The magnetic field
| English | Chinese | Pinyin |
|---|---|---|
| magnetic field | 磁场 | cí chǎng |
| current | 电流 | diànliú |
| permanent magnets | 永磁体 | yǒng cí tǐ |
| iron filings | 铁屑 | tiě xiè |
| field lines | 场线 | chǎng xiàn |
| right-hand grip rule | 右手定则 | yòu shǒu dìng zé |
| solenoid | 螺线管 | luó xiàn guǎn |
| iron core | 铁芯 | tiě xīn |
| electromagnets | 电磁铁 | diàn cí tiě |
A field that ignores anything standing still
- Hold a stationary charge next to a powerful magnet and absolutely nothing happens. No force, no push, nothing.
- Set the same charge moving and it is suddenly shoved sideways, at right angles both to its motion and to the field.
- That selectivity is what makes a magnetic field different from an electric one, and it is the first thing the examiner asks you to state.
- This lesson is what a magnetic field 磁场 is, what produces one, and the field-line patterns you must be able to draw.
What a magnetic field is
- A magnetic field is a region in which a force acts on a moving charge, a current-carrying conductor or a magnetic material.
- The mark scheme wants the word force and one of those three objects. "A region around a magnet" on its own scores nothing.
- Note what is missing: a charge at rest feels no magnetic force at all. An electric field acts on any charge; a magnetic field is choosy.
A magnetic field is a region where:
Magnetic forces act only on moving charges (and currents) — a charge at rest feels nothing.
A magnetic field is a region in which a force acts on which of these? Select all that apply.
A stationary charge feels no magnetic force at all, which is exactly what distinguishes a magnetic field from an electric one.
What produces one
- Moving charges, which in practice means a current 电流 in a wire.
- Permanent magnets 永磁体, where the field comes from tiny atomic currents inside the material.
- Those are the same cause underneath. A permanent magnet is moving charge too, just on an atomic scale.
Iron filings 铁屑 turn an invisible field into something you can photograph
Which of these produce a magnetic field?
Moving charges and currents make magnetic fields; a permanent magnet does so via atomic currents. A still charge makes only an electric field.
Field lines
- Outside a magnet, field lines 场线 run from N to S, and from S to N inside it, so they always form closed loops.
- Lines never cross, and closer lines mean a stronger field. Every line carries an arrow.
- The lines are strongest, that is closest together, near the poles.
Closed loops, crowded at the poles
Magnetic field lab
Move between magnetic arrangements and see how field patterns change.
Outside a magnet, the field lines run:
Lines leave the N pole and enter the S pole outside, forming closed loops through the magnet.
Magnetic field lines never cross.
If they crossed, the field would point two ways at once — impossible. Closer lines just mean a stronger field.
Field lines drawn closer together mean a ____ field.
Line spacing shows strength — closer lines, stronger field.
Worked example: drawing the field of a straight wire
- Draw four field lines around a long straight wire carrying current into the page.
- The lines are concentric circles centred on the wire. That is the first mark.
- Their spacing increases with distance, because the field weakens as you move away. That is the second.
- Every line carries an arrow, and for current into the page the direction is clockwise, by the right-hand grip rule 右手定则: thumb along the current, fingers curl the way the field points.
The patterns to know
- Bar magnet: curved lines from N to S outside, closest near the poles.
- Long straight wire: concentric circles, spacing widening with distance.
- Flat circular coil: the loops around the two sides reinforce through the centre, so the field there is at right angles to the plane of the coil and strongest at the centre. Seen from one face the coil is a north pole, from the other a south pole.
- Long solenoid 螺线管: many such coils in a row, so the field inside is nearly uniform and parallel to the axis, like a stretched bar magnet, and it falls off fast outside.
Uniform down the middle, and a bar magnet's field at the ends
Drawing field lines round a straight wire is marked on which points? Select all that apply.
Circles, widening spacing, arrows. For current into the page the direction is clockwise, by the right-hand grip rule.
The iron core
- Putting an iron core 铁芯, or any ferrous material, inside a solenoid greatly increases the field.
- The reason, and this is the marked answer: the core becomes magnetised by the solenoid's field, and its own field adds to the field of the current.
- This is why electromagnets 电磁铁 and transformers are built around iron cores rather than air.
What is the magnetic field like inside a long solenoid?
A solenoid is many coils in a row, so the field inside is uniform like a stretched bar magnet's, and it falls off fast outside.
Match each arrangement to the shape of its magnetic field.
All four are worth being able to draw from memory, with arrows and with spacing that shows where the field is strong.
Marks that slip away
- The definition needs the word force and one of: moving charge, current-carrying conductor, magnetic material. A stationary charge feels nothing.
- Field lines form closed loops: N to S outside, S to N inside. They never simply stop.
- A wire's field lines are concentric circles whose spacing widens, and each needs an arrow.
- An iron core works because it is magnetised and adds its own field, not because it "concentrates" or "attracts" the field.
Why does an iron core greatly increase the field of a solenoid?
The core's atomic magnets line up and contribute their own field. That is the marked wording, and it is why electromagnets and transformers use iron cores.
You've got it
- a magnetic field is a region where a force acts on a moving charge, a current-carrying conductor or a magnetic material; a stationary charge feels nothing
- it is produced by moving charges or by permanent magnets, which are atomic currents
- field lines run N to S outside and S to N inside, forming closed loops; they never cross, and closer means stronger
- patterns: bar magnet, concentric circles round a wire by the right-hand grip rule, a coil acting as a small magnet, and a solenoid uniform inside; an iron core is magnetised and adds its own field