Primary & Middle School Physics · Grades 1–9
25Magnets and the Compass
Years ago you discovered that a magnet’s two ends are different: one way two magnets snap together, flipped around they push apart. We promised those ends had names and a famous job — guiding travelers across the world. Today the promise is kept: welcome to the poles, and to the little instrument that found the way for every explorer before satellites existed.
25.1 The two poles
Definition 25.1 (Poles)
The two special ends of a magnet — where its pull is strongest, where the paperclips cluster — are its poles. They are named the north pole (often painted red, marked N) and the south pole (marked S). Every magnet has both: one of each, always.
Proposition 25.2 (The rule of the poles)
Opposite poles attract each other: N snaps to S. Like poles repel each other: N pushes N away, S pushes S away. This one rule explains every snap and every springy refusal you have ever felt between two magnets.
Example 25.3 (Reading old experiments)
When your two magnets snapped together, an N was facing an S. When they refused springily, N faced N (or S faced S). And the paperclips always clustered at the ends because the poles are where a magnet’s power lives — its middle is the calm between them.
25.2 The Earth is a magnet
Example 25.4 (The greatest magnet of all)
Hang a bar magnet from a thread so it can turn freely, wait for it to settle — and something wonderful happens: it always comes to rest pointing the same way, one pole toward the far north of the world, the other toward the far south. Some giant, invisible magnet must be pulling its poles into line. That giant is the Earth itself: our whole planet is a magnet, with its magnetic poles near the top and bottom of the globe.
Definition 25.5 (Compass)
A compass is a small, light magnet needle balanced on a fine pivot so it can swing freely. Left in peace, the needle obeys the Earth-magnet: its marked end settles pointing north. Under the needle, a card shows the four cardinal directions: North, East, South, West.
Method 25.6 (Using a compass)
- Hold the compass flat and steady, far from magnets and big iron things (fences, cars — they distract the needle);
- wait for the needle to stop swinging;
- turn the compass card gently until its N sits under the needle’s marked end;
- read your directions: East on your right when facing north, West on your left, South at your back.
Check with the sky: around here, the midday shadow of the sundial chapter points north — shadow and needle should agree.
Example 25.7 (Make one yourself)
Stroke a steel sewing needle with one pole of a magnet, twenty times, always the same way, lifting the magnet away between strokes: the needle becomes a small magnet itself. Lay it gently on a slice of cork floating in a bowl of water — and watch the cork slowly turn until the needle points north–south. You have built the sailor’s compass of a thousand years ago.
Remark 25.8 (The instrument that opened the oceans)
Before the compass, sailors hugged the coasts and prayed for clear nights to steer by stars. With a floating needle, they could hold a course through fog, storm and moonless dark — across whole oceans. Few instruments have redrawn the map of the world so completely. And one puzzle remained open for centuries: why does the Earth act as a magnet? The full story waits in the university years of this course — it is worth the wait.
25.3 Exercises
Exercise 25.1 ★
What are the poles of a magnet, and what are their names? Where on a bar magnet do paperclips cluster?
Solution
Solution of Exercise 25.1.
The poles are the magnet’s two strong ends: the north pole (N, often red) and the south pole (S). Paperclips cluster at the poles — the ends — not the middle.
Exercise 25.2 ★
State the rule of the poles. Two magnets snap together: which poles were facing?
Exercise 25.3 ★
The N pole of one magnet approaches the N pole of another. What happens? And N toward S?
Solution
Solution of Exercise 25.3.
N toward N: they push apart (like poles repel). N toward S: they pull together and snap.
Exercise 25.4 ★
Why does a freely hanging bar magnet always settle pointing the same way? What does this reveal about the Earth?
Exercise 25.5 ★
Name the four cardinal directions in order, starting from North and turning right. Facing north, which direction is on your right?
Solution
Solution of Exercise 25.5.
North, East, South, West. Facing north, East is on your right.
Exercise 25.6 ★
Why must a compass be kept away from magnets, iron fences and cars while you read it?
Solution
Solution of Exercise 25.6.
The needle is a tiny magnet and obeys the nearest strong pull. A magnet or big iron mass nearby out-pulls the distant Earth-magnet, and the needle lies about north.
Exercise 25.7 ★
Describe how to turn a sewing needle into a compass needle, and how to let it swing freely without a pivot.
Solution
Solution of Exercise 25.7.
Stroke the needle with one pole of a magnet, twenty times in the same direction, lifting the magnet between strokes — it becomes a little magnet. Lay it on a cork slice floating in a bowl of water: the floating cork is a pivot made of water, and the needle swings to point north–south.
Exercise 25.8 ★
From the sundial chapter: which direction does the midday shadow point around here? How can this check a compass — or replace a lost one?
Exercise 25.9 ★
Why was the compass such a treasure for sailors, compared with steering by the coastline or the stars?
Solution
Solution of Exercise 25.9.
It works in fog, under clouds, on moonless nights and far from any coast — the needle asks nothing of the sky. Sailors could finally hold a course across open oceans in any weather.
Exercise 25.10 ★★
A hiker walks toward a mountain hut that lies due north. Her compass needle, held properly, points at her friend’s belt buckle instead of north whenever he stands close on her left. What is happening, in which direction is she in danger of drifting, and what is the one-step fix?
Solution
Solution of Exercise 25.10.
The friend’s iron belt buckle out-pulls the Earth-magnet at close range: the needle points at the buckle, not north. Trusting it, she would drift — the false “north” lies toward her left, so she would bend her path leftward, to the west. The fix: read the compass away from the buckle (a few steps apart), then walk.
Exercise 25.11 ★★
A magnet is sawed in half between its two poles, hoping to get a lone N in one hand and a lone S in the other. Sailors’ lore — and every experiment ever tried — says each half turns out to be a complete little magnet with both poles. Test the claim in thought with the compass-needle recipe: the stroked sewing needle is short — does it have one pole or two? What does this suggest about hunting a lone pole by cutting?
Solution
Solution of Exercise 25.11.
The short stroked needle has two poles — its ends behave like N and S (float two such needles and they snap end to end, opposite ends attracting). So shortening a magnet does not remove a pole, and cutting one in half just makes two complete short magnets: the hunt for a lone pole by cutting never ends — each cut hands you both poles again.