Which of the following statements best describes the behavior of magnetic field lines around a straight current-carrying conductor?
They form concentric circles centered on the conductor.
The correct answer is that the field lines form concentric circles centred on the conductor. This was first demonstrated by Hans Christian Oersted, who observed that a compass needle placed near a current-carrying wire deflects, proving that an electric current produces a magnetic field around it.
Around a long, straight wire the magnetic field lines lie in planes that are perpendicular to the wire, wrapping around it as a series of closed circular loops. The direction of these loops is given by the right-hand thumb rule: if you grip the wire with your right hand so the thumb points along the direction of conventional current, the curled fingers show the direction in which the circular field lines point.
The strength of the field is given by the formula:
B = μ₀I / (2πr)
where I is the current, r is the perpendicular distance from the wire, and μ₀ is the permeability of free space. This shows the loops are densest (strongest field) close to the wire and grow weaker as r increases, which is why the circles are drawn farther apart with distance.
Why the other options are wrong: Lines that radiate straight outward describe the electric field of a point charge, not the magnetic field of a current. Straight lines running parallel to the wire would carry no circulating sense and cannot represent the actual looping field. A field that changes direction randomly along the wire contradicts the steady, symmetric pattern produced by a constant current. The circular, concentric pattern is the only correct description.
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