Electromagnetic Induction | IGCSE AQA Physics Revision | 电磁感应考点精讲

📚 Electromagnetic Induction | IGCSE AQA Physics Revision | 电磁感应考点精讲

Electromagnetic induction is the process where a voltage (and current if a complete circuit is present) is induced in a conductor when it experiences a changing magnetic field. This phenomenon is the foundation for generation of electrical power and operation of transformers, motors, and many other devices. In this article, we will explore the key concepts, laws, and applications of electromagnetic induction required for the IGCSE AQA Physics examination, with bilingual explanations to strengthen your understanding.

电磁感应是指导体在变化的磁场中产生电动势（若电路闭合则产生感应电流）的现象。它是发电、变压器、电动机等设备运行的基础。本文将以中英双语形式梳理 IGCSE AQA 物理考点，涵盖法拉第定律、楞次定律、发电机、变压器等核心内容，帮助你系统掌握电磁感应知识。

1. What is Electromagnetic Induction? | 什么是电磁感应？

Electromagnetic induction occurs when there is relative motion between a conductor and a magnetic field, or when the magnetic field around a conductor changes. A voltage, known as induced electromotive force (emf), is generated across the ends of the conductor. If the conductor forms part of a complete circuit, an induced current flows.

当导体与磁场之间存在相对运动，或导体周围的磁场发生变化时，就会发生电磁感应。导体的两端会产生电压，称为感应电动势。如果导体构成闭合回路的一部分，则会有感应电流流过。

A simple demonstration involves moving a bar magnet into and out of a coil of wire. The galvanometer connected to the coil shows a deflection, indicating the presence of an induced current. The direction of the current reverses when the magnet is pulled out.

一个简单的演示实验：将条形磁铁插入线圈再拔出，连接在线圈上的检流计指针会发生偏转，表明产生了感应电流。当磁铁拔出时，电流方向反转。

In generators, coils rotate in a magnetic field; in transformers, an alternating current in one coil creates a changing magnetic field that links to another coil. Both are practical applications of electromagnetic induction.

在发电机中，线圈在磁场中旋转；在变压器中，一个线圈中的交流电产生变化的磁场，该磁场耦合到另一个线圈。两者都是电磁感应的重要应用。

2. Faraday’s Law of Electromagnetic Induction | 法拉第电磁感应定律

Faraday’s law states that the induced emf in a circuit is directly proportional to the rate of change of magnetic flux linkage through the circuit. For a coil of N turns, the induced emf (ε) can be expressed as:

法拉第定律指出，电路中感应电动势的大小与穿过该电路的磁通量变化率成正比。对于匝数为 N 的线圈，感应电动势 ε 可表示为：

ε ∝ N (ΔΦ/Δt)

where Φ is the magnetic flux, t is time, and ΔΦ/Δt is the rate of change of flux. A larger number of turns, a stronger magnet, or faster relative motion all increase the induced emf.

其中 Φ 为磁通量，t 为时间，ΔΦ/Δt 是磁通量变化率。线圈匝数越多、磁铁磁性越强、相对运动越快，感应电动势就越大。

Flux linkage is the product of the magnetic flux and the number of turns. It is effectively the total flux passing through the coil. The unit of magnetic flux is the weber (Wb).

磁链是磁通量与匝数的乘积，代表穿过线圈的总磁通。磁通量的单位是韦伯（Wb）。

In the exam, you need to know that the induced emf can be increased by:

using a stronger magnetic field

使用更强的磁场
moving the magnet or coil more quickly

加快磁铁或线圈的相对运动速度
increasing the number of turns on the coil

增加线圈匝数
winding the coil around an iron core to concentrate the magnetic field

将线圈绕在铁芯上以集中磁场

在考试中，你需要了解增加感应电动势的方法，包括：使用更强的磁场、加快相对运动速度、增加线圈匝数、以及使用铁芯集中磁场。

3. Lenz’s Law and Direction of Induced Current | 楞次定律与感应电流方向

Lenz’s law gives the direction of the induced current: the induced current flows in a direction such that its magnetic effect opposes the change that produced it. This is a consequence of the conservation of energy.

楞次定律确定了感应电流的方向：感应电流的方向总是使得它的磁效应阻碍引起感应电流的变化。这是能量守恒定律的体现。

For example, when the north pole of a magnet approaches a coil, the induced current in the coil produces a north pole facing the magnet to repel it. When the north pole is withdrawn, the coil produces a south pole to attract the magnet back, thus opposing the change in flux.

例如，当磁铁 N 极靠近线圈时，线圈中感应电流产生的磁场会在靠近磁铁的一端形成 N 极，以排斥磁铁。当 N 极离开时，线圈则形成 S 极，吸引磁铁返回，从而阻碍磁通量的变化。

To determine the direction, you can use the right‑hand grip rule or Fleming’s right‑hand rule for generators. Lenz’s law also explains why eddy currents produce damping effects.

判断方向可采用右手螺旋定则或发电机右手定则。楞次定律还解释了涡流产生阻尼效应的原因。

A key exam point: if the change is an increase in magnetic flux, the induced current creates a field opposite to the applied field; if the change is a decrease, the induced current creates a field in the same direction as the applied field.

考试重点：若磁通量增加，感应电流产生的磁场与原磁场方向相反；若磁通量减少，则感应电流的磁场与原磁场方向相同。

4. Factors Affecting the Magnitude of Induced EMF | 影响感应电动势大小的因素

The magnitude of the induced emf depends on several factors, all linked to the rate of change of magnetic flux linkage. Understanding these factors helps in designing more efficient generators and transformers.

感应电动势的大小取决于几个因素，均与磁链的变化率相关。理解这些因素有助于设计更高效的发电机和变压器。

Experimental investigations show that:

The speed of relative motion: faster movement produces a greater rate of flux change and a larger emf.

相对运动速度：运动越快，磁通量变化率越大，电动势越大。
The strength of the magnetic field: stronger magnets increase the flux density, hence the flux linkage change per unit time.

磁场强度：磁铁磁性越强，磁通密度越大，单位时间内磁链变化越大。
The number of turns of the coil: more turns multiply the flux linkage, boosting the induced emf.

线圈匝数：匝数越多，磁链成倍增加，电动势增大。
The angle at which the coil cuts the magnetic field lines: maximum emf is induced when the conductor moves perpendicular to the field lines.

导体切割磁力线的角度：当导体运动方向与磁感线垂直时，感应电动势最大。

The use of a soft iron core intensifies the magnetic field through the coil, increasing the flux linkage without changing the current or speed.

使用软铁芯可以增强通过线圈的磁场，在不改变电流或速度的情况下增加磁链。

In a graph of flux versus time, the induced emf corresponds to the negative gradient of the flux‑time graph. A steep slope means a large emf.

在磁通量-时间图上，感应电动势对应磁通量曲线的负斜率。斜率越大，电动势越大。

5. Generators: AC and DC | 发电机：交流发电机与直流发电机

Generators convert mechanical energy into electrical energy using electromagnetic induction. A coil rotates in a magnetic field, inducing an alternating emf across its ends. The basic structure includes a coil (armature), magnets, slip rings or split‑ring commutator, and carbon brushes.

发电机利用电磁感应将机械能转化为电能。线圈在磁场中旋转，两端产生交变电动势。基本结构包括线圈（电枢）、磁铁、滑环或换向器以及碳刷。

In an AC generator (alternator), the ends of the coil are connected to two separate slip rings, which rotate with the coil. Brushes maintain continuous contact, delivering alternating current to the external circuit.

在交流发电机中，线圈两端分别连接到两个独立的滑环，滑环随线圈转动。碳刷始终保持接触，将交流电输送到外电路。

The induced emf varies sinusoidally. The emf is maximum when the coil is horizontal (plane parallel to the magnetic field) because the rate of flux change is greatest. The emf is zero when the coil is vertical (plane perpendicular to the field) because the rate of flux change is momentarily zero.

感应电动势呈正弦变化。当线圈水平（平面平行于磁场）时，磁通量变化率最大，电动势最大。当线圈垂直（平面垂直于磁场）时，磁通量变化率为零，电动势瞬时为零。

A DC generator uses a split‑ring commutator instead of slip rings. The commutator reverses the coil connections every half turn, so the external current always flows in one direction, producing a rectified output.

直流发电机使用换向器（整流子）代替滑环。换向器每半圈调换线圈与外电路的连接方向，使得外电路中电流始终单向流动，输出脉动直流电。

6. The Split‑Ring Commutator | 换向器（整流子）的作用

The split‑ring commutator is a mechanical rectifier in a DC generator. It consists of two separate halves of a ring, each connected to one end of the coil, and rotating with the coil. Carbon brushes press against the commutator segments.

换向器是直流发电机中的机械整流装置。它由两个彼此绝缘的半环组成，每个半环连接线圈的一端，随线圈一起转动。碳刷压在半环上保持接触。

As the coil rotates, each brush remains connected to the same side of the external circuit, but the commutator swaps which end of the coil is connected to which brush every half revolution. This ensures that the induced current in the external circuit always flows in the same direction.

线圈旋转时，每个碳刷始终与外电路的同一侧相连，但换向器每半圈将线圈两端与碳刷的连接互换一次，从而确保外电路中的感应电流方向始终不变。

In an AC generator, slip rings maintain the same coil‑brush connections, so the external current alternates. The commutator is what distinguishes a DC generator from an AC generator at the IGCSE level.

在交流发电机中，滑环保持线圈与碳刷的连接不变，因此外电路中的电流是交流的。IGCSE 阶段，区分交直流发电机的关键就在于是否使用了换向器。

Be prepared to label a diagram and explain why the commutator reverses the connections. A common exam question asks you to predict the direction of current or to complete a graph of output voltage against time.

需要能够标注示意图并解释换向器为何要交换连接。常见的考题包括判断电流方向或补全输出电压-时间图像。

7. Transformers: Structure and Working Principle | 变压器：结构与工作原理

A transformer changes the voltage of an alternating current supply. It consists of two coils, the primary and secondary, wound on a common soft iron core. The coils are electrically insulated from each other.

变压器用于改变交流电的电压。它由两个线圈——初级线圈和次级线圈——绕在同一个软铁芯上构成。线圈之间彼此绝缘。

An alternating current in the primary coil produces a changing magnetic field. The soft iron core channels this changing magnetic flux to the secondary coil. According to Faraday’s law, this changing flux induces an alternating emf across the secondary coil.

初级线圈中的交流电产生变化的磁场。软铁芯将这一变化的磁通引导至次级线圈。根据法拉第定律，变化的磁通在次级线圈两端感应出交变电动势。

The iron core is laminated (made of thin sheets insulated from each other) to reduce eddy current losses. The transformer works only with alternating current because a steady direct current would produce a constant flux, giving no induction in the secondary.

铁芯采用叠片结构（由相互绝缘的薄片叠成）以减少涡流损耗。变压器只能使用交流电工作，因为稳定的直流电产生恒定磁通，无法在次级线圈中感应出电压。

8. The Transformer Equation | 变压器公式

For an ideal transformer (100% efficiency), the ratio of the voltages is equal to the ratio of the number of turns:

对于理想变压器（效率 100%），电压比等于匝数比：

Vₚ / Vₛ = Nₚ / Nₛ

where Vₚ and Vₛ are the primary and secondary voltages, and Nₚ and Nₛ are the number of turns on the primary and secondary coils respectively.

其中 Vₚ 和 Vₛ 分别是初级和次级电压，Nₚ 和 Nₛ 分别是初级和次级的匝数。

If the secondary has more turns than the primary (Nₛ > Nₚ), the transformer is a step‑up transformer, and the secondary voltage is higher. If Nₛ < Nₚ, it is a step‑down transformer, and the secondary voltage is lower.

如果次级匝数大于初级（Nₛ > Nₚ），则为升压变压器，次级电压升高。如果 Nₛ < Nₚ，则为降压变压器，次级电压降低。

For an ideal transformer, the power input equals power output: Vₚ × Iₚ = Vₛ × Iₛ. Hence, a step‑up transformer increases voltage but decreases current, and a step‑down transformer decreases voltage but increases current.

对于理想变压器，输入功率等于输出功率：Vₚ × Iₚ = Vₛ × Iₛ。因此，升压变压器提高电压但降低电流，降压变压器降低电压但提高电流。

This equation is frequently tested; you should be able to calculate an unknown quantity given three others.

该公式是高频考点，你要能够根据已知的三个量计算第四个未知量。

9. Step‑up and Step‑down Transformers in Use | 升压与降压变压器的实际应用

Step‑up transformers are used in power stations to raise the voltage for long‑distance transmission. Higher voltage means lower current for the same power, which reduces heat losses (I²R losses) in the cables.

升压变压器用于发电站，提高电压以实现远距离输电。在相同功率下，电压越高，电流越小，从而减少输电线中的热损耗（I²R 损耗）。

Step‑down transformers are used in substations to reduce the high transmission voltage to safer, usable levels for homes, industry, and offices. Typical domestic supplies are 230 V in many countries, stepped down from hundreds of kilovolts.

降压变压器用于变电站，将高电压降至家庭、工业和办公场所安全可用的电压水平。许多国家民用电为 230 V，由数百千伏的高电压逐步降压得到。

Transformers also appear in power adapters for electronic devices, battery chargers, and in microwave ovens to supply the magnetron with high voltage.

变压器还广泛用于电子设备的电源适配器、充电器以及微波炉中，为磁控管提供高压。

In all cases, the use of transformers greatly improves the efficiency of electricity distribution networks. The national grid relies on high‑voltage transmission and local step‑down transformers.

在所有应用中，变压器都大幅提升了电力配送网络的效率。国家电网依赖高压输电和本地降压变压器的配合。

10. Energy Losses in Transformers and How to Minimize Them | 变压器的能量损耗及减少方法

Real transformers are not 100% efficient. Energy losses arise from several sources, and understanding them is part of the IGCSE specification.

实际变压器的效率并非 100%。能量损耗有多个来源，理解这些损耗是 IGCSE 考纲的要求。

Eddy currents are loops of induced current flowing within the iron core. They produce heat and reduce efficiency. Laminating the core with thin, insulated layers significantly reduces eddy currents.

涡流是在铁芯内部流动的感应电流回路。它们产生热量，降低效率。将铁芯用绝缘的薄片叠成叠片结构可有效减少涡流。

Resistance of the coils causes heating as current flows (copper losses). Using thick copper wire reduces the resistance and thus the heating.

线圈的电阻导致电流流过时发热（铜耗）。使用粗铜线可降低电阻，减少发热。

Hysteresis losses occur because the core material’s magnetization lags behind the magnetising field when the direction of magnetisation is repeatedly reversed. Soft iron has a narrow hysteresis loop and is chosen to minimise this loss.

磁滞损耗是由于铁芯材料在反复磁化时，磁化强度滞后于磁场变化而产生的。软铁具有狭窄的磁滞回线，选中它可以减少此类损耗。

Some flux may ‘leak’ out of the core and not link both coils perfectly. Using a closed, well‑designed core shape (like a ring) improves flux linkage.

部分磁通可能从铁芯“泄漏”，未能完全耦合两个线圈。采用闭合且设计良好的铁芯形状（如环形）可改善磁链。

Exam questions often ask you to identify one method to improve efficiency and explain how it works. Lamination is the most common answer for eddy currents.

考试中常要求你提出一种提高效率的方法并解释原理。针对涡流的最常见答案是叠片铁芯。

11. Eddy Currents and Their Applications | 涡流及其应用

Eddy currents are circular electric currents induced in a conductor when it is exposed to a changing magnetic field. While they cause energy losses in transformers, they also have useful applications.

涡流是导体处于变化的磁场中时感应出的环形电流。虽然它们在变压器中造成能量损耗，但也有许多有益的应用。

Electromagnetic braking uses strong magnets near a rotating metal disc. The eddy currents induced in the disc create a magnetic force that opposes the motion (Lenz’s law), providing smooth, contactless braking in trains and roller coasters.

电磁制动利用靠近旋转金属盘的强力磁铁。盘中感应出的涡流产生阻碍运动的磁力（楞次定律），可为火车和过山车提供平稳的非接触式制动。

Induction cookers use a rapidly changing magnetic field to induce eddy currents in the base of a metal pan, heating it directly without heating the hob surface.

电磁炉利用快速变化的磁场在金属锅底感应出涡流，直接加热锅具而不加热灶面。

Metal detectors and security gates use eddy current sensing: when a metal object passes through a varying magnetic field, eddy currents in the object alter the field, triggering an alarm.

金属探测器和安检门利用涡流感应原理：金属物体通过变化的磁场时，内部涡流会改变磁场，触发报警。

At IGCSE, you need to describe eddy currents, explain how they are reduced (lamination) and give at least one useful application.

IGCSE 阶段要求你描述涡流，解释如何减少涡流（叠片），并能举出至少一个有益的应用实例。

12. Summary of Key Points | 考点总结

Electromagnetic induction is a cornerstone of IGCSE AQA Physics. The key points to remember are:

An induced emf is produced by a changing magnetic field.

变化的磁场产生感应电动势。
Faraday’s law: induced emf ∝ rate of change of flux linkage.

法拉第定律：感应电动势与磁链变化率成正比。
Lenz’s law: induced current opposes the change producing it.

楞次定律：感应电流阻碍引起它的变化。
Generator types: AC uses slip rings, DC uses split‑ring commutator.

发电机类型：交流用滑环，直流用换向器。
Transformer equation: Vₚ/Vₛ = Nₚ/Nₛ; for ideal transformer, power in = power out.

变压器公式：Vₚ/Vₛ = Nₚ/Nₛ；理想变压器输入功率=输出功率。
Minimise transformer losses: lamination, thick wire, soft iron core.

减少变压器损耗：叠片、粗导线、软铁芯。
Eddy currents: undesirable in cores but useful in brakes and cookers.

涡流：在铁芯中不利，但在制动器和电磁炉中有益。

Practice drawing flux linkage graphs, interpreting voltage‑time graphs for AC and DC generators, and applying the transformer equation. Remember the importance of conserving energy in all explanations.

勤加练习绘制磁链图像，分析交直流发电机的电压-时间图像，运用变压器公式。在所有解释中，务必体现能量守恒这一核心思想。

Published by TutorHao | Physics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Electromagnetic Induction | IGCSE AQA Physics Revision | 电磁感应考点精讲