NEET 2022 · PhysicsAmpere thick wirePrevious Year Question
From Ampère's circuital law for a long straight wire of circular cross-section carrying a steady current, the variation of magnetic field in the inside and outside region of the wire is:
Answer: (C) A linearly increasing function of distance r up to the boundary of the wire and then decreasing one with 1/r dependence for the outside region. Answer: (C) Solution: By Ampère's law for a uniformly-current wire of radius R: Inside (r < R): B = μ₀ I r/(2πR²), so B increases linearly with r.
- A.Uniform and remains constant for both the regions
- B.A linearly increasing function of distance up to the boundary of the wire and then linearly decreasing for the outside region
- C.A linearly increasing function of distance r up to the boundary of the wire and then decreasing one with 1/r dependence for the outside region✓
- D.A linearly decreasing function of distance up to the boundary of the wire and then a linearly increasing one for the outside region
Correct Answer
(C) A linearly increasing function of distance r up to the boundary of the wire and then decreasing one with 1/r dependence for the outside region
Solution & Explanation
Answer: (C) Solution: By Ampère's law for a uniformly-current wire of radius R: Inside (r < R): B = μ₀ I r/(2πR²), so B increases linearly with r. Outside (r > R): B = μ₀ I/(2πr), so B decreases as 1/r. B peaks at the surface r = R. This matches: linearly increasing up to the boundary, then 1/r decreasing outside → option C.
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