Question

A long straight wire of radius a carries a steady current I. The current is uniformly distributed over its cross section. The ratio of magnetic fields B1 and B2 at radial distances a/2 and 2a respectively, from the axis of the wire is 

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Answer 1

 1 is the axis oft wire

Answer 2

Answer:

option (C) is correct answer.

Explanation:

Consider two amperian loops of radius a/2 and 2a as shown in the diagram.

Applying ampere's circuital law for these loops, we get

∮B.dL=μ0Ienclosed

For the smaller loop,

⇒B×2π2a=μ0×πa21×(2a)2

=μ0I×41=4μ0I

⇒B1=4πaμ0I

B′×2π(2a)=μ0I

B′B=4πaμ0I×μ0I4πa=1

Hence,

option (C) is correct answer.