NEET 2026 (Phase 1) · PhysicsAmpere thick wirePrevious Year Question
The figure shows a long straight solid wire of circular cross-section of radius 'a' carrying steady current I. The current I is uniformly distributed across its cross-section. The plot which correctly represents the variation of magnetic field (B) with distance (r) from the axis of the conductor is:
- A.B rises linearly for r < a, then falls as 1/r for r > a✓
- B.B falls as 1/r for r < a, then rises linearly for r > a
- C.B is constant for r < a, then falls as 1/r for r > a
- D.B rises linearly for r < a, then stays constant for r > a
Correct Answer
(A) B rises linearly for r < a, then falls as 1/r for r > a
Solution & Explanation
Answer: (A) Solution: Apply Ampère's law. Inside (r < a) the enclosed current ∝ r²: B(2πr) = μ₀ I (r²/a²) → B = μ₀ I r /(2π a²), so B ∝ r (linear rise from 0). Outside (r > a) full current is enclosed: B(2πr) = μ₀ I → B = μ₀ I /(2π r), so B ∝ 1/r (decay). B is maximum at r = a. The correct plot rises linearly to r = a, then decays as 1/r.
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