Resistance vs. Resistivity
Electrical resistance and resistivity.
Electrical resistance of an electrical conductor depends on
 the length of the conductor
 the material of the conductor
 the temperature of the material
 the crosssectional area of the conductor
and can be expressed as
R = ρ L / A (1)
where
R = resistance of the conductor (ohms, Ω)
ρ = resistivity of the conductor material (ohm metre, Ω m)
L = length of conductor (m)
A = crosssectional area of conductor (m^{2})
Resistivity of some Common Conductors
 Aluminum: 2.65 x 10^{8} Ω m (0.0265 μΩ m)
 Carbon: 10 x 10^{8} Ω m (0.10 μΩ m)
 Copper: 1.724 x 10^{8} Ω m (0.0174 μΩ m)
 Iron: 10 x 10^{8} Ω m (0.1 μΩ m)
 Silver: 1.6 x 10^{8} Ω m (0.0265 μΩ m)
Note that resistivity depends on temperature. The values above are for temperatures 20 ^{o}C.
Resistivity of some Common Insulators
 bakelite: 1 x 10^{12} Ω m
 glass: 1 x 10^{10}  1 x 10^{11} Ω m
 marble: 1 x 10^{8} Ω m
 mica: 0.9 x 10^{13} Ω m
 paraffin oil: 1 x 10^{16} Ω m
 paraffin wax (pure): 1 x 10^{16} Ω m
 plexiglass: 1 x 10^{13} Ω m
 polystyrene: 1 x 10^{14} Ω m
 porcelain: 1 x 10^{12} Ω m
 pressed amber: 1 x 10^{16} Ω m
 vulcanite: 1 x 10^{14} Ω m
 water, distilled: 1 x 10^{10} Ω m
Note that good conductors of electricity have low resistivity and good insulators have high resistivity.
Example  Resistance of a Conductor
The resistance of 10 meter gauge 17 copper wire with cross sectional area 1.04 mm^{2} can be calculated as
R = (1.7 x 10^{8} Ω m) (10 m) / ((1.04 mm^{2})(10^{6} m^{2}/mm^{2}))
= 0.16 Ω
Example  Crosssectional area and Resistance
The copper wire above is reduced to gauge 24 and crosssectional area 0.205 mm^{2}. The increase in resistance can be calculated to
R = (1.7 x 10^{8} Ω m) (10 m) / ((0.205 mm^{2})(10^{6} m^{2}/mm^{2}))
= 0.83 Ω
Convert between Electrical Resistivity Units
 1 Ω m = 10^{2} Ω cm = 2.54 10^{2} Ω inch = 3.048 10^{1} Ω foot
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