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What is conductance ? Give the unit of conductivity .
Ans.
Conductance(G) is reciprocal of resistance. Whereas resistance of a conductor measures the opposition which it offers to the flow of current, the conductance measures the inducement which it offers to flow of current.
We have R = þ( L/A ) , R = þ
Conductance, G = 1/R
G = 1/ (þ L/A )
G = A/ þ L ( Since, 1/ þ = σ )
So, G = σ A/L
Where σ ( Greek letter sigma ) is called the conductivity or specific conductance of the material. The unit of conductance is mho i.e., ohm spelt backward.
So the unit of conductivity, σ = (mho × metre)/(square metre)
= mho/metre
The above little reflection shows that SI unit of conductivity is mho/metre . Now-a-days, the siemen (s) is used as the unit of conductance & conductivity is expressed as siemen/metre.
What are the effects of temperature on resistance ?
Ans.
Generally, the resistance of a material changes with the change in temperature. The effect of temperature on resistance can be written as follow……….
(a) If the resistance of a pure metal or a conductor increases, the resistance of the conductor varies directly.
So metals have positive temperature co-efficient of resistance. Graphically it can be shown as below..........
(b) If the temperature of a semiconductors, electrolytes, insulators etc increases, the resistance of these substance decreases. Hence these materials have negative temperature co-efficient of resistance.
(c) In case of alloys, if the temperature increases, the increases in resistance is relatively small and irregular. For some high resistance alloys (example, Eureka, manganin, constantan etc), the change in resistance is practically negligible over a wide range of temperatures.
