Effect of variation of temperature on resistance
Content :
👉 Explain on what factors electrical resistance of a conductor depends
👉 State the temperature co-efficient of resistance.
The resistance of material largely depends on temperature and varies according to the material.
When resistance r is a constant depending on the nature of the material of the conductor and known as its specific resistance or resistivity.
Effect of temperature on resistance:
Actually, the relative values of resistance that were given earlier apply to the metals when they are at about room temperature.
At higher or lower temperatures, the resistances of all materials change. In most cases, when the temperature of a material goes up, its resistance goes up too.
But with some other materials, increased temperature causes the resistance to go down.
The amount by which the resistance is affected by each degree of temperature change is called the temperature coefficient. And the words positive and negative are used to show whether the resistance goes up or down with the temperature.
When the resistance of the material goes up as temperature is increased, it has a positive temperature coefficient.
It is appropriate in the case of pure metals such as silver, copper, aluminum, brass etc.
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| Effect of variation of temperature on resistance |
In the case of certain alloys such as eureka, manganin, etc. increase in resistance due to increase in temperature is relatively less and irregular.
When a material's resistance goes down as the temperature is increased, it has a negative temperature coefficient.
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| negative temperature coefficient |
This applies in the case of electrolytes, insulators such as paper, rubber, glass, mica etc. and partial conductors i.e semiconductors such as carbon.
Temperature coefficient of resistance of a conductor
Let a metallic conductor, having a resistance of Ro at 0°C, be heated to t°C and let its resistance at this temperature be Rt. Then, considering normal ranges of temperature, it is found that the increase in resistance depends:
• directly on its initial resistance (Ro)
• directly on the rise in temperature (t)
• on the nature of the material of the conductor
Hence (Rt - Ro) = Ro t α
where α (alpha) is constant and is known as the temperature coefficient of resistance of the conductor.
Rearranging Eq, we get
α = R t- Ro / Ro x t = ΔR / Ro x t
If Ro= 1Ω, t = 1°C, then α = ΔR = Rt - Ro.
Hence, the temperature-coefficient of a material may be defined as: the change in resistance in ohm per °C rise in temperature.
we find that RT = Ro(1+α t)
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