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.
When a steady electric current flows through a long, straight conductor, it produces a magnetic field in the surrounding space. The correct description is that the field lines form concentric circles centred on the conductor, lying in planes that are perpendicular to the wire.
This pattern follows from Oersted's discovery and is captured by the right-hand thumb rule (Maxwell's corkscrew rule): if you grip the wire with your right hand so the thumb points along the direction of conventional current, the curled fingers point in the direction of the circular magnetic field lines. Above the wire and below the wire the field therefore points in opposite tangential directions, but always along circles.
The magnitude of the field is given by Ampère's law for a long straight wire:
B = μ₀I / (2πr)
where I is the current, r is the perpendicular distance from the wire, and μ₀ is the permeability of free space. Because B depends on 1/r, the circles are crowded (strong field) close to the wire and spread out (weaker field) farther away — but they remain circles at every distance.
The other statements are incorrect:
- Lines that radiate straight outward describe the electric field of a point charge, not the magnetic field of a current; magnetic field lines never begin or end on a source, they close on themselves.
- Lines parallel to the wire would imply the field points along the current, which contradicts the experimentally observed circular (tangential) pattern.
- The field does not change direction randomly; for a steady current its direction at every point is fixed and fully determined by the right-hand rule.
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Select the correct answer using the code given below.
Which one of the following is a reason why astronomical distances are measured in light-years?
