It forms concentric circles around the conductor.
Explanation
When an electric current flows through a conductor, it produces a magnetic field in the surrounding space. The pattern and direction of this magnetic field depend on the shape of the conductor and the direction of the current. For a straight current-carrying conductor, the magnetic field lines are circular and centered on the wire.
Option Analysis:
- It is parallel to the direction of the current. is Incorrect: The magnetic field lines are not parallel to the direction of the current. Instead, they lie in a plane perpendicular to the conductor.
- It forms concentric circles around the conductor. is Correct: The magnetic field lines form concentric circles around the conductor. The center of these circles lies on the axis of the conductor. The direction of these lines can be determined using the Right-Hand Thumb Rule (if the thumb points in the direction of the current, the curled fingers show the direction of the magnetic field).
- It exists only if the conductor is made of iron. is Incorrect: The generation of a magnetic field is a fundamental property of moving charges (electric current). It occurs in any conductor (copper, aluminum, etc.) carrying current, not just iron. While the nature of the core material affects the field strength in solenoids, the field around a straight wire exists regardless of the wire's material.
- It is independent of the magnitude of the current. is Incorrect: The magnitude of the magnetic field ($B$) produced by a straight current-carrying conductor is directly proportional to the magnitude of the current ($I$) flowing through it ($B \propto I$). It is inversely proportional to the distance ($r$) from the conductor ($B \propto 1/r$).
Key Takeaway:
The magnetic field due to a straight current-carrying conductor consists of concentric circles in a plane perpendicular to the wire, with the field strength decreasing as the distance from the wire increases.