Right-Hand Rule for Magnetic Field Around a Current-Carrying Wire

Steps:

• Magnetic field lines form concentric circles around a straight wire carrying direct current.
• Field direction follows the right-hand rule: thumb points in current direction, fingers curl in field direction.
• Unknown current direction allows clockwise or counterclockwise field patterns, both valid if circular.
• Diagram 1 shows incorrect linear or radial pattern; diagrams 2 and 3 show valid circular patterns with opposite directions.

Why B is correct:

• Diagrams 2 and 3 match the Ampere's law pattern of circular fields, with directions reversible by the right-hand rule for opposite currents.

Why the others are wrong:

• A includes diagram 1, which violates the circular field pattern required by Ampere's law.
• C excludes diagram 2, a valid circular field consistent with the right-hand rule.
• D excludes diagram 2, ignoring one possible field direction for unknown current.

Final answer: B