alleryThe magnetic field at distance r from a long straight wire carrying current I is
Aμ₀ I r / (2π)
Bμ₀ I / (4π r²)
C0 by symmetry
Dμ₀ I / (2π r), circling the wire
Answer & Solution
Correct answer: D. μ₀ I / (2π r), circling the wire
Ampere's law on a circle of radius r gives B · 2πr = μ₀ I
⇒ B = μ₀ I / (2π r).
Field lines are concentric circles.
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