Magnetic Poles and Forces
FoundationA magnet has a north and a south pole: like poles repel, unlike poles attract, and the force is strongest at the poles. Magnetic force passes through paper and plastic to attract magnetic materials, can magnetise a nail and can show direction.
- A magnet has a north and a south pole: like repels, unlike attracts.
- The force is strongest at the poles and weakest in the middle.
- Magnetic force passes through non-magnetic things such as paper, card and plastic.
文體小知識：趣味實驗說明文用容易做的小實驗講科學，幾塊磁鐵就能發現磁力的規律。
§Text Passage
1A magnet attracts certain materials with an invisible pull called magnetic force. Every magnet has two poles: the end that points north when freely suspended is the north pole, and the end pointing south is the south pole. The force is strongest at the poles and weakest in the middle: the middle of a bar magnet can hardly lift a paper clip, while the ends can lift a whole chain of them.
2The poles obey one rule: like poles repel, unlike poles attract. Bring the north poles of two magnets close together and you feel a push; bring a north pole to a south pole and they snap together. A compass shows direction because the Earth itself acts like a giant magnet, and the needle's north pole swings towards the north.
3Magnetic force passes through non-magnetic materials. A paper clip stays stuck to the magnet with a sheet of paper between them, and even through thin card or a plastic board. But magnets do not attract everything: the materials drawn to magnets are mainly iron, cobalt and nickel, while plastic, wood, glass, copper and aluminium are not attracted.
4A magnet can also pass its magnetism to an iron nail. Rub the magnet along the nail in one direction dozens of times and the nail becomes a temporary magnet that can lift paper clips. Lay the magnetised needle across a small piece of foam floating on water and it slowly swings to point north and south — a homemade compass.
1Summary 重點摘要
A magnet has a north and a south pole: like poles repel, unlike poles attract, and the force is strongest at the poles. Magnetic force passes through paper and plastic to attract magnetic materials, can magnetise a nail and can show direction.
2Vocabulary cards Vocabulary
The force by which a magnet attracts magnetic materials and pushes or pulls another magnet's poles.
The two ends of a magnet where the force is strongest: north and south.
The end of a magnet that points north when free to turn, marked N.
The end of a magnet that points south when free to turn, marked S.
The pushing apart that happens when like poles come close.
The pulling together that happens when unlike poles come close.
Making something such as an iron nail temporarily magnetic.
A material drawn to magnets, mainly iron, cobalt and nickel.
A direction-finding tool using the Earth's magnetism; the needle's north pole points north.
3Step by step Steps
- Step 1: Find the two polesEvery magnet has two poles, north and south. Hang a bar magnet level on a cotton thread and let it swing freely; the end that settles pointing north is the north pole, the other end is the south pole, and a red sticker on the north pole marks it. Knowing the two poles stops confusion in the like-and-unlike pole experiments later. Keep the test away from other magnets and steel furniture, so nothing disturbs the free swinging and the poles you find are accurate.
- Step 2: Prove like repels, unlike attractsThere is an important rule between poles: like poles repel and unlike poles attract. Bring the north poles of two magnets close together and your hands feel a push; bring a north pole near the other magnet's south pole and the two snap together at once. Trying both with your own hands fixes the rule firmly in your memory. Lay the magnets on a smooth table and bring them close; one slides away by itself, and the repulsion is plain to see.
- Step 3: Find where the force is strongestThe magnetic force is not equally strong everywhere on a magnet. Use the middle and then the ends of a bar magnet to lift paper clips: the middle cannot lift even one, while each end lifts five at once, proving that the force is strongest at the poles and weakest in the middle. Knowing where the force concentrates, use the ends when you pick things up. Hang a chain of clips from the middle and from the ends and compare the lengths — the difference in strength shows up even more clearly.
- Step 4: Test which materials are attractedA magnet cannot attract everything; it mainly attracts iron and similar materials. Test a paper clip, a copper coin, aluminium foil, a plastic ruler and a wooden pencil one by one: only the iron clip is held tightly, while the copper, aluminium, plastic and wood do not move at all. Remember this result, and you will know a magnet can only pick out things made of iron. The materials drawn to magnets are mainly iron, cobalt and nickel; coins are mostly copper-nickel alloys, so the pull on them is weak.
- Step 5: Test force through materialsMagnetic force is invisible, yet it works through some materials. Put a sheet of paper between the magnet and the clip and the clip stays firmly stuck; slide the magnet under a plastic board and the iron beads on top follow it like dancers. This proves the force can pass through paper and plastic and attract iron across a gap. But through materials that are too thick the force weakens and the clip may not hold, so test by adding one layer at a time.
- Step 6: Magnetise a nail to make a compassIron that touches a magnet temporarily becomes magnetic — this is called magnetisation. Rub the magnet along an iron nail in one direction thirty times and the nail becomes a magnet; lay it across a small piece of foam floating on water and set it straight gently. The tip slowly turns until it points north and south, and a simple compass is made. The rubbing must go one way only — rubbing back and forth cancels the magnetism; a knock or a drop makes the magnetised nail lose it again.
4Diagrams Diagrams
5Worked example Example
Example: Chai keeps two bar magnets in his pencil box. Every time he closes the box, the magnets push apart and slide away. Why does this happen, and how can he make them hold together?
- The ends facing each other are like poles, and like poles repel with a pushing force.
- When the box closes, that push slides the two magnets apart.
- The fix: turn one magnet around so that unlike poles face each other.
- Unlike poles attract, so the magnets hold tightly together and stay put in the pencil box.
- Answer: The magnets push apart because like poles are facing each other. Turn one magnet around so that unlike poles meet, and the two magnets will attract each other and stay firmly fixed in the pencil box.
6Common mistakes Mistakes
7Quick check Quick check
Q1What two poles does a magnet have?
Q2What happens when like poles come close? And unlike poles?
Q3Which part of a magnet is strongest?
Q4Name two materials not attracted to magnets.
Q5Why can a compass show north and south?
8Recap Recap
- A magnet has a north and a south pole: like repels, unlike attracts.
- The force is strongest at the poles and weakest in the middle.
- Magnetic force passes through non-magnetic things such as paper, card and plastic.
- Magnets attract only magnetic materials such as iron, cobalt and nickel.
- Magnetise a nail by rubbing, float it on foam, and you have a simple compass.