Magnetism
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| magnet/ˈmæɡnɪt/ | nam châm |
| poles/pəʊlz/ | cực từ |
| magnetic field/mæɡˈnetɪk fiːld/ | từ trường |
| permanent magnet/ˈpɜːmənənt ˈmæɡnɪt/ | nam châm vĩnh cửu |
| induced magnetism/ɪnˈdjuːst ˈmæɡnɪtɪzəm/ | từ tính cảm ứng |
| soft iron/sɒft ˈaɪən/ | sắt mềm |
| field lines/fiːld laɪnz/ | đường sức từ |
| electromagnets/ɪˌlektrəʊˈmæɡnɪts/ | nam châm điện |
The pull of a magnet 磁体
- A fridge magnet sticks; a compass needle always swings to point north.
- A magnet has two ends, called poles 磁极: a north (N) and a south (S).
- Let's see the rules magnets follow — and how to picture their invisible field.
Poles and magnetic materials
- The pole rule is like the charge rule:
- Same poles (N–N or S–S) repel (push apart).
- Different poles (N–S) attract (pull together).
- A magnet attracts magnetic materials: iron, steel, cobalt, nickel. Copper, plastic and wood are not magnetic.
Iron filings line up along the magnetic field 磁场 of a bar magnet
Make the magnetic field visible
Compass needles trace the field from N to S; switch between attract and repel and watch the whole pattern reorganise, with a neutral point appearing for like poles.
Two north poles are brought close together. They:
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Like poles repel. Only different poles (N and S) attract.
Which of these is a magnetic material (attracted by a magnet)?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Iron (also steel, cobalt, nickel) is magnetic. Copper, plastic and wood are not.
Induced, temporary and permanent magnets 永磁体
- Put iron near a magnet and the iron becomes a magnet itself — induced magnetism 感应磁性. The near end gains the opposite pole, so they attract.
- Soft iron 软铁 magnetises easily but loses it quickly → a temporary magnet (great for electromagnet cores).
- Steel is harder to magnetise but keeps it → a permanent magnet (great for bar magnets).
The magnetic field of a bar magnet
Which material is best for a permanent magnet?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Steel is hard to magnetise but keeps it, so it makes a permanent magnet. Soft iron loses magnetism quickly, so it suits temporary magnets and electromagnet cores.
Match each magnetism idea to what it means.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Each item links the term to its correct meaning.
Magnetic fields
- A magnetic field is a region where a magnetic pole feels a force.
- We draw it with field lines 磁感线 that come out of N and go into S.
- Where the lines are close together, the field is strong.
- Show the pattern with iron filings or a small plotting compass (the needle points along the field).
Magnetic field lines around a bar magnet point:
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
By convention, field lines come out of N and go into S; the closer together they are, the stronger the field.
Where the field lines are closer together, the magnetic field is stronger.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Closely spaced field lines mean a strong field — that is why the field is strongest near the poles.
Uses of magnets
- Permanent magnets: fridge catches, compass needles, loudspeakers.
- Electromagnets 电磁铁 (magnetic only while a current flows): scrap-yard cranes, electric bells, relays.
- An electromagnet's big advantage: you can switch it on and off and make it stronger — a permanent magnet cannot.
Like poles repel; unlike poles attract
What can an electromagnet do that a permanent magnet cannot?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
An electromagnet is magnetic only while a current flows, so it can be switched on/off and made stronger by increasing the current or turns.
You've got it
- like poles repel, unlike poles attract; magnetic materials = iron, steel, cobalt, nickel
- soft iron = temporary magnet; steel = permanent magnet
- field lines go out of N, into S; closer lines = stronger field
- electromagnets can be switched on/off and made stronger
Write from 4 poles
- One bar magnet is cut into 2 pieces.
- Say how many poles there are. Say why an electromagnet core is soft iron rather than steel.
A bar magnet is cut into 2 pieces. How many poles are there in total, and why is soft iron used for an electromagnet core?
Two magnets have two poles each. Field lines leave a north pole and enter a south pole.
What does cutting the magnet into 2 pieces produce?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
2 times 2 is 4 poles, and each piece still has both poles.