Explain the term 'drift velocity' of electrons in a conductor. Hence obtain the expression for the current through a conductor in terms of 'drift velocity'.

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The average velocity of all the free electrons in the conductor is called the drift velocity of free electrons of the conductor. When a conductor is connected to a source of emf an electric field is established in the conductor, such that
When = potential difference across the conductor and =length of the conductor 
The electric field exerts an electrostatics force '' on each free electron in the conductor 
The acceleration of each electron is given by 

Where, =electric charge on the electron and 
=mass of electron 
Acceleration and electric field are in opposite directions, so the electrons attain a velocity in addition to thermal velocity in the direction opposite to that of electric field. 
Where =relaxation time between two successive collision 
Let =number density of electrons in the conductor 
N0. of free electrons in the conductor =
Total charge on the conductor ,
Time taken by this charge to cover the length of the conductor,


Using equation (i) and (ii), we get that 

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