📚 GCSE CIE Physics: Electromagnetic Induction Key Points | GCSE CIE 物理：电磁感应考点精讲
Electromagnetic induction is a core topic in the CIE IGCSE Physics syllabus (0625), explaining how electricity can be generated by changing magnetic fields. Understanding the principles, factors, and applications—such as generators and transformers—is essential for success in the exam.
电磁感应是 CIE IGCSE 物理大纲 (0625) 的核心主题，解释如何通过变化的磁场产生电流。理解其原理、影响因素以及发电机和变压器等应用，对考试成功至关重要。
1. The Phenomenon of Electromagnetic Induction | 电磁感应现象
An e.m.f. is induced across a conductor whenever it experiences a changing magnetic field. This can be achieved by moving a magnet into or out of a coil, or by moving a conductor through a magnetic field. No battery is needed.
当导体处于变化的磁场中时，导体两端会产生感应电动势。这可以通过将磁体移入或移出线圈，或让导体在磁场中运动来实现，无需电池。
If the conductor is part of a complete circuit, the induced e.m.f. drives an induced current. The effect is temporary and only lasts while the magnetic field or the motion is changing.
如果导体是闭合电路的一部分，感应电动势会驱动感应电流。这种效应是暂时的，仅在磁场或运动发生变化时存在。
2. Factors Affecting Induced e.m.f. | 影响感应电动势的因素
The induced e.m.f. can be increased by moving the magnet faster relative to the coil. A higher speed means a greater rate of cutting magnetic field lines, leading to a larger e.m.f.
增大感应电动势的一种方法是让磁体相对线圈运动得更快。速度越快，切割磁感线的速率越大，导致电动势越大。
Using a stronger magnet also produces a larger induced e.m.f. because the magnetic flux density is higher, so each movement cuts more magnetic field lines.
使用更强的磁体也会产生更大的感应电动势，因为磁通密度更高，每次运动会切割更多的磁感线。
Increasing the number of turns on the coil results in a greater induced e.m.f. Each turn contributes to the total voltage induced.
增加线圈匝数会使感应电动势更大。每匝线圈都会贡献一部分感应电压。
Inserting a soft iron core inside the coil concentrates the magnetic field and increases the magnetic flux linkage, further boosting the induced e.m.f.
在线圈内插入软铁芯可集中磁场、增大磁通链，从而进一步提高感应电动势。
3. Faraday’s Law and Lenz’s Law | 法拉第定律与楞次定律
Faraday’s law states that the magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage (the number of turns N multiplied by the change in magnetic flux ΔΦ per unit time).
法拉第定律指出，感应电动势的大小与磁通匝链数的变化率成正比（即线圈匝数 N 乘以单位时间内的磁通量变化 ΔΦ/Δt）。
ε ∝ N ΔΦ / Δt
Lenz’s law determines the direction of the induced current: it always flows in a direction that opposes the change in magnetic field that produced it. This is a consequence of the conservation of energy.
楞次定律决定了感应电流的方向：感应电流的方向总是阻碍引起它的磁场变化。这是能量守恒的结果。
For example, if the north pole of a magnet moves towards a coil, the induced current creates a north pole at the near end of the coil to repel the approaching magnet, opposing the increase in flux.
例如，磁体的 N 极靠近线圈时，感应电流使线圈近端成为 N 极，以排斥靠近的磁体，阻碍磁通量的增加。
4. The A.C. Generator (Alternator) | 交流发电机
An a.c. generator consists of a coil rotating in a fixed magnetic field. The ends of the coil are connected to slip rings, which maintain continuous contact with carbon brushes to deliver an alternating e.m.f. to the external circuit.
交流发电机由一个在固定磁场中旋转的线圈组成。线圈两端连接到滑环，滑环与碳刷保持连续接触，向外部电路提供交变电动势。
As the coil rotates, the induced e.m.f. varies sinusoidally. Every half turn, the direction of the e.m.f. reverses, producing an alternating current (a.c.). The voltage–time graph is a sine wave crossing zero.
当线圈旋转时，感应电动势按正弦规律变化。每半圈，电动势方向反转一次，产生交流电。其电压-时间图是一条过零点的正弦波。
5. The D.C. Dynamo | 直流发电机
A d.c. dynamo uses a split-ring commutator instead of slip rings. The commutator reverses the connections to the external circuit every half turn, so the current flows in only one direction, giving a rectified output.
直流发电机使用裂环换向器代替滑环。换向器每半圈反转外部电路连接，因此电流只沿一个方向流动，输出整流波形。
| Feature | A.C. Generator | D.C. Dynamo |
|---|---|---|
| Commutation | Two slip rings | Split-ring commutator |
| Output waveform | Alternating current (sine wave) | Direct current (varying, always positive) |
| Current direction | Reverses every half turn | Always same direction, but pulsates |
交流发电机使用两个滑环，产生方向交替变化的正弦波交流电；直流发电机使用裂环换向器，产生方向不变但大小波动的直流电。
6. Structure of a Transformer | 变压器结构
A transformer consists of two coils of wire, the primary coil and the secondary coil, wound around a common soft iron core. The core concentrates the magnetic field and improves the linkage between the coils.
变压器由两个线圈（初级线圈和次级线圈）绕在一个共用软铁芯上组成。铁芯集中磁场，增强线圈之间的磁通链。
An alternating voltage applied to the primary coil creates a changing magnetic field in the core, which then induces an alternating voltage in the secondary coil.
在初级线圈上施加交变电压，会在铁芯中产生变化的磁场，进而在次级线圈中感应出交变电压。
7. The Transformer Equation | 变压器方程
The ratio of the voltages across the primary and secondary coils is equal to the ratio of the number of turns:
初级与次级线圈的电压比等于其匝数比：
Vₚ / Vₛ = Nₚ / Nₛ
If Nₛ > Nₚ, the transformer steps up the voltage (Vₛ > Vₚ). If Nₛ < Nₚ, it steps down the voltage (Vₛ < Vₚ).
如果 Nₛ > Nₚ，变压器升压（Vₛ > Vₚ）；如果 Nₛ < Nₚ，则降压（Vₛ < Vₚ）。
In exam questions, you may be given three of the four values and asked to calculate the unknown. Always substitute carefully and use the same units for turns.
考试题目中，常给出四个量中的三个，求未知量。代入时务必仔细，并保持匝数单位一致。
8. Ideal Transformer and Power | 理想变压器与功率守恒
For an ideal transformer with 100% efficiency, the power delivered to the primary coil equals the power taken from the secondary coil: Pₚ = Pₛ, so Vₚ Iₚ = Vₛ Iₛ.
对于效率100%的理想变压器，输入到初级线圈的功率等于次级线圈输出的功率：Pₚ = Pₛ，因此 Vₚ Iₚ = Vₛ Iₛ。
Vₚ Iₚ = Vₛ Iₛ
Therefore, a step-up transformer increases voltage but decreases current, and a step-down transformer decreases voltage but increases current, assuming constant power.
因此，升压变压器增大电压但减小电流，降压变压器降低电压但增大电流，假设功率不变。
From the two equations we can derive Iₚ / Iₛ = Nₛ / Nₚ. This shows that current is inversely proportional to the turns ratio (and to the voltage ratio) in an ideal transformer.
由两个公式可推出 Iₚ / Iₛ = Nₛ / Nₚ，表明理想变压器中电流与匝数比（及电压比）成反比。
9. Power Transmission and Applications | 输电与变压器应用
Transformers are essential in the National Grid. Electricity is generated at about 25 kV, stepped up to 400 kV or 132 kV for transmission over long distances, then stepped down for safe domestic use (230 V). High voltage reduces current, minimizing energy wasted as heat in transmission lines (P = I²R).
变压器在国家电网中至关重要。发电厂以约25 kV 发电，升压至 400 kV 或 132 kV 进行远距离输电，然后在用户端降压至安全的 230 V。高电压降低了电流，从而减少了传输线的热损耗（P=I²R）。
Step-down transformers are also used in
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