A long straight wire with a circular crosssection having radius $R$, is carrying a steady current $I$. The current I is uniformly distributed across this cross-section. Then the variation of magnetic field due to current I with distance $r\;( r < R )$ from its centre will be
JEE MAIN 2022, Easy
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Use Ampere's law
$\text { B. } 2 \pi r =\mu_{0} \cdot \frac{ I }{\pi R ^{2}} \cdot \pi r ^{2}$
Thus $B \propto r$
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