Electrical Resistance
The obstruction offered by any conductor in the path of flow of current is called its electrical resistance .
Electrical resistance , R = V / I Its SI unit is ohm ( Ω ) and its dimensional formula is [ ML 2 T -3 A -2 ]. Electrical resistance of a conductor R = ρl / A where , l = length of the conductor , A = cross-section area and ρ = resistivity of the material of the conductor .
Resistivity Resistivity of a material of a conductor is given by
ρ = m / n 2 τ where , n = number of free electrons per unit volume .
Resistivity of a material depend on temperature and nature of the material . It is independent of dimensions of the conductor , i . e ., length , area of
cross-section etc . Resistivity of metals increases with increase in temperature as
ρ t = ρ o ( 1 + αt )
where ρ o and ρ t are resistivity of metals at O ° C and t ° C and α temperature coefficient of resistivity of the material .
For metals α is positive , for some alloys like nickel , manganese and constantan , α is positive but very low .
For semiconductors and insulators . α is negative .
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