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[Date (January 26, 2015)]
Resistance and Resistivity
Resistance is the property of a substance which makes it oppose the flow of an electric current through it. Generally, metals acids and liquid salts and salt solutions are very good conductors of electricity. Metals with higher numbers of delocalized electrons have higher conductivities than those with fewer delocalized electrons. For example, aluminium is more conductive than copper which is also more conductive than copper. When an electric current is applied to a conductor, the delocalized electrons assume a directed motion depending on the polarity of the potential difference. As the electrons move, they collide and bombard with atoms and molecules of the conducting material, thereby creating heat (T. B. Theraja).
Materials which offer higher hindrance to the flow of electrons on application of a potential difference are called poor conductors of electricity. Examples of poor conductors are: Polyvinyl chloride (PVC), dry wood, rubber, glass and mica. Insulators are used in the coating of conductors to prevent electrical shocks .Resistance is measured in ohms denoted Ω. Depending on the degree of resistances other commonly used units are; mega-ohms (M Ω), kilo-ohms (k Ω), mill-ohms (m Ω) and micro-ohms (μ Ω) (T. B. Theraja).
Resistance of a material is derived from the factors affecting it. According to the laws of resistance, resistance is directly proportional to length, l, and inversely proportional to the cross-sectional area of the conductor. It also depends on the nature of the material and the temperature of the material. Thus resistance of a conductor is given by:
R= ρlA ρ is the resistivity of the conductor material, l is the length of the conductor, and A is the cross-sectional area of the conductor in this formula. In this formula, for resistance to be measured in Ohms, the length is measured in metres, resistivity is measured in Ohm-metres and cross-sectional area is measured in square metres (A. Theraja).

Works Cited

Theraja, A.K. T.B. Of Electrical Technology (Multi Colour). Vol. 1. S. Chand & Company Ltd, 2005.
Theraja, T. B. Fundamentals of electrical engineering and electronics. S. Chand, 2006.