 The resistance of a particular metal / conductor depends upon its length, area and nature of material and temperature.
 The resistance is directly proportional to length and inversely proportional to area.
R α L
R α 1 / a
Combining these
two
R α L / a
R = ρL / a ………. ( 1 )
Where
ρ = Electrical
resistivity of conductor, constant
L = Length of
conductor
a = Area of
conductor
The unit of resistance is ohm.
from equation ( 1
)
ρ = Ra / L
The unit of
resistivity ρ is = unit of resistance × Unit of area / Unit of length
=
Resistance × meter^{2} / meter
=
Resistance × meter
If L = 1, a = 1,
then ρ = R
Definition of resistivity
 The resistivity of any material is equal is the resistance of that material having unit length and unit cross section area.
Resistivity of material
 The conductivity of material like conductor, semiconductor and insulator depends upon their resistivities.
 The metals have low resistivities in the range of 10 ^{– 8} ohm meter to 10 ^{– 6} ohm meter.
 The insulators have resistivities 10^{18} times greater than metal whereas the resistivities of semiconductor lies between insulator and conductor.
 As the temperature increases, the resistivity decreases.
 The resistivity of semiconductor material is also affected by the small amount of impurities whether it is trivalent or pentavalent.
Material 
Resistivity in ohm meter at 0 ^{o}C 
Temperature coefficient of resistance ( Per degree
centigrade ) at 0 ^{o}C 
Metal 


Silver 
1.6 × 10 ^{– 8} 
0.0041 
Copper 
1.7 × 10 ^{– 8} 
0.0068 
Aluminum 
2.7 × 10 ^{– 8} 
0.0043 
Tungsten 
5.6 × 10 ^{– 8} 
0.0045 
Iron 
10 × 10 ^{– 8} 
0.0065 
Platinum 
11 × 10 ^{– 8} 
0.0039 
Mercury 
98 × 10 ^{– 8} 
0.0009 
Nichrome 
100 × 10 ^{– 8} 
0.0004 
Manganin 
48 × 10 ^{– 8} 
0.002 × 10 ^{– 3} 
Semiconductor 


Carbon 
3.5 × 10 ^{– 5} 

Germanium 
0.46 

Silicon 
2300 

Insulators 


Pure water 
2.5 × 10 ^{5} 

Glass 
10^{10} to 10^{14} 

Hard rubber 
10^{13} to 10^{16} 

NaCL 
10^{14} 

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