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The magnitude of the magnetic field at perpendicular distance r from an infinite straight wire carrying current I is
Aμ₀I/(2πr)
Bμ₀I/(4πr)
Cμ₀Ir/2
Dμ₀I/(2r²)
Answer & Solution
Correct answer: A. μ₀I/(2πr)
1. Use Ampere's law on a circle of radius r centred on the wire.
2. By symmetry, B is tangential and constant on the circle: ∮B·dl = B·(2πr).
3. Enclosed current is I, so B·(2πr) = μ₀I.
4. Therefore B = μ₀I/(2πr).
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", §4.7_
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