| 1resistivity/specific resistance 2resistance 3opposed 4reciprocal 5conductivity 6conductance | A B C D E , F |
2 
Read the text and decide whether the following statements are true or false. Give your reasons.
1 Resistivity is the resistance of a length of conductor at a given temperature to voltage.
2 A substance having a high resistivity will have high conductivity as well.
3 Resistivity of conductors doesnt depend on temperature.
4 Conductors have lower values of resistivity than insulators.
5 Conductance of a material is the reciprocal of its conductivity.
6 The unit for resistivity is siemens per metre.
Resistivity
Resistivity is the resistance in ohms offered by a unit cube of a substance to the flow of electric current.
Resistivity (also known as specific resistance) is a measure indicating how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electrons. The SI unit for resistivity is the ohm metre.
The resistance of a conductor of a uniform cross-section varies directly as the product of the length and the resistivity of the conductor and inversely as the cross-sectional area of the conductors. Therefore, you can calculate the resistance of a conductor if you know the length, cross-sectional area and resistivity of the substance.
Resistivity is given the Greek symbol ρ (pronounced rho). The resistance of a material is usually defined by:
where
ρ is the resistivity (measured in ohm metres)
R is the electrical resistance (measured in ohms)
l is the length (measured in metres)
A is the cross-sectional area (measured in square metres)
Typical values for the resistivity of materials used in electrical engineering are given in Table 1. The materials manganin and constantan have high resistivity combined with a low temperature coefficient of resistance, and are widely used in ammeter shunts and in voltmeter multiplier resistors. Nichrome has a very high resistivity and is used as a conductor in heating elements in fires and ovens, and operates satisfactorily at temperatures up to 1100oC.
It should be noted that Table 1 may vary with temperature. For example, the resistivity of copper increases to 1,73×10-8 Wm at 20oC, but the resistivity of constantan remains unchanged when the temperature changes from 0o to 20oC.
Table 1. Resistivity of metals at 0oC
| Material | Resistivity (Wm) |
| Silver | 1.47×10-8 |
| Copper | 1.55×10-8 |
| Aluminium | 2.5×10-8 |
| Zinc | 5.5×10-8 |
| Nickel | 6.2×10-8 |
| Iron | 8.9×10-8 |
| Manganin | 41.5×10-8 |
| Constantan | 49.0×10-8 |
| Nichrome | 108.3×10-8 |
Insulators have much higher values of resistivity, typical values in Wm being
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|
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Glass 109-1012
Mica and mineral oil 1011-1015
Plastic 107-109
Wood 108-1011
Water (distilled) 102-105
Resistivity is the reciprocal of conductivity. It is given the Greek symbol σ (sigma) and has the units siemens per metre (s/m).
A substance that has a high resistivity will have a low conductivity, and vice versa.
3 
Solve the problem and comment on its solution.
Problem Determine the resistance of a 200m length of copper wire of diameter 1 mm at a temperature of 20oC. The resistivity of copper at this temperature is 1.7310-8 Wm.
4 
Fill in the gaps in the sentences with an appropriate word. Compare your answers with your partner.
1 Resistivity indicates how strongly a material the flow .
2 Resistance can be calculated if you know the length, the resistivity and of a conductor.
3 You should remember that resistivity varies with .
4 In the table you can find typical for the resistivity of materials.
5 Cross-sectional area of a material is measured in .
5 
Speaking task. Put the questions in the correct order and ask your partner.
1 be how can defined resistivity?
2 to the calculate what resistance are factors used?
3 at copper what 0oC have resistivity does?
4 conductivity is of what measurement unit of the?
5 temperature is between what resistivity relationship and the?
