Earth's magnetic field
Every compass you have ever used works because of something invisible and enormous: the Earth itself behaves like a giant magnet. Long before anyone understood electricity, sailors trusted a floating needle to find north, and no one knew why it worked. In 1600 the English scientist William Gilbert proposed the answer that still stands: the Earth itself is a huge magnet, with a magnetic field wrapped around the whole planet, and a compass needle is simply a small magnet responding to it. That single idea explains why a compass behaves the same way anywhere on Earth's surface, and it sets up the twist at the heart of this chapter — when a current-carrying wire is placed near a compass, Oersted's field competes with Earth's, and the needle visibly moves.
The Earth behaves like a giant magnet surrounded by its own magnetic field, so a freely-pivoting compass needle — itself a tiny magnet — always aligns with that field and settles pointing north-south unless a nearer field, such as that of a current-carrying wire, competes with it.
The core idea
A magnet always seeks north-south. A bar magnet suspended so that it can turn freely does not settle in a random direction — it always comes to rest pointing roughly north-south. This single, repeatable fact, familiar from earlier grades, is the starting clue for everything that follows: something around the whole Earth must be pulling every freely-pivoting magnet into that same orientation.
The Earth itself is a giant magnet. The explanation is that the Earth behaves like an enormous bar magnet, generating a magnetic field that surrounds the entire planet, from deep underground out into space. A compass needle is nothing more than a small, lightweight bar magnet balanced on a pivot so it can rotate freely. Placed anywhere on Earth's surface, away from other magnets or currents, the needle simply lines up with this surrounding field — the same way an ordinary bar magnet's needle lines up with the field of any nearby stronger magnet. This is why a compass reliably points north-south wherever it is used.
Field lines make the invisible field visible. Physicists describe a magnetic field using imaginary field lines that show its direction at every point. For Earth's field, these lines behave much like those of a giant bar magnet buried inside the planet — emerging near one pole, curving around the outside, and re-entering near the other. A compass needle at any point aligns itself along the field line passing through that point, which is why the needle's resting direction changes only gradually as you carry it from place to place, never randomly.
This is the "default" state that Oersted's discovery disturbs. A compass on a table, far from any other magnet or wire, is already responding to a field — Earth's own — even though nothing seems to be happening. This quiet, ever-present background is exactly what makes Oersted's observation dramatic: bringing a current-carrying wire close enough introduces a second field strong enough, at that short range, to rival Earth's, and the needle swings to a new resting direction that balances both.
Why it works
There is no formula behind this idea — it is a description of cause and effect, and it is worth walking through carefully because exams often probe whether a student truly understands the mechanism rather than just the fact. Start with what a magnet does to another magnet: any freely-pivoting magnet feels a turning effect from a nearby magnetic field and rotates until it lines up with that field. This is true of two ordinary bar magnets on a table, and it is equally true of a compass needle sitting inside Earth's field — the needle is not special, it is just small and light enough to respond visibly. Next, ask what field a compass "sees" when nothing else is around. Because Earth's own field exists everywhere on and above the surface, a compass is never truly in a field-free zone. In the everyday case — no other magnets, no current flowing anywhere nearby — the only field acting on the needle is Earth's, so the needle simply obeys it and settles north-south. This is why the "resting" direction of a compass is not arbitrary: it is the direction of Earth's local field. Finally, ask what changes when a current-carrying wire is brought close. A current-carrying conductor creates its own magnetic field around itself — this is Oersted's discovery, the central new idea of the chapter. Near the wire, that field can be comparable to or even stronger than Earth's field at that same spot, because magnetic field strength from a wire grows the closer you get to it. The needle no longer has only Earth's field to obey; it responds to the combined effect of both fields acting at once, and rotates to a new direction that balances them. Move the wire away, or increase the distance, and Earth's field regains the upper hand; switch the current off entirely, and the wire's field vanishes, leaving Earth's field to pull the needle back to its original north-south rest position. Nothing about Earth's field itself changed during any of this — only whether it had competition.
Worked example
Question: A straight wire is laid horizontally, running north-south, directly above a compass whose needle is already resting along the same north-south line. A switch in the circuit is initially open. Describe, stage by stage, what the compass needle does as the switch is closed and then reopened, and explain each stage using Earth's magnetic field.
Step 1. Before the switch is closed, no current flows in the wire, so the wire produces no magnetic field of its own. The only field acting on the needle is Earth's magnetic field, so the needle rests in its usual position, aligned north-south — exactly as it would on a table far from any wire.
Step 2. The switch is closed and current begins to flow. A current-carrying conductor generates a magnetic field around itself (Oersted's discovery), so a second field now exists in the space around the needle, in addition to Earth's.
Step 3. The needle is now under the influence of two fields at once: Earth's field, which still favours the north-south direction, and the wire's field, which favours a different direction close to the wire. The needle rotates away from north-south and settles in a new direction that reflects the combined pull of both fields — this visible deflection is the evidence Oersted originally observed.
Step 4. The switch is reopened and the current stops flowing. The wire's magnetic field disappears immediately because it only exists while current flows. With the wire's field gone, Earth's magnetic field is once again the only field acting on the needle, so the needle rotates back and settles into its original north-south position.
Final answer: the needle points north-south with the switch open, deflects to a new direction when the switch is closed because the wire's field now acts alongside Earth's field, and returns to north-south when the switch is reopened because only Earth's field remains.
Common traps
- Thinking the needle points north-south because of something special about the compass itself — it is entirely due to Earth's own, always-present magnetic field, not any property unique to that particular needle.
- Assuming the wire's field is "stronger than Earth's" in some absolute, permanent sense — really, it is only strong enough at close range to rival Earth's field there; move far enough from the wire and Earth's field wins again.
- Forgetting that Earth's magnetic field is present all the time, even when the needle looks perfectly "normal" and undisturbed — a field is easy to notice only when something visibly deflects because of it.
- Confusing Earth's own magnetism, which explains why a compass works at all, with the magnetic effect of current, which is the new field created only while current flows through a nearby conductor.
Ask the tutor: Want to see how the direction of deflection changes if the wire is placed below the compass instead of above it, or run north-east to south-west instead of north-south?
If Earth's magnetic field is always there, why don't I feel it or see it doing anything most of the time?
Magnetic fields are invisible and do not produce any sensation you can feel directly — you only notice a field's effect when something responds to it visibly, like a compass needle rotating or iron filings arranging themselves along field lines. Earth's field is constantly acting on a compass needle even when the needle looks perfectly still, because "resting north-south" is itself the needle responding to the field — it is just that a field doing its steady job looks like nothing happening at all.
Is Earth's magnetic field the same thing as the magnetic field created by a current-carrying wire?
No, they are two separate fields with different origins, even though both are magnetic fields and both can make a compass needle move. Earth's field is always present, everywhere on the planet, and exists whether or not any current is flowing anywhere. The wire's field exists only while current actually flows through that particular conductor, and it appears only in the space around that wire. Near a switched-on wire, both fields act on a nearby needle together, which is why the needle's direction reflects a combination of the two.
Why does the compass needle move to a new direction instead of just pointing straight at the wire when the current is switched on?
The needle does not simply obey the wire's field alone — it is still sitting inside Earth's field too, and a freely-pivoting magnet always settles in the direction that balances every field acting on it at once. So the final resting direction is neither purely north-south (Earth's preference) nor purely the wire's own direction, but somewhere in between, depending on how strong the wire's field is at that point compared with Earth's. That is exactly why Oersted's original deflection was a clue that two fields, not one, were now involved.