One of the characteristics of any electrically conductive material is the dependence of resistance on temperature. If it is depicted as a graph on where the time intervals (t) are marked on the horizontal axis and the value of the ohmic resistance (R) on the vertical axis, then we get a broken line. The dependence of resistance on temperature schematically consists of three sections. The first corresponds to a slight heat - at this time the resistance changes very little. This happens until a certain point, after which the line on the graph goes up sharply - this is the second section. The third, last component is a straight line, going upwards from the point at which growth R stopped, at a relatively small angle to the horizontal axis.

The physical meaning of this graph is as follows: the dependence of the resistance on the temperature of the conductor is described as simple as long as the heating value does not exceed a certain value characteristic of this material. Let us give an abstract example: if at a temperature of + 10 ° C the resistance of a substance is 10 ohms, then up to 40 ° C the value of R practically does not change, remaining within the limits of measurement error. But already at 41 ° C there will be a surge of resistance to 70 ohms. If the further temperature rise does not stop, then for each successive degree there will be an additional 5 ohms.

This property is widely used in various electrical devices, so it is natural to give data on copper as one of the most common materials in So, for copper conductor heating for each additional degree leads to an increase in resistance by half a percent from a specific value (can be found in reference tables, is given for 20 ° C, 1 m length with a section of 1 sq. Mm).

When a metallic conductor appears, an electric current appears - directed movement of elementary particles with a charge. The ions located in the metal nodes are not able to hold electrons in their outer orbits for a long time, so they move freely throughout the volume of the material from one node to another. This chaotic movement is due to external energy - heat.

Although the fact of movement is evident, it is not directional, therefore it is not considered as a current. When an electric field appears, the electrons are oriented in accordance with its configuration, forming directional motion. But since the thermal effect has not disappeared anywhere, the randomly moving particles collide with directional fields. The dependence of the resistance of metals on temperature shows the magnitude of interference with the passage of current. The higher the temperature, the higher the R of the conductor.

The obvious conclusion: reducing the degree of heating, you can reduce the resistance. (about 20 ° K) is precisely characterized by a significant decrease in the thermal chaotic motion of particles in the structure of a substance.

The considered property of conductive materials has found wide application in electrical engineering. For example, the dependence of conductor resistance on temperature is used in electronic sensors. Knowing its value for any material, you can make a thermistor, connect it to a digital or analog reader, perform the appropriate graduation of the scale and use as an alternative. Most modern thermal sensors are based on this principle, because the reliability is higher and the design is simpler.

In addition, the dependence of resistance on temperature makes it possible to calculate the heating of the windings of electric motors.

There are various conditions under which charge carriers pass through certain materials. And the direct influence on the charge of an electric current is resistance, which is dependent on the environment. The factors that change the flow of electric current include temperature. In this article we consider the dependence of the conductor resistance on temperature.

Metals

How does temperature affect metals? To find out this dependence, the following experiment was carried out: a battery, an ammeter, a wire and a torch are connected to each other with the help of wires. Then it is necessary to measure the current reading in the circuit. After the readings have been taken, bring the torch to the wire and heat it. When heated wire can be seen that the resistance increases, and the conductivity of the metal decreases.

1. Metal wire
2. Battery
3. Ammeter

Dependence is indicated and justified by the formulas:

From these formulas it follows that R of the conductor is determined by the formula:

An example of the dependence of the resistance of metals on temperature is provided in the video:

You also need to pay attention to such properties as superconductivity. If the ambient conditions are normal, then by cooling, the conductors reduce their resistance. The graph below shows how temperature and resistivity in mercury depend.

Superconductivity is a phenomenon that occurs when the material reaches the critical temperature (Kelvin is closer to zero), at which the resistance drops sharply to zero.

Gases

Gases play the role of dielectric and can not conduct electric current. And in order for it to be formed, charge carriers are needed. Their role is played by ions, and they arise due to the influence of external factors.

Dependence can be seen by example. For the experiment, the same construction as in the previous experiment is used, only the conductors are replaced with metal plates. There must be a small space between them. The ammeter should indicate no current. When placing the burner between the plates, the device will indicate the current that passes through the gas medium.

Below is a graph of the current-voltage characteristics of the gas discharge, where it can be seen that the increase in ionization at the initial stage increases, then the dependence of the current on the voltage remains unchanged (that is, as the voltage increases, the current remains the same) and a sharp increase in current, which leads to the dielectric layer breakdown .

Consider the conductivity of gases in practice. The passage of electric current in gases is used in fluorescent lamps and lamps. In this case, the cathode and the anode, two electrodes are placed in a flask, inside which there is an inert gas. How does such a phenomenon depend on gas? When the lamp turns on, the two filaments are heated, and thermoelectronic emission is created. Inside the flask is covered with phosphor, which emits the light that we see. How does mercury depend on phosphor? Mercury vapors, when electrons bombard them, form infrared radiation, which in turn emits light.

If you apply a voltage between the cathode and the anode, then there is a conductivity of gases.

Liquids

Current conductors in a liquid are anions and cations that move due to an electric external field. Electrons provide little conductivity. Consider the dependence of resistance on temperature in liquids.

1. Electrolyte
2. Battery
3. Ammeter

The dependence of the effect of electrolytes on heating is prescribed by the formula:

Where a is a negative temperature coefficient.

How R depends on heating (t) is shown in the graph below:

Such a relationship should be taken into account when charging batteries and batteries.

Semiconductors

And how does the resistance depend on heating in semiconductors? To begin, let's talk about thermistors. These are devices that change their electrical resistance under the influence of heat. This semiconductor temperature coefficient of resistance (TKS) is much higher than metals. Both positive and negative conductors, they have certain characteristics.

Where: 1 is TKS less than zero; 2 - TKS is greater than zero.

In order for conductors such as thermistors to start working, they take as a basis any point on the I – V characteristic:

• if the element temperature is less than zero, then such conductors are used as a relay;
• to control the changing current, as well as what temperature and voltage, use the linear section.

Thermistors are used when checking and measuring electromagnetic radiation, which is carried out at ultrahigh frequencies. Because of this, these conductors are used in systems such as fire alarms, heat testing and control of the use of bulk solids and liquids. Those thermistors, in which TKS is less than zero, are used in cooling systems.

Now about thermoelements. How does the Seebeck effect on thermoelements? Dependence is that such conductors function on the basis of this phenomenon. When the temperature of the junction rises when heated, an emf appears at the junction of the closed circuit. Thus, their dependence is manifested and thermal energy is converted into electricity. To fully understand the process, I recommend to study our instructions on how

Many metals, for example, such as copper, aluminum, silver, have the property of conduction of electric current due to the presence of free electrons in their structure. Also, metals have some resistance to current, and each has its own. The resistance of a metal is strongly dependent on its temperature.

You can understand how metal resistance depends on temperature, if you increase the temperature of the conductor, for example, in the area from 0 to t2 ° C. With increasing conductor temperature, its resistance also increases. Moreover, this dependence is almost linear.

From a physical point of view, an increase in resistance with increasing temperature can be explained by an increase in the amplitude of oscillations of the nodes of the crystal lattice, which in turn makes the passage of electrons more difficult, that is, the resistance to electric current increases.

Looking at the graph you can see that at t1 the metal has a much lower resistance than, for example, at t2. With a further decrease in temperature, you can come to the point t0, where the conductor resistance will be almost zero. Of course, his resistance is zero can not be, but only tends to him. At this point, the conductor becomes a superconductor. Superconductors are used in strong magnets as a winding. In practice, this point lies much further, in the region of absolute zero, and it is impossible to determine it according to this schedule.

For this graph, you can write the equation

Using this equation, you can find the resistance of the conductor at any temperature. Here we need the point t0 obtained earlier in the graph. Knowing the temperature at this point for a particular material, and the temperature t1 and t2, we can find resistance.

The change in resistance with temperature is used in any electric machine where direct access to the winding is not possible. For example, in an asynchronous motor, it is enough to know the stator resistance at the initial moment of time and at the moment when the engine is running. By simple calculations, it is possible to determine the temperature of the engine, which is done automatically in production.

« Physics - Grade 10

What physical quantity is called resistance
What and how does the resistance of the metallic conductor depend on?

Different substances have different resistivity. Does the resistance depend on the condition of the conductor? from its temperature? The answer must give experience.

If you pass the current from the battery through a steel coil, and then begin to heat it in the flame of the burner, then the ammeter will show a decrease in current. This means that as the temperature changes, the resistance of the conductor changes.

If at a temperature equal to 0 ° С, the resistance of the conductor is equal to R 0, and at temperature t it is equal to R, then the relative change in resistance, as experience shows, is directly proportional to the change in temperature t:

The coefficient of proportionality α is called the temperature coefficient of resistance.

Temperature coefficient of resistance  - the value equal to the ratio of the relative change in the resistance of the conductor to the change in its temperature.

It characterizes the dependence of the resistance of a substance on temperature.

The temperature coefficient of resistance is numerically equal to the relative change in the resistance of the conductor when heated by 1 K (by 1 ° C).

For all metallic conductors, the coefficient α\u003e 0 and varies slightly with temperature. If the temperature change interval is small, then the temperature coefficient can be considered constant and equal to its average value in this temperature range. Pure metals

In electrolyte solutions, the resistance with increasing temperature does not increase, but decreases. For them α< 0. Например, для 10%-ного раствора поваренной соли α = -0,02 К -1 .

When the conductor is heated, its geometrical dimensions change slightly. Conductor resistance varies mainly due to changes in its resistivity. You can find the dependence of this resistivity on temperature, if in the formula (16.1) to substitute the values The calculations lead to the following result:

ρ = ρ 0 (1 + αt), or ρ = ρ 0 (1 + αΔТ), (16.2)

where ΔT is the change in absolute temperature.

Since a varies little with the temperature of the conductor, we can assume that the resistivity of the conductor depends linearly on temperature (Fig. 16.2).

The increase in resistance can be explained by the fact that with increasing temperature the amplitude of oscillations of ions in the nodes of the crystal lattice increases, so free electrons collide with them more often, losing the direction of motion. Although the coefficient a is rather small, taking into account the dependence of resistance on temperature when calculating the parameters of heating devices is absolutely necessary. Thus, the resistance of a tungsten filament of an incandescent lamp increases when current passes through it due to heating by more than 10 times.

In some alloys, for example, in a copper-nickel alloy (Constantin), the temperature coefficient of resistance is very small: α ≈ 10 -5 K -1; Constantine resistivity is large: ρ ≈ 10 -6 Ω m. Such alloys are used to make reference resistors and additional resistors to measuring instruments, i.e., in those cases where it is required that the resistance does not noticeably change with temperature fluctuations.

There are also such metals, for example, nickel, tin, platinum, etc., whose temperature coefficient is much higher: α ≈ 10 -3 K -1. The dependence of their resistance on temperature can be used to measure the temperature itself, which is carried out in resistance thermometers.

Devices based on temperature are based on devices made of semiconductor materials, - thermistors. They are characterized by a large temperature coefficient of resistance (tens of times higher than that of metals), the stability of the characteristics over time. The nominal resistance of thermistors is significantly higher than that of metallic resistance thermometers, it is usually 1, 2, 5, 10, 15 and 30 kΩ.

Usually, platinum wire is taken as the main working element of a resistance thermometer, its dependence on temperature is well known. Temperature changes are judged by changes in the resistance of the wire, which can be measured. Such thermometers can measure very low and very high temperatures when ordinary liquid thermometers are unsuitable.

Superconductivity.

The resistance of metals decreases with decreasing temperature. What happens when the temperature tends to absolute zero?

In 1911, the Dutch physicist X. Kamerlingh Onnes discovered a remarkable phenomenon - superconductivity. He found that when mercury cooled in liquid helium, its resistance initially changes gradually, and then at a temperature of 4.1 K it drops very sharply to zero (Fig. 16.3).

The phenomenon of falling to zero conductor resistance at a critical temperature is called superconductivity.

The discovery of Kamerlingh Onnes, for which in 1913 he was awarded the Nobel Prize, led to the study of the properties of substances at low temperatures. Later many other superconductors were discovered.

Superconductivity of many metals and alloys is observed at very low temperatures - starting at about 25 K. The reference tables give the transition temperatures to the superconducting state of certain substances.

The temperature at which a substance enters a superconducting state is called critical temperature.

The critical temperature depends not only on the chemical composition of the substance, but also on the structure of the crystal itself. For example, gray tin has a diamond structure with a cubic crystal lattice and is a semiconductor, and white tin has a tetragonal unit cell and is a silvery-white, soft, ductile metal, capable of transitioning to a superconducting state at a temperature of 3.72 K.

For substances in the superconducting state, sharp anomalies of magnetic, thermal, and a number of other properties were noted, so it is more correct to speak not of the superconducting state, but of the special state of matter observed at low temperatures.

If a current is created in the superconducting ring conductor and then the current source is removed, then the strength of this current does not change indefinitely. In the usual (non-superconducting) conductor, the electric current in this case is terminated.

Superconductors are widely used. So, they build powerful electromagnets with a superconducting winding, which create a magnetic field for long periods of time without energy. After all no heat is generated in the superconducting winding.

However, to obtain an arbitrarily strong magnetic field using a superconducting magnet is impossible. A very strong magnetic field destroys the superconducting state. Such a field can also be created by a current in the superconductor itself. Therefore, for each conductor in the superconducting state, there is a critical value of current, which cannot be exceeded without breaking the superconducting state.

Superconducting magnets are used in accelerators of elementary particles, magnetohydrodynamic generators, which convert the mechanical energy of a jet of red-hot ionized gas moving in a magnetic field into electrical energy.

Explanation of superconductivity is possible only on the basis of quantum theory. It was given only in 1957 by American scientists J. Bardin, L. Cooper, J. Schrieffer and Soviet scientists, academician N. N. Bogolyubov.

In 1986, high-temperature superconductivity was discovered. Complex oxide compounds of lanthanum, barium and other elements (ceramics) with a transition temperature to the superconducting state of about 100 K were obtained. This is higher than the boiling point of liquid nitrogen at atmospheric pressure (77 K).

In the near future, high-temperature superconductivity will most likely lead to a new technical revolution in all electrical engineering, radio engineering, and computer design. Now progress in this area is hampered by the need to cool the conductors to the boiling point of expensive gas - helium.

The physical mechanism of superconductivity is rather complicated. In a very simplistic way, it can be explained as follows: electrons unite in the right rank and move without colliding with a crystal lattice consisting of ions. This motion is significantly different from the usual thermal motion, in which the free electron moves chaotically.

Hopefully, it will be possible to create superconductors at room temperature. Generators and electric motors will become extremely compact (they will decrease several times) and economical. Electricity can be transferred to any distance without loss and accumulate in simple devices.

\u003e\u003e Physics: Dependence of conductor resistance on temperature

Different substances have different resistivity (see § 104). Does the resistance depend on the condition of the conductor? from its temperature? The answer must give experience.
  If you pass the current from the battery through the steel coil, and then begin to heat it in the flame of the burner, the ammeter will show a decrease in current. This means that as the temperature changes, the resistance of the conductor changes.
  If at a temperature of 0 ° C, the resistance of the conductor is R 0, and at a temperature t  it is equal R, then the relative change in resistance, as experience shows, is directly proportional to the change in temperature. t:

Coefficient of proportionality α   called temperature coefficient of resistance. It characterizes the dependence of the resistance of a substance on temperature. The temperature coefficient of resistance is numerically equal to the relative change in the resistance of the conductor when heated by 1 K. For all metallic conductors, the coefficient α   \u003e 0 and varies slightly with temperature. If the temperature change interval is small, then the temperature coefficient can be considered constant and equal to its average value in this temperature range. Pure metals α ≈ 1/273 K -1. Have electrolyte solutions resistance with increasing temperature does not increase, but decreases. For them α < 0. Например, для 10%-ного раствора поваренной соли α ≈ -0.02 K -1.
  When the conductor is heated, its geometrical dimensions change slightly. Conductor resistance varies mainly due to changes in its resistivity. You can find the dependence of this resistivity on temperature, if in the formula (16.1) to substitute the values
. The calculations lead to the following result:

Because α   changes little with the temperature of the conductor, we can assume that the resistivity of the conductor depends linearly on temperature ( rice.16.2).

The increase in resistance can be explained by the fact that with increasing temperature the amplitude of oscillations of ions in the lattice sites increases, so free electrons collide with them more often, losing the direction of motion. Although the coefficient α   rather small, taking into account the dependence of resistance on temperature when calculating heating devices is absolutely necessary. Thus, the resistance of a tungsten filament of an incandescent lamp increases when more than 10 times the current passes through it.
  In some alloys, for example in copper-nickel (constantan), the temperature coefficient of resistance is very small: α   ≈ 10 -5 K -1; Constantan resistivity is large: ρ   ≈ 10 -6 ohm m. Such alloys are used for the manufacture of reference resistances and additional resistances to measuring instruments, i.e., in those cases where it is required that the resistance does not change appreciably with temperature fluctuations.
  The dependence of metal resistance on temperature is used in resistance thermometers. Usually, platinum wire is taken as the main working element of such a thermometer, its dependence on temperature is well known. Temperature changes are judged by the change in the resistance of the wire, which can be measured.
  Such thermometers can measure very low and very high temperatures when conventional liquid thermometers are unsuitable.
The resistivity of metals increases linearly with increasing temperature. In electrolyte solutions, it decreases with increasing temperature.

???
  1. When does a light bulb consume more power: immediately after switching it on to the network or after a few minutes?
  2. If the resistance of a coil of a cooker did not change with temperature, then its length at rated power should be greater or smaller?

G.Ya.Myakishev, B.B. Bukhovtsev, N.N.Sotsky, Physics 10th grade

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