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 - these are 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 electrical current resistance?

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The technique still uses an outdated unit of measuring resistivity.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, constant resistors are indicated as follows:

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.

Any physical body through which the directional movement of ions occurs, has a certain resistance. The property of each type of material of the conductor does not give the charged particles through it - the electrical resistance. Taking into account the differences of countering the movement of ions, many fundamental schemes are built, some of which are used in electronic computing machines, for example, these are triggers in household appliances, headphones.

What is electrical resistance

What is resistance in physics? Resistance is physical importance that describes the properties of a conductive material to prevent the passage of charged particles through it. According to the Ohm's law, this value is equal to the value of the voltage at the end portions of the conductor, divided by the strength of the current in the amperer passing through it. The value of countering the directional current of charged particles for the chains with alternating current and the fields of electromagnetic type is characterized by fields of the wave obstacle to the change in the potential and impedance.

Interesting. Based on this characteristic, the radio metal resistor was also obtained, from the English Resistance resistance. This part is required to be introduced into the power supply chain of the active obstacle to electric current.

Definition of a unit of resistance– Oh.

What is the measuring current resistance? Unit of measurement in the International System System. This value is equal to the resistance in the chain between the extreme sections, between which the voltage of the value of 1 V with the strength of the current in 1a. The word was obtained by the surname of the scientist - Georg Oma. It was accepted as a unit of measurement in the sixties of the twentieth century together with the international system of UN units.

Resistance reproduction tools

To determine the measure of electrical resistance use:

• The resistance store is a special set of radio elements of various nominal. These components are specially manufactured in such a way as to contain the reference resistance of the conductors. When connecting an electrical conductory with a constant or alternating current to the resistance store, you can choose a suitable resistor in the size and obtain a certain voltage at the output, which can then be measured using a voltmeter;
• The coil is a device that works on a similar to the store principle. When you connect to the input of the device, you can adjust the existing levers and switches to adjust the range of the unit and get the required voltage at the output.

State standard resistance

This state standard under the index of GET 14-91 is customary as follows.

The values \u200b\u200band characteristics of reference resistance

Feature name	Value for state standard 14-91
Reproducible value in Omach	6453 and 12906.
Stored value in Omah	1
Inaccuracies in the first type (a) in billion dollars	25
Inaccuracies in the second type (B) in billion dollars	35
The amount of standard uncertainty, PPB	45
Increased uncertainty with a coefficient equal to two, PPB	90

Static and dynamic resistance

According to the theory of nonlinear chains, separated the resistance value to the static and dynamic. The first is identical to the law of Ohm and is equal to the ratio of the voltage on the element to the current strength of the current. The dynamic value of the element to be inherent in the signs of nonlinearity is the value obtained by dividing the minimum increase in voltage to the appropriate increase in the current force.

The dependence of the values \u200b\u200bfrom the characteristics of the conductor

In the conductor carriers of the electric current are free adversely charged particles. The behavior in the substance is like gas. The density of free particles depends on the density of the medium. Based on this, the density and structure of the crystal lattice are determined by the type of conductive material and its dimensions. Because of this, the conduction is influenced by the cross-sectional area and temperature. Resistance through the cross-sectional area is considered a calculated value.

Resistance to human body

This value is non-linear, depends on many parameters and cannot be considered ohomic. The value may vary in time, decreases relative to a person who is excited and sweated. In addition, the influence of the environment is influenced by this magnitude. A dry dermis value may exceed 10 thousand Ohm * meter. Therefore, a temporary graph of a person in humans can have a different look.

Devices for measuring resistance (DC)

To measure resistance, you can apply:

• Ommeter - directly allows you to show the load level;
• Witston Bridge;
• It is possible to calculate according to the obtained data of the ammeter and the voltmeter on simple formulas.

Explorer when it goes on it

During the passage of the electric charge on the conductor, the heat of thermal energy occurs. In this case, the conductor can be very hot. Energy is calculated by the formula:

A \u003d p * t, where p is the power, calculated by the formula P \u003d U * i.

Typical case - heating aluminum under high voltage.

Effect of temperature specific resistance

The specific resistance characteristic with increasing temperature also rises. This is due to an increase in the rate of movement of charged particles in the metal, with an increase in temperature. The specific resistance of substances conductive electric current and coal during heating, respectively, decreases due to an increase in the number of free electrons per unit volume.

Indicators for solid-state materials

The resistivity of alloys and solid-state metals is practically not changed when the temperature is raised or reduced. This is due to the density of the crystal lattice. Characteristic is inherent in Constantane, manganin and other dense alloys. This feature requires an increased specific value relative to the components.

Communication with specific conductivity

Electrical conductivity is a characteristic of the medium to carry out charged particles, as well as a change in the properties of bodies or medium, due to which the movement of charged particles occurs under the influence of the electromagnetic field. This value is considered to be reverse by the resistance of the conductor.

Indicators for liquid conductors

Indicators of electrical resistance of salts and alkalis solutions are dynamic. Values \u200b\u200bdepend on the composition, the concentration of the substance. In this case, the effect of temperature, inverse metals. During heating due to diffusion effect, the value falls and vice versa. At too low temperatures, the electrolyte can go to a solid aggregate state and not conducted. So, water that crystallized is not a conductor. Hydraulic obstruction of the movement of particles occurs due to the presence of derivatives of derivatives that are conductors in the liquid.

Dependence of the resistivity of deformation

With cold processing of conductors, plastic deformation of the raw material occurs with the subsequent distortion of the crystal lattice, which significantly increases the level of resistivity.

Electrical resistance is the property of any substance to prevent the movement of ions. The characteristic is dynamic and depends on several factors. Isolation and some materials have a resistance level at which the electric current is not capable of passing through the substance. This can characterize some substances as poorly conductive current due to a small volume of ions. What is the conductor resistance? The value due to which the loss of power occurs when the electricity passing.

Video

Introduction………………………………………………………………………………2

Measurement of resistance at constant current…………………..…….3

Method ammeter-voltmeter ......................................................... 3

Method of direct assessment ...................................................... ..4

Bridges for measuring resistance on constant current .................. ... 6

Measurement of very large resistance .......................................... 9

Measurement of resistance with alternating current………………….…...10

Impitance meter ............................................................................... ... 10

Measuring line ....................................................................11

Measuring ultramal resistance ............................................ 13

conclusions………………………………………………………………….………..…14

Introduction

Electrical resistance is the main electrical characteristics of the conductor, the value characterizing the countering the electrical circuit or its plot of electrical current. Also resistance can be called the item (it is more often called the resistor) the current rendering of the current. Electrical resistance is due to the transformation of electrical energy into other types of energy and is measured in Oma.

Resistance (often indicated by the letter R) is considered at certain limits, a constant value for this conductor and it can be defined as

R - resistance;

U is the difference of electrical potentials at the ends of the conductor, is measured in volts;

I - current flowing between the edges of the conductor under the action of the potential difference is measured in amperes.

For practical measurement of resistance, many different methods are used, depending on the conditions of measurement and nature of objects, from the required accuracy and speed of measurements. For example, there are methods for measuring resistance at constant current and with a variable, measuring large resistance, resistances of small and ultramal, straight and indirect, etc.

The purpose of the work is to identify the main most common in practice, resistance measurement methods.

Measurement of resistance at constant current

The main methods of measuring constant current resistance are the indirect method, the method of direct assessment, as well as the bridge method. The choice of measurement method depends on the expected value of the measured resistance and the required measurement accuracy. From indirect methods, the most versatile is a voltmeter ammeter method.

Voltmeter ammeter method

This method is based on measuring the current flowing through the measured resistance and the voltage drop on it. Apply two measurement schemes: measurement of large resistance (A) and measurement of low resistance (b). According to the measurement of current and voltage, the desired resistance is determined.

For the scheme (a), the desired resistance and relative methodological error can be determined by formulas:

where RX is the measured resistance, and RA is the resistance of the ammeter.

For the scheme (b), the desired resistance and the relative methodological measurement error are determined by the formulas:

From the formula, it can be seen that when counting the desired resistance along the approximate formula, an error occurs, because when measuring currents and stresses in the second scheme, an ammeter takes into account that current that passes through the voltmeter, and in the first scheme, the voltmeter measures the voltage in addition to the resistor also on the ammeter .

From the definition of relative methodological errors, it follows that the measurement according to the scheme (A) provides a smaller error when measuring large resistance, and the measurement according to the scheme (b) - when measuring low resistance. Measurement error according to this method is calculated by expression:

"The instruments used in measurement must have accuracy class not more than 0.2. The voltmeter is connected directly to the measured resistance. The current when measuring should be such that the readings are counted on the second half of the scale. In accordance with this, the shunt is selected, used for the possibility of measuring the current to the device of class 0.2. In order to avoid the heating of the resistance and, accordingly, reducing the accuracy of measurements, the current in the measurement circuit should not exceed 20% of the nominal ".

The advantage of the method of the method of measurement ammeter and voltmeter is that the resistor with measurable resistance can be skiing the same current as in the condition of its operation, which is important when measuring the resistance, the values \u200b\u200bof which depend on the current.

Direct evaluation method.

The method of direct assessment involves measuring the resistance of a direct current with an ohmmeter. Ommeter is called a measuring device of a direct reference to determine the electrical active (active resists also refer to ohmic resistances) of resistance. Typically, the measurement is performed at a DC, however, in some electronic homwheels it is possible to use AC. The varieties of oxmeters: megohommeters, teraommeters, gigammeters, milliimeters, microgrammeters, differing ranges of measured resistance.

On the principle of operation, the melometers can be divided into magnetoelectric - with a magnetoelectric meter or magnetoelectric logater (megoometers) and electronic, which are analog or digital.

"The action of a magnetoelectric ommeter is based on measuring the current flow occurring through the measured resistance at a constant voltage of the power supply. To measure resistances from hundreds of Ohm to several mega, the meter and the measured resistance of the RX is activated sequentially. In this case, the current I strength in the measuring and deviation of the moving part of the device A is proportional: i \u003d u / (R0 + Rx), where U is the power supply voltage; R0 - resistance of the meter. With small Rx values \u200b\u200b(up to several Ohm), the meter and Rx are included in parallel. "

The basis of the logometric megaommeters is taken by a logometer, to whose shoulders are connected in different combinations (depending on the measurement limit) exemplary internal resistors and measurable resistance, the expiration of the logometer depends on the ratio of these resistance. As a high voltage source required for carrying out such measurements, a mechanical inductor is usually used in such devices - a manual electric generator, in some meganers instead of the inductor, a semiconductor voltage converter is used.

The principle of operation of electronic hommeters is based on the transformation of the measured resistance in proportional to it voltage using the operational amplifier. The measured resistor is included in the feedback circuit (linear scale) or to the amplifier input. The digital ohmmeter is a measuring bridge with automatic balancing. The balancing is made by the digital control device by selecting precision resistors in the bridge shoulders, after which the measuring information from the control device is supplied to the indication unit.

"When measuring small resistance, an additional error may occur due to the effect of transitional resistance at the connection points. To avoid this use the so-called four-wire connection method. The essence of the method is that two pairs of wires are used - one pair for the measured object is given a current of a certain force, with the help of another pair from the object to the device, the voltage drop proportional to the current and the resistance of the object is supplied. The wires are connected to the outputs of the measured two-pole so that each of the current wires does not touch the voltage directly corresponding to it, and it turns out that the transition resistance in the contact places are not included in the measuring chain. "

Concept of electrical resistance and conductivity

Any body by which the electric current flows has a certain resistance.The property of the conductor material to prevent the passage through it is called electrical resistance.

The electronic theory explains the essence of the electrical resistance of metal conductors. Free electrons when moving along the conductor, countless atoms and other electrons are met in their path on their way and, interacting with them, inevitably lose part of their energy. Electrons experience resistance to their movement. Various metal conductors having a different atomic structure have different electrical current resistance.

Exactly the same explains the resistance of liquid conductors and gases by passing the electric current. However, we should not forget that in these substances are not electrons, but the charged particles of molecules meet resistance at their movement.

Resistance is indicated by Latin letters R or R.

Om accepted per unit of electrical resistance.

OM is the resistance of the Rtuchi post 106.3 cm high with a cross section of 1 mm2 at 0 ° C.

If, for example, the electrical resistance of the conductor is 4 ohms, then it is written as follows: r \u003d 4 Ohm or R \u003d 4Ω.

To measure the resistance of a large magnitude, a unit is adopted, called Megoma.

One meg is equal to one million ohm.

The more the resistance of the conductor, the worse it spends an electric current, and, on the contrary, the smaller the resistance of the conductor, the easier it is to pass through this conductor through this conductor.

Therefore, for the characteristics of the conductor (from the point of view of passing through it, it is possible to consider not only its resistance, but also the magnitude, inverse resistance and called conduction.

Electrical conductivity It is called the ability of the material to pass through electric current.

Since the conductivity is the magnitude, inverse resistance, then it is expressed as 1 / r, denotes the conductivity of the Latin letter G.

The influence of the material of the conductor, its size and the ambient temperature by the amount of electrical resistance

The resistance of various conductors depends on the material from which they are made. The concept of so-called specific resistance is introduced for the characteristics of the electrical resistance of various materials.

Specific resistance The resistance of the conductor with a length of 1 m and cross-sectional area is 1 mm2. The resistivity is indicated by the letter of the Greek alphabet p. Each material from which the conductor is manufactured has its specific resistance.

For example, the specific resistance of the copper is 0.017, i.e. the copper conductor with a length of 1 m and a cross section of 1 mm2 has a resistance of 0.017 ohms. The resistivity of aluminum is 0.03, the resistivity of the iron is 0.12, the resistivity of the constantan - 0.48, the resistivity of the nichrome is 1-1.1.

The resistance of the conductor is directly proportional to its length, i.e., the longer the conductor, the greater its electrical resistance.

The resistance of the conductor is inversely proportional to the area of \u200b\u200bits cross-section, i.e., the thickness of the conductor, the resistance is less, and, on the contrary, the thinner the conductor, the resistance is more.

To better understand this dependence, imagine two pairs of reporting vessels, and one pair of vessels connecting the tube is thin, and the other is thick. It is clear that when filled with water of one of the vessels (each pair), the transition of it to another vessel in a thick tube will occur much faster than fine, i.e. the thick tube will have a smaller resistance to the flow of water. Similarly, the electric current is easier to go through a thick conductor than in a thin, that is, the first has a smaller resistance to it than the second.

The electrical resistance of the conductor is equal to the resistivity of the material from which this conductor is made multiplied by the length of the conductor and divided into the area of \u200b\u200bthe cross section of the conductor:

R \u003d P L / S,

Where - R is the resistance of the conductor, Ohm, L is the length in the conductor in M, S - the cross-sectional area of \u200b\u200bthe conductor, mm 2.

Cross section area of \u200b\u200ba round conductor Calculated by the formula:

S \u003d Pi x D 2/4

Where p. - constant value equal to 3.14; D - conductor diameter.

And this is determined by the length of the conductor:

l \u003d S r / p,

This formula makes it possible to determine the length of the conductor, its cross section and resistivity, if the remaining values \u200b\u200bincluded in the formula are known.

If it is necessary to determine the cross-sectional area of \u200b\u200bthe conductor, then the formula leads to the following form:

S \u003d P L / R

Converting the same formula and deciding equality regarding P, we will find the specific resistance of the conductor:

r \u003d R S / L

The last formula has to use in cases where the resistance and size of the conductor are known, and its material is unknown and besides, it is difficult to determine in appearance. To do this, it is necessary to determine the resistivity of the conductor and using the table, find a material with such a resistivity.

Another reason affecting the resistance of the conductors is the temperature.

It has been established that with an increase in temperature, the resistance of metal conductors increases, and decreases with a decrease. This increase or decrease in resistance for conductors from pure metals is almost the same and on average equals 0.4% per 1 ° C. The resistance of liquid conductors and coal with increasing temperature decreases.

The electronic theory of the structure of the substance gives the following explanation to an increase in the resistance of metal conductors with an increase in temperature. When heated, the conductor receives thermal energy, which is inevitably transmitted to all atoms of the substance, resulting in the intensity of their movement. The increased movement of atoms creates greater resistance to the directional movement of free electrons, which increases the resistance of the conductor. With a decrease in temperature, the best conditions for the directional movement of electrons are created, and the conductor resistance decreases. This explains an interesting phenomenon - superconductance of metals.

Superconductivity, i.e., a decrease in metal resistance to zero, occurs at a huge negative temperature - 273 ° C, called absolute zero. At an absolute zero temperature, the metal atoms seem to be frozen in place, absolutely not preventing the movement of electrons.

Among other indicators characterizing the electrical circuit, the conductor, it is worth highlighting electrical resistance. It determines the ability of the material atoms to prevent the directional passage of electrons. Assistance in determining this value can provide both a specialized device - an ohmmeter and mathematical calculations on the basis of knowledge of the relationship between the values \u200b\u200band physical properties of the material. The measurement of the indicator is made in Omah (OM), the symbol R.

OMA's law is a mathematical approach when determining resistance

The ratio set by the George Omom determines the relationship between the voltage, current force, resistance based on the mathematical relationship of concepts. The validity of a linear relationship is R \u003d U / I (voltage ratio to current) - noted in all cases.
Unit of measurement [R] \u003d b / a \u003d Ohm. 1 Ohm - the resistance of the material by which current is 1 amp at a voltage of 1 volt.

Empirical resistance calculation formula

The objective data on the conductivity of the material is followed from its physical characteristics, defining both its own properties and reactions to external influences. Based on this, conductivity depends on:

• Size.
• Geometry.
• Temperatures.

The atoms of the conductive material are faced with directional electrons, preventing further promotion. At high concentration of the latter, atoms are not able to resist them and conductivity is high. Large resistance values \u200b\u200bare characteristic of dielectrics that differ almost zero conductivity.

One of the defining characteristics of each conductor is its specific resistance - ρ. It determines the dependence of resistance from the material of the conductor and exposure from the outside. This is fixed (within the same material) the value that represents the data of the conductor of the following dimensions - length 1 m (ℓ), the cross-sectional area of \u200b\u200b1 sq.m. Therefore, the relationship between these values \u200b\u200bis expressed by the relation: R \u003d ρ * ℓ / S:

• The conductivity of the material falls as its length increases.
• An increase in the cross-sectional area of \u200b\u200bthe conductor entails a decrease in its resistance. Such a pattern is due to a decrease in the density of electrons, and, consequently, the contact of the particles of the material with them becomes more rare.
• The growth of the material temperature stimulates the growth of resistance, while the temperature drop entails its decline.

The calculation of the cross-sectional area is advisable to produce according to the formula S \u003d πd 2/4 in determining the length of the roulette will help.

Power relationship (P)

Based on the formula of the Ohm, U \u003d I * R and P \u003d I * U. Consequently, p \u003d i 2 * R and p \u003d u 2 / r.
Knowing the amount of current and power, resistance can be defined as: R \u003d P / I 2.
Knowing the amount of voltage and power, resistance is easy to calculate according to the formula: R \u003d U 2 / p.

The resistance of the material and the magnitude of other concomitant characteristics can be obtained using special measuring instruments or on the basis of established mathematical patterns.