Resistivity is a physical quantity used to indicate the electrical resistance characteristics of various materials. The resistance of a wire made of a material that is 1 meter long and has a cross-sectional area of ​​1 square millimeter is called the resistivity of this material.
Unit of resistivity
In the International System of Units, the unit of resistivity is ohm·meter (Ω·m or ohmm), and the common unit is ohm·mm 2 /m.
Resistivity calculation formula
The formula for calculating the resistivity is: Ï = RS / L. p is the resistivity, s is the cross-sectional area, R is the resistance value, and L is the length of the wire.
Description of resistivity
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, 1OO W lamp filament is 484 ohms when energized, and only about 40 ohms when not energized.
3 resistivity and resistance are two different concepts. Resistivity is a property that reflects the effect of a substance on the current. Resistance is a reflection of the object's resistance to current.
4 The use of resistivity, the resistivity divided by the thickness of the metal film to obtain the so-called sheet resistance, commonly used in engineering applications to measure the amount of resistance is one of the resistivity is a sheet resistance; the size of the resistor is the square resistance multiplied by the length and width of the metal block ratio.
Resistivity of metal conductors (table)
Resistivity of several metal conductors at 20 ° C
Material resistivity (Ω m)
(1) Silver 1.6 × 10-8 (5) Platinum 1.0 × 10-7 (9) Constantan 5.0 × 10-7
(2) Copper 1.7 × 10-8 (6) Iron 1.0 × 10-7 (10) Nickel-chromium alloy 1.0 × 10-6
(3) Aluminum 2.9 × 10-8 (7) Mercury 9.6 × 10-7 (11) Iron-chromium aluminum alloy 1.4 × 10-6
(4) Tungsten 5.3 × 10-8 (8) Manganese copper 4.4 × 10-7 (12) Alnico 1.6 × 10-6
(13) Graphite (8 ~ 13) × 10-6
It can be seen that the resistivity of the metal is small, the resistivity of the alloy is large, the non-metal and some metal oxides are larger, and the electrical resistivity of the insulator is extremely large. The resistivity of germanium, silicon, selenium, copper oxide, boron, etc. is smaller than that of the insulator and larger than that of the metal. We call these materials semiconductors.
Summary: Under normal conditions (known from the table), the best conductivity is silver, copper, aluminum. These three materials are the most commonly used, and are often used as wires, among which copper is the most widely used, almost the current wire. All copper (exact instruments, except for special occasions) aluminum wire has been eliminated due to chemical instability and oxidation. Silver has the best conductivity but is rarely used due to its high cost. It is only used in high-demand applications such as precision instruments, high-frequency oscillators, and aerospace. By the way, gold is used. In some cases, the contacts on the instrument are also useful in gold. That is because the chemical nature of gold is stable, not because its electrical resistivity is small.
Other metal resistivity
Metal temperature (0 ° C) Ï Î±o , 100
Zinc 20 × 10-3 × 10-3
5.9 4.2
Aluminum (soft) 20 2.75 4.2
Aluminum (soft) –78 1.64
Alumel Alloy 20 33 1.2
锑0 38.7 5.4
铱20 6.5 3.9
Indium 0 8.2 5.1
Invar 0 75 2
锇20 9.5 4.2
Cadmium 20 7.4 4.2
Potassium 20 6.9 5.11
Calcium 20 4.6 3.3
Gold 20 2.4 4.0
Silver 20 1.62 4.1
Chrome (soft) 20 17
Nickel-chromium alloy (Klummer) — 70—110 .11—.54
Cobalt a 0 6.37 6.58
Connaught Copper — 50 –.04–1.01
Zirconium 30 49 4.0
Brass – 5–7 1.4–2
Mercury 0 94.08 0.99
Mercury 20 95.8
Tin 20 11.4 4.5
锶0 30.3 3.5
Bronze – 13–18 0.5
铯20 21 4.8
é“‹20 120 4.5
é“Š20 19 5
Tungsten 20 5.5 5.3
Tungsten 1000 35
Tungsten 3000 123
Tungsten –78 3.2
é’½20 15 3.5
Metal temperature (0 ° C) Ï Î±o , 100
Dura Aluminum (Soft) — 3.4
Iron (pure) 20 9.8 6.6
Iron (pure) – 78 4.9
Iron (steel) — 10-20 1.5-5
Iron (cast) — 57-114
Copper (soft) 20 1.72 4.3
Copper (soft) 100 2.28
Copper (soft) –78 1.03
Copper (soft) –183 0.30
é’20 18 2.4
Sodium 20 4.6 5.51
Lead 20 21 4.2
Nickel-chromium alloy (without iron) 20 109 .10
Nickel-chromium alloy (iron-containing) 20 95—104 .3—.5
Nickel-chromium alloy - 27-45 .2—.34
Nickel (soft) 20 7.24 6.7
Nickel (soft) – 78 3.9
Platinum 20 10.6 3.9
Platinum 1000 43
Platinum–78 6.7
Platinum rhodium alloy 2 20 22 1.4
Palladium 20 10.8 3.7
Arsenic 20 35 3.9
Nickel copper and zinc resistance wire — 34—41 .25—.32
é“ (soft) 20 6.4
Magnesium 20 4.5 4.0
Manganese copper 20 42-48 –03—+.02
Molybdenum 20 5.6 4.4
Foreign silver — 17—41 .4—.38
Lithium 20 9.4 4.6
Phosphor bronze - 2-6
é“·20 12.5 5.5
é“‘20 5.1 4.4
The average temperature coefficient between 10 ° C and the melting point is 2 platinum 90%, 铑 10% * If the resistivity unit is expressed in ohm cm (Ωcm ), the value in the table should be expanded by 100 times.
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