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Define relaxation time of the free electrons drifting in a conductor: How is it related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.

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Relaxation time is the time interval between two successive collisions of electrons in a conductor, when current flows. 
 
= no. of free electrons 
=area of conductor
=drift velocity 
=charge of electron 
                = field      =mass of electron       = Relaxation time
From this, you can find expression of relaxation time
 
Field is   L=Length and
From this you can modify the expression in terms of resistivity.

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