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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