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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 a/2 and 2a is

Asked in JEE Main 2007 · Thick wires and cables

Answer: (4) 1

Step-by-step solution

Inside: B=(μ₀ i r)/(2π a²), at a/2: (μ₀ i)/(4π a).

Outside: B=(μ₀ i)/(2π r), at 2a: (μ₀ i)/(4π a).

Ratio =1

Why the other options are wrong

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