Question
Why does a freely suspended compass needle always point in the north-south direction? Explain using the concept of Earth’s magnetic field.
(NCERT Class 6, Chapter 13 — Fun with Magnets)
Solution — Step by Step
A compass needle is a thin, magnetised piece of metal that can rotate freely on a pivot. Like every magnet, it has a north pole and a south pole.
The Earth has a magnetic field around it, as if there’s a huge bar magnet buried inside it. This magnetic field has a north magnetic pole (near the geographic South Pole) and a south magnetic pole (near the geographic North Pole).
Yes, that’s a bit confusing — the Earth’s magnetic south pole is near the geographic North Pole.
We know that opposite magnetic poles attract each other. The north pole of the compass needle is attracted towards the Earth’s magnetic south pole (which is near geographic north). So the needle swings and settles pointing roughly towards geographic north.
That’s why we call it the “north-seeking pole” or simply the “north pole” of the magnet — it’s the end that seeks the north direction.
Why This Works
Every magnet, when free to rotate, aligns itself with the external magnetic field it sits in. The compass needle is sitting in Earth’s magnetic field, so it aligns along the magnetic field lines — pointing from magnetic south to magnetic north. Since Earth’s magnetic poles are close to (but not exactly at) the geographic poles, the compass gives us a good approximation of the north-south direction.
The slight difference between magnetic north and geographic north is called magnetic declination. In most of India, this angle is small (about 1-2°), so for practical purposes, a compass points “north” accurately enough.
Alternative Method — Think of field lines
Imagine magnetic field lines coming out of the Earth near the geographic South Pole, looping through space, and entering the Earth near the geographic North Pole. The compass needle simply aligns along these invisible field lines, like a tiny arrow showing the direction of the field at that point.
Here’s a fun experiment from your NCERT book: hang a bar magnet from a string so it can rotate freely. It will always settle in the north-south direction — just like a compass. This proves the magnet is responding to Earth’s magnetic field, not to some force from the geographic pole itself.
Common Mistake
Students often think the Earth’s magnetic north pole is at the geographic North Pole. It’s actually the opposite — the Earth’s magnetic south pole is near the geographic North Pole. That’s why the north pole of the compass (which seeks south magnetic poles) points towards geographic north. The naming is historical and confusing, but this distinction is regularly tested in exams.