The reciprocal of resistance is called conductance and the reciprocal of resistivity. is called conductivity .The
SI unit of conductance is seimen and that for conductivity is seimen per meter.
Temperature Dependence Of Resistance
Ohm's law itself says that resistance is a constant only if temperature remains unchanged. The resistance Of a metallic conductor increases with temperature. For a given conductor the increases in resistance is directly proportional to the original resistance is directly proportional to the original resistance and also on the change in temperature. The temperature coefficient of resistance of a material is defined as the increase in resistance per unit resistance per unit rise in temperature. Commercially available resistors used in electronics
circuits are carbon resistors . Carbon also has a negative temperature coefficient of resistance.
However for semiconductors resistance increase with the decrease in temperature. That is semiconductor have negative temperature coefficient of resistance.
Super Conductivity
The phenomenon by which the resisitance of certain substance reduces to zero when cooled to extremely low temperature is called super conductivity and the temperature below which superconductivity occurs is called critical temperature. Mercury is not the only substance which exhibits superconductivity.Now researches are going on to obtain superconducting materials with critical temperature of the order of room temperature.Most of the good conductors do not exhibit supercondivity.
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