📚 IGCSE Physics: Magnetism and Electromagnetism Essentials | IGCSE物理：磁与电磁学要点
This article summarises the core ideas of magnetism and electromagnetism for the Edexcel IGCSE Physics syllabus. You will need to explain concepts clearly, draw simple field diagrams, and apply the right-hand rules in exam questions.
本文为 Edexcel IGCSE 物理课程总结磁与电磁学的核心要点。在考试中，你不仅需要清晰解释概念，还要会画简单的磁场图，并正确运用右手定则分析问题。
1. Properties of Magnets | 磁体的基本性质
Every magnet has two poles: a north pole (N) and a south pole (S). Magnetic force is strongest at the poles. Like poles repel each other, while unlike poles attract each other. A freely suspended bar magnet will always come to rest pointing roughly north-south because the Earth behaves like a giant magnet.
每个磁体都有两个磁极：北极（N）和南极（S）。磁力在磁极处最强。同名磁极相互排斥，异名磁极相互吸引。自由悬挂的条形磁铁最终会大致指向南北方向，这是因为地球本身就像一个巨大的磁体。
Magnetic materials are attracted to magnets, but they do not repel. The ends of a magnet are often marked N and S. If a magnet is broken in half, each piece becomes a complete magnet with its own N pole and S pole.
磁性材料会被磁体吸引，但不会被磁体排斥。磁铁的两端通常标有 N 和 S。如果把磁铁摔成两半，每一段都会成为拥有独立 N 极和 S 极的完整磁体。
2. Magnetic Fields and Field Lines | 磁场与磁力线
A magnetic field is the region around a magnet where another magnet or magnetic material experiences a magnetic force. Magnetic field lines are drawn from the north pole to the south pole outside the magnet. Inside the magnet they go from S to N.
磁场是磁体周围能对其他磁体或磁性材料产生磁力作用的区域。在磁体外部，磁力线从北极出发回到南极；在磁体内部，磁力线从南极指向北极。
Rules for field lines:
画磁力线时必须遵守以下规则：
- Field lines never cross each other. 磁力线永不相交。
- The closer the lines, the stronger the magnetic field. 磁力线越密，磁场越强。
- Arrows show the direction a free north pole would move. 箭头方向表示自由北极在该点所受磁力的方向。
Near the poles, field lines are crowded together, so the field is strongest there. In plotting compass tests, the needle of the compass lines up along the magnetic field direction.
在磁极附近，磁力线最密集，因此磁场最强。在用指南针探测磁场时，小磁针会沿着该点的磁场方向排列。
3. Magnetic Materials and Magnetisation | 磁性材料与磁化
The common magnetic materials are iron, steel, cobalt and nickel. Iron and steel are the most important in IGCSE questions. Soft iron magnetises easily and loses its magnetism quickly when removed from a strong field. Steel is harder to magnetise but keeps its magnetism for longer.
常见磁性材料有铁、钢、钴和镍。其中铁和钢在 IGCSE 题目中最为重要。软铁容易被磁化，但一旦离开强磁场，磁性也会很快消失；钢较难被磁化，但磁化后能较长时间保持磁性。
| Property | 性质 | Soft iron | 软铁 | Steel | 钢 |
| Easy to magnetise? 容易磁化吗？ | Yes 是 | Harder 较难 |
| Retains magnetism? 保持磁性吗？ | Poorly 不持久 | Well 持久 |
| Main use 主要用途 | Electromagnet cores 电磁铁铁芯 | Permanent magnets 永磁体 |
To make a magnet, you can stroke a steel bar with a magnet in one direction, or place the material inside a solenoid carrying a direct current. Demagnetisation can be done by heating, hammering, or placing the magnet in a solenoid carrying an alternating current and slowly withdrawing it.
制造磁铁的方法有两种：用磁体沿同一方向反复摩擦钢条，或者将材料放入通有直流电的螺线管中。退磁则可以通过加热、敲击，或将磁体放入通有交流电的螺线管中并缓慢移出来实现。
4. Magnetic Effect of a Current | 电流的磁效应
When an electric current flows through a wire, a magnetic field is produced around the wire. The field lines are concentric circles centred on the wire. The direction of the field depends on the direction of the current.
当电流通过导线时，导线周围会产生磁场。磁力线是以导线为圆心的同心圆。磁场方向取决于电流方向。
Use the right-hand grip rule: point the thumb of your right hand in the direction of conventional current, and your fingers curl in the direction of the magnetic field lines.
使用右手螺旋定则：右手拇指指向传统电流方向，四指弯曲的方向即为磁场方向。
A solenoid is a long coil of wire. Its magnetic field is similar to that of a bar magnet. Inside the solenoid the field is strong and almost uniform; outside it is like the field around a bar magnet.
螺线管是长直线圈，其磁场与条形磁铁相似。螺线管内部磁场强且近似均匀，外部磁场则类似条形磁铁周围的磁场。
Field strength increases with current, number of turns per metre, or a soft iron core inside the coil.
磁场强度会随电流增大、单位长度匝数增加，或在线圈中放入软铁芯而增强。
5. Electromagnets and Applications | 电磁铁及其应用
An electromagnet is made by wrapping a solenoid around a soft iron core. Its main advantage over a permanent magnet is that it can be switched on and off. Its strength can also be controlled by changing the current or the number of turns.
电磁铁由绕在软铁芯上的螺线管构成。相比永磁体，它最大的优点是可以控制磁性的有无，还可以通过改变电流大小或匝数来控制磁性强弱。
Applications include the electric bell, the circuit breaker, and the electromagnetic crane used in scrapyards. In each case, the magnet attracts a piece of iron or steel when switched on, and releases it when the current is switched off.
电磁铁的应用包括电铃、断路器和废品回收站使用的电磁起重机。工作时，通电后电磁铁吸引铁制或钢制物件；断电后磁性消失，物件被释放。
In exams, you may be asked to explain how an electric bell works. The key steps are: the switch is pressed, current flows, the electromagnet attracts the hammer, the hammer strikes the bell, the circuit breaks, and the spring returns the contact, repeating the cycle.
考试中常要求解释电铃的工作原理。关键步骤是：按下开关，有电流通过，电磁铁吸引小锤，小锤敲击铃铛，电路断开，弹簧使触点复位，如此反复循环。
6. The Motor Effect | 电动机效应
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is called the motor effect. The direction of the force can be found using Fleming’s left-hand rule.
当通电导体置于磁场中时，导体会受到力的作用，这就是电动机效应。力的方向可以用弗莱明左手定则判断。
- First finger: direction of the magnetic field, N to S. 食指：磁场方向，从 N 到 S。
- Second finger: direction of conventional current. 中指：传统电流方向。
- Thumb: direction of the force or motion. 拇指：力或运动的方向。
The size of the force increases if the magnetic flux density increases, if the current increases, or if the length of wire in the field increases. For a wire at right angles to the field:
力的大小会随着磁通密度增大、电流增大，或导体在磁场中有效长度增大而增大。当导线与磁场方向垂直时：
F = B I l
力 = 磁通密度 × 电流 × 导体有效长度
An electric motor uses this effect. A coil of wire in a magnetic field experiences a pair of forces that rotate the coil. A split-ring commutator reverses the current every half turn so the rotation continues in the same direction.
电动机利用这一效应工作。磁场中的线圈受到一对力偶的作用而转动。换向器每半圈改变一次电流方向，使线圈持续朝同一方向旋转。
7. Electromagnetic Induction | 电磁感应
Electromagnetic induction happens when a conductor cuts magnetic field lines, or when the magnetic field through a coil changes. This induces an electromotive force (e.m.f.) across the conductor. If the circuit is complete, an induced current flows.
当导体切割磁力线，或穿过线圈的磁场发生变化时，就会发生电磁感应，并在导体两端产生感应电动势（e.m.f.）。如果电路闭合，就会产生感应电流。
The size of the induced e.m.f. can be increased by moving the magnet faster, using a stronger magnet, or increasing the number of turns on the coil.
感应电动势的大小可以通过更快地移动磁体、使用更强的磁体或增加线圈匝数来增大。
The direction of the induced current always opposes the change that caused it. This is Lenz’s law. If a north pole approaches a coil, the coil creates its own north pole to repel it.
感应电流的方向总是阻碍引起感应电流的磁通量变化，这就是楞次定律。例如，当磁铁 N 极靠近线圈时，线圈会产生一个 N 极来排斥它。
Faraday’s law states that the induced e.m.f. is proportional to the rate of change of magnetic flux linkage.
法拉第电磁感应定律指出：感应电动势的大小与磁通链的变化率成正比。
Induced e.m.f. ∝ rate of change of magnetic flux linkage
感应电动势 ∝ 磁通链变化率
8. The Generator Principle | 发电机原理
A generator converts kinetic energy into electrical energy. The simplest generator is a coil rotating in a magnetic field. As the coil rotates, the magnetic flux through it changes continuously, so an alternating e.m.f. is induced.
发电机把动能转化为电能。最简单的发电机是在磁场中转动的线圈。当线圈转动时，穿过线圈的磁通量不断变化，因此会产生交变电动势。
In an AC generator, the coil is connected to slip rings and carbon brushes. Each half-turn, the current in the external circuit changes direction, so the output is alternating current.
交流发电机中，线圈连接到滑环和碳刷上。每转过半圈，外部电路中的电流方向就改变一次，因此输出为交流电。
In a DC generator, a split-ring commutator is used instead of slip rings. The commutator swaps the connections every half-turn, so the direction of current in the external circuit stays the same.
直流发电机使用换向器代替滑环。换向器每半圈交换一次连接，使外部电路中的电流方向保持不变。
9. Alternating Current and Direct Current | 交流电与直流电
Direct current (DC) flows in one direction only. Cells and batteries supply direct current. Alternating current (AC) reverses its direction many times per second. Mains electricity in the UK is AC with a frequency of 50 Hz.
直流电（DC）只沿一个方向流动，电池和蓄电池提供的是直流电。交流电（AC）每秒多次改变方向。英国家庭用电是频率为 50 Hz 的交流电。
An oscilloscope trace shows AC as a sinusoidal wave, while DC appears as a horizontal straight line. A frequency of 50 Hz means the current completes 50 full cycles every second.
示波器屏幕上，交流电显示为正弦波形，直流电显示为水平直线。50 Hz 的频率意味着电流每秒完成 50 个完整周期。
AC is easier to generate and its voltage can be changed easily using transformers. For these reasons, mains electricity uses AC rather than DC.
交流电容易产生，并且可以通过变压器方便地改变电压。因此，家庭供电系统使用交流电而非直流电。
10. Transformers | 变压器
A transformer changes the voltage of an alternating current supply. It has two coils wound on a soft iron core. The primary coil is connected to the AC input, and the secondary coil is connected to the output. It only works with alternating current.
变压器可以改变交流电的电压。它由绕在软铁芯上的两个线圈组成：原线圈接交流输入，副线圈接输出。变压器只能使用交流电。
For an ideal transformer:
对于理想变压器：
Vₛ / Vₚ = Nₛ / Nₚ
副线圈电压 / 原线圈电压 = 副线圈匝数 / 原线圈匝数
In an ideal transformer, no power is lost, so VₚIₚ = VₛIₛ. A step-up transformer has more turns on the secondary coil; a step-down transformer has fewer turns on the secondary coil.
在理想变压器中，没有功率损失，因此 VₚIₚ = VₛIₛ。升压变压器副线圈匝数多于原线圈；降压变压器副线圈匝数少于原线圈。
Energy losses in real transformers are reduced by using a laminated soft iron core to reduce eddy currents, and by using thick low-resistance copper wires to reduce heating losses.
实际变压器中，能量损失可以通过使用叠片软铁芯来减小涡流，以及使用粗而低电阻的铜线来减少发热损耗。
Published by TutorHao | Physics Revision Series | aleveler.com
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