Resistivity, and hence the resistance of metals, depends on temperature, increasing with its growth. The temperature dependence of the conductor resistance is explained by the fact that
- the scattering intensity (number of collisions) of charge carriers increases with increasing temperature;
- their concentration changes when the conductor is heated.
Experience shows that when not too high and not too low temperatures dependencies resistivity and the resistance of the conductor from temperature are expressed by the formulas:
where are the specific resistances of the conductor substance, respectively, at 0 ° C and t ° C; R 0, R t - conductor resistance at 0 ° С and t ° С, - temperature coefficient of resistance: measured in SI in Kelvin to the minus first degree (K -1). For metallic conductors, these formulas are applicable from temperatures of 140 K and above.
Substances are characterized by the dependence of the change in resistance upon heating on the type of substance. It is numerically equal to the relative change in the resistance (resistivity) of the conductor when heated by 1 K.
where is the average value temperature coefficient resistance in the interval.
For all metallic conductors> 0 and varies slightly with temperature. For pure metals = 1/273 K -1. In metals, the concentration of free charge carriers (electrons) is n = const and the increase occurs due to an increase in the intensity of scattering of free electrons on the ions of the crystal lattice.
For electrolyte solutions 0, for example, for a 10% sodium chloride solution = -0.02 K -1. The resistance of electrolytes decreases with increasing temperature, since the increase in the number of free ions due to the dissociation of molecules exceeds the increase in the scattering of ions in collisions with solvent molecules.
The formulas for the dependence of and R on temperature for electrolytes are similar to the above formulas for metal conductors. It should be noted that this linear dependence remains only in a small range of temperature variation, in which = const. At large intervals of temperature variation, the dependence of the resistance of electrolytes on temperature becomes nonlinear.
Graphically, the dependence of the resistance of metal conductors and electrolytes on temperature is shown in Figures 1, a, b.
At very low temperatures, close to absolute zero (-273 ° C), the resistance of many metals abruptly drops to zero. This phenomenon is called superconductivity. The metal goes into a superconducting state.
The dependence of the resistance of metals on temperature is used in resistance thermometers. Usually, a platinum wire is taken as the thermometric body of such a thermometer, the dependence of the resistance of which on temperature has been sufficiently studied.
Temperature changes are judged by the change in wire resistance that can be measured. Such thermometers allow you to measure very low and very high temperatures when conventional liquid thermometers are not suitable.
One of the characteristics of any conductive material is the dependence of resistance on temperature. If you depict it in the form of a graph on where the time intervals (t) are marked on the horizontal axis, and the ohmic resistance value (R) is marked along the vertical axis, then you get a broken line. The dependence of resistance on temperature schematically consists of three sections. The first corresponds to a slight heating - at this time, the resistance changes very slightly. This happens until a certain moment, after which the line on the chart goes up sharply - this is the second section. The third, last component is a straight line going up from the point at which the growth of R stopped, below the relatively slight 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 some value characteristic of this particular 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 will practically not change, remaining within the measurement error. But already at 41 ° C, there will be a jump in resistance up to 70 ohms. If the further rise in temperature does not stop, then for each subsequent degree there will be an additional 5 ohms.
This property is widely used in various electrical devices, therefore, it is natural to give data on copper as one of the most common materials in So, for a copper conductor, heating for each additional degree leads to an increase in resistance by half a percent of the specific value (can be found in reference tables, given for 20 ° C, 1 m length with a cross section 1 sq. Mm).
When it occurs in a metal conductor, an electric current appears - a directed movement of elementary particles that have a charge. The ions located in the nodes of the metal are not able to keep electrons in their outer orbits for a long time, therefore 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 obvious, it is not directional, therefore, it is not considered as a current. When the appearance electric field electrons are oriented in accordance with its configuration, forming a directional movement. But since the thermal effect has not disappeared anywhere, chaotically moving particles collide with directed fields. The dependence of the resistance of metals on temperature shows the magnitude of the interference with the passage of current. The higher the temperature, the higher the R of the conductor.
The obvious conclusion: by reducing the degree of heating, you can also reduce resistance. (about 20 ° K) is precisely characterized by a significant decrease in the thermal chaotic motion of particles in the structure of matter.
The considered property of conducting materials found wide application in electrical engineering. For example, the dependence of the resistance of a conductor 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 reading device, perform the appropriate scale calibration and use it as an alternative.Most modern temperature sensors are based on this principle, because the reliability is higher and the design is simpler.
In addition, the dependence of the resistance on temperature makes it possible to calculate the heating of the electric motor windings.
Thermal resistance, thermistor or thermistor are three names for the same device, the resistance of which changes depending on its heating or cooling.
Advantages of a thermistor:
- ease of manufacture;
- excellent performance under heavy loads;
- stable work;
- the small size of the product allows it to be used in miniature sensors;
- low thermal inertness.
Types of thermistors and how they work
The basis of the sensor is a resistive element, for the manufacture of which semiconductors, metals or alloys are used, that is, elements in which there is a pronounced dependence of resistance on temperature. All materials used in their creation must have a high specific temperature coefficient of resistance.
For the production of thermistors used the following materials and their oxides:
- platinum;
- nickel;
- copper;
- manganese;
- cobalt.
Certain metal halides and chalcogenides can also be used.
If a metal resistive element is used, then it is made in the form of a wire. If semiconductor, then - most often in the form of a plate.
Important! The materials from which the thermal resistance is made must have a large negative temperature (NTC) or positive (PTK) coefficient of resistance.
If the coefficient is negative, then when heated, the resistance of the thermistor decreases, if it is positive, it increases.
Metal thermistors
The current in metals is formed due to the movement of electrons. Their concentration does not increase when heated, but the speed of chaotic movement increases. Thus, when heated, the value of the resistivity of the conductor increases.
The dependence of the resistance of metals on temperature is nonlinear and has the form:
Rt = R0 (1 + A t + B t2 + ...), where:
- Rt and R0 are the resistance of the conductor at a temperature of t and 0 ° С, respectively,
- A, B - coefficients that depend on the material. Coefficient A is called the temperature coefficient.
If the temperature does not exceed 100 ° C, then the resistance of the conductor is calculated using the following formula:
Rt = R0 (1 + A t),
and the rest of the coefficients are neglected.
Each type of thermistor has certain limitations for its use. So, for example, copper sensors can be used in temperature range from -50 ° С to + 180 ° С, platinum - from -200 to + 650 ° С, nickel devices - up to 250-300 ° С.
Semiconductor thermistors
For the manufacture of thermistors, oxides CuO, CoO, MnO, etc. are used. During manufacture, the powder is sintered into a detail the desired shape... So that during operation the resistive element is not damaged, it is covered with a protective layer.
In semiconductor devices, the dependence of the resistivity on temperature indicators is also not linear. With its increase in the sensor, the value of R drops sharply due to an increase in the concentration of electric charge carriers (holes and electrons). In this case, we speak of sensors with a negative temperature coefficient. However, there are PTC thermistors that behave like metals when heated, i.e. R increases. Such sensors are called PTC sensors.
The formula for the dependence of the resistance of a semiconductor thermistor on temperature is:
where:
- A - constant characterizing the resistance of the material at t = 20 ° C;
- T is the absolute temperature in degrees Kelvin (T = t + 273);
- B is a constant depending on physical properties semiconductor.
Construction of metal thermistors
There are two main types of instrument design:
- winding;
- thin-braided.
In the first case, the sensor is made in the form of a spiral. The wire is either wound on a cylinder made of glass or ceramic, or placed inside it. If the winding is carried out along the cylinder, then on top it is necessarily covered with a protective layer.
In the second case, a thin substrate made of ceramics, sapphire, copper oxide, zirconium, etc. is used. Metal is sprayed on it thin layer, which is additionally insulated from above. The metal layer is made in the form of a track and is called a meander.
For your information. To protect the thermistor, it is placed in a metal case or covered with a special insulating layer on top.
There are no fundamental differences in the operation of both types of sensors, but film devices operate in a narrower temperature range.
The devices themselves can be made not only in the form of rods, but also beads, discs, etc.
Thermistor Applications
If the thermal resistance is placed in any medium, then its temperature will depend on the intensity of heat transfer between it and the environment. It depends on a number of factors: the physical properties of the medium (density, viscosity, etc.), the speed of the medium, the initial ratio of the temperature indicators of the medium and the thermistor, etc.
Thus, knowing the dependence of the resistance of the conductor on temperature, it is possible to determine quantitative indicators of the medium itself, for example, speed, temperature, density, etc.
One of important characteristics thermistor is its measurement accuracy, that is, how much the real readings of the thermistor differ from laboratory readings. The accuracy of the device is characterized by the tolerance class, which determines the maximum deviation from the declared indicators. The tolerance class is specified as a function of temperature. For example, the tolerance values of class AA platinum sensors are ± (0.1 + 0.0017 | T |), class A - ± (0.15 + 0.002 | T |).
Important! Naturally, when creating a thermal resistance, developers strive to minimize the losses associated with thermal conductivity and radiation of the device itself during operation.
Thermistors are widely used in electronics, systems thermal control, fire systems, etc.
Video
Exists different conditions in which charge carriers pass through certain materials. And on charge electric current direct influence has resistance, which has a dependence on environment... The factors that change the flow of electric current include temperature. In this article, we will look at the dependence of the resistance of a conductor on temperature.
Metals
How does temperature affect metals? To find out this dependence, such an experiment was carried out: a battery, an ammeter, a wire and a torch are connected to each other using wires. Then you need to measure the current reading in the circuit. After the readings have been taken, you need to bring the burner to the wire and heat it up. When the wire is heated, it can be seen that the resistance increases, and the conductivity of the metal decreases.
- Metal wire
- Battery
- Ammeter
Dependency is indicated and justified by the formulas:
From these formulas it follows that the 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 a property as superconductivity. If the ambient conditions are normal, then by cooling, the conductors decrease their resistance. The graph below shows how temperature and resistivity in mercury depends.
Superconductivity is a phenomenon that occurs when a material reaches critical temperature(Kelvin closer to zero), at which the resistance drops sharply to zero.
Gases
Gases play the role of a dielectric and cannot conduct electric current. And in order for it to form, charge carriers are needed. Ions play their role, and they arise due to the influence of external factors.
Dependency can be seen with an example. For the experiment, the same design is used as in the previous experiment, only the conductors are replaced with metal plates. There must be small space... The ammeter should indicate no current. When placing the burner between the plates, the device will indicate the current that flows through the gas medium.
Below is a graph of the current-voltage characteristic gas discharge, where it can be seen that the growth of ionization at the initial stage increases, then the dependence of the current on voltage remains unchanged (that is, with an increase in voltage, the current remains the same) and a sharp increase current strength, which leads to breakdown of the dielectric layer.
Let's 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 anode, two electrodes are placed in a flask, inside which there is inert gas... How does such a phenomenon depend on gas? When the lamp is turned on, the two filaments are heated and thermionic emission is generated. The inside of the bulb is coated with a phosphor that emits the light that we see. How does mercury depend on a phosphor? Mercury vapors, when bombarded with electrons, form infrared radiation, which in turn emits light.
If a voltage is applied between the cathode and the anode, then gas conductivity arises.
Liquids
Current conductors in a liquid are anions and cations, which move due to an external electric field. Electrons provide negligible conductivity. Consider the dependence of resistance on temperature in liquids.
- Electrolyte
- Battery
- Ammeter
The dependence of the effect of electrolytes on heating is prescribed by the formula:
Where a is the negative temperature coefficient.
How R depends on heating (t) is shown in the graph below:
This relationship must be taken into account when charging batteries and batteries.
Semiconductors
And how does the resistance depend on heating in semiconductors? First, let's talk about thermistors. These are devices that change their electrical resistance when exposed to heat. This semiconductor has a temperature coefficient of resistance (TCR) an order of magnitude higher than metals. Both positive and negative conductors, they have certain characteristics.
Where: 1 is TCS less than zero; 2 - TCS is greater than zero.
In order for conductors such as thermistors to start working, any point on the I - V characteristic is taken as a basis:
- if the element temperature is less than zero, then such conductors are used as relays;
- to control the changing current, as well as what temperature and voltage, use a linear section.
Thermistors are used when checking and measuring electromagnetic radiation, which is carried out at ultrahigh frequencies. Due to this, these conductors are used in systems such as fire alarm, checking heat and monitoring the use of bulk media and liquids. Those thermistors with a TCR less than zero are used in cooling systems.
Now about thermoelements. How does the Seebeck phenomenon affect thermoelements? Dependency lies in the fact that such conductors function on the basis of a given phenomenon. When the temperature of the junction rises during heating, an EMF appears at the junction of the closed circuit. Thus, their dependence is manifested and thermal energy turns into electricity. To fully understand the process, I recommend that you study our instructions on how
As the temperature of the conductor rises, the number of collisions of free electrons with atoms increases. Therefore, decreases average speed directional movement of electrons, which corresponds to an increase in the resistance of the conductor.
On the other hand, as the temperature rises, the number of free electrons and ions per unit volume of the conductor increases, which leads to a decrease in the resistance of the conductor.
Depending on the predominance of one factor or another, as the temperature rises, the resistance either increases (metals), or decreases (coal, electrolytes), or remains almost unchanged (metal alloys, for example, mangaiin).
With slight changes in temperature (0-100 ° C), the relative increase in resistance corresponding to heating by 1 ° C, called the temperature coefficient of resistance a, remains constant for most metals.
Having designated - resistance at temperatures, we can write an expression for the relative increment of resistance with increasing temperature from to:
Temperature Coefficient of Resistance Values for various materials are given in table. 2-2.
From expression (2-18) it follows that
The resulting formula (2-20) makes it possible to determine the temperature of the wire (winding) if you measure its resistance at given or known values.
Example 2-3. Determine the resistance of air sticky wires at temperatures if the line length is 400 m, and the cross-section copper wires
Line wire resistance at temperature
