Resistivity is low for metals, more for semiconductors and very high alloys like nichrome, constantan etc.
( In magnetic field the resistivity of metals increases. But resistivity of ferromagnetic materials such as iron, nickel, cobalt etc decreases in magnetic field.)
Electrical Conductivity
The reciprocal of resistivity is called electrical conductivity. Electrical conductivity( σ) = 1 / ρ = 1 / RA = ne 2 τ / m Its SI units is ohm-1 m-1 or mho m-1 or siemen m-1. Relation between current density( J) and electrical conductivity( σ) is given by J = σ E where, E = electric field intensity.
Ohmic Conductors
Those conductors which obey Ohm’ s law, are called ohmic conductors e. g., all metallic conductors are ohmic conductor.
For ohmic conductors V – I graph is a straight line.
Non-ohmic Conductors
Those conductors which do not obey Ohm’ s law, are called non-ohmic conductors. e. g., diode valve, triode valve, transistor, vacuum tubes etc.
For non-ohmic conductors V – I graph is not a straight line.
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