Magnetic Field and Oersted's Experiment

The compass needle deflects because the current in the wire produces a magnetic field around it. This field exerts a force on the compass needle, pulling it away from north.

Oersted's experiment showed that the deflection only occurs while current flows. The magnetic field from the wire exists only when current is present.

When the current is switched off, the wire's magnetic field disappears. The only remaining magnetic field is Earth's magnetic field, so the compass needle returns to pointing toward geographic north.

The compass needle returns to pointing toward geographic north — exactly what Oersted observed during his 1820 experiment when he switched the current off.

The question asks what happens to a compass needle after the current is switched off — so we need to think about what causes the deflection in the first place.

The option 'It remains deflected in the direction it moved' is incorrect. The deflection is caused by the wire's magnetic field; once the current stops, the field disappears and there is nothing to keep the needle deflected.

'It spins continuously until the current is switched back on' is incorrect. There is no physical mechanism that would cause the needle to spin — it would simply settle in response to whatever field is present.

'It points toward the nearest end of the wire' is incorrect. A wire carrying no current has no magnetic poles and produces no magnetic field, so the needle has no reason to point at the wire.

The correct answer is 'It returns to pointing toward geographic north'. With no current, the only magnetic field present is Earth's field, which aligns the compass toward north.

When electric current flows through a straight wire, the resulting magnetic field forms concentric circular loops centred on the wire. This is fundamentally different from the field around a bar magnet.

Unlike a bar magnet, a straight current-carrying wire has no north or south poles. The field lines are complete circles — they do not start or end at poles.

The circles are closer together near the wire (stronger field) and spread out farther away (weaker field). The direction of circulation is determined by the Right-Hand Grip Rule based on the current direction.

The correct description is 'Concentric circles centred on the wire' — this is a defining feature of the magnetic field around a straight current-carrying conductor.

The question asks about the shape of magnetic field lines around a straight current-carrying wire — specifically asking which description is correct.

'Straight lines parallel to the wire' describes the field inside a solenoid, not around a single straight wire. This option is incorrect.

'Curved lines from a north pole to a south pole' describes the external field of a bar magnet or solenoid. A straight wire has no poles, so this is incorrect.

'Straight lines radiating outward like spokes' would describe an electric field from a charged wire, not a magnetic field. Magnetic field lines around a wire are circular, not radial.

The only correct option is 'Concentric circles centred on the wire', which is the well-established shape of the magnetic field around a straight current-carrying conductor.