---
title: "Magnets and the Compass"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 25
exercises: 11
source: https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass
---

# Chapter 25 — Magnets and the Compass

Years ago you discovered that a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet)’s two ends are different: one way two [magnets](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) 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](#def-g4-magnets-and-compass-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](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-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](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) has both: one of each, always.

**Proposition 25.2 (The rule of the poles).**

Opposite [poles](#def-g4-magnets-and-compass-poles) [attract](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) each other: N snaps to S. Like [poles](#def-g4-magnets-and-compass-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](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet).

![The rule of the poles: opposites attract, likes repel — the grown-up name for the game you played in an earlier year.](https://one-course.com/images/onecourse/chapters/physics-1/g4-magnets-and-compass/fig-4b1828e0b507.svg)

*The rule of the [poles](#def-g4-magnets-and-compass-poles): opposites [attract](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), likes repel — the grown-up name for the game you played in an earlier year.*

**Example 25.3 (Reading old experiments).**

When your two [magnets](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) 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](#def-g4-magnets-and-compass-poles) are where a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-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](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-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](#def-g4-magnets-and-compass-poles) toward the far north of the world, the other toward the far south. Some giant, invisible [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) must be pulling its [poles](#def-g4-magnets-and-compass-poles) into line. That giant is the Earth itself: our whole planet is a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), with its magnetic [poles](#def-g4-magnets-and-compass-poles) near the top and bottom of the globe.

**Definition 25.5 (Compass).**

A *compass* is a small, [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) needle balanced on a fine [pivot](https://one-course.com/books/physics/1/en/chapter/17-levers-and-balance-scales#def-g3-levers-and-scales-lever) so it can swing freely. Left in peace, the needle obeys the [Earth-magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet): its marked end settles pointing north. Under the needle, a card shows the four *cardinal directions*: North, East, South, West.

![A compass: a magnet needle riding a pivot. Wherever you stand on Earth, the needle finds north for you.](https://one-course.com/images/onecourse/chapters/physics-1/g4-magnets-and-compass/fig-a8122edd45fd.svg)

*A [compass](#def-g4-magnets-and-compass-compass): a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) needle riding a [pivot](https://one-course.com/books/physics/1/en/chapter/17-levers-and-balance-scales#def-g3-levers-and-scales-lever). Wherever you stand on Earth, the needle finds north for you.*

![A real pocket compass: the red half of the needle points north, and the card underneath carries the cardinal directions. Photo: Evan-Amos (public domain).](https://one-course.com/images/onecourse/chapters/physics-1/g4-magnets-and-compass/img-c52946ef6b63.jpg)

*A real pocket [compass](#def-g4-magnets-and-compass-compass): the red half of the needle points north, and the card underneath carries the [cardinal directions](#def-g4-magnets-and-compass-compass). *Photo: Evan-Amos (public domain).**

**Method 25.6 (Using a compass).**

1. Hold the [compass](#def-g4-magnets-and-compass-compass) flat and steady, far from [magnets](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) and big iron things (fences, cars — they distract the needle);
2. wait for the needle to stop swinging;
3. turn the [compass](#def-g4-magnets-and-compass-compass) card gently until its N sits under the needle’s marked end;
4. 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](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) of the [sundial](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-sundial) chapter points north — [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) and needle should agree.

**Example 25.7 (Make one yourself).**

Stroke a steel sewing needle with one [pole](#def-g4-magnets-and-compass-poles) of a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), twenty times, always the same way, lifting the [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) away between strokes: the needle becomes a small [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-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](#def-g4-magnets-and-compass-compass) of a thousand years ago.

**Remark 25.8 (The instrument that opened the oceans).**

Before the [compass](#def-g4-magnets-and-compass-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](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-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](#def-g4-magnets-and-compass-poles) of a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), and what are their names? Where on a bar [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) do paperclips cluster?

**Solution of Exercise 25.1.**

The [poles](#def-g4-magnets-and-compass-poles) are the [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet)’s two strong ends: the [north pole](#def-g4-magnets-and-compass-poles) (N, often red) and the [south pole](#def-g4-magnets-and-compass-poles) (S). Paperclips cluster at the [poles](#def-g4-magnets-and-compass-poles) — the ends — not the middle.

**Exercise 25.2 ★.**

State the rule of the [poles](#def-g4-magnets-and-compass-poles). Two [magnets](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) snap together: which [poles](#def-g4-magnets-and-compass-poles) were facing?

**Solution of Exercise 25.2.**

Opposite [poles](#def-g4-magnets-and-compass-poles) [attract](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet); like [poles](#def-g4-magnets-and-compass-poles) repel. A snap means opposite [poles](#def-g4-magnets-and-compass-poles) were facing: an N toward an S.

**Exercise 25.3 ★.**

The N [pole](#def-g4-magnets-and-compass-poles) of one [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) approaches the N [pole](#def-g4-magnets-and-compass-poles) of another. What happens? And N toward S?

**Solution of Exercise 25.3.**

N toward N: they push apart (like [poles](#def-g4-magnets-and-compass-poles) repel). N toward S: they pull together and snap.

**Exercise 25.4 ★.**

Why does a freely hanging bar [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) always settle pointing the same way? What does this reveal about the Earth?

**Solution of Exercise 25.4.**

Because a giant [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) is pulling its [poles](#def-g4-magnets-and-compass-poles) into line: the Earth itself is a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), with magnetic [poles](#def-g4-magnets-and-compass-poles) near the top and bottom of the globe.

**Exercise 25.5 ★.**

Name the four [cardinal directions](#def-g4-magnets-and-compass-compass) in order, starting from North and turning right. Facing north, which direction is on your right?

**Solution of Exercise 25.5.**

North, East, South, West. Facing north, East is on your right.

**Exercise 25.6 ★.**

Why must a [compass](#def-g4-magnets-and-compass-compass) be kept away from [magnets](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), iron fences and cars while you read it?

**Solution of Exercise 25.6.**

The needle is a tiny [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) and obeys the *nearest* strong pull. A [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) or big iron [mass](https://one-course.com/books/physics/1/en/chapter/20-measuring-mass#def-g3-measuring-mass-mass) nearby out-pulls the distant [Earth-magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), and the needle lies about north.

**Exercise 25.7 ★.**

Describe how to turn a sewing needle into a [compass](#def-g4-magnets-and-compass-compass) needle, and how to let it swing freely without a [pivot](https://one-course.com/books/physics/1/en/chapter/17-levers-and-balance-scales#def-g3-levers-and-scales-lever).

**Solution of Exercise 25.7.**

Stroke the needle with one [pole](#def-g4-magnets-and-compass-poles) of a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), twenty times in the same direction, lifting the [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) between strokes — it becomes a little [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet). Lay it on a cork slice floating in a bowl of water: the floating cork is a [pivot](https://one-course.com/books/physics/1/en/chapter/17-levers-and-balance-scales#def-g3-levers-and-scales-lever) made of water, and the needle swings to point north–south.

**Exercise 25.8 ★.**

From the [sundial](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-sundial) chapter: which direction does the midday [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) point around here? How can this check a [compass](#def-g4-magnets-and-compass-compass) — or replace a lost one?

**Solution of Exercise 25.8.**

It points north. A [compass](#def-g4-magnets-and-compass-compass) whose needle agrees with the midday [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) is telling the truth — and on a sunny day the [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) alone can serve as a rough [compass](#def-g4-magnets-and-compass-compass).

**Exercise 25.9 ★.**

Why was the [compass](#def-g4-magnets-and-compass-compass) such a treasure for sailors, compared with steering by the coastline or the stars?

**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](#def-g4-magnets-and-compass-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 of Exercise 25.10.**

The friend’s iron belt buckle out-pulls the [Earth-magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-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](#def-g4-magnets-and-compass-compass) away from the buckle (a few steps apart), then walk.

**Exercise 25.11 ★★.**

A [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) is sawed in half between its two [poles](#def-g4-magnets-and-compass-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](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) with *both* [poles](#def-g4-magnets-and-compass-poles). Test the claim in thought with the compass-needle recipe: the stroked sewing needle is short — does it have one [pole](#def-g4-magnets-and-compass-poles) or two? What does this suggest about hunting a lone [pole](#def-g4-magnets-and-compass-poles) by cutting?

**Solution of Exercise 25.11.**

The short stroked needle has *two* [poles](#def-g4-magnets-and-compass-poles) — its ends behave like N and S ([float](https://one-course.com/books/physics/1/en/chapter/5-floating-and-sinking#def-g1-floating-and-sinking-words) two such needles and they snap end to end, opposite ends attracting). So shortening a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) does not remove a [pole](#def-g4-magnets-and-compass-poles), and cutting one in half just makes two complete short [magnets](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet): the hunt for a lone [pole](#def-g4-magnets-and-compass-poles) by cutting never ends — each cut hands you both [poles](#def-g4-magnets-and-compass-poles) again.