A long straight wire of radius a carries a steady current i. The current is uniformly distributed across its cross-section. The ratio of the magnetic field at 2 a and 2a is-
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A long straight wire of radius a carries a steady current i. The current is uniformly distributed across its cross-section. The ratio of the magnetic field at 2 a and 2a is-

G1 Physics · Accepted Answer

Inside a uniformly current-filled wire, B(r)=μ₀Ir/(2πa²), so at r=a/2, B=μ₀I/(4πa). Outside, B=μ₀I/(2πr), so at r=2a the same value is obtained. Ratio=1, hence C.