Practice portal › Moving Charges and Magnetism › Biot-Savart Law: Straight Wires and Arcs
Asked in NEET 2022 Re-exam · Straight wire segments
Idea: the wire looks the same from every direction around it, so the field pattern must share that symmetry.
From the Biot-Savart law, d⃗B∝ d⃗l×̂r, so ⃗B is perpendicular both to the wire and to the line from the wire to the point.
A curve everywhere perpendicular to those two directions is a circle in the plane perpendicular to the wire.
Ampere's law agrees: B(2π r)=μ₀I gives the same magnitude at every point of such a circle, so the lines are closed circles centred on the wire.
Their sense is given by the right-hand grip rule.
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