$A$ uniform copper wire carries a current $i$ amperes and has $p$ carriers per meter$^3$. The length of the wire is $\lambda$ meters and its cross-section area is $s$ meter $^2$. If the charge on a carrier is $q$ coulombs, the drift velocity in $ms^{-1}$ is given by
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The current flowing through the wire is given by
$I=n e A v$
Where I = current
$\mathrm{n}=$ charge carrier current density
$\mathrm{e}=$ charge on the carriers
$A=$ cross sectional area
$\mathrm{v}=$ drift velocity
In the above example
$l=$ current $=\mathrm{i}$
$\mathrm{n}=$ charge carrier current density $=\mathrm{p}$
$\mathrm{e}=$ charge on the carriers $=\mathrm{q}$
$A=$ cross sectional area $=s$
$\mathrm{v}=$ drift velocity $=\mathrm{v}$
hence $i=p q s v$
$\therefore v=\frac{i}{p q{s}}$
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