📚 IGCSE Edexcel Physics: Electromagnetic Induction | IGCSE Edexcel 物理：电磁感应考点精讲
Electromagnetic induction is the process of generating an electromotive force (e.m.f.) across a conductor when it experiences a changing magnetic field. This fundamental principle underpins the operation of generators, transformers, and many modern electrical devices. In the Edexcel IGCSE Physics specification, a clear understanding of Faraday’s law, Lenz’s law, generators, and transformers is essential for exam success. This revision guide breaks down every key concept, equation, and experiment you need to master.
电磁感应是当导体处于变化的磁场中时，在其两端产生电动势（e.m.f.）的过程。这一基本原理是发电机、变压器和许多现代电气设备运作的基础。在Edexcel IGCSE物理考试中，对法拉第定律、楞次定律、发电机和变压器的清晰理解是取得高分的必要条件。本考点精讲将逐一剖析你需要掌握的每一个核心概念、方程式和实验。
1. What is Electromagnetic Induction? | 什么是电磁感应？
Electromagnetic induction is the production of a potential difference (voltage) across a conductor when it is exposed to a changing magnetic field. This effect was discovered by Michael Faraday in 1831 and is the basis for generating most of the world’s electricity.
电磁感应是指在导体处于变化的磁场中时，其两端产生电势差（电压）的现象。该效应由迈克尔·法拉第于1831年发现，是全球大部分电力生产的基础。
If a magnet is moved into a coil of wire, a voltage is induced across the coil. If the coil is part of a complete circuit, an induced current flows. The induced current flows only while the magnet is moving relative to the coil.
如果将磁铁移入线圈，线圈两端就会产生感应电压。如果线圈是闭合回路的一部分，就会有感应电流流过。感应电流只在磁铁相对于线圈运动时才会产生。
Similarly, moving a straight wire across a magnetic field also induces an e.m.f. The wire must cut through magnetic field lines for the effect to occur.
同样地，使一根直导线在磁场中运动也会感应出电动势。导线必须切割磁感线才会产生此效应。
2. Faraday’s Law of Induction | 法拉第电磁感应定律
Faraday’s law states that the magnitude of the induced e.m.f. across a conductor is directly proportional to the rate of change of magnetic flux linkage. For a coil with N turns, the induced e.m.f. is given by:
法拉第定律指出：导体中感应电动势的大小与磁通链的变化率成正比。对于匝数为N的线圈，感应电动势由下式给出：
ε = − N ΔΦ / Δt
Where ε is the induced e.m.f. (in volts, V), N is the number of turns on the coil, ΔΦ is the change in magnetic flux (in webers, Wb), and Δt is the time taken for the change (in seconds, s). The negative sign indicates the direction of the induced e.m.f. as described by Lenz’s law.
其中ε为感应电动势（单位伏特，V），N为线圈匝数，ΔΦ为磁通量变化量（单位韦伯，Wb），Δt为变化所需时间（单位秒，s）。负号表示感应电动势的方向，由楞次定律描述。
3. Factors Affecting Induced EMF | 影响感应电动势的因素
The size of the induced e.m.f. can be increased by any of the following:
感应电动势的大小可以通过以下任一方式增大：
- Increasing the speed of relative motion between the magnet and the coil (or between the conductor and the magnetic field).
- 增大磁铁与线圈（或导体与磁场）之间的相对运动速度。
- Using a stronger magnet, which produces a greater magnetic flux density.
- 使用更强的磁铁，以产生更大的磁通密度。
- Increasing the number of turns on the coil.
- 增加线圈的匝数。
- Winding the coil around a soft iron core, which concentrates the magnetic flux.
- 将线圈绕在软铁芯上，以集中磁通量。
4. Lenz’s Law | 楞次定律
Lenz’s law states that the direction of the induced current is always such that it opposes the change in magnetic flux that produced it. This law is a consequence of the conservation of energy and explains the negative sign in Faraday’s equation.
楞次定律指出：感应电流的方向总是使其阻碍产生该电流的磁通量变化。这条定律是能量守恒的体现，解释了法拉第公式中的负号。
For example, when a magnet is pushed into a coil, the induced current creates a magnetic field that repels the magnet, opposing the motion. When the magnet is pulled out, the induced current creates a field that attracts the magnet, again opposing the change.
例如，将磁铁推入线圈时，感应电流产生的磁场会排斥磁铁，阻碍其运动。当磁铁被拉出时，感应电流产生的磁场会吸引磁铁，同样阻碍变化。
This principle is crucial for understanding the behaviour of generators and for applying the right-hand rule to determine the direction of induced current.
这一原理对理解发电机的工作方式以及运用右手定则判断感应电流方向至关重要。
5. The AC Generator (Alternator) | 交流发电机
An AC generator converts mechanical energy into alternating current. It consists of a coil rotating within a uniform magnetic field. The ends of the coil are connected to two slip rings, which rotate with the coil. Carbon brushes press against the slip rings to carry the current to an external circuit.
交流发电机将机械能转换为交流电。它由一个在匀强磁场中旋转的线圈构成。线圈的两端连接到两个随线圈一起转动的滑环上。碳刷压在滑环上，将电流导出至外电路。
As the coil rotates, it changes the angle at which it cuts the magnetic field lines. This produces an induced e.m.f. that varies sinusoidally. The output voltage alternates in polarity, giving an alternating current (AC).
当线圈转动时，其切割磁感线的角度不断变化，从而产生按正弦规律变化的感应电动势。输出电压的极性交替变化，形成交流电（AC）。
V = Vₚₑₐₖ sin(θ)
The peak voltage is reached when the plane of the coil is parallel to the magnetic field (maximum rate of flux cutting). The voltage is zero when the coil is perpendicular to the field.
当线圈平面与磁场平行时（磁通量切割速率最大），电压达到峰值。当线圈垂直于磁场时，电压为零。
6. The DC Generator (Dynamo) | 直流发电机
A DC generator has a similar structure but uses a split-ring commutator instead of slip rings. The commutator reverses the connection of the coil to the external circuit every half turn. This ensures that the current flows in only one direction through the load, producing a direct current (DC).
直流发电机结构与交流发电机相似，但使用换向器（分裂环）代替滑环。每转过半圈，换向器就调换线圈与外电路的连接方向。这样，通过负载的电流只沿一个方向流动，从而输出直流电（DC）。
The output voltage from a simple DC generator is not perfectly steady; it varies from zero to a peak but always remains positive. By using multiple coils and a smooth commutator, a smoother DC output can be achieved.
简单的直流发电机输出电压并非完全平稳，它从零变化到峰值，但始终保持正极性。通过使用多个线圈和光滑的换向器，可以获得更平稳的直流输出。
7. Transformers: Structure and Operation | 变压器：结构与原理
A transformer is a device used to change the voltage of an alternating current. It consists of two insulated coils (primary and secondary) wound around a common soft iron core. An alternating current in the primary coil produces a changing magnetic field in the core, which induces an alternating e.m.f. across the secondary coil.
变压器是一种用来改变交流电压的装置。它由两个绝缘线圈（初级和次级）绕在一个共同的软铁芯上构成。初级线圈中的交流电在铁芯中产生变化的磁场，该磁场在次级线圈两端感应出交变电动势。
Transformers operate only on AC because a changing magnetic field is essential for induction. A steady direct current in the primary would produce a constant magnetic field and no induced voltage in the secondary.
变压器只能工作于交流电，因为变化的磁场是感应现象的必要条件。初级线圈中的恒定直流电会产生恒定磁场，次级线圈中不会产生感应电压。
8. The Transformer Equation | 变压器方程式
For an ideal transformer with 100% efficiency, the relationship between the primary and secondary voltages and the number of turns on each coil is given by:
对于效率为100%的理想变压器，初级、次级电压与各线圈匝数之间满足的关系如下：
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. If Nₛ > Nₚ, the transformer is a step-up transformer (Vₛ > Vₚ). If Nₛ < Nₚ, it is a step-down transformer (Vₛ < Vₚ).
其中Vₚ为初级电压，Vₛ为次级电压，Nₚ为初级线圈匝数，Nₛ为次级线圈匝数。若Nₛ > Nₚ，则为升压变压器（Vₛ > Vₚ）；若Nₛ < Nₚ，则为降压变压器（Vₛ < Vₚ）。
This equation can be rearranged to find any unknown quantity. For example, if a transformer with 500 primary turns and 50 secondary turns is connected to a 230 V mains supply, the secondary voltage is:
该公式可经变形求解任意未知量。例如，一个初级线圈500匝、次级线圈50匝的变压器接到230 V市电上，次级电压为：
Vₛ = (Nₛ / Nₚ) × Vₚ = (50 / 500) × 230 V = 23 V
9. Transformer Efficiency | 变压器效率
In practice, transformers are not 100% efficient due to energy losses. For an ideal transformer, the input power equals the output power:
实际上，由于存在能量损耗，变压器效率并非100%。对理想变压器而言，输入功率等于输出功率：
Vₚ × Iₚ = Vₛ × Iₛ
This means that if the voltage is stepped up, the current is stepped down proportionally, and vice versa. The efficiency of a transformer is calculated as:
这意味着电压升高时，电流会按比例降低，反之亦然。变压器的效率计算公式为：
η = (Pₒᵤₜ / Pᵢₙ) × 100% = (Vₛ Iₛ / Vₚ Iₚ) × 100%
Energy losses in a real transformer arise from: heating of the coils (Joule heating), eddy currents induced in the iron core, and hysteresis in the core material. Laminated cores and low-resistance copper coils reduce these losses.
实际变压器中的能量损耗来自：线圈发热（焦耳热）、铁芯中感生的涡流以及铁芯材料的磁滞。采用叠片铁芯和低电阻铜线圈可降低这些损耗。
10. Power Transmission and Transformers | 输电与变压器
Transformers are essential in the national electricity grid. Power stations generate electricity at about 25 kV. Step-up transformers raise this voltage to 400 kV or more for long-distance transmission. High voltage means low current for the same power, which drastically reduces energy lost as heat in the transmission cables (power loss = I²R).
变压器在国家级电网中不可或缺。发电站发出的电力约25 kV，升压变压器将此电压升至400 kV甚至更高，用于远距离输电。相同功率下，高电压意味着低电流，从而大幅降低输电电缆中的热量损耗（功率损耗 = I²R）。
Near homes and factories, step-down transformers reduce the voltage to safe levels (e.g., 230 V in the UK). This entire process would be impossible without electromagnetic induction.
在住宅和工厂附近，降压变压器将电压降至安全水平（如英国的230 V）。没有电磁感应，这一整套过程便无法实现。
11. Induced EMF in a Straight Wire | 直导线中的感应电动势
Electromagnetic induction can also be demonstrated with a single straight wire moving through a magnetic field. When the wire cuts magnetic field lines at right angles, an e.m.f. is induced across its ends. The magnitude of this e.m.f. is given by:
电磁感应也可用一根在磁场中运动的直导线来演示。当导线垂直切割磁感线时，其两端便会感应出电动势。该电动势的大小由下式给出：
ε = B L v
Where B is the magnetic flux density (T), L is the length of the wire within the field (m), and v is the component of velocity perpendicular to the field (m/s). The direction of the induced current can be found using Fleming’s right-hand rule for generators: thumb – motion, first finger – field, second finger – current.
其中B为磁通密度（T），L为导线在磁场中的有效长度（m），v为导线速度在垂直于磁场方向上的分量（m/s）。感应电流的方向可用发电机右手定则判断：拇指——运动方向，食指——磁场方向，中指——电流方向。
12. Key Experiments and Demonstrations | 关键实验与演示
Experiment 1: Electromagnetic induction using a coil and magnet. Connect a coil to a sensitive centre-zero galvanometer. Push a bar magnet into the coil, and observe the pointer deflect in one direction. Pull the magnet out, and the pointer deflects in the opposite direction. Fast motion produces a larger deflection.
实验1：利用线圈和磁铁演示电磁感应。将线圈与一个灵敏的零点居中检流计相连。将条形磁铁推入线圈，观察指针向一侧偏转。拉出磁铁时，指针向另一侧偏转。快速运动产生更大的偏转。
Experiment 2: Investigating factors affecting induced e.m.f. Use the same setup and systematically change the magnet strength, number of coils, and speed of motion. Record the galvanometer readings to confirm Faraday’s law qualitatively.
实验2：探究影响感应电动势的因素。使用同样装置，依次改变磁铁强度、线圈匝数和运动速度。记录检流计读数，定性验证法拉第定律。
Experiment 3: A model transformer. Wind two separate coils on a soft iron U-core and close the core with a yoke. Apply an AC voltage to the primary and measure the secondary voltage with a voltmeter. Verify the transformer equation for different turn ratios.
实验3：模型变压器。在U型软铁芯上分别绕两个线圈，用轭铁闭合磁路。给初级线圈加上交流电压，用电压表测量次级电压。验证不同匝数比下的变压器方程式。
Published by TutorHao | Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply