1. R=L/S (R resistance S sectional area L length resistivity)
2. Knowledge expansion:
3. Resistivity is a physical quantity used to express the resistance characteristics of various substances. The resistance of a conductor made of a material with a cross-sectional area of 1 m and 1 mm at room temperature (20) is called the resistivity of the material.
4. At a certain temperature, there is a formula R=l/s, where is the resistivity, l is the length of the material, and s is the area. It can be seen that the resistance of the material is proportional to its length and inversely proportional to its area. The above formula gives the definition of resistivity:=Rs/l resistivity is not only related to the conductor material, but also related to the conductor temperature. In the range of small temperature change, the resistivity of almost all metals changes linearly with temperature, that is,=o (1 at). Where, t is the temperature in degrees Celsius, o is the resistivity at 0, and A is the temperature coefficient of resistivity.
5. Since the resistivity changes with temperature, it is necessary to explain the physical state of the resistance of some electrical appliances. For example, when the power is on, the resistance of the 220 V low-voltage filament is 484 ohms, while when the power is off, it is only about 40 ohms. Resistivity and resistance are two different concepts. Resistivity is an attribute that reflects the blocking effect of material on current, and resistance is an attribute that reflects the blocking effect of material on current. The resistivity of common metal conductor at 20 is (m) (1) silver 1.6510-8 (2) copper 1.7510-8 (3) aluminum 2.8310-8 (4) tungsten 5.4810-8 (5) iron 9.78 10-8.
6. (9) Kangtong 5.010-7 (10) Ni Cr alloy 1.010-6 (11) Fe Cr Al alloy 1.410-6
7. (12) Al Ni Fe alloy 1.610-6 (13) graphite (8~13) 10-6.