Electrical Engineering ⇒ Topic : Effect of Temperature on Resistance

Sunita
 
Effect of Temperature on Resistance As already studied in Chap. , an increase in temperature increases the resistance of pure metals. If a conductor having a resistance of R_{o} at 0°C is heated to a temperature 1°C, its resistance increases. Let the new resistance be R_{T}. The net increase in resistance is (R_{T}  R_{0}). This can be written as: (R_{T } R_{O}) = α R_{O} T .............. (1) where, α (Greek symbol, Alpha) is a constant and is called the temperature coefficient of resistance of the material of the conductor. From Eq. (1) we find that R_{T} = R_{O} (l + αT) .............. (2) where R_{T} is the final resistance, R_{O} is the initial resistance, T is the difference between the final and the initial temperatures and α is the temperature coefficient of resistance of the material. The temperature coefficient of some metals which are widely used as conductor is given in Table (a). TABLE (a) Temperature coefficient of resistance of different metals So far, we have assumed that a does not change with temperature, but in most metals it does change with temperature. If α_{1 }is the temperature coefficient at temperature T_{1} and α_{2} is the temperature coefficient at temperature T_{2} and if we know α_{1}, T_{1} and T_{2}, we can calculate α_{2} from the following equation .............. (3) Similarly, resistivity or specific resistance of a metal also changes with temperature.The relationship between ρ_{o }and ρ_{T}, can be written as ρ_{T} = ρ_{0}(l+ αT) ................. (4) Equations (2), (3) and (4) describe the variation of resistance, temperature coefficient and resistivity of a material with variation in its temperature  
 
Seema
 
Effect of Temperature on Resistance In general, the resistance of a material changes with the change in temperature. The effect of temperature upon resistance varies according to the type of material as discussed below
figure (a) Figure (a) shows temperature/resistance graph for copper which is a straight line. If this line is extended backward, it would cut the temperature axis at 234.5°C. It means that theoretically, the resistance of copper wire is zero at 234.5°C. However, in actual practice, the curve departs (point A) from the straight line path at very low temperatures  
 
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