Which of the following statements about the direction of the $B$ field (magnetic field) around a straight, current-carrying wire is correct?

• A. The $B$ field is parallel to the direction of the current along the wire.
• B. The $B$ field does not exist outside the wire.
• C. The $B$ field forms concentric circles around the wire, with the direction given by the right-hand rule.
• D. The $B$ field points radially outward from the wire at all points.

Answer: (C)

Verified step by step guidance

1. Recall that the magnetic field \( \mathbf{B} \) around a straight current-carrying wire is generated by the moving charges (current) in the wire.
2. Understand that the magnetic field lines form closed loops around the wire, meaning they do not run parallel or radially outward from the wire but instead wrap around it.
3. Apply the right-hand rule to determine the direction of the magnetic field: if you point the thumb of your right hand in the direction of the conventional current, your fingers curl in the direction of the magnetic field lines.
4. Recognize that the magnetic field exists outside the wire and decreases in magnitude with distance from the wire, but it does not vanish outside the wire.
5. Conclude that the correct description is that the magnetic field forms concentric circles around the wire, with the direction given by the right-hand rule.