Magnetic Fields
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| magnetic field/mæɡˈnetɪk fiːld/ | từ trường |
| pole/pəʊl/ | cực từ |
| tesla/ˈteslə/ | tesla |
| monopoles/ˈmɒnəʊpəʊlz/ | đơn cực từ |
Why does a compass needle always swing to the north?
- Let a compass settle and it lines up along an invisible direction.
- A magnetic field 磁场 fills the space around every magnet.
- The needle is a tiny magnet turning to follow that field.
- This unit builds the whole story of magnetism from this field.
Poles and field lines
- Every magnet has a north and a south pole 磁极.
- Field lines run from N to S outside the magnet (and S to N inside).
- The field $\vec B$ is a vector; its unit is the tesla 特斯拉 (T).
- Where the lines crowd, the field is strongest.
See a magnet's field
Explore the field lines looping from north to south around a bar magnet.
Outside a bar magnet, the field lines run:
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Outside, field lines run from the north pole to the south pole.
The SI unit of magnetic field strength is the ____.
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Magnetic field $B$ is measured in tesla (T).
Like poles repel
- Bring two like poles together and they push apart.
- Opposite poles pull together — just like charges.
- The force acts through the field each magnet makes.
- A compass needle turns until its N points along the field.
Two north poles brought close together will:
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Like poles repel, just like like charges.
No magnetic monopoles
- You can never isolate a single N or S pole.
- Cut a magnet in half and each piece grows a new pair of poles.
- There are no magnetic monopoles 磁单极 — poles always come in pairs.
- This is why magnetic field lines always form closed loops.
Cut a bar magnet in half. You get:
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No monopoles: each half grows its own N and S.
Magnetic field lines always form closed loops because there are no magnetic monopoles.
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With no monopoles, lines have nowhere to start or stop — they close.
Select all true statements about magnetic fields.
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N-to-S outside, no monopoles, closed loops. A single pole can never be isolated.
The Earth is a giant magnet
- The Earth makes its own magnetic field, roughly like a bar magnet.
- A compass needle aligns with it, pointing toward magnetic north.
- The field also shields us from some of the Sun's charged particles.
- Sailors used this long before anyone understood why it worked.
You cut a bar magnet exactly in half at its middle.
- You do not get a separate N piece and S piece.
- Each half becomes a smaller magnet with its own N and S.
Magnetic field lines always form closed loops — they never start or stop on a pole the way electric lines start on charge. That is because there are no magnetic monopoles; N and S always come together.
A magnetic field $\vec B$ (in tesla) surrounds every magnet, with lines running N to S outside. Like poles repel, opposite attract. There are no magnetic monopoles, so field lines always form closed loops. The Earth itself acts as a giant magnet.
Write from 6 poles
- One bar magnet is cut across into 3 pieces.
- Say how many poles there are, and whether any piece is a north pole alone.
A bar magnet is cut into 3 separate pieces. How many poles are there in total, and can one piece be a north pole by itself?
Three magnets have two poles each. Cutting a magnet does not separate its poles.
What does cutting the magnet into 3 pieces produce?
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3 times 2 is 6 poles, and each piece still has both poles.