Magnetic Fields
Introduced| English |
|---|
| magnetic field/mæɡˈnetɪk fiːld/ |
| pole/pəʊl/ |
| tesla/ˈteslə/ |
| monopoles/ˈmɒnəʊpəʊlz/ |
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:
Outside, field lines run from the north pole to the south pole.
The SI unit of magnetic field strength is the ____.
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:
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:
No monopoles: each half grows its own N and S.
Magnetic field lines always form closed loops because there are no magnetic monopoles.
With no monopoles, lines have nowhere to start or stop — they close.
Select all true statements about magnetic fields.
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.