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  • (a) Define the term 'drift velocity' and 'relaxation time' giving their physical significance. (b) A conductor of length L is connected across a DC source of emf E. If the conductor is replaced by another of the same material

(a) Define the term 'drift velocity' and 'relaxation time' giving their physical significance.

(b) A conductor of length L is connected across a DC source of emf E. If the conductor is replaced by another of the same material and area of cross-section but length 5L, by what factor will be the drift velocity change? 

 

 
 
 
 
 

Answers (1)

Drift velocity 

when an electric field is applied across the conductor, the charge carriers move with a certain velocity along the direction of positive potential. That is called Drift velocity.

v_{d }= \frac{-e\overrightarrow{E}}{m}\tau

where, v_{d } - drift velocity

            \tau - relaxation time

Relaxation time is the average time between two successive collisions of an electron with lattice ions/ atoms.

(b) 

We know that,

R= \rho \frac{l}{A}

and v_{d}= \frac{I}{n_{e}A}

I = \frac{V}{R}

so, v_{d} = \frac{\frac{V}{R}}{n_{e}A}=\frac{\frac{VA}{\rho l}}{n_{e}A}

which means v_{d} \propto \frac{1}{l}

Therefore, 

\frac{v_{d}^{'}}{v_{d}}= \frac{L}{5L}

v_{d}^{'}= \frac{v_{d}}{5}

Drift velocity becomes \frac{1}{5}th time of the initial drift velocity.

Posted by

Safeer PP

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