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What is the relation between resistance and resistivity?
The property of a conductor due to which it opposes the flow of current through it is called resistance.The resistance of a conductor is numerically equal to the ratio of the potential difference across its ends to the current flowing through it.
i.e;                       Resistance = Potential difference / Current 
or                                      R = V / I  
The resistance of a conductor depends on length ,thickness , nature of material and temperature ,of the conductor.
The SI unit of resistance is ohm .
It has been found by experiments that on increasing the length of a wire,its resistance increases and on decreasing the length of the wire,its resistance  decreases.Actually,the resistance of a conductor is directly proportional to its length.
ie; Resistance, R ? l
where l is the length of the conductor
Also it has been found by experiments that the resistance of a conductor is inversely proportional to its area of cross section.
ie; Resistance,               R ? 1/A
where A is the area of cross section
 Thus we have :
                              R ? l                   ...........(1)
                             R ? 1/A               ...........(2)
By combining the relations (1) and  ( 2),we get :
                                 R ?  l/A
or                              R = ? x l / A
where   (rho) is a constant known as resistivity of the material of the conductor.Resistivity is also known as specific resistance.


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