NEET 2021 · PhysicsAmpere thick wirePrevious Year Question
A thick current cable of radius R carries current 'I' uniformly distributed across its cross-section. The variation of magnetic field B(r) due to the cable with the distance 'r' from the axis of the cable is represented by:
Answer: (A) (A). Answer: (A) Solution: Apply Ampère's law to the thick cable with uniform current density.
- A.(A)✓
- B.(B)
- C.(C)
- D.(D)
Correct Answer
(A) (A)
Solution & Explanation
Answer: (A) Solution: Apply Ampère's law to the thick cable with uniform current density. Inside (r < R): enclosed current ∝ r², so B = μ₀ I r/(2πR²), i.e. B ∝ r (rises linearly from 0). Outside (r > R): full current enclosed, B = μ₀ I/(2πr), i.e. B ∝ 1/r (decays). B is maximum at the surface r = R. The correct graph rises linearly up to r = R, then decreases as 1/r → option A.
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