Resistivity is a physical quantity used to indicate the electrical resistance characteristics of various materials. The ratio of the product of the resistance of an original made of a substance (20 ° C at normal temperature) to the cross-sectional area is called the resistivity of the substance. The resistivity is independent of the length of the conductor, the cross-sectional area, etc., and is the electrical property of the conductor material itself, determined by the material of the conductor and temperature dependent.
The unit of resistivity in the International System of Units is Ω·m, which is pronounced as ohmic meters, or Omega. The common unit is "ohm·cm".
basic introduction
definition:
Resistivity is a physical quantity used to indicate the electrical resistance characteristics of various materials.
In the case of a constant temperature, there is a formula R = Ïl / S where Ï 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 the length of the material and inversely proportional to its cross-sectional area. From the above formula, the definition of resistivity is known: Ï=RS/l
Application range
Substances with lower resistivity are called conductors. Common conductors are mainly metals, and silver is the most conductive in nature. When an external electric field is present, the free electrons of the metal collide with the positive ions of the thermal oscillator at the lattice nodes during the movement, which hinders the movement of the electrons and thus has a certain resistance. Other non-conductive materials such as glass, rubber, etc., have a high electrical resistivity and are generally referred to as insulators. A substance (such as silicon) between a conductor and an insulator is called a semiconductor. The scientific symbol of resistivity is Ï(Rho). The resistance of the known object can be calculated from the resistivity Ï, the length l and the cross-sectional area A: Ï=RA/I, in which the resistance R is in ohms, the length l is in meters, and the cross-sectional area A is in square meters. The resistivity Ï is in ohm·meter (Ω·m)
company name
In the International System of Units, the unit of resistivity is ohm·meter (Ω·m or ohmm), and the common units are ohm·mm and ohm·m.
Calculation formula
The formula for calculating the resistivity is: Ï=RS/L
Ï is the resistivity - the common unit Ω·m
S is the cross-sectional area - common unit m2
R is the resistance value - common unit Ω
L is the length of the wire - common unit m
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Another formula for calculating resistivity is: Ï=E/J
Ï is the resistivity - the common unit Ω·m
E is the electric field strength - commonly used unit N / C
J is the current density - commonly used unit A / m2
(E, J can be a vector)
For the same substance, under the same conditions: the resistivity is constant, that is, it is only related to the temperature of the object of the type of conductor (such as copper, aluminum), and to the shape of the object (such as length, Cross-sectional area, etc.) are irrelevant. Because it describes the conductivity of the substance. R=p*l/s This formula is based on the relationship between the resistance of the conductor and the length, cross-sectional area, and resistivity under the same conditions. It is mainly used to find the resistance. For example, the resistance is like chemistry. The amount of matter, and the resistivity corresponds to the amount of matter. The concentration of the substance is only related to which kind of solution, but it has nothing to do with its volume. You said, a bottle of Cu2SO4 solution, I took a test tube from it, did the amount of the substance change? No, I think this Can you understand? But the amount of its substance has changed. Similarly, a copper wire, you cut it in half, make it thinner, only the size of its resistance, and can not change its resistivity. The amount of matter = the concentration of the substance *V only the parameters of this formula are small, and the resistance is much more. If you want to know the formula of resistivity, it is related to temperature (substantially the speed of electron movement), the number of protons in the nucleus, the number of neutrons, the number of electrons in the atom, and the distribution of electrons in the electron. You have to learn some electron dynamics. ... a lot of micro mechanics.
Resistivity unit, formula and conversion
Resistivity
Unit of resistivity: In the International System of Units, the unit of resistivity is ohm·m (Ω.cm), and the common unit is ohm·mm 2 /m.
Conductor resistivity
The resistivity r of a point in the conductor material is defined as the ratio of the magnitude of the electric field strength E at that point to the magnitude of the current density j at the same point, ie
(10-10)
A conductor of uniform cross-section made of a certain material, if the length is l and the cross-sectional area is S, the resistance of the conductor can be proved by the formula (10-10).
.(10-11)
The electrical resistivity of the conductor material is determined by the nature of the material itself. The resistivity of various materials varies with temperature. In the normal temperature range, the resistivity of the metal material changes linearly with temperature, and the relationship of change can be expressed as
r=r0( 1+at) , (10-12)
Where r is the resistivity at t ° C, r 0 is the resistivity at 0 ° C, and a is called the temperature coefficient of resistance. The r0 and a values ​​for some metals, alloys, and carbon are listed in Table 10-1.
Table 10-1 r0 and a values ​​for some materials
material
R0/(W×m)
a/°C-1
silver
1.49×10-8
4.3×10-3
copper
1.55×10-8
4.3×10-3
aluminum
2.50×10-8
4.7×10-3
Carbon (amorphous)
3500×10-8
-4.6×10-4
Nickel-chromium alloy
(60% Ni, 15% Cr, 25% Fe)
110×10-8
1.6×10-4
It can be seen from the data in the table that the a value of pure metal is about 0.4%, which means that the electrical resistivity increases by about 0.4% for every 1 °C increase in temperature. The linear expansion coefficient of these materials is much smaller, and the linearity increases by only about 0.001% for every 1 °C increase in temperature. Therefore, when considering the resistance of the metal conductor as a function of temperature, the change in length l and cross-sectional area S can be ignored. Multiplying the two sides of the formula (10-12) by l/S, the relationship between the resistance of the metal conductor and the temperature is obtained.
R=R0(1+at) , (10-13)
Where R is the resistance of t ° C and R 0 is the resistance of 0 ° C. According to this linear relationship, a resistance thermometer can be made for temperature measurement. Commonly used resistance thermometers are copper resistance thermometers (-50 ° C ~ 150 ° C) and platinum resistance thermometers (-200 ° C ~ 500 ° C).
In the International System of Units, the unit of resistivity is W × m (ohm × m). The reciprocal of resistivity is called conductivity, which is usually expressed by s, and its unit is S × m-1 (Siemens / meter).
The resistivity of some materials will decrease to near zero below their specific temperature TC. This phenomenon is called superconductivity, and the material in superconducting state is called superconductor. The temperature TC is called the superconducting transition temperature, and different materials have different transition temperatures. The transition temperature of titanium is 0.39 K, the aluminum is 1.19 K, the lead is 7.2 K, the Nb3Sn is 18.1 K, the Nb3Ge is 23.2 K, and the La-Ba-Cu-O oxide is 46. The K, Y-Ba-Cu-O system oxide is 90 K, the Tl-Ba-Ca-Cu-O system oxide is 125 K, and the Hg-Ba-Ca-Cu-O system oxide is 134 K.
Special Instructions
1. The resistivity Ï is not only related to the material of the conductor but also to the temperature of the conductor. In the range where the temperature does not change much: the resistivity of almost all metals changes linearly with temperature, that is, Ï = Ïo (1 + at). Where t is the Celsius temperature, Ïo is the resistivity at 0 °C, and a is the resistivity temperature coefficient.
2. Since the resistivity changes with temperature, the resistance of some appliances must be stated in their physical state. For example, the resistance of a 220 V -100 W lamp filament is 484 ohms when energized and only about 40 ohms when not energized.
3. Resistivity and resistance are two different concepts. The resistivity is an attribute that reflects the effect of the substance on the current. The resistance is an attribute that reflects the object's resistance to current.
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