What is Electrical Resistivity?
Electrical resistivity is a property of materials and is different from electrical resistance which is a property of a component. Resistivity is also known as Specific Electrical Resistance or Volume Resistivity. Resistivity can also be defined as the inverse of the conductivity of a material - ρ = 1 / σ.It is a measure of how a material opposes the flow of electric current. The SI unit of resistivity is ohm meter (Ω.m). Resistivity, ρ (Greek letter rho), is defined as:
ρ = E / J
where ρ is the static resistivity (It’s measured in V m/A), E is the electric field's magnitude (measured in V/m), J is the current density's magnitude (measured in A/m²).
It can also be defined as:-
ρ = R x A / L
where 'R' is the electrical resistance (measured in Ω), 'L' is the length of material (measured in m), and 'A' is the cross-sectional area (in m²).
There are applications where the weight of a conductor is important. Here the resistivity and density combined are more important than resistivity alone. As an example long distance overhead cables are often made out of aluminium rather than copper because it is lighter and this offsets the slightly higher resistivity.
Generally, the resistivity of metals increases with temperature. Semiconductors, as used in active electronic components such as transistors and diodes, experience a decrease in resistivity as temperature increases. At high temperatures the resistance of a metal increases linearly with increase in temperature, while at lower temperatures the temperature dependence of resistivity follows a power law function of temperature.
When the temperature of a metal is reduced, the resistivity reaches a constant value which is known as the residual resistivity. The value is dependent on the type of metal, purity and thermal history. Some metals lose all electrical resistivity at very low temperatures due to superconductivity.