R=ρl/A
Where, ρ is a proportionality constant and it is called specific resistance or resistivity. See figure 15 to get clear picture of this equation.
specific resistance depends on the temperature and nature of the material where as the resistance of the conductor depends on nature of material,temperature and geometrical factors like length and cross section area of the conductor.
fig-15
The SI unit of resistivity is Ω - m
The reciprocal of resistivity is called conductivity (σ). The values of resistivity of material determine their conductivity. Metals with low resistivity behave as good conductors.
Table 3
Resistivity of various materials
| Material | ρ(Ω-m) at 20 °C |
|---|---|
| Silver | 1.59 × 10-8 |
| Copper | 1.68 × 10-8 |
| Gold | 2.44 × 10-8 |
| Aluminium | 2.82 × 10-8 |
| Calcium | 3.36 × 10-8 |
| Tungsten | 5.60 × 10-8 |
| Zinc | 5.90 × 10-8 |
| Nickel | 6.99 × 10-8 |
| Iron | 1.00 × 10-7 |
| Lead | 2.20 × 10-7 |
| Nichrome | 1.10 × 10-6 |
| Carbon (Graphite) | 2.50 × 10-6 |
| Germanium | 4.60 × 10-1 |
| Drinking water | 2.00 × 10-1 |
| Silicon | 6.40 × 102 |
| Wet wood | 1.00 × 103 |
| Glass | 10.0 × 1010 |
| Rubber | 1.00 × 1013 |
| Air | 1.30 × 1016 |
Metals such as copper are therefore used for making electric wires. The filament of an electric bulb is Table3 usually made of tungsten, because of its higher Resistivity of various materials resistivity values and melting point (34220C).
The value of resistivity of insulator are very high of the order of 1014 to 1016Ω-m. Alloys like Nichrome (Nickel, chromium and CERT TELANGAN iron) and Manganin (86% copper, 12% manganese, 2% nickel) have 30-100 times larger values of resistivity than those of metals. This makes them suitable for use in the heating elements of electric irons, toasters etc. These alloys also have the advantage that their resistance varies very little with temperature and they do not oxidise easily. The resistivity of materials such as silicon and germanium are 105 to 1010 times more than that of metals, but 1015 to 1016 times less than that of insulators. Such materials are called semiconductors. Semiconductors are used to make diodes, transistors and integrated Circuits (ICs). ICs are used in all sorts of electronic devices, including computer, TV, mobile phones etc.
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