Drift Velocity - Practice Questions & MCQ

Relaxation time ($\tau$): The time interval between two successive collisions of electron with the Positive ions.

Mean Free path: The path between two consecutive collisions is called free path. The average length of these free paths is called “Mean Free Path”. 

Drift velocity

Drift velocity is the average velocity that a particle such as an electron attains in a material due to an electric field.

$
v_d=\frac{-e \vec{E}}{m} \tau
$
Where $v_d$ is the drift velocity, E is the electric field applied, e and m are the charge and mass of electrons respectively and $\tau$ is the relaxation time.
- $v_\mathrm{d}$ is directly proportional to $\mathrm{E}: v_d \propto E$ when the temperature is constant, the greater the electric field larger will be the drift velocity

Drift velocity and current

$
\begin{aligned}
& J=\frac{I}{A}=\frac{n e A v_d}{A} \\
& J=n e v_d
\end{aligned}
$

• Current independent of Area: Current does not change with change in cross-sectional area
• $v_\mathrm{~d}$ is inversely proportional to area: $v_d \propto \frac{1}{A}$ :

• Drift velocity varies inversely with the area of cross-section

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