Which option correctly describes the magnetic field lines around a straight current-carrying conductor?

• A. Parallel lines
• B. Helical lines
• C. Concentric circles
• D. Radial spokes

Answer: (C)

Magnetic field lines around a straight current-carrying conductor form closed loops that encircle the wire, lying in planes perpendicular to the wire. These lines are concentric circles centered on the wire, with the direction given by the right-hand rule: point the thumb of your right hand along the current, and the curled fingers show the magnetic field direction around the wire. The field strength decreases with distance from the wire, following B = μ0 I / (2π r), so closer to the wire the lines are denser.

This is why the description of concentric circles is the best fit. Parallel lines would imply a uniform field, which isn’t what happens around a straight current. Helical lines would require a component of the field along the wire, as seen in other setups like solenoids or moving charges in a field. Radial spokes would indicate lines radiating outward from the wire, which magnetic fields around a straight conductor do not do.