One of the characteristics of any material that conducts electric current is the dependence of resistance on temperature. If it is depicted in the form of a graph on the coordinate plane, where time intervals (t) are marked on the horizontal axis and ohmic resistance value (R) on the vertical axis, then a broken line will be obtained. The temperature dependence of the resistance consists schematically of three sections. The first corresponds to a slight heating - at this time, the resistance changes very slightly. This happens until a certain point, after which the line on the chart goes up sharply - this is the second section. The third, last component is a straight line, extending 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 dependence of the resistance on temperature of the conductor is described by a simple linear equation until the value of heating exceeds some 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 R value will not practically 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 increase in temperature does not stop, then an additional 5 Ohms will fall on each subsequent degree.
This property is widely used in various electrical devices, therefore, it is logical to give data on copper as one of the most common materials in electric machines. So, for a copper conductor, heating for every 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 cross-section of 1 square mm).
When an electromotive force EMF appears in a metal conductor, an electric current appears - a directed movement of elementary particles with a charge. Ions located in the nodes of the crystal lattice of a metal are not able to hold electrons in their outer orbits for a long time, so they freely move throughout the entire volume of the material from one node to another. This chaotic movement is caused by 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 directional movement. But since the thermal effect has not disappeared anywhere, randomly moving particles collide with the directed field. The dependence of the resistance of metals on temperature shows the amount of interference with 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 decrease in the chaotic thermal motion of particles in the structure of matter.
The considered property of conductive materials has found wide application in electrical engineering. For example, the temperature dependence of the resistance of a conductor is used in electronic sensors. Knowing its value for any material, it is possible to manufacture a thermistor, connect it to a digital or analog reader, perform the appropriate graduation of the scale and use it as an alternative to mercury thermometers. Most modern temperature sensors are based on just such a principle, because reliability is higher and the design is simpler.
In addition, the temperature dependence of the resistance makes it possible to calculate the heating of the motor windings.