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Physics · 2025 · 2 marks
CBSE 2025 · Region 6 · Set 3 · Q18
(a) What is the difference between 'velocity' and 'drift velocity' of electrons in a current-carrying conductor. (b) A copper wire of uniform…
(a)
What is the difference between 'velocity' and 'drift velocity' of electrons in a current-carrying conductor.
(b)
A copper wire of uniform cross-sectional area carries a current of $\displaystyle 3.4$ A . The drift velocity of conduction electrons is $\displaystyle 0.2 \mathrm{~mm} / \mathrm{s}$. If the number density of electrons in copper is $\displaystyle 8.5 \times 10^{28} \mathrm{~m}^{-3}$, find the area of cross-section of the wire.
Marking-scheme solution
(a) Difference between velocity and drift velocity of electrons in a
(a) Velocity is the rate of change of displacement with time. Drift velocity is the average velocity of free electrons under the influence of external electric field.
(b) \(\displaystyle I=\) neAv \(\displaystyle _{d}\)
\[\begin{aligned}
& A=\frac{I}{n e \mathrm{v}_{d}} \\
& A=\frac{3.4}{8.5 \times 10^{28} \times 1.6 \times 10^{-19} \times 0.2 \times 10^{-3}} \\
& A=1.25 \times 10^{-6} \mathrm{~m}^{2}
\end{aligned}
\]
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