Define Resistivity | Resistivity | Unit of Resistivity
Resistivity | Define Resistivity :- In the previous article “Resistance” we saw that the resistance of a conductor is given by the following formula :
Here we say that ρ is a proportionality constant.
In physics, to define a proportionality constant we use a method in which, whatever quantities are in the multiplication or division of the proportionality constant, we take them as units.
So in equation (1) putting
Therefore, we can define resistivity as the resistance of a conductor having unit length and unit cross-sectional area.
Definition : The resistivity (specific resistance) of a material is equal to the resistance of a wire of unit length and unit cross-sectional area of that material.
Dependency of Resistivity(Define Resistivity)
- On the nature of material &
- Temperature of material
Resistivity (ρ) does not depend on the shape and size of the material as it is a characteristic property of a conducting material.
Specific use of some conducting materials on the basis of their Resistivity
- Resistance wire of rheostat and resistance box are made of are made of constantan(copper/nickel alloy) or manganin(alloy of copper, manganese, and nickel) because they have moderate resistivity(constantan-49 × 10-8 Ωm & manganin-48.2 × 10-8 Ωm) and very small temperature coefficient of resistance. Their resistivity is nearly independent of temperature.
- The heating element of electric devices like heater, immersion heater, geyser, electric iron etc are made of Nichrome (nickel/chromium alloy, 80 % nickel and 20 % chromium) wire. This is because it has a high resistivity(100 × 10-8 Ωm) and it does not oxidize and burn easily at high temperature, i.e. it has higher melting and boiling point than metals.
- The filament of bulb is made up of tungsten(alloy of various combinations of iron, copper, nickel, cobalt and/or molybdenum) because it has high resistivity(5.6× 10-8 Ωm), very high melting point of 3300 K and gives light at 2400 K. At high temperature tungsten reacts with air forming oxides so bulb is filled with an inert gas.
- Fuse wire is made of tin lead alloy because it has low melting point and low resistivity. The fuse is used in series in electrical circuits and when current exceeds the safety limit, it melts to produce open circuit.
- The connection wires are made of copper because it has low resistivity.
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