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Physics · PYQ NEET UG 2016(1)
हिन्दीA long straight wire of radius \( a \) carries a steady current \( I \). The current is uniformly distributed over its cross-section. The ratio of the magnetic fields \( B \) and \( B' \) at radial distances \( \frac{a}{2} \) and \( 2a \) respectively, from the axis of the wire is
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Correct answer · Option B
1
Explanation
Inside the wire (\( r = a/2 \)), \( B = \frac{\mu_0 I r}{2\pi a^2} = \frac{\mu_0 I (a/2)}{2\pi a^2} = \frac{\mu_0 I}{4\pi a} \). Outside (\( r = 2a \)), \( B' = \frac{\mu_0 I}{2\pi(2a)} = \frac{\mu_0 I}{4\pi a} \). Therefore, \( B / B' = 1 \).
