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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