing the frequency of the pulses in the power inverter.
In the literature it is said that Nikola Tesla proposed an alternating current of 220 V at 60 Hz as optimal for the operation of its own two-phase motors( invented asynchronous machines, proved that at 60 Hz the maximum economic effect from the use of own developments is achieved).Due to a number of discrepancies in the coordination and lobbying of private interests, the parameters are different in the USA and Europe.
Nicola Tesla is considered the father of AC and asynchronous motors. The mentioned type of movement of charge carriers differs from the constant one: “Alternating current refers to the flow of charge carriers alternately in both directions along the conductor.”
Definition can be attributed to fluids. Alternating current is formed by one charge, then by another. In practice, this is usually referred to as a stream of electrons, twice changing the direction in a period. The frequency of the process is measured in Hz, the graph( particle flux density) is close to a sine wave. In industrial networks there are three phases( the ancestor - M. O. Dolivo-Dobrovolsky, the first to find errors in the theoretical limits on the efficiency of multiphase current motors).Imagine as independent sinusoids shifted evenly relative to each other by 120 degrees. While one chart goes through zero, the second is already going through a third of the period, and the remaining one - two thirds.
Three phases in industrial units allow you to create a rotating magnetic field( the brainchild of Nikola Tesla), which drives the rotors of electric motors. In this case, it is possible to save considerably on the copper of the neutral wire( neutral), most of the current leaves the installation via the phase wires, where the potential is lower at this cycle time. Circuit design of 380 V networks differs significantly from 220.
Phenomena related to electric current
It has already been observed that the electric current flowing through a conductor creates a magnetic field around it. The principle of operation of devices is based on this principle. Among the number mentioned, electric locks are considered the simplest. Creating coils of wire, it is possible to add the effect of the resulting field from each turn. What they use in practice, forming a small current the force of attraction of the armature of the lock of several centners with a ridiculous rated power of ten watts. This is how most intercom systems work.
Similarly, the effect of the appearance of a magnetic field is applied to:
- Loading and unloading of ferrous metal at receiving and processing points.
- A variety of relays.
- Electric engines of all modifications.
The current during the flow through a conductor causes a heating effect. The phenomenon is described by the Joule-Lenz law, which states that the thermal effect is directly proportional to the square of the electric current and the resistance of the conductor. The main misunderstanding of technology by beginners is based on this. At low voltage, most devices to maintain the same power consume more current. A striking example would be the incandescent bulbs, where at the voltage of 27 V the former intensity is achieved only by a tenfold increase in current.
This causes shock overheating of the power cord. According to the Joule-Lenz effect, it turns out that the power depends on the square of the current. And when the latter is increased by 10 times, the thermal effect increases by two orders of magnitude( 100 times).This explains such a high heating of the place of the welding arc, although the power cord remains cold. The transmitted power remains the same, but the voltage on the electrode is much lower than the input 220 V. The effect of temperature rise occurs.
The thermal effect is applied in heaters, where it is considered a side effect, but useful. As for the bulbs with filaments, here most of the energy is wasted. The thread is heated by the flow of electric current, but low energy is converted into light. Mass is transmitted by radiation in the infrared, invisible spectrum. This complexity is solved in energy-saving light bulbs, where the current arcs in a gaseous medium or emits photons, passing through the pn-junction of a special design.
In electric heaters, they try to increase efficiency by creating directivity properties with the help of mirrors and other reflectors.
Transmission of information
It has been observed that the high frequency current spreads predominantly along the surface of the conductor, and not in the thickness. As a result, the metal rod actively radiates energy into space. In conventional wires to block the effect of the screen is used, if it is intentionally removed, it turns out the antenna. This is used in transmitting information over the air. Nikola Tesla planned to transmit energy to the distance using the described method. But the research remained classified by the FBI, and publicly announced that the latest work of the scientist can not solve the task.