Conductor resistance - capacity of the material impede the flow of electric current. Including the case of the skin effect of high-frequency variable voltage.
Materials are divided into classes according to the resistivity. Considered value - the resistance - is considered a key, will perform gradation of all the substances found in nature:
- Conductors -. Materials having a resistivity of up to 10 m mO Affected most metals, graphite.
- Dielectrics - 100 megohms resistivity. M - 10 m POM Peta prefix is used in the context of the fifteenth tens degrees.
- Semiconductors - a group of electrical material with a resistivity in the range of conductors to dielectrics.
Resistivity is called, allowing the cutting wire to characterize the parameters length of 1 meter, an area of 1 square meter. Often inconvenient to use numbers. Real-section of the cable is much smaller. For example, the PV-3 for an area of tens of millimeters. The calculation is simplified if we use the units ohm mm² / m (see. Fig.).
Specific resistance of metal
The resistivity is denoted by the Greek letter "rho", to obtain the resistance value of the length indicator multiply, dividing by the sample area. Translation between standard units of measurement ohm m increasingly used for the calculation shows that the relationship is established through the sixth degree of tens. Sometimes it will be possible to find among the tabulated values of information concerning the resistivity of copper:
- 168 micromhos m;
- 0.00175 ohms sq. mm / m.
It is easy to verify the figures differ by about 4%, make, run the units. So, the numbers given gauge copper. If necessary, accurate calculations issue further specified separately. Information about the resistivity of the sample are purely empirically. Cut-off wire to the known section length is connected to the contacts of the meter. For Answer required readings divided by the sample length, multiplied by the cross section area. In tests, the sample is assumed to choose longer, minimizing the error. A significant part of the testers is endowed with sufficient accuracy for fit values.
So fearful of physicists, desperate to learn Chinese multimeters work with resistivity uncomfortable. It is much easier to take the finished cut (of greater length), to evaluate the option of full piece. In practice, the proportion of Ohm's play a small role, the following steps are performed for the assessment of losses. Directly determined resistance active circuit area and current depend quadratically. Having said that, we note: Guides in electrical engineering can be divided into two categories of applicability:
- Materials of high conductivity, high resistance. First used to create cables, the second - the resistance (resistors). In the tables there is no clear distinction is taken into account practicality. Silver low resistance to create a wire is not used at all for devices contacts - is rare. For obvious reasons.
- Alloys with high elasticity are used to create flexible current-carrying parts, springs working parts contactors. Resistance usually should be minimized. Clearly, for this purpose fundamentally unsuitable ordinary copper, which is inherent a large degree of plasticity.
- Alloys with high or low thermal expansion coefficient. First serve basis for the creation of bimetallic plates structurally underpin thermal and launch protection relays. The second image group invar alloys. It is often required, which is important geometric shape. The holder of the filament in a normal light bulb (replacement of expensive tungsten) and vacuum-tight junction at the intersection of the glass. But more often Invar alloys no relation to electricity are not used as part of machines, appliances.
Formula communication resistivity ohmic
Physical basis of the electrical conductivity
Resistance of the conductor is recognized by the reciprocal of electrical conductivity. In the modern theory is not established thoroughly how the current formation process. Physicists often rested against the wall, watching the phenomenon, which could not possibly be explained in terms of positions previously put forward concepts. Today is considered to be the dominant band theory. Required to give a brief digression from the concepts of the structure of matter.
Initially it was assumed: the substance is presented substance, charged positively, electrons floating in it. I thought so notorious Lord Kelvin (nee Thomson), which is named in honor of the unit of measurement of absolute temperature. For the first time I made an assumption about the structure of the Rutherford planetary atoms. The theory put forward in 1911 year, was built on a large dispersion fact alpha radiation deflection substances (individual particles varied flight angle on a very large value). On the basis of the existing preconditions author concluded, the positive charge of the atom is concentrated in a small region of space, called the kernel. The fact that individual cases of strong deflection angle of flight due to the fact that the path of the particles lay in the vicinity of the nucleus.
So installed outside the geometric dimensions of the individual elements and for different substances. We concluded that the diameter of the gold core fits region 3 pm (pico - a prefix to the negative twelfth tens degree). Further development of the theory of the structure substances fulfilled Bor in 1913. Based on observations of behavior of hydrogen ions concluded: unit charge of an atom is, the mass was defined to be approximately one sixteenth the weight oxygen. Bohr suggested an electron is held by attractive forces determined Pendant. Therefore, something keeps from falling into the nucleus. Boron assumed fault centrifugal force arising during rotation of the particle orbit.
Important amendment to the model introduced by Sommerfeld. Made elliptic orbits introduced two quantum numbers describing the trajectory - n and k. Bohr remarked, Maxwell's theory for the model fails. Moving particle is required to generate the magnetic field in space, then gradually the electron would fall into the nucleus. Therefore, we have to admit: there are the orbits in which the energy of the radiation in space is not happening. It is easy to see: the assumptions contradict each other, once again reminding: conductor resistance as a physical quantity, now unable to explain physics.
Why? Band theory of the chosen basis of Bohr's postulates, which read: position orbits are discrete, are calculated in advance, the geometric parameters are connected by certain relations. Conclusions scientists have come to complement the wave mechanics, as made by mathematical models were powerless to explain certain phenomena. Modern theory says, for each substance is provided in the electron state of three zones:
- The valence electron band, fixedly related atoms. More energy is required - to break the link. The electrons of the valence band to the conduction is not involved.
- The conduction band electrons in the substance when a field forming the electric current (the orderly movement of charge carriers).
- The forbidden zone - area of energy states where electrons can not be in normal conditions.
Inexplicable experience Jung
According band theory, the conductor area overlaps the conduction valence. Electron cloud formed easily entrained by the electric field, forming a current. For this reason, the resistance conductor has such a small value. And scientists are making futile efforts to explain what is an electron. It is only known: an elementary particle exhibit wave and corpuscular properties. Heisenberg's uncertainty principle puts the facts on the ground: it is impossible to have a 100% at the same time to determine the location of an electron and energy.
As for the empirical part, the scientists noticed: Young's experiment, done with electrons, gives a curious result. Scientific missed photons flow through the two closest shield slit interference pattern was obtained, composed next strips. Offered to do a test with electrons, there was a collapse:
- If electrons pass the beam passing two slit interference pattern is formed. It occurs if the photons move.
- If the electrons shoot one, nothing changes. Therefore... one particle is reflected from myself there in multiple places?
- Then we began to try to fix the time of the passage of the electron shield plane. And... the interference pattern disappeared. There were two spots in front of the slots.
Effect powerless to explain from a scientific point of view. So, electrons "guess" of an ongoing monitoring cease exhibit wave properties. It shows the limitations of the modern concepts of physics. Well, if it can be satisfied! Another man of science has offered to monitor the particles, when they have passed through the slit (flew in a certain direction). And what? Again the electrons are no longer exhibit wave properties.
Is obtained, the elementary particles are returned back in time. At a time when the gap went. Penetrated the mystery of the future to learn whether or not to monitor. Depending on the facts corrected behavior. Clearly, the answer may not be hitting the bull's eye. Riddle is awaiting approval to this day. By the way, Einstein's theory, put forward at the beginning of the XX century, is now refuted: found particles that are larger than the speed of light.
As the resistance of the conductors is formed
Modern views say, free electrons are moved through the conductor at a speed of about 100 km / s. Under the action occurring within a field drift ordered. carriers move along the lines of tension rate is low, makes units of centimeters per minute. During the motion of the electrons collide with lattice atoms, a certain fraction of the energy is converted into heat. And measure of this conversion is called a conductor resistance. The higher, the more electrical energy is converted into heat. It based on this principle of the heaters.
In parallel, the context is the numerical expression of the conductivity of the material, which can be seen in the figure. For resistance relies unit divided into a specified number. The course of further transformations discussed above. It is seen that the resistance depends on the parameters - the thermal motion of the electrons and their mean free path that leads directly to the crystal lattice structure of the substance. Explanation - resistance conductors differs. At lower copper aluminum.