How do magnetic field lines behave around a long, straight current carrying wire?

Introduction / Context:
A current carrying conductor produces a magnetic field in the space around it. For a long, straight wire carrying a steady current, the pattern of the magnetic field lines is simple and is a standard result in electromagnetism. This question checks whether you know the shape of these field lines and how they are arranged around the wire.

Given Data / Assumptions:
- The conductor is a long, straight wire carrying a steady current in one direction.
- The environment is otherwise empty space or air, with no nearby magnetic materials.
- We consider the magnetic field produced by the current, not by external magnets.
- Right hand rule conventions are used to determine the direction of the field.

Concept / Approach:
According to Ampere's law and the Biot Savart law, the magnetic field around a long, straight current carrying wire forms concentric circles around the wire. The direction of the field lines can be found using the right hand grip rule: if you grasp the wire with your right hand so that your thumb points in the direction of the current, your fingers curl in the direction of the magnetic field lines. The field strength decreases with distance from the wire, but the lines form closed loops circling the wire, not radial spokes.

Step-by-Step Solution:
Step 1: Recall that a steady current in a straight wire produces a magnetic field in the surrounding space. Step 2: Use the right hand grip rule: thumb in the direction of the current, fingers curling around the wire. Step 3: Recognise that the curling of the fingers represents magnetic field lines forming circles centered on the wire. Step 4: Understand that each field line is a closed loop that circles the wire and does not start or end at the wire. Step 5: Note that there is no radial component pointing directly away from or toward the wire in this idealised case. Step 6: Conclude that the correct description is that the magnetic field lines circle the wire in closed loops concentric with the wire.

Verification / Alternative check:
Experiments using iron filings and a compass around a current carrying wire show that compass needles align tangentially to circles around the wire. If you sprinkle iron filings on a card pierced by a vertical wire carrying current and gently tap the card, the filings arrange themselves in circular patterns centered on the wire. This experimental evidence matches the theoretical prediction of circular field lines.

Why Other Options Are Wrong:
They extend radially outward from the wire: Radial field lines would point straight away from or toward the wire, which is not observed for a current carrying conductor.
They both circle and extend radially outward: In the idealised case of a long straight wire in free space, the field is purely circular, not partially radial.
They do not exist around the wire at all: This contradicts basic electromagnetic theory and many experiments showing that currents create magnetic fields.

Common Pitfalls:
Some learners imagine electric field lines, which can be radial around a point charge, and incorrectly transfer that picture to magnetic fields around a wire. Another confusion is to think that current produces only electric effects and to forget the associated magnetic field. Keeping separate mental pictures for electric fields of charges and magnetic fields of currents, and using the right hand grip rule consistently, helps avoid these errors.

Final Answer:
Magnetic field lines around a long, straight current carrying wire circle the wire in closed loops that are concentric with the wire.