📚 Alternating Current | 交流电 考点精讲
Alternating current (AC) is the type of electricity that powers our homes and schools. Unlike direct current (DC), which always flows in one direction, AC repeatedly reverses its direction. This article covers the key WJEC IGCSE Physics concepts on AC, including its generation, waveform, transformers, and rectification, helping you master every detail for the exam.
交流电（AC）是为家庭和学校供电的电流类型。与始终沿一个方向流动的直流电（DC）不同，交流电会周期性地改变方向。本文涵盖 WJEC IGCSE 物理中交流电的核心概念，包括其产生、波形、变压器和整流，助你全面掌握每个考点细节。
1. AC vs DC | 交流电与直流电
Direct current (DC) flows steadily in one direction. The current from a battery or a solar cell is DC. In DC circuits, the polarity of the supply is fixed. Alternating current (AC), however, changes direction periodically. The voltage of an AC supply rises and falls, and the current reverses many times each second. In the UK, the mains electricity is AC with a frequency of 50 Hz and a nominal voltage of 230 V.
直流电（DC）稳定地沿一个方向流动。电池或太阳能电池提供的电流是直流电。在直流电路中，电源的极性是固定的。然而，交流电（AC）会周期性地改变方向。交流电源的电压会升高和降低，电流每秒多次反向。在英国，市电是频率为 50 Hz、标称电压为 230 V 的交流电。
On an oscilloscope, DC appears as a straight horizontal line at a constant voltage, whereas AC appears as a wave that oscillates above and below zero. The key advantage of AC is that its voltage can be easily changed using a transformer, making it efficient for long-distance power transmission.
在示波器上，直流电显示为一条恒定电压的水平直线，而交流电则显示为在零电压上下振荡的波形。交流电的主要优点是它的电压可以通过变压器轻松改变，从而适合远距离高效输电。
2. Generating Alternating Current | 交流电的产生
AC is generated by the process of electromagnetic induction. When a coil of wire is rotated in a magnetic field, or a magnet is moved in and out of a coil, a voltage is induced across the ends of the coil. This induced voltage changes direction as the coil moves past the magnetic poles, producing an alternating current if the coil is part of a complete circuit.
交流电是通过电磁感应过程产生的。当线圈在磁场中旋转，或磁铁移入和移出线圈时，线圈两端会产生感应电压。当线圈经过磁极时，感应电压会改变方向，如果线圈构成完整回路，则产生交流电。
The size of the induced voltage can be increased by using a stronger magnet, adding more turns to the coil, or rotating the coil faster. In a power station, turbines spin large electromagnets inside stationary coils to generate AC on a massive scale.
感应电压的大小可以通过使用更强的磁铁、增加线圈匝数或加快线圈转速来提高。在发电站中，涡轮机带动大型电磁铁在固定线圈内旋转，从而大规模产生交流电。
3. The AC Generator (Alternator) | 交流发电机
A simple AC generator consists of a rectangular coil mounted on an axle so that it can rotate between the poles of a permanent magnet. The ends of the coil are connected to slip rings, which press against carbon brushes to collect the induced current. Unlike a DC generator, which uses a split-ring commutator, the slip rings ensure the output current reverses direction each half-turn, maintaining an alternating current.
简单的交流发电机由一个安装在轴上的矩形线圈组成，线圈可以在永磁体两极之间旋转。线圈的两端连接到集电环上，集电环压住碳刷来收集感应电流。与使用分半换向器的直流发电机不同，集电环确保输出电流每半圈改变一次方向，保持交流电输出。
As the coil rotates from 0° to 360°, the induced voltage follows a sine curve. The voltage is maximum when the coil is horizontal (cutting field lines at the fastest rate) and zero when the coil is vertical (moving parallel to the field lines). This produces the characteristic sinusoidal waveform of AC.
当线圈从 0° 旋转到 360° 时，感应电压遵循正弦曲线变化。线圈水平时电压最大（以最快速度切割磁力线），线圈垂直时电压为零（平行于磁力线运动）。这产生了交流电标志性的正弦波形。
4. Sinusoidal Waveform of AC | 交流电的正弦波形
The voltage and current of mains AC follow a sine function. If we plot voltage against time, we get a smooth wave that rises to a positive peak, falls through zero to a negative peak, and returns to zero. This is why we call it a sinusoidal waveform. One complete cycle includes one positive half and one negative half.
市电交流电的电压和电流遵循正弦函数变化。如果绘制电压随时间的变化图，会得到一个光滑的波形，它上升到正峰值，穿过零降至负峰值，再回到零。这就是为什么称它为正弦波形。一个完整的周期包括一个正半周和一个负半周。
The amplitude of the wave represents the peak voltage. The horizontal axis represents time, and the distance between two successive crests is the period. The steepness of the wave changes with frequency; a higher frequency gives a shorter period and a more tightly packed waveform on the oscilloscope screen.
波形的幅度代表峰值电压。横轴表示时间，两个连续波峰之间的距离就是周期。波形的陡峭程度随频率变化；频率越高，周期越短，示波器屏幕上的波形就越密集。
5. Frequency and Period | 频率和周期
Frequency (f) is the number of complete cycles per second, measured in hertz (Hz). The period (T) is the time taken for one complete cycle. They are related by the equation:
频率（f）是每秒的完整周期数，单位是赫兹（Hz）。周期（T）是完成一个完整周期所需的时间。它们的关系式为：
f = 1 / T
.02 s (or 20 ms). If an oscilloscope time base is set to 5 ms/div, one complete cycle of mains AC would span 4 divisions.
对于英国市电，频率为 50 Hz，因此周期 T = 1/50 = 0.02 秒（即 20 毫秒）。如果示波器的时间基准设为 5 ms/格，则一个完整的市电交流周期将占 4 格。
In an AC generator, doubling the rotation speed doubles the frequency. Higher frequencies are used in some appliances, but mains electricity is standardised to 50 Hz or 60 Hz (in other countries) for safety and compatibility.
在交流发电机中，转速加倍会使频率加倍。某些电器使用更高频率，但市电为安全性和兼容性而标准化为 50 Hz（其他国家为 60 Hz）。
6. Peak Voltage and RMS Voltage | 峰值电压和均方根电压
The peak voltage (Vpeak) is the maximum value of the AC voltage, measured from the zero line to the top of the wave. However, the average power delivered by AC depends on a kind of average voltage called the root mean square (RMS) voltage (Vrms). For a sinusoidal waveform, the relationship is:
峰值电压（Vpeak）是交流电压的最大值，从零线到波峰测量。然而，交流电提供的平均功率取决于一种称为均方根（RMS）电压（Vrms）的平均电压。对于正弦波形，关系为：
Vrms = Vpeak / √2
as 230 V, which is the RMS value. This means the peak voltage is about 325 V (since 230 × √2 ≈ 325 V). When using an oscilloscope, you measure the peak-to-peak voltage (2 × Vpeak) and then divide by 2 to obtain Vpeak before converting to RMS.
英国市电标称值为 230 V，这是 RMS 值。这意味着峰值电压约为 325 V（因为 230 × √2 ≈ 325 V）。使用示波器时，你先测量峰峰值电压（2 × Vpeak），然后除以 2 得到 Vpeak，再转换为 RMS。
The RMS value is crucial because it allows us to compare AC with DC in terms of heating effect. 1 A RMS AC current produces the same heating in a resistor as 1 A DC current.
RMS 值至关重要，因为它允许我们在热效应方面将交流电与直流电进行比较。1 A RMS 交流电流在电阻中产生的热量与 1 A 直流电流相同。
7. Oscilloscope Traces | 示波器波形
An oscilloscope is the best tool for visualising AC waveforms. The vertical scale (volts/div) tells you the voltage per division on the screen, and the horizontal scale (time/div) gives the time base. By counting divisions and multiplying by the scales, you can calculate peak voltage and period.
示波器是显示交流波形的最佳工具。垂直刻度（伏特/格）告诉你屏幕上每格代表的电压，水平刻度（时间/格）给出时间基准。通过数格数并乘以相应的刻度，你可以计算峰值电压和周期。
For example, if the peak of an AC trace is 3 divisions above the centre line and the vertical scale is 2 V/div, then Vpeak divisions on a time base of 5 ms/div, the period T = 4 × 5 = 20 ms, so f = 1/0.02 = 50 Hz.
例如，如果交流波形的波峰在中心线以上 3 格，垂直刻度为 2 V/格，则 Vpeak = 3 × 2 = 6 V。如果完整周期在时间为 5 ms/格下占 4 格，周期 T = 4 × 5 = 20 ms，因此 f = 1/0.02 = 50 Hz。
You must also be able to adjust the controls to make the waveform clearer. Changing the volts/div changes the height of the wave; changing the time/div stretches or compresses the wave horizontally.
你还必须能够调整控制旋钮使波形更清晰。改变伏特/格会改变波高；改变时间/格会在水平方向拉伸或压缩波形。
8. Transformers | 变压器
A transformer is a device that changes the voltage of an alternating supply. It consists of two coils of insulated wire wound around a laminated soft iron core. An alternating current in the primary coil produces a changing magnetic field, which the iron core channels to the secondary coil, inducing an alternating voltage across it.
变压器是一种改变交流电源电压的装置。它由两个缠绕在叠片软铁芯上的绝缘线圈组成。初级线圈中的交流电产生变化的磁场，铁芯将该磁场引导至次级线圈，从而在其次两端感应出交流电压。
Transformers only work with AC because a changing magnetic field is needed to induce a voltage. If DC were applied, there would be no continuous change in the magnetic field, and no voltage would be induced in the secondary coil once the initial connection is made.
变压器仅适用于交流电，因为需要变化的磁场来感应电压。如果接入直流电，磁场就不会持续变化，一旦初始连接建立，次级线圈就不会再感应出电压。
The iron core is laminated (made of thin layers separated by insulation) to reduce eddy currents, which would waste energy as heat. Step-up transformers increase voltage (more turns on secondary), while step-down transformers decrease voltage (fewer turns on secondary).
铁芯是叠片式的（由绝缘隔开的薄层组成），以减少涡流，涡流会以热量形式浪费能量。升压变压器升高电压（次级匝数更多），降压变压器降低电压（次级匝数更少）。
9. Transformer Equation | 变压器公式
For an ideal transformer (100% efficient), the ratio of the voltages is equal to the ratio of the number of turns on each coil. This is given by the transformer equation:
对于理想变压器（效率 100%），电压比等于各线圈的匝数比。由变压器公式给出：
Vp / Vs = Np / Ns
Where Vp and Np are the voltage and number of turns on the primary coil, and Vs and Ns and the primary voltage is 230 V, the secondary voltage will be (500/2000) × 230 = 57.5 V.
其中 Vp 和 Np 是初级线圈的电压和匝数，Vs 和 Ns 是次级的电压和匝数。如果一个变压器初级 2000 匝，次级 500 匝，初级电压 230 V，则次级电压为 (500/2000) × 230 = 57.5 V。
Assuming 100% efficiency, power input equals power output: Vp Ip = Vs Is. This means a step-up transformer (Vs > Vp) will deliver a smaller current in the secondary coil. This is the principle behind high-voltage power transmission.
假设效率 100%，输入功率等于输出功率：Vp Ip = Vs Is。这意味着升压变压器（Vs > Vp）将在次级线圈中输出更小的电流。这正是高压输电原理之所在。
10. Power Transmission | 电力传输
Electricity is transmitted over long distances at very high voltages, often 132 kV, 275 kV or 400 kV, to reduce energy losses. The power lost in the transmission cables due to their resistance R is given by Ploss rent is reduced for the same power (P = VI), dramatically lowering the I²R heating losses.
电力以非常高的电压（通常为 132 kV、275 kV 或 400 kV）进行远距离传输，以减少能量损失。由于电缆电阻 R 造成的功率损耗由 Ploss = I²R 给出。通过升高电压，在相同功率下电流减小（P = VI），从而显著降低 I²R 热损耗。
A national grid uses step-up transformers at the power station to raise the voltage, then step-down transformers near homes and schools to lower it to a safe level (230 V). Without high-voltage transmission, the current would be massive, causing excessive heating, wasted energy, and the need for very thick, expensive cables.
国家电网在发电站使用升压变压器升高电压，然后在家庭和学校附近使用降压变压器将电压降至安全水平（230 V）。如果没有高压输电，电流将非常大，导致过热、能量浪费，并且需要很粗且昂贵的电缆。
11. Rectification (Converting AC to DC) | 整流（交流转直流）
Many electronic devices, such as phone chargers and laptops, need DC to operate. Rectification is the process of converting AC into DC using diodes. A diode is a component that only allows current to flow in one direction. In a simple half-wave rectifier circuit, a single diode is placed in series with the load. It blocks the negative half-cycles, allowing only the positive half-cycles to pass.
许多电子设备（如手机充电器和笔记本电脑）需要直流电才能工作。整流是利用二极管将交流电转换为直流电的过程。二极管是一种只允许电流单向流动的元件。在简单的半波整流电路中，一个二极管与负载串联。它会阻隔负半周，只让正半周通过。
Half-wave rectification gives a bumpy DC output with large gaps between peaks. A full-wave rectifier uses four diodes arranged in a bridge configuration (bridge rectifier) to flip the negative half-cycles into positive ones, producing a smoother DC output with twice the frequency of ripples.
半波整流会产生波峰之间有较大间隙的脉动直流输出。全波整流器使用四只二极管组成桥式结构（桥式整流），将负半周翻转为正半周，产生更平滑且波纹频率加倍的直流输出。
To smooth out the ripples and obtain a nearly steady DC voltage, a capacitor is connected across the output. The capacitor charges up near the peaks and discharges between them, reducing the voltage variation. Larger capacitance values give better smoothing.
为了平滑波纹并得到近似稳定的直流电压，在输出端并联一个电容。电容在波峰附近充电，并在波峰之间放电，从而减小电压波动。电容值越大，平滑效果越好。
You may be asked to draw and interpret voltage-time graphs for half-wave and full-wave rectified outputs with and without smoothing.
你可能会被要求绘制和解释半波和全波整流输出在有和没有滤波时的电压-时间图像。
12. Summary and Exam Tips | 总结与应试技巧
AC is essential for modern life, and the WJEC IGCSE Physics exam will test your ability to describe its generation, measure it with an oscilloscope, use the transformer equation, and explain the advantages of high-voltage transmission. Always remember the difference between AC and DC, the link between frequency and period, and how a diode can be used for rectification.
交流电是现代生活不可或缺的一部分，WJEC IGCSE 物理考试将测试你描述其产生、用示波器测量、运用变压器公式以及解释高压输电优势的能力。永远牢记交流电与直流电的区别、频率和周期的关系，以及如何使用二极管进行整流。
In calculations, show all steps: Vp/Vs = Np/Ns, Vrms = Vpeak/√2, and Ploss stematic with divisions and scales. Understand that transformers require a changing magnetic field, which is why they only work with AC. Focus on clear, labelled diagrams and precise terminology in explanation questions.
在计算中，展示所有步骤：Vp/Vs = Np/Ns、Vrms = Vpeak/√2 和 Ploss = I²R。处理示波器波形时，要系统地使用格数和刻度。理解变压器需要变化的磁场，这就是为什么它只适用于交流电。在解释题中，注重清晰、标有注释的示意图和准确的术语。
Keep practising past paper questions on AC and rectification, and you'll walk into the exam with confidence.
持续练习关于交流电和整流的历年真题，你将充满信心地走进考场。
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