Which of the followings is true of the magnetic field produced by a current in a long-straight wire?

When an electric current flows through a long, straight wire, it produces a magnetic field around the wire. According to the right-hand rule, if you curl the fingers of your right hand around the wire in the direction of the magnetic field, your thumb points in the direction of the current. The magnetic field lines form concentric circles around the wire, lying in planes perpendicular to the length of the wire. This circular pattern of magnetic field lines is a fundamental characteristic of the magnetic field produced by a long straight current-carrying conductor.

Mathematically, the magnetic field \( B \) at a distance \( r \) from a long straight wire carrying current \( I \) is given by Ampère's law:

where \( \mu_0 \) is the permeability of free space.