📚 Electricity and Magnetism: Key Exam Points | 电与磁 考点精讲
This comprehensive revision guide unpacks every essential concept in the IGCSE OCR Science topic of electricity and magnetism. From permanent magnets and field lines to electromagnetic induction and transformers, you will find clear explanations, paired English–Chinese paragraphs, helpful tables, and key equations using Unicode symbols. Use this resource to consolidate your understanding and prepare confidently for your exam.
这篇全面的复习指南梳理了 IGCSE OCR 科学科目中电与磁专题的每一个核心概念。从永磁体和磁感线，到电磁感应和变压器，你将看到清晰的解释、中英对照段落、实用表格以及用 Unicode 符号表示的关键方程式。利用本资源巩固你的理解，为考试做好充分准备。
1. Magnets and Magnetic Materials | 磁铁与磁性材料
Magnets have two poles, north (N) and south (S). Like poles repel each other; unlike poles attract. Only a few materials – mainly iron, steel, nickel, and cobalt – are ferromagnetic and can be magnetised or attracted by a magnet.
磁铁有两个极，北极 (N) 和南极 (S)。同名磁极相互排斥，异名磁极相互吸引。只有少数材料——主要是铁、钢、镍和钴——属于铁磁性材料，能够被磁化或被磁铁吸引。
Permanent magnets are made of magnetically hard materials, such as steel, which retain their magnetism once magnetised. Temporarily magnetised objects are made of magnetically soft materials, like pure iron, which easily lose their magnetism.
永磁体由磁硬材料（如钢）制成，一旦磁化便能长时间保持磁性。暂时磁化的物体由磁软材料（如纯铁）制成，容易失去磁性。
You can test for magnetism by repulsion. Only a magnet will repel another magnet. Attraction alone is not enough, because an unmagnetised piece of iron will also be attracted to a magnet.
可以通过排斥现象来检验磁性。只有磁体才会排斥另一个磁体。单凭吸引不足以判断，因为未被磁化的铁片同样会被磁体吸引。
2. Magnetic Fields and Field Lines | 磁场与磁感线
A magnetic field is a region around a magnet where a magnetic force can be detected. The field is strongest at the poles. Magnetic field lines show the direction of the force: they point from the north pole to the south pole outside the magnet, and continue inside from south to north.
磁场是磁铁周围能够检测到磁力的区域。磁场在磁极处最强。磁感线标示力的方向：在磁体外部，磁感线从北极指向南极；在磁体内部，从南极指向北极，形成闭合回路。
The field lines never cross. The closer the lines are, the stronger the magnetic field. A uniform magnetic field, found between two opposite poles aligned face-to-face, has evenly spaced, parallel lines.
磁感线永不相交。线越密集，磁场越强。在两个异名磁极面对面放置时，中间会产生匀强磁场，磁感线等距且平行排列。
You can plot field lines using a plotting compass or iron filings. The compass needle aligns with the field, with its north-seeking pole pointing in the direction of the field lines.
可以使用指南针或铁屑来绘制磁感线图。指南针的指针会沿磁场方向排列，其指北极指向磁感线的切线方向。
3. Magnetic Field around a Current-carrying Wire | 载流导线周围的磁场
When an electric current flows through a straight wire, it generates a circular magnetic field around the wire. The direction of the field can be determined using the right-hand grip rule: point your right thumb in the direction of conventional current (positive to negative), and your curled fingers show the direction of the magnetic field lines.
当电流通过直导线时，会在导线周围产生环形磁场。磁场方向可用右手握拳定则判断：伸直右手拇指指向电流方向（正电荷流动方向），弯曲的四指所指即为磁感线方向。
A solenoid (a long coil of wire) produces a magnetic field similar to that of a bar magnet when a current passes through it. Inside the solenoid, the field is strong and uniform. The strength of the magnetic field can be increased by:
- increasing the current
- increasing the number of turns (coils)
- inserting a soft iron core
螺线管（长线圈）通电后会产生与条形磁铁相似的磁场。在螺线管内部，磁场强且均匀。增强磁场的方法包括：
- 增大电流
- 增加线圈匝数
- 插入软铁芯
4. Electromagnets and Their Uses | 电磁铁及其应用
An electromagnet is a solenoid with an iron core. It is a temporary magnet: its magnetic field exists only while current is flowing. Electromagnets are used in relays, electric bells, loudspeakers, and scrapyard lifting cranes because their strength can be adjusted and they can be switched on and off.
电磁铁是带有铁芯的螺线管。它是一种暂时磁体：只在通电时产生磁场。电磁铁广泛应用于继电器、电铃、扬声器和废料场起重机，因为其磁场强度可调且可以随时开关。
In a relay, a small current in one circuit operates an electromagnet to close a switch in a second circuit, allowing a low-voltage circuit to safely control a high-voltage circuit. In electric bells, an electromagnet attracts a hammer to strike the bell, breaking the circuit in the process so the hammer returns, creating a repeated ring.
在继电器中，一个小电流通过电磁铁吸合另一个电路中的开关，实现低压电路安全控制高压电路。在电铃中，电磁铁吸引小锤敲击铃铛，同时切断电路，小锤弹回，如此反复发出连续响声。
5. The Motor Effect and Fleming's Left-Hand Rule | 电机效应与弗莱明左手定则
When a current-carrying conductor is placed in an external magnetic field, the two magnetic fields interact, producing a force on the conductor. This is called the motor effect. The force is maximum when the current is perpendicular to the magnetic field, and zero when they are parallel.
当载流导线置于外部磁场中时，两个磁场相互作用，使导线受到力的作用，这就是电机效应。当电流方向与磁场方向垂直时，力最大；两者平行时，力为零。
The magnitude of the force can be calculated using:
F = BIL
where F is the force (N), B is the magnetic flux density (T, tesla), I is the current (A), and L is the length of conductor in the field (m). B is measured in tesla and represents the strength of the magnetic field.
力的大小可用公式计算：F = BIL，其中 F 为力（牛顿），B 为磁通量密度（特斯拉，T），I 为电流（安培），L 为导线在磁场中的有效长度（米）。B 的单位特斯拉表示磁场强度。
Fleming's left-hand rule gives the direction of the force: hold your left thumb, forefinger, and second finger mutually at right angles. The thuMb indicates Motion (force), the Forefinger indicates Field (N to S), and the seCond finger indicates Current (conventional).
弗莱明左手定则用于判断力的方向：伸出左手，让拇指、食指和中指互成直角。拇指表示力的方向（Motion），食指表示磁场方向（Field，从 N 到 S），中指表示电流方向（Current，正电荷方向）。
6. Electric Motors | 电动机
A simple d.c. motor consists of a coil of wire placed in a uniform magnetic field (between the poles of a permanent magnet). A split-ring commutator connects the coil to the power supply. The commutator reverses the current in the coil every half-turn, ensuring the coil continues to spin in the same direction.
简单的直流电动机由放置在匀强磁场（永磁体两极之间）中的线圈组成。一个换向器（裂环式）将线圈与电源连接。换向器每半圈反转线圈中的电流方向，确保线圈持续朝同一方向转动。
When current flows, opposite sides of the coil experience forces in opposite directions according to Fleming's left-hand rule, producing a turning effect (a couple). The speed of rotation can be increased by:
- increasing the current
- increasing the number of turns on the coil
- increasing the magnetic field strength
- using a soft iron core to concentrate the field
通电时，线圈对边根据左手定则受到方向相反的力，形成力偶，产生转动效果。提高转速的方法包括：
- 增大电流
- 增加线圈匝数
- 增强磁场强度
- 使用软铁芯集中磁场
7. Electromagnetic Induction | 电磁感应
Electromagnetic induction is the process of generating a potential difference (p.d.) across a conductor when it experiences a changing magnetic field. This p.d., often called an induced e.m.f., can produce a current if the conductor is part of a complete circuit.
电磁感应是指在导体所处的磁场发生变化时，导体两端产生电势差（电压）的过程。这个电势差常称为感应电动势。如果导体构成闭合回路，就会产生感应电流。
Induction occurs when there is relative motion between a conductor and a magnetic field, or a change in the magnetic field around a conductor. The direction of the induced current opposes the change that caused it – this is Lenz's law.
当导体与磁场之间存在相对运动，或者导体周围的磁场发生变化时，就会发生电磁感应现象。感应电流的方向总是试图阻碍引起感应的变化——这就是楞次定律。
The size of the induced p.d. can be increased by:
- moving the conductor (or magnet) faster
- using a stronger magnetic field
- using a coil with more turns
- inserting a soft iron core inside the coil
增大感应电动势的方法有：
- 加快导体（或磁体）的运动速度
- 使用更强的磁场
- 增加线圈匝数
- 在线圈中插入软铁芯
8. Generators and Alternators | 发电机与交流发电机
A generator converts kinetic energy into electrical energy by electromagnetic induction. In a simple a.c. generator (alternator), a coil rotates in a magnetic field, connected to an external circuit via slip rings and brushes. Slip rings maintain continuous contact but do not split the circuit, so the current alternates direction each half-turn, producing alternating current (a.c.).
发电机通过电磁感应将动能转化为电能。在简单的交流发电机中，线圈在磁场中旋转，通过滑环和电刷与外电路连接。滑环保持持续接触但不切换电路，因此电流每半圈改变方向，产生交流电 (a.c.)。
A d.c. generator (dynamo) uses a split-ring commutator instead of slip rings, so the induced current is always delivered in one direction – a pulsating direct current. On a graph, the output of an alternator is a sine wave; a dynamo produces a varying but unidirectional wave.
直流发电机（dynamo）使用换向器而非滑环，因此输出的电流方向始终不变，呈脉动直流电。在图像上，交流发电机的输出是正弦波；直流发电机产生的是波动但单向的波形。
| Feature | Alternator (a.c.) | Dynamo (d.c.) |
|---|---|---|
| Current type | Alternating | Pulsating direct |
| Connection | Slip rings and brushes | Split-ring commutator |
| Direction change | Every half-rotation | Never (output is unidirectional) |
特征 | 交流发电机 | 直流发电机
电流类型 | 交流电 | 脉动直流
连接装置 | 滑环和电刷 | 换向器
方向变化 | 每半圈改变 | 始终不变（单向）
9. Transformers | 变压器
A transformer changes the size of an alternating voltage. It consists of two coils wound on a common laminated soft iron core. An alternating current in the primary coil produces a changing magnetic field, which links through the core to induce an alternating voltage in the secondary coil.
变压器用于改变交流电压的大小。它由绕在公共叠片软铁芯上的两个线圈组成。初级线圈中的交流电产生变化的磁场，通过铁芯耦合，在次级线圈中感应出交流电压。
The transformer equation relates voltage and turns:
Vₚ / Vₛ = Nₚ / Nₛ
where Vₚ is the primary voltage, Vₛ is the secondary voltage, Nₚ is the number of turns on the primary coil, and Nₛ is the number of turns on the secondary coil.
变压器方程联系了电压与匝数：Vₚ / Vₛ = Nₚ / Nₛ，其中 Vₚ 为初级电压，Vₛ 为次级电压，Nₚ 为初级匝数，Nₛ 为次级匝数。
ewer turns on the secondary (Nₛ < Nₚ), reducing the voltage. For an ideal transformer (100% efficiency), power in equals power out: IₚVₚ = IₛVₛ.
升压变压器的次级匝数多于初级 (Nₛ > Nₚ)，所以 Vₛ > Vₚ。降压变压器次级匝数较少 (Nₛ < Nₚ)，电压降低。对于理想变压器（效率100%），输入功率等于输出功率：IₚVₚ = IₛVₛ。
10. The National Grid and Energy Transmission | 国家电网与电能输送
The National Grid is a system of cables and transformers that distributes electrical energy from power stations to homes and industries across the country. Step-up transformers raise the voltage to hundreds of thousands of volts for long-distance transmission.
国家电网是由电缆和变压器组成的系统，将发电站的电能输送到全国各地的家庭和工厂。升压变压器把电压升高到数十万伏，用于远距离输电。
High voltage is used because, for a given power, a higher voltage means a lower current. Since the heat losses in the cables are proportional to I²R, reducing the current dramatically reduces energy wasted as heat. Step-down transformers then lower the voltage to safe levels (e.g., 230 V) for consumer use.
使用高电压是因为在相同功率下，电压越高，电流越小。电缆中的热损耗正比于 I²R，因此减小电流可极大降低以热量形式浪费的电能。随后降压变压器将电压降至安全水平（如 230 伏）供用户使用。
11. Comparing Motor Effect and Generator Effect | 电机效应与发电机效应比较
It is easy to confuse the motor effect with electromagnetic induction. The motor effect converts electrical energy into kinetic energy – a current in a magnetic field experiences a force. Electromagnetic induction converts kinetic energy into electrical energy – movement of a conductor in a magnetic field generates a voltage.
电机效应和电磁感应现象容易混淆。电机效应将电能转化为动能——磁场中的载流导体受到力的作用。电磁感应则将动能转化为电能——导体在磁场中运动产生电压。
In the motor effect, you provide an electric current and get motion. In the generator effect, you provide motion and get an induced current. This distinction is fundamental for answering OCR exam questions about energy transfers.
电机效应：输入电流，得到运动。发电机效应：输入运动，得到感应电流。这一区别是 OCR 考试中回答能量转化题目的基础。
12. Key Equations and Units Summary | 关键方程式与单位汇总
| Equation | Symbols and Units |
|---|---|
| F = BIL | F: force (N), B: magnetic flux density (T or N/A·m), I: current (A), L: length (m) |
| Vₚ / Vₛ = Nₚ / Nₛ | V: voltage (V), N: number of turns (no unit) |
| IₚVₚ = IₛVₛ (ideal transformer) | I: current (A), V: voltage (V) |
方程 | 符号与单位
F = BIL | F: 力 (N), B: 磁通量密度 (T 或 N/A·m), I: 电流 (A), L: 长度 (m)
Vₚ / Vₛ = Nₚ / Nₛ | V: 电压 (V), N: 匝数 (无单位)
IₚVₚ = IₛVₛ (理想变压器) | I: 电流 (A), V: 电压 (V)
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