Formulate the definition of electrical resistance. Electricity, current, voltage, resistance and power. Resistors in electrical circuit

- Electrotechnical value, which characterizes the property of the material to prevent the flow of electric current. Depending on the type of material, the resistance can strive for zero - to be minimal (miles / micro ohms - conductors, metals), or be very large (gig oh - isolation, dielectric). The magnitude of the reverse electrical resistance is.

unit of measurement electrical resistance - ohm. Denotes the letter R. The dependence of the resistance from the current and in the closed circuit is determined.

Ommeter- Device for direct measurement of chain resistance. Depending on the range of the measured value, are divided into gigammeters (for large resistance - when measuring insulation), and on micro / miliommeters (for small resistances - when measuring the transitional resistances of contacts, engine windings, etc.).

There is a wide variety of hommeters by constructive different manufacturers, From electromechanical to microelectronic. It is worth noting that the classic ohmmeter measures the active part of the resistance (the so-called oomics).

Any resistance (metal or semiconductor) in the variable current circuit is active and reactive component. The amount of active and reactive resistance is constituted full resistance of the variable circuitand calculated by the formula:

where, z is the impedance of the AC circuit;

R is the active resistance of the variable circuit;

XC is capacitive reactive resistance of the AC circuit;

(Conduction, W - Angle Speed \u200b\u200bof AC)

XL is an inductive reactive resistance of the AC circuit;

(L- inductance, W - Angle speed of alternating current).

Active resistance- this is part of the full resistance electrical chainwhose energy is fully converted to other types of energy (mechanical, chemical, thermal). A distinctive feature of the active component is the full consumption of all electricity (the energy is not refunded back to the network), and the reactive resistance returns part of the energy back to the network (negative property of the reactive component).

Physical meaning of active resistance

Each environment where pass electric charges, creates obstacles on their paths (it is believed that these are crystalline lattice nodes), in which they struggle both and lose their energy that is released in the form of heat.

Thus, the fall (loss of electrical energy) occurs, part of which is lost due to the internal resistance of the conductive medium.

A numerical value that characterizes the ability of the material to prevent the passage of charges and is called resistance. It is measured in Omah (OM) and is inversely proportional electrical conductivity of the magnitude.

Different elements of the periodic Mendeleev system have different electrical resistances (P), for example, the smallest UD. Resistance has silver (0.016 ohms * mm2 / m), copper (0.0175 Ohm * mm2 / m), gold (0.023) and aluminum (0.029). They are used in industry as the main materials on which all electrical engineering and energy are being built. Dielectrics, on the contrary, possess high UD. Resistance and used for isolation.

The resistance of the conductive medium can change significantly depending on the cross section, temperature, values \u200b\u200band current frequency. In addition, different media possess different charge carriers (free electrons in metals, ions in electrolytes, "holes" in semiconductors), which are determining resistance factors.

Physical meaning of reactive resistance

In coils and capacitors, an energy accumulation is accumulated in the form of magnetic and electric fields, which requires some time.

Magnetic fields in alternating current networks are changed after the changing direction of the movement of charges, while having additional resistance.

In addition, there is a steady shift of phases and current strength, and this leads to additional electricity losses.

Resistivity

How to find out the resistance of the material if it does not flow on it and we do not have an ohmmeter? There is a special value for it - specific electrical resistance Material in

(These are table values \u200b\u200bthat are defined by the experimental means for most metals). With this value and physical values \u200b\u200bof the material, we can calculate the resistance by the formula:

where, p.- resistivity (units of measurement OM * M / mm 2);

l - Explorer length (m);

S is a cross section (mm 2).

The essence of the concept of "resistance"

Definition 1.

The resistance is the physical value characterizing the medium (conductor) through which the electric current flows.

From a physical point of view, the resistance is due to the fact that the charged particles, moving from one end of the conductor to another, are faced with atoms of its crystal lattice or other elementary particles of the medium. Therefore, current flow under normal conditions is associated with the allocation of a certain amount of heat due to such collisions, i.e. With loss of energy.

Note 1.

When the conductors are cooled to ultra-low temperatures, there is a phenomenon of superconductivity, when the resistance becomes zero.

Resistance depends on the following factors:

material (for example, tungsten resistance is higher than in copper);
geometric shape (the longer the conductor and thinner its cross section - the more resistance);
temperature (than it is higher, the more resistance), etc.

From the Ohm law, resistance can be expressed as

$ R \u003d \\ FRAC (U) (I) $

where $ U $ - voltage, $ I $ - current power.

Unit of measurement of resistance

In the system, the resistance is measured in Oma.

Note 2.

The measurement unit of OM is named after German Physics Georg Ohm (1787 - 1854), which made a great contribution to the development of electrical engineering.

The SC system was introduced in 1960. IN Russian Federation GOST 8.417-2002 operates, in which an electrical resistance is also specified as an electrical resistance unit.

OM is a derivative of a unit equal to the resistance of the conductor, according to which a current of 1 amp is flowing, causing a voltage drop at the ends of this volt conductor. Volt for SI is an extra system unit, therefore, ohms are expressed in kilograms, seconds and amps:

$ Om \u003d \\ FRAC (m ^ 2 \\ Cdot kg) (C ^ 3 \\ Cdot A ^ 2) $.

In the system "centimeter, gram, second" (SGS), the resistance unit has no own name, as well as a unit of current and voltage. To recalculate resistance between SGS and C systems, the ratio is used:

$ 1 unit. SGS \u003d 9 \\ CDOT 10 ^ 11 Ohm $.

In the SGSE system and the Gauss system, the resistance is measured in stats. Stat is a private from dividing the voltage expressed in the stavols, for the strength of the current, expressed in the stampers.

$ 1 STAT \\ APPROX 8,99 \\ CDOT 10 ^ 11 Ohm $.

In the SGSM system, the resistance is measured in abomas (voltage - in abvolts, current strength - in abamper):

$ 1 abom \u003d 1put \u003d 10 ^ (- 9) OM $.

For measuring electrical resistance, ometers are used - instruments equipped with its own current sources. Modern devices of this type show the measurement result on the electronic scoreboard. Old ohmeters showed the result by means of mechanical shooter, which is less practical, but clearly demonstrates the nature of the measured value.

The arrow of the classical ohmmeter is attached to the conductive spring-loaded frame rotating in a constant magnetic field, when the current is passed through which the electromagnetic force occurs, which is interacting with the magnetic field. The greater the current flow through the conductor, the more stronger the arrow and, therefore, less resistance. Therefore, the testimony on the scales of such devices is often counted without from left to right, but to the right left.

Figure 1. Ommeter scale (top) sample from right to left. Author24 - Student Internet Exchange

In practice, several units of measurement - kilometers, megaoms are often used.

To marking resistors - electronic components with a predetermined resistance - the system of color strips is used, which allows not to apply poorly readable text to the details.

Electrical resistance - The physical quantity characterizing the properties of the conductor to prevent the passage of the electric current and equal to the ratio of the voltage at the ends of the conductor to the strength of the current flowing through it.

Resistance for alternating current circuits and for variables of electromagnetic fields is described by the concepts of impedance and wave resistance. Resistance (resistor) is also called radio component intended for administration to the electrical circuits of active resistance.

Resistance (often denoted by the letter R or R) is considered at certain limits, a constant value for this conductor; It can be calculated as

R \u003d u i, (\\ displayStyle R \u003d (\\ FRAC (U) (I)),) R - resistance, Ohm; U - the difference of electrical potentials (voltage) at the ends of the conductor, in; I - the current of the current flowing between the edges of the conductor under the action of the potential difference, A.

Encyclopedic YouTube.

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✪ 8 CL - 129. Operation and power of electric current

✪ Lesson 358. Active resistance in the AC circuit. Active current and voltage

✪ Lesson 305. Electric current in semiconductors. Own and impurity conductivity.

✪ Lesson 296. Temperature dependence of metals resistance. Superconductivity

✪ 8 CL - 110. Electric chain and its components

Subtitles

Units and dimension

static (in the SSSE and Gaussian system, 1 STATω \u003d (10 9 -2) / cm \u003d 898 755 178 736,818 Ohm (exactly) ≈ 8,98755 · 10 11 ohms, equal to the resistance of the conductor, through which, under the stress 1, the stamper flow flows 1);
abom (in SGSM, 1 ABω \u003d 1 · 10 -9 Ohm \u003d 1 nano, is equal to the resistance of the conductor through which the voltage of 1 abavolt flows 1 abamper).

The dimension of resistance in the SSSE and the Gaussian system is equal to TL -1 (that is, coincides with the dimension of the reverse speed, CM), in the SGSM - Lt. -1 (i.e., coincides with the speed dimension, cm / s).

The return value with respect to the resistance is the electrical conductivity, the unit of measurement of which in the Siemens system serves as Siemens (1 cm \u003d 1 Ohm -1), in the SSSE system (and Gaussian) Statsimenx and in the SGSM - Absimenses.

Physics phenomena

High electrical conductivity of metals is associated with the fact that they have a large number of current carriers - electric conductivitygenerated from valence electrons of metal atoms that do not belong to a specific atom. Electric current in the metal occurs under the action of an external electric field, which causes an ordered movement of electrons. The electrons moving under the action are scattered on the heterogeneities of the ion lattice (on impurities, lattice defects, as well as disorders of the periodic structure associated with thermal oscillations of ions). At the same time, the electrons lose the pulse, and the energy of their movement is converted into the internal energy of the crystalline lattice, which leads to the heating of the conductor when the electric current is passed.

The resistivity is a scalar physical quantity, numerically equal to the resistance of a uniform cylindrical conductor of a single length and a single cross-sectional area.

Metal resistance decreases with a decrease in temperature; At temperatures of the order of several Kelvins, the resistance of most metals and alloys tends or becomes zero (the effect of superconductivity). On the contrary, the resistance of semiconductors and insulators with a decrease in temperature (in some range) is growing. Resistance also changes as the current / voltage occurring through the conductor / semiconductor.

The dependence of the resistance from the material, the length and the cross-sectional area of \u200b\u200bthe conductor

In the metal, movable charge carriers are free electrons. We can assume that with its chaotic movement, they behave like gas molecules. Therefore, in classical physics, free electrons in metals are called electronic gas and in the first approximation it is believed that the laws established for the ideal gas are applicable to it.

The density of electron gas and the structure of the crystal lattice depends on the type of metal. Therefore, the resistance of the conductor should depend on the genus of its substance. In addition, it should still depend on the length of the conductor, its cross-sectional area and on temperature.

The effect of the conductor cross-section on its resistance is explained by the fact that with a decrease in the cross section of the electron flow in the conductor at the same time the current becomes more dense, therefore the interaction of electrons with particles of the substance in the conductor becomes stronger.

Resistance comes from the word "resist". In electronics there is such a thing as Ohm.What is it and what is it eating with? For a more detailed answer, let's consider this scheme:

Letters in circles - it measuring instruments

The voltmeter serves to measure the voltage, and ammeter - to measure the current force. How they are correctly used in this article.

So, if you skip the electric current with a current in 1 ampere on the wire, and at the ends of this wire we will have a voltage of 1 volt, this means that our wire has resistance in 1 ohm.

In electrical engineering and electronics, the resistance is indicated by the letter R.. For example, a person's body has resistance from several hundred ohm and up to 100 com. For calculations take 1 com. It depends on many factors such as the floor, age, skin condition, the strength of touching the conductors to the skin, the level of alcohol in the blood and so on.The copper wire in the meter long and the cross section of 1 mm 2 has a resistance of 0.1 Ohm.

From which resistance depends

Which of the items will have greater resistance electric Toku.?

garden hose

or oil highway?

Of course, the garden hose. Why? Yes, because its diameter is much smaller than the oil highway.

And now answer such a question, what hose will have a greater resistance, given the fact that their lengths and diameters are equal?

Corrugated

or smooth?

Of course corrugated. His walls will prevent the flow of water.

And one more nuance. We have a garden corrugated hose. We cut a small length from it, but still still a big hose skeleton remained

Which hose is greater resistance to the flow of water? I think that that is longer.

Resistance formula

Oddly enough, but things with wire are in the same way. The thinner and longer wire, the more its electrical current resistance. The material from which it is made is also played. Various materials in different ways are carried out electric current. There are those that remarkably conduct a current, type of silver, and there are those that almost do not pass through themselves electric current, such as porcelain.

Therefore, the formula will have this kind:

The technique still uses an outdated unit of measuring resistivity. Ohm x mm 2 / m.To translate B.Om x M., it is enough to multiply by 10 -6, as 1 mm 2 \u003d 10 -6 m 2.

As you see from the table above, silver has the smallest resistance, so the silver wire will be the best conductor in the design of radio-electronic devices. Well, the most common and cheap is copper and aluminum. It is these two metal that are mainly used in the entire electronic and electrical industry.

Substances that have the smallest electrical current resistance and have very low resistance called conductors, and substances that have a very large resistance to electric current and almost miss it through themselves, are called dielectrics. Between them stands class semiconductors.

Resistors

Electronics already have special radio-electronic components. They are called .

There are permanent resistors who have practically no resistance:

and there are also variable resistors:

With the help of them, you can change resistance in any specific range.

Consecutive and parallel compound of resistors

IN electrical circuits Permanent resistors are referred to as:

variables look a little different

All described resistors can be connected in parallel or sequentially. With parallel connection, the conclusions of the resistors are connected in the common points.

In this case, to learn the overall resistance of all resistors in the chain, it will be enough to use the formula where the value between points A and B (R AB) is the most r total:

With a consistent connection, the ratings of the resistors are simply summed up

In this case

Summary

Resistance plays a major role in electronics and electrical engineering. Any material in the universe has electrical current resistance. Some materials are very poorly passed through themselves electric current, and some materials, such as silver and copper, have a very low resistance and perfectly pass through electric current.

The resistance also affects such parameters as the material, the cross-sectional area of \u200b\u200bthe material, as well as its length. Materials that are perfectly carried out through themselves the electric current are called conductor, and which prevent the flow of electric current - dielectrics.

Resistors - special radio elements in electronics, which have a certain saturation rating and perform various functions.

For safe operation on power supply networks, as well as an elementary understanding of the laws of the electric field, it is necessary to have at least the primary knowledge of the basic laws of physics and know the determination of the current voltage and the resistance of the materials. This article discusses obstacles to passing the current, which is resistance, and the main formula for its calculation, and what is a resistor, and what is necessary for which it is necessary.

Definition of resistance

The physics benefits provide the following definition of the specified phenomenon. The electrical resistance of the conductor is a physical value that indicates the properties of the material to prevent the free passage of the current from the source point to the consumer. This indicator is equal to the ratio ratio at the ends of the conductor to the current flowing through the cable. There are several types of resistance based on material properties, such types include:

Resistance to conductors close to zero. In this case, the ability to prevent the free movement of the current on the cable is very low, to materials with similar characteristics include metal wiring made of color;
Minimum conductor resistance. The current on such a material flows, but meets a certain obstacle that lowers the voltage and makes it difficult to work the uninterrupted operation of the electric field. As a rule, it is not intended for use as conductor objects, for example, metal tools or building materials having a different configuration and section;
High electrical resistance of the subject. Such products are called completely dielectric, as their material has the properties of the full resistance to the flow of current on its surface. Frequently specified properties have plastic and rubber parts, cable insulation or wooden tool handles. The current according to these subjects does not pass, but its voltage remains unchanged.

All listed resistance types are the main and, relying on them, manufacturers of any products indicate in the product passport, the level of protection of the conductor from the current action. This is necessary to comply with the safety rules during the use of such objects and tools in construction and everyday life.

What is measured by the resistance of the conductor

The electrical resistance is indicated by the letter R and is measured in Oma. The resistance indicator depending on the type of current and its voltage in the closed chain is based on the Ohm law, which allows you to calculate the real value of the current passing. Thus, to calculate the resistance, it is necessary to substitute the data under the main formula:

R is the designation of resistance;
U - voltage in volts;
I - current in amperes.

What is measured by resistance

The device for measuring the resistance of the electric is called an ohmmeter, it connects to the conductor with the power on and automatically measure the resistance indicators. As a rule, when measuring the resistance, the device converts an alternating current to constant and only after that performs other actions. For professional use, multifunctional tools are often used, in the set of which several devices that measure voltage, current and material resistance are included.

There is also a classification of diagnostic devices by mobility criterion. Based on such an indicator, there are:

Stationary Ommers - are often used in laboratories or installed electrical cabinets, with numerous equipment. Such instruments show the data constantly and can be connected to alarm alarm that will notify the service personnel about the safety threat;
Mobile complexes or tools are most often used when carrying out repair or mounting work on the line to determine the class of the conductor and its integrity.

Despite the seeming simplicity, this device requires care and compliance with safety rules when used. Since all actions are carried out on working network, then you need to carefully examine the instruction instructions and do not attempt to connect it with a circuit violation.

Resistors in electrical circuit

Resistor with different resistance is the device electrical network With a passive action, which is designed to absorb certain energy and hold its unlimited time. This product does not carry out any work, therefore it is considered a passive part, but its use is necessary in almost all power schemes, as well as in closed conductors. The resistor is the most common element, it is used more often than any other components in most chips.

Resistors are of different marking, which is applied with paint to the outer case. The higher the figure on the shell, the more resistance has the product. The principle of operation of the part is based on the properties of the material to absorb the charge and dispel it in the form of an electric field, without increasing the voltage within the network of conductors.

The resistor on the basis of working material is a few species, but the scheme of its action does not change in principle. There are photoresistors, alternating and direct current devices, each of the types of parts performs its function and ensures the normal operation of the flaked electrical circuit.

This type of product, depending on the need, can be completed on a serial or parallel scheme. With a consistent arrangement, the overall resistance will be equal to their sum, and with parallel - the resistance will be the sum of the indicators of all resistors located in the chain.

What affects resistance indicators

There are several factors that affect the level of resistance of one or another conductor. For this principle, three main indicators can be distinguished:

Explorer length. The greater the cable, the higher it will be its resistance, and this also applies to the materials with a zero indicator, and the conductor with the average resistance;
Cross section area. Unlike the length of the network, the area of \u200b\u200bthe material affects reverse side: than it is more, the smaller the resistance of the conductor;
Cable material characteristics. There is such a concept as conductivity or specific resistance of the material, based on this parameter, one or another cable of the same area and length can have different resiliques, depending on impurities in the composition of the metal.

These factors are the main, but sometimes it can be attributed to the outer temperature of the medium in which the conductor is located, since with a significant cold the electric field has the properties to dissipate, which is lost intensity, therefore, the resistance will fluctuate.

Thus, in the understanding of the basic laws of physics and determining the voltage and resistance of materials, even a novice master will be able to perform work on the installation and maintenance of networks, repair of equipment and successfully apply the knowledge gained in practice.