📚 Electromagnetic Induction (4.7.2) | 电磁感应（4.7.2）
Electromagnetic induction is the process by which an electromotive force (EMF) is generated in a conductor when it experiences a changing magnetic field. This principle, first discovered by Michael Faraday in 1831, forms the foundation of most modern electricity generation and distribution. In the Edexcel IGCSE Science curriculum, understanding electromagnetic induction is essential for explaining how generators, transformers, and many everyday devices work.
电磁感应是指当导体处于变化的磁场中时，在导体内产生电动势（EMF）的过程。这一原理由迈克尔·法拉第于 1831 年首次发现，是现代大部分发电与电力输送的基础。在 Edexcel IGCSE 科学课程中，理解电磁感应对于解释发电机、变压器及许多日常设备的工作原理至关重要。
1. What is Electromagnetic Induction? | 什么是电磁感应？
Electromagnetic induction occurs whenever a conductor cuts across magnetic field lines, causing a potential difference to be induced across its ends. If the conductor forms part of a complete circuit, an induced current flows. The effect can be produced either by moving a magnet into a coil or by moving a coil in a magnetic field.
只要导体切割磁感线，就会在其两端产生感应电势差，这就是电磁感应。如果导体构成完整回路的一部分，就会产生感应电流。无论是将磁铁移入线圈，还是让线圈在磁场中运动，都能产生这种效应。
In the IGCSE specification, you are expected to recall that a change in the magnetic field around a conductor is necessary. No change means no induction – a stationary magnet inside a stationary coil produces no EMF.
在 IGCSE 考试中，你需要记住导体周围的磁场必须发生变化。没有变化就没有感应——静止的磁铁放在静止的线圈内不会产生电动势。
2. Magnetic Flux and Flux Linkage | 磁通量与磁链
weber (Wb). Flux linkage is flux multiplied by the number of turns N in a coil, i.e., NΦ. A changing flux linkage induces an EMF.
为了定量描述感应现象，我们引入磁通量（Φ）的概念，它是磁通密度（B）与垂直于磁场方向的面积（A）的乘积：Φ = B × A，单位是韦伯（Wb）。磁链是磁通量与线圈匝数 N 的乘积，即 NΦ。磁链发生变化时就会产生电动势。
In IGCSE, you do not need to perform complex flux calculations, but you must understand that increasing the speed of relative motion, the strength of the magnet, or the number of coil turns will increase the induced EMF.
在 IGCSE 阶段，你不需要进行复杂的磁通量计算，但必须理解：增大相对运动的速度、增强磁铁强度或增加线圈匝数都会使得感应电动势增大。
3. Faraday's Law of Electromagnetic Induction | 法拉第电磁感应定律
Faraday's law states that the magnitude of the induced EMF is directly proportional to the rate of change of flux linkage. Mathematically, this can be written as:
法拉第定律指出，感应电动势的大小与磁链的变化率成正比。用数学式可表示为：
ε = –N × (ΔΦ ÷ Δt)
where ε is the induced EMF, N is the number of turns, ΔΦ is the change in magnetic flux, and Δt is the time taken for that change. The negative sign relates to Lenz's law, which we will discuss next. For the IGCSE exam, you should be able to describe how doubling the speed or the number of turns doubles the induced EMF.
式中 ε 是感应电动势，N 为线圈匝数，ΔΦ 是磁通量的变化量，Δt 是变化所用的时间。负号与楞次定律有关，我们将在下一节讨论。在 IGCSE 考试中，你需要能解释如何通过加倍速度或匝数来使感应电动势加倍。
4. Lenz's Law and Energy Conservation | 楞次定律与能量守恒
Lenz's law gives the direction of the induced current: the induced current always flows in a direction such that it opposes the change that produced it. This is a direct consequence of the conservation of energy. If the induced current aided the change, energy would be created from nothing, which is impossible.
楞次定律给出了感应电流的方向：感应电流总是朝着抵抗引起它的变化的方向流动。这是能量守恒的直接结果。如果感应电流助长变化，能量就会无中生有，这绝不可能。
For example, when a north pole of a magnet moves into a coil, the induced current creates its own north pole at the end of the coil to repel the incoming magnet, thus opposing the motion. You must be able to predict the direction of induced current using the right‑hand grip rule or by considering the pole induced on the coil.
例如，当磁铁的北极移入线圈时，感应电流会在线圈近端产生一个北极以排斥靠近的磁铁，从而抵抗运动。你需要能够利用右手定则或通过判断线圈上感生的磁极来预测感应电流的方向。
5. The AC Generator (Alternator) | 交流发电机
A simple AC generator consists of a coil of wire rotating within a uniform magnetic field. As the coil rotates, the flux linkage changes continuously, inducing an alternating EMF. Slip rings and brushes connect the coil to an external circuit without tangling the wires, allowing the alternating current to be output.
简单的交流发电机由一个在匀强磁场中旋转的线圈构成。线圈旋转时，磁链不断变化，从而产生交变电动势。滑环和电刷将线圈与外部电路连接起来，又不会使导线缠绕，从而输出交流电。
When the coil is parallel to the magnetic field, the rate of cutting field lines is greatest, so the induced EMF is maximum. When the coil is perpendicular to the field, the rate of cutting is momentarily zero, so the EMF is zero. You should be able to sketch a graph of EMF against time, showing a sinusoidal pattern.
当线圈平面与磁场平行时，切割磁感线的速率最大，感应电动势达到峰值。当线圈平面与磁场垂直时，切割速率瞬间为零，电动势为零。你需要能够画出电动势随时间变化的正弦曲线图。
6. Back EMF in DC Motors | 直流电动机中的反电动势
Although a DC motor is designed to convert electrical energy into kinetic energy, it also acts as a generator once it spins. The rotating coil cuts magnetic field lines and induces an EMF that opposes the applied voltage; this is called back EMF. Back EMF explains why a running motor draws less current than a stalled motor.
尽管直流电动机的设计目的是将电能转化为动能，一旦它开始旋转，也会起到发电机的作用。旋转的线圈切割磁感线，产生的电动势与外加电压方向相反，称为反电动势。反电动势解释了为什么运转中的电动机比被堵转时电流更小。
The IGCSE syllabus highlights back EMF as a practical demonstration of electromagnetic induction. Understanding back EMF also reinforces Lenz's law, because the induced EMF opposes the change in current that drives the motor.
IGCSE 大纲将反电动势列为电磁感应的实际例证。理解反电动势也能加深你对楞次定律的认识，因为感应电动势总是抵抗驱动电动机的电流变化。
7. Transformers: Structure and Principles | 变压器：结构与原理
A transformer consists of two coils, the primary and secondary, wound around a laminated soft iron core. An alternating current in the primary coil produces a constantly changing magnetic field, which is guided through the core to the secondary coil. By electromagnetic induction, an alternating EMF is induced in the secondary coil.
变压器由缠绕在分层软铁心上的两个线圈——初级线圈和次级线圈组成。初级线圈中的交流电产生持续变化的磁场，该磁场通过铁心传导至次级线圈。根据电磁感应原理，次级线圈中便感应出交变电动势。
The laminated core reduces energy losses due to eddy currents. Transformers work only with AC, because a steady DC would produce a constant magnetic field with no flux change, and therefore no induction. This is a key point frequently tested.
分层铁心可以减小涡流引起的能量损耗。变压器只适用于交流电，因为恒定的直流电产生的是稳定的磁场，磁通量不发生变化，因而无法感应出电动势。这是经常考查的关键点。
8. The Transformer Equation and Calculations | 变压器方程与计算
For an ideal transformer (100% efficient), the ratio of the voltages is equal to the ratio of the number of turns:
对于理想变压器（效率为100%），电压之比等于匝数之比：
Vp ÷ Vs = Np ÷ Ns
Also, since power input equals power output, we have:
同时，由于输入功率等于输出功率，我们还有：
Ip × Vp = Is × Vs
where Vp and Vs are primary and secondary voltages, Np and Ns are the numbers of turns, and Ip and Is are the currents. A step‑up transformer has more turns on the secondary side, increasing voltage but decreasing current. A step‑down transformer does the opposite. You must be able to rearrange these equations confidently.
式中 Vp 和 Vs 分别是初级与次级电压，Np 和 Ns 是匝数，Ip 和 Is 是电流。升压变压器在次级侧有更多匝数，可升高电压但降低电流；降压变压器则相反。你必须能熟练地变换这些等式。
9. Power Transmission via the National Grid | 电力输送与国家电网
Electricity is generated at power stations at around 25 kV. To minimise energy losses due to heating in transmission lines, the voltage is stepped up to 400 kV or more using step‑up transformers. A higher voltage means a lower current for the same power, and since power loss in a cable is given by I²R, a reduced current dramatically lowers the heat loss.
发电站发出的电力大约在 25 kV。为减少传输线上因发热造成的能量损耗，会使用升压变压器将电压升高至 400 kV 甚至更高。对于相同功率，电压越高，电流越小，而电缆中的功率损耗公式为 I²R，因此减小电流能显著降低热损耗。
Close to homes and industries, step‑down transformers reduce the voltage to safe levels (230 V in the UK). This entire system, called the National Grid, relies entirely on electromagnetic induction and the use of transformers. IGCSE exam questions often ask you to explain why high‑voltage transmission is efficient.
在接近用户和工厂时，降压变压器再将电压降至安全水平（英国为 230 V）。这整个系统称为国家电网，完全依赖于电磁感应和变压器。IGCSE 试题常会要求你解释为什么高压输电更高效。
10. Practical Investigations and Safety | 实验探究与安全注意事项
In the laboratory, electromagnetic induction can be demonstrated by moving a bar magnet in and out of a solenoid connected to a sensitive milliammeter. The meter needle deflects back and forth, showing an alternating induced current. You can also investigate factors affecting the induced EMF by changing the speed of magnet movement, the magnet strength, or the number of turns on the coil.
在实验室中，可以通过将条形磁铁在连接灵敏毫安表的螺线管中来回移动来演示电磁感应。指针来回偏转，显示出交变感应电流。你也可以改变磁铁移动的速度、磁铁强度或线圈匝数，探究影响感应电动势大小的因素。
Always use appropriate safety measures: avoid high currents that may cause overheating, ensure equipment is well‑insulated, and never open a live transformer without supervision. A common investigation also includes building a model transformer to measure the input and output voltages and verify the turns ratio equation.
请务必采取适当的安全措施：避免过高的电流导致过热，确保设备绝缘良好，且切勿在无监督的情况下打开带电的变压器。常见的探究活动还包括制作一个变压器模型，测量输入和输出电压，以此验证匝数比公式。
| Factor (因素) | Effect on Induced EMF (对感应电动势的影响) |
|---|---|
| Faster relative motion (更快的相对运动) | Increases (增大) |
| Stronger magnet (更强的磁铁) | Increases (增大) |
| More coil turns (更多线圈匝数) | Increases (增大) |
| Stationary magnet (静止的磁铁) | No EMF (无电动势) |
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图