--- title: "Magnets" book: "Primary & Middle School Physics" subject: physics language: en chapter: 6 exercises: 9 source: https://one-course.com/books/physics/1/en/chapter/6-magnets --- # Chapter 6 — Magnets On the refrigerator door, little figures hold up drawings without glue, without tape, without hooks. Pull one off: nothing sticky on its back — yet it jumps back and holds on. These little figures hide a [magnet](#def-g1-magnets-magnet), and [magnets](#def-g1-magnets-magnet) are full of surprises worth a whole chapter of experiments. ## 6.1 What a magnet grabs **Definition 6.1 (Magnets).** A *magnet* is a special piece of stone or metal that pulls certain things toward itself without touching them. We say the magnet *attracts* them. The things it attracts stick to it; the rest do not care about it at all. **Example 6.2 (Try it: the grabbing test).** Walk around your home with a [magnet](#def-g1-magnets-magnet) and try it on: a paperclip, a nail, the refrigerator door, a wooden spoon, a plastic brick, a sheet of paper, a glass, a cork. The [magnet](#def-g1-magnets-magnet) grabs the paperclip, the nail and the refrigerator door — and ignores every one of the others. Only some things obey [magnets](#def-g1-magnets-magnet). ![The grabbing test: the paperclip and the nail rush to the magnet; cork and plastic stay exactly where they are.](https://one-course.com/images/onecourse/chapters/physics-1/g1-magnets/fig-6f1d7473c125.svg) *The grabbing test: the paperclip and the nail rush to the [magnet](#def-g1-magnets-magnet); cork and plastic stay exactly where they are.* **Example 6.3 (The magnet’s favorite metal).** Look at what got grabbed: paperclip, nail, refrigerator door. They are all made of *iron* (or of steel, which is mostly iron). Wood, plastic, paper, glass, stone: never. And here is the fine surprise: even some *metals* are ignored — try a coin or a sheet of kitchen foil. A [magnet](#def-g1-magnets-magnet) is not a metal-lover; it is an iron-lover. ## 6.2 The magnet’s reach **Example 6.4 (Try it: grabbing through things).** Put a paperclip on the table and hold your [magnet](#def-g1-magnets-magnet) *under* the tabletop, just below the clip. Move the [magnet](#def-g1-magnets-magnet): the clip slides around the table as if by magic — the pull goes right through the wood. It also works through paper, through cardboard, through a glass of water: drop a clip in the glass and walk it up the side with the [magnet](#def-g1-magnets-magnet), keeping your fingers dry. ![A paperclip on a thread, held straining toward the magnet below — across a gap of air, with nothing touching.](https://one-course.com/images/onecourse/chapters/physics-1/g1-magnets/fig-57ab40c2f6d0.svg) *A paperclip on a thread, held straining toward the [magnet](#def-g1-magnets-magnet) below — across a gap of air, with nothing touching.* **Remark 6.5 (Pulling without touching).** Think how strange this is. To move a toy, you must touch it: push it, pull it, kick it. The [magnet](#def-g1-magnets-magnet) moves the clip *across empty air*, through wood, through water, touching nothing. Only very few things in nature can act without touching — keep this wonder in a corner of your head, for it will come back, much later, as one of the deepest ideas in this course. ![Real magnets at work: paperclips hang from the tip of a magnetized rod. The pink clip is coated in plastic — the pull passes straight through its coat. Photo: Donald Olszewski, CC BY 4.0.](https://one-course.com/images/onecourse/chapters/physics-1/g1-magnets/img-eb3a72a20bde.jpg) *Real [magnets](#def-g1-magnets-magnet) at work: paperclips hang from the tip of a magnetized rod. The pink clip is coated in plastic — the pull passes straight through its coat. *Photo: Donald Olszewski, CC BY 4.0.** **Method 6.6 (Magnet fishing).** A game for two, and a fair test of the reach: 1. Tie a [magnet](#def-g1-magnets-magnet) to a string — this is the fishing rod; 2. scatter paperclips on the floor — these are the fish; 3. lower the [magnet](#def-g1-magnets-magnet) slowly from high above one clip; 4. stop lowering the moment the clip jumps up to the [magnet](#def-g1-magnets-magnet) . The jump happens only when the [magnet](#def-g1-magnets-magnet) is close: the pull is strong near the [magnet](#def-g1-magnets-magnet) and fades quickly farther away. ## 6.3 Two magnets together **Example 6.7 (Try it: pull or push?).** Take two [magnets](#def-g1-magnets-magnet), one in each hand, and bring their ends slowly together. Sometimes they leap at each other and snap together. Now turn one [magnet](#def-g1-magnets-magnet) around, other end first: bring them together again, and this time you feel a soft, springy *push* — the [magnets](#def-g1-magnets-magnet) refuse to touch, as if an invisible cushion sat between them. Same two [magnets](#def-g1-magnets-magnet): one way they pull, turned around they push. ![Two magnets with colored ends, twice. When two different ends face each other, the magnets pull together; flip one so the same ends face, and they push each other away.](https://one-course.com/images/onecourse/chapters/physics-1/g1-magnets/fig-3014e7ea4a22.svg) *Two [magnets](#def-g1-magnets-magnet) with colored ends, twice. When two *different* ends face each other, the [magnets](#def-g1-magnets-magnet) pull together; flip one so the *same* ends face, and they push each other away.* **Remark 6.8 (The two ends).** So the two ends of a [magnet](#def-g1-magnets-magnet) are not the same, even when they look alike! Each [magnet](#def-g1-magnets-magnet) has two different ends, and the game of pull and push depends on *which* ends meet. Try the paperclip test too: clips cluster at the two ends of a bar [magnet](#def-g1-magnets-magnet) and mostly ignore its middle. These two special ends have names and a famous job — guiding travelers across the world — but that story waits for a later year of this book. ## 6.4 Exercises **Exercise 6.1 ★.** Sort into “the [magnet](#def-g1-magnets-magnet) grabs it” and “the [magnet](#def-g1-magnets-magnet) ignores it”: a nail; a cork; a paperclip; a plastic duck; a steel spoon; a sheet of paper. **Solution of Exercise 6.1.** Grabbed: the nail, the paperclip, the steel spoon. Ignored: the cork, the plastic duck, the sheet of paper. **Exercise 6.2 ★.** Which stuff does a [magnet](#def-g1-magnets-magnet) love? Name two metals or materials the [magnet](#def-g1-magnets-magnet) ignores anyway. **Solution of Exercise 6.2.** Iron (and steel, which is mostly iron). Ignored anyway: a coin and kitchen foil — metals, but not iron. **Exercise 6.3 ★.** A drawing holds on the refrigerator door under a [magnet](#def-g1-magnets-magnet) figure. What is between the [magnet](#def-g1-magnets-magnet) and the door? What does this show about the [magnet](#def-g1-magnets-magnet)’s pull? **Solution of Exercise 6.3.** The sheet of paper (the drawing) is between them. The [magnet](#def-g1-magnets-magnet)’s pull goes right through the paper to the iron door — it works through things. **Exercise 6.4 ★.** A paperclip falls into a glass full of water. How can you get it out with a [magnet](#def-g1-magnets-magnet), without wetting your fingers? **Solution of Exercise 6.4.** Slide the [magnet](#def-g1-magnets-magnet) against the *outside* of the glass, next to the clip, and walk it slowly up the side: the clip follows inside, up to the rim, pulled through the glass and the water. **Exercise 6.5 ★.** In the [magnet](#def-g1-magnets-magnet) fishing game of [Method 6.6](#met-g1-magnets-fishing), the clip jumps only when the [magnet](#def-g1-magnets-magnet) comes close. What does this tell you about the pull far from the [magnet](#def-g1-magnets-magnet)? **Solution of Exercise 6.5.** Far from the [magnet](#def-g1-magnets-magnet) the pull is too weak to move the clip; it grows strong only close up. The [magnet](#def-g1-magnets-magnet)’s reach fades quickly with distance. **Exercise 6.6 ★.** Bring two [magnets](#def-g1-magnets-magnet) end to end: they snap together. What single move makes the same two [magnets](#def-g1-magnets-magnet) push apart instead? **Solution of Exercise 6.6.** Turning one of the two [magnets](#def-g1-magnets-magnet) around, so that the other end comes first. Then they push apart instead of snapping together. **Exercise 6.7 ★.** Where do paperclips cluster on a bar [magnet](#def-g1-magnets-magnet): at the middle, or at the ends? What does this say about where a [magnet](#def-g1-magnets-magnet) is strongest? **Solution of Exercise 6.7.** At the ends — clips mostly ignore the middle. A [magnet](#def-g1-magnets-magnet) is strongest at its two ends. **Exercise 6.8 ★.** Name one way a [magnet](#def-g1-magnets-magnet) is like your hand moving a toy, and one way it is completely different. (Think of [Remark 6.5](#rem-g1-magnets-notouch).) **Solution of Exercise 6.8.** Alike: both can pull a thing toward you. Different: your hand must touch the toy, while the [magnet](#def-g1-magnets-magnet) pulls the clip across empty air, through wood and through water, touching nothing. **Exercise 6.9 ★★.** Tom says: “[Magnets](#def-g1-magnets-magnet) [attract](#def-g1-magnets-magnet) everything made of metal.” Design a little experiment with a coin, a nail and kitchen foil to test Tom’s sentence, say what will happen — and correct his sentence. **Solution of Exercise 6.9.** Hold the [magnet](#def-g1-magnets-magnet) near each object in turn. The nail jumps to it; the coin and the foil do not move, though both are metal. So Tom’s sentence is wrong as stated; corrected: “[Magnets](#def-g1-magnets-magnet) [attract](#def-g1-magnets-magnet) things made of iron (or steel), not every metal.”