Dependence of resistance on temperature

One of the characteristics of any conductive material is the resistance to temperature. If it is depicted as a graph on the coordinate plane, where time intervals (t) are marked along the horizontal axis and the value of the ohmic resistance (R) is vertical, a broken line is obtained. The temperature dependence of the resistance consists schematically of three sections. The first corresponds to a slight heating - at this time the resistance varies very slightly. This happens until a certain point, after which the line on the chart abruptly goes up - this is the second section. The third, the last component - is a straight line that extends upward 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 resistance versus temperature dependence of the conductor is described by a simple linear equation until the heating value exceeds some value characteristic for the given material. Let's give an abstract example: if at a temperature of + 10 ° C the resistance of the substance is 10 Ohm, then up to 40 ° C the value of R practically does not change, remaining within the error of measurement. But already at 41 ° C there will be a jump in resistance up to 70 Ohm. If the further increase 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 cite data on copper as one of the most common materials in electric machines. Thus, for a copper conductor, heating for each additional degree leads to an increase in resistance by half a percentage of the specific value (can be found in the reference tables, given for 20 ° C, 1 m in length with a section of 1 mm2).

When an EMF electromotive force appears in the metallic conductor, an electric current appears-a directional movement of elementary particles possessing a charge. Ions that are in the nodes of the crystal lattice of a metal can not hold electrons for long in their outer orbits, so they freely move throughout the volume of material from one node to another. This chaotic movement is due to external energy - heat.

Although the fact of displacement is obvious, 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 a directed motion. 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 to the passage of current. The higher the temperature, the higher the R conductor.

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

This property of conductive materials has found wide application in electrical engineering. For example, the dependence of the 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 reading device, perform the appropriate scale calibration and use as an alternative to mercury thermometers. At the heart of most modern thermo sensors this is the principle, because reliability is higher, and design is simpler.

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