Electricity

The resistance of a conductor is affected by the following factors:

 Length of the Conductor

Resistance of a conductor is directly proportional to the length of the wire i.e., longer the wire greater will be the resistance and shorter the wire smaller will be the resistance. If L represents the length of the uniform wire, then

Area of the Cross-Section of the Conductor

Resistance of a conductor is inversely proportional to the area of the cross-section of the uniform wire. That means thinner the wire greater the resistance and thicker the wire lower the resistance. If A is the area of cross-section of the uniform wire, then

By combining both the factors,

where is the constant of proportionality called the resistivity of the material of the wire.

Resistivity of a material is the resistance between the ends of a conductor of length one metre and area of cross-section of one square - metre. Unit of (resistivity) is Ohm-metre

In other words, resistivity of a material is the resistance between the opposite faces of a unit cube of that material.

Electricity Resistivity of Some Substances at 20^O C is listed in the table given below.

	Material	Resistivity (W m)
Conductors	Silver	1.60 x 10-8
	Copper	1.62 x 10-8
	Aluminium	2.63 x 10-8
	Tungsten	5.20 x 10-8
	Nickel	6.84 x 10-8
	Iron	10.0 x 10-8
	Chromium	12.9 x 10-8
	Mercury	94.0 x 10-8
	Manganese	1.84 x 10-6
Alloys	Constantan (alloy of Cu and Ni)	49 x 10-6
	Manganin (alloy of Cu, Mn and Ni)	44 x 10-6
	Nichrome (alloy of Ni, Cr, Mn and Fe)	100 x 10-6
Insulators	Glass	1010 - 1014
	Hard rubber	1013 - 1016
	Ebonite	1015 - 1017
	Diamond	1012 - 1013
	Paper (dry)	1012

Note

Good conductors possess low resistivity.

Effect of Temperature

The effect of temperature has already been discussed earlier under limitations of ohm's law. In general for metallic conductors higher the temperature larger is the resistance.