📚 Mastering AC for IGCSE AQA Physics | IGCSE AQA 物理：交流电 考点精讲
Alternating current (AC) is a fundamental concept in IGCSE AQA Physics that describes the type of electricity supplied to homes and industry. Unlike direct current (DC), where charges flow in one direction only, AC periodically reverses its direction. This article breaks down every key point you need to know for your exam, from waveforms on an oscilloscope to transformers and the national grid.
交流电（AC）是 IGCSE AQA 物理中一个基本概念，它描述了供给家庭和工业的电力类型。与直流电（DC）电荷只朝一个方向流动不同，交流电会周期性地改变方向。本文逐一解析考试所需掌握的所有关键知识点，从示波器上的波形到变压器与国家电网。
1. AC vs DC: The Basic Difference | 交流电与直流电的基本区别
Direct current (DC) is the unidirectional flow of electric charge. Batteries and solar cells supply DC. The current flows from positive to negative terminals in a circuit and its magnitude remains constant over time.
直流电（DC）是电荷单向流动。电池和太阳能电池提供直流电。电路中电流从正极流向负极，其大小随时间保持不变。
Alternating current (AC) is the flow of electric charge that periodically reverses direction. In the UK, mains electricity is AC at a frequency of 50 Hz, meaning the current changes direction 100 times per second (50 full cycles).
交流电（AC）是电荷周期性地改变方向的流动。在英国，市电是频率为 50 Hz 的交流电，意味着电流每秒改变方向 100 次（50 个完整周期）。
The voltage-time graph for DC is a horizontal straight line (if steady DC). For AC, it is a sinusoidal wave that oscillates about the time axis.
直流电的电压-时间图像是一条水平直线（如果是稳定直流）。交流电的图像是围绕时间轴振荡的正弦波。
2. Displaying AC and DC on an Oscilloscope | 用示波器显示交流和直流
A cathode-ray oscilloscope (CRO) can be used to display voltage as a function of time. The vertical axis (y-gain) represents voltage, and the horizontal axis (time-base) represents time.
阴极射线示波器（CRO）可用于显示电压随时间的变化。垂直轴（y 增益）代表电压，水平轴（时基）代表时间。
With a steady DC input and the time-base switched on, the trace is a horizontal line displaced from the centre. If the DC voltage is positive, the line shifts upward; if negative, it shifts downward.
当输入稳定直流电且时基开启时，轨迹是一条偏离中心的水平线。若直流电压为正，线向上偏移；若为负，则向下偏移。
With an AC input, the trace becomes a sine wave. The peak of the wave corresponds to the maximum voltage, and the period (horizontal width of one complete wave) allows you to calculate the frequency.
当输入交流电时，轨迹变为正弦波。波的峰值对应最大电压，周期（一个完整波的水平宽度）可用于计算频率。
If the time-base is switched off, AC produces a vertical line because the voltage variation is too fast for the eye to follow different horizontal positions.
如果关闭时基，交流电产生一条竖直线，因为电压变化太快，眼睛无法追踪不同的水平位置。
3. Frequency, Period and the 50 Hz Mains | 频率、周期与 50 Hz 市电
The frequency (f) of an AC supply is the number of complete cycles per second, measured in hertz (Hz). The period (T) is the time for one complete cycle: T = 1 / f.
交流电源的频率（f）是每秒完整周期的数量，单位为赫兹（Hz）。周期（T）是一个完整周期所需的时间：T = 1 / f。
In the UK, mains electricity has a frequency of 50 Hz, so the period is T = 1/50 = 0.02 s (20 ms). This means one full oscillation of voltage takes 20 milliseconds.
在英国，市电频率为 50 Hz，因此周期为 T = 1/50 = 0.02 s（20 毫秒）。这意味着电压的一个完整振荡耗时 20 毫秒。
On an oscilloscope, if the time-base setting is known, you can calculate the period by counting horizontal divisions for one complete wave and multiplying by the seconds-per-division setting. Then f = 1/T.
在示波器上，若已知时基设置，可以通过计算一个完整波的水平格数并乘以每格秒数设置来计算周期。进而 f = 1/T。
4. Peak Voltage and RMS Voltage | 峰值电压与有效值电压
The peak voltage (Vpeak) is the maximum voltage reached by an AC supply, measured from the zero line to the top of the waveform.
峰值电压（Vpeak）是交流电源达到的最大电压，从零线到波形顶点的测量值。
For a sinusoidal AC supply, the root mean square (RMS) voltage is the effective DC voltage that would deliver the same average power to a resistor. For IGCSE, you use the relationship:
对于正弦交流电源，均方根（RMS）电压是能向电阻器提供相同平均功率的等效直流电压。在 IGCSE 中，你使用如下关系：
Vrms = Vpeak / √2
The UK mains is quoted as 230 V, which is the RMS value. The peak voltage is therefore 230 × √2 ≈ 325 V.
英国市电标称 230 V，这是有效值。因此峰值电压为 230 × √2 ≈ 325 V。
Similarly, Irms = Ipeak / √2. This concept is only applied to sinusoidal AC; for other waveforms, the factor is different but not required at IGCSE.
类似地，Irms = Ipeak / √2。此概念仅适用于正弦交流电；其他波形的因子不同，但 IGCSE 不要求。
5. The AC Generator (Alternator) | 交流发电机
A simple AC generator consists of a coil of wire rotating in a uniform magnetic field. Slip rings and carbon brushes connect the rotating coil to an external circuit without tangling the wires.
一个简单的交流发电机由在均匀磁场中转动的线圈组成。滑环和碳刷将旋转线圈连接到外部电路，而不会使导线缠绕。
As the coil rotates, the magnetic flux linkage changes sinusoidally, inducing an electromotive force (EMF) that alternates in direction every half-turn. This produces an alternating output voltage.
当线圈旋转时，磁通链按正弦规律变化，感应出每半圈改变方向的电动势（EMF）。这产生了交变输出电压。
The size of the induced voltage can be increased by: using stronger magnets, increasing the number of turns on the coil, winding the coil on a soft iron core, or rotating the coil faster.
可通过以下方式增大感应电压：使用更强的磁铁、增加线圈匝数、将线圈绕在软铁芯上或加快线圈旋转速度。
The graph of output voltage against time is a sine wave. When the coil is perpendicular to the field (maximum flux), the induced voltage is zero because the rate of change of flux is zero.
输出电压-时间图像是正弦波。当线圈与磁场垂直（磁通量最大）时，感应电压为零，因为磁通量变化率为零。
6. Transformer Principles | 变压器原理
A transformer changes the size of an alternating voltage. It consists of a laminated soft iron core with two coils wound around it: the primary coil connected to the input voltage and the secondary coil delivering the output voltage.
变压器改变交流电压的大小。它由一个层压软铁芯和绕在其上的两个线圈组成：初级线圈连接输入电压，次级线圈输出电压。
The alternating current in the primary coil produces a changing magnetic field, which is concentrated by the iron core and passes through the secondary coil. This changing magnetic field induces an alternating voltage across the secondary coil by electromagnetic induction.
初级线圈中的交流电产生变化的磁场，该磁场被铁芯集中并穿过次级线圈。这个变化的磁场通过电磁感应在次级线圈两端感应出交流电压。
Transformers only work with AC because a changing magnetic field is necessary to induce a voltage. A steady DC supply produces a constant magnetic field and no induction.
变压器仅适用于交流电，因为需要变化的磁场来感应电压。稳定的直流电源产生恒定磁场，没有感应。
7. The Transformer Equation | 变压器方程
For an ideal transformer (assumed 100% efficient), the ratio of voltages is equal to the ratio of turns:
对于理想变压器（假设 100% 效率），电压比等于匝数比：
Vp / Vs = Np / Ns
where Vp and Vs are primary and secondary voltages, Np and Ns are primary and secondary turns.
其中 Vp 和 Vs 是初级和次级电压，Np 和 Ns 是初级和次级匝数。
A step-up transformer increases voltage: Ns > Np, so Vs > Vp. A step-down transformer decreases voltage: Ns < Np, so Vs < Vp.
升压变压器增加电压：Ns > Np，因此 Vs > Vp。降压变压器降低电压：Ns < Np，因此 Vs < Vp。
Assuming 100% efficiency, input power equals output power: Ip Vp = Is Vs. This means if the voltage is stepped up, the current is stepped down, and vice versa.
假设 100% 效率，输入功率等于输出功率：Ip Vp = Is Vs。这意味着若电压升高，电流降低，反之亦然。
8. Why High Voltage is Used for Transmission | 为什么输电要用高电压
Electricity is transmitted across the national grid at very high voltages (e.g., 400,000 V) to minimise energy losses. The power lost in the transmission cables due to their resistance is given by Plost = I²R.
电力在国家电网中以非常高的电压（例如 400,000 V）输送，以尽量减少能量损失。输电线路因电阻造成的功率损失由 Plost = I²R 给出。
For a given amount of power (P = IV), a higher voltage means a lower current. Since the loss depends on the square of the current, reducing the current dramatically reduces heating losses in the cables.
对于给定功率（P = IV），电压越高，电流越低。由于损耗与电流的平方成正比，降低电流将大幅减少电缆中的发热损耗。
Step-up transformers are used at power stations to raise the voltage for transmission. Step-down transformers then reduce the voltage in stages to safer levels (230 V) for domestic and industrial use.
发电站使用升压变压器提高电压以进行输电。然后降压变压器分阶段将电压降低到供家庭和工业使用的安全水平（230 V）。
9. Rectification: Converting AC to DC | 整流：将交流电转换为直流电
Rectification is the process of converting alternating current into direct current using diodes. A diode allows current to flow in only one direction.
整流是利用二极管将交流电转换为直流电的过程。二极管只允许电流朝一个方向流动。
Half-wave rectification uses a single diode. It blocks the negative half-cycle of the AC supply, so only the positive half-cycles appear across the load. The output is a pulsating DC with gaps.
半波整流使用单个二极管。它阻断交流电源的负半周，因此仅正半周出现在负载上。输出是带有间隙的脉动直流电。
Full-wave rectification uses four diodes in a bridge arrangement. It inverts the negative half-cycles so that both halves of the AC waveform are made positive, producing a smoother fluctuating DC output.
全波整流使用桥式排列的四个二极管。它将负半周反转，使得交流波形的两个半周都变为正半周，产生更平滑的波动直流输出。
A capacitor can be added across the output to smooth the voltage by charging when the voltage rises and discharging when it falls. This reduces ripple.
可以在输出端并联一个电容器以平滑电压，当电压升高时充电，电压下降时放电。这减少了纹波。
10. Comparing AC and DC Appliances | 交流与直流电器的比较
Many household appliances use AC directly from the mains (e.g., heaters, incandescent lamps). Motors in vacuum cleaners and drills often use universal motors that can run on AC or DC, but induction motors are designed for AC only.
许多家用电器直接使用市电交流电（如加热器、白炽灯）。吸尘器和电钻中的电动机通常使用可以在交流或直流下运行的通用电机，但感应电机仅设计用于交流电。
Electronic devices such as laptops and mobile phones require low-voltage DC. They use a power adapter containing a transformer, rectifier and smoothing circuit to convert 230 V AC mains to the required dc voltage.
笔记本电脑和手机等电子设备需要低压直流电。它们使用含有变压器、整流器和平滑电路的电源适配器，将 230 V 交流市电转换为所需的直流电压。
Battery-operated devices are inherently DC. When charged, the charging circuit converts AC to DC. This is why most chargers feel warm – there are energy losses in the conversion process.
电池供电设备本质上是直流电。充电时，充电电路将交流电转换为直流电。这就是为什么大多数充电器会发热——转换过程中存在能量损失。
11. Safety and the Three-Pin Plug | 安全与三脚插头
AC mains electricity can be lethal. The three-pin plug used in the UK provides safety features: the live wire (brown) carries the alternating supply, the neutral wire (blue) completes the circuit, and the earth wire (green/yellow) is a safety path to the ground.
交流市电可能是致命的。英国使用的三脚插头提供了安全功能：火线（棕色）承载交流电源，零线（蓝色）构成回路，地线（绿/黄）是通往大地的安全路径。
If a fault occurs, such as a live wire touching the metal case of an appliance, current flows through the earth wire to ground. This low-resistance path causes a large current that blows the fuse or trips the circuit breaker, disconnecting the appliance.
如果发生故障，例如火线接触电器的金属外壳，电流通过地线流向大地。这种低电阻路径产生大电流，熔断保险丝或使断路器跳闸，断开电器。
The fuse rating is chosen just above the normal operating current of the appliance. Double-insulated appliances (marked with a square within a square) do not require an earth wire.
保险丝的额定值选择略高于电器的正常工作电流。双重绝缘电器（标有套叠方框符号）不需要地线。
12. Key Equations Summary | 关键公式总结
| Equation | Explanation | 公式 | 说明 |
| T = 1 / f | Period and frequency relationship | T = 1 / f | 周期与频率的关系 |
| Vrms = Vpeak / √2 | RMS voltage for sinusoidal AC | Vrms = Vpeak / √2 | 正弦交流电的有效值电压 |
| Vp / Vs = Np / Ns | Transformer voltage ratio | Vp / Vs = Np / Ns | 变压器的电压比 |
| Ip Vp = Is Vs | Ideal transformer power equality | Ip Vp = Is Vs | 理想变压器的功率等式 |
| Plost = I²R | Power loss in transmission cables | Plost = I²R | 输电电缆中的功率损失 |
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