📚 Alternating Current (AC) – GCSE CIE Physics Revision Guide | GCSE CIE 物理：交流电 考点精讲
Alternating current is the backbone of modern electrical power systems. From the electricity in your home to the operation of transformers, a solid understanding of AC is essential for GCSE CIE Physics. This guide covers all key concepts, graphs, and equations you need to know, with clear bilingual explanations.
交流电是现代电力系统的支柱。从家里的电源到变压器的工作，掌握交流电知识是 GCSE CIE 物理的关键。本指南涵盖了所有核心概念、图形和公式，并提供清晰的中英双语讲解。
1. What is Alternating Current? | 什么是交流电？
An alternating current (AC) is an electric current that periodically reverses direction. The magnitude of the current also varies continuously with time. In contrast, a direct current (DC) flows in one fixed direction only.
交流电（AC）是指电流方向周期性地发生逆转的电流，其大小也随时间不断变化。而直流电（DC）的电流方向则始终固定不变。
Most mains electricity supplies are AC because it can easily be stepped up or down in voltage using a transformer, making long-distance transmission efficient.
大多数市电都是交流电，因为它可以方便地通过变压器升压或降压，从而实现高效长距离输电。
2. Direct Current (DC) vs Alternating Current (AC) | 直流电与交流电对比
DC is supplied by cells and batteries. Electrons move steadily in one direction from the negative terminal to the positive terminal. In AC, electrons oscillate back and forth, generating a changing current.
直流电由电池和干电池提供，电子从负极稳定地流向正极。而在交流电中，电子前后振荡，产生大小和方向都变化的电流。
| Feature | Alternating Current (AC) | Direct Current (DC) |
| Direction of flow | Reverses periodically | Constant single direction |
| Source | AC generator (alternator), mains | Batteries, solar cells |
| Voltage–time graph | Sine wave, crosses zero | Straight horizontal line |
| Use with transformer | Yes – essential for voltage change | No – cannot use standard transformer |
上表比较了交流电与直流电的主要区别。交流电方向周期性变化，波形为正弦波，适合变压器；直流电方向不变，波形为水平直线。
3. Producing Alternating Current – The AC Generator | 产生交流电 – 交流发电机
An AC generator (alternator) converts mechanical energy into electrical energy by electromagnetic induction. A coil of wire is rotated in a magnetic field. As the coil cuts magnetic field lines, an alternating voltage is induced across its ends.
交流发电机通过电磁感应将机械能转化为电能。线圈在磁场中旋转，当线圈切割磁感线时，其两端就会感应出交变电压。
The direction of the induced current reverses every half‑turn because each side of the coil moves up and then down through the magnetic field. This produces a current that regularly changes direction.
感应电流的方向每半圈反转一次，因为线圈的每一边都会先向上运动再向下运动穿过磁场，从而产生周期性改变方向的电流。
4. Why Slip Rings Instead of a Commutator? | 为什么使用滑环代替换向器？
In an AC generator, the ends of the rotating coil are connected to two separate slip rings. Carbon brushes press against these rings to take off the current. The connections to the external circuit do not swap every half‑turn – hence, the current keeps alternating.
在交流发电机中，旋转线圈的两端分别连接到两个独立的滑环上。碳刷压在滑环上来输出电流。由于与外电路的连接不随每半圈而交换，所以电流始终是交变的。
In contrast, a DC generator uses a split‑ring commutator that swaps connections every half‑turn, giving a unidirectional but pulsating current. The use of slip rings is exactly what makes the output truly alternating.
相比之下，直流发电机使用裂环换向器，每半圈交换一次连接，产生方向不变但脉动的电流。滑环的使用正是实现真正交流输出的关键。
5. The AC Waveform: Sine Wave | 交流电波形：正弦波
When an AC generator rotates at a steady speed, the induced voltage varies sinusoidally. The graph of voltage against time is a sine wave. The voltage rises to a maximum, drops to zero, reverses polarity, reaches a negative maximum, and returns to zero.
当交流发电机匀速旋转时，感应出的电压随时间呈正弦变化。电压-时间图像是一个正弦波，电压升至最大值后降至零，再反向升至负的最大值，最后回到零。
A full cycle corresponds to one complete wave (e.g. from a positive peak to the next positive peak). The shape is smooth and repetitive, demonstrating the alternating nature of the current.
一个完整周期对应一个完整的波形（例如从正峰到下一个正峰）。波形平滑且周期重复，直观地显示了电流的交流特性。
6. Interpreting AC Graphs: Peak Voltage, Period, Frequency | 解读交流电图形：峰值电压、周期、频率
From an AC voltage–time graph you can extract:
从交流电压-时间图像中，你可以读出：
- Peak voltage (V₀) – the maximum voltage in either the positive or negative direction.
峰值电压（V₀） – 正或负方向的最大电压。 - Peak‑to‑peak voltage – the total voltage change from the positive peak to the negative peak (2 × V₀).
峰‑峰值电压 – 从正峰到负峰的总电压变化（等于 2 × V₀）。 - Time period (T) – the time taken for one complete cycle, measured in seconds.
周期（T） – 完成一个完整循环所需的时间，单位为秒。 - Frequency (f) – the number of cycles per second, measured in hertz (Hz). It is calculated using:
频率（f） – 每秒钟的周期数，单位为赫兹（Hz）。计算公式如下：
f = 1 / T
For the UK mains supply, f = 50 Hz, so T = 1/50 = 0.02 s (20 ms).
英国市电频率为 50 Hz，因此周期 T = 1/50 = 0.02 秒（20 毫秒）。
7. Mains Electricity: AC at Home | 市电：家庭交流电
The mains electricity supplied to homes in many countries, including the UK, is an AC supply with a stated voltage of about 230 V and a frequency of 50 Hz. This 230 V is the root‑mean‑square (rms) voltage, not the peak voltage. The peak voltage of the UK mains is approximately 325 V (since V₀ = √2 × Vᵣₘₛ).
包括英国在内的许多国家，家庭市电为交流电，标称电压约为 230 V，频率为 50 Hz。这里的 230 V 是均方根（rms）电压，而非峰值电压。英国市电的峰值电压约为 325 V（因 V₀ = √2 × Vᵣₘₛ）。
In the GCSE exam you are expected to know that mains electricity is AC, its approximate voltage and frequency, and why the supply is alternating.
在 GCSE 考试中，你需要知道市电是交流电、其大致电压和频率，以及为什么采用交流供电。
8. Transformers: Why AC is Essential | 变压器：为什么交流电至关重要
A transformer changes an alternating voltage by electromagnetic induction. It consists of two coils wound on a shared iron core. An alternating current in the primary coil produces a changing magnetic field, which induces a voltage in the secondary coil.
变压器通过电磁感应来改变交流电压。它由绕在共用铁芯上的两个线圈组成。初级线圈中的交流电产生变化的磁场，从而在次级线圈中感应出电压。
The voltage ratio is equal to the turns ratio:
电压比等于匝数比：
V₁ / V₂ = N₁ / N₂
where V₁ and V₂ are the primary and secondary voltages, and N₁ and N₂ are the numbers of turns on the primary and secondary coils. Transformers will not work with a steady DC supply because a constant current cannot maintain a changing magnetic field.
其中 V₁ 和 V₂ 分别是初级和次级电压，N₁ 和 N₂ 为初级和次级的匝数。变压器无法在稳定的直流电下工作，因为恒定的电流无法维持变化的磁场。
9. Using an Oscilloscope to Display AC and DC | 使用示波器显示交流电和直流电
An oscilloscope shows how voltage varies with time. For AC, a sine wave is displayed, centred on the zero‑voltage line. The vertical gain (volt/div) and time base (time/div) settings allow you to measure peak voltage and period.
示波器可以显示电压如何随时间变化。对于交流电，屏幕上会出现以零电压线为中心的正弦波。通过垂直增益（伏/格）和时基（时间/格）设置，可以测量峰值电压和周期。
For DC, the trace is a straight horizontal line displaced above or below the centre line, indicating a constant voltage in one direction. When the time base is switched off, AC appears as a vertical line due to the rapid to‑and‑fro motion of the spot.
直流电的迹线是一条偏离中心线的水平直线，表示方向不变的恒定电压。当时基关闭时，交流电呈现为一条竖直线，这是因为光点在快速来回运动。
10. Advantages of AC over DC for Power Distribution | 交流电在电力输送中相比直流电的优势
AC can easily be stepped up to very high voltages using transformers. High‑voltage transmission reduces current for the same power (P = IV), which in turn decreases energy losses as heat in the cables (Pₗₒₛₛ = I²R). At the consumer end, the voltage is stepped down to safer levels.
交流电可以便捷地通过变压器升压至很高的电压。在相同功率下，高压传输可以降低电流（P = IV），从而减少输电线的热损耗（Pₗₒₛₛ = I²R）。到达用户端后，电压再被降低到安全水平。
Although modern high‑voltage DC (HVDC) is used for some long‑distance links, AC remains dominant because of the simplicity and efficiency of voltage conversion with transformers.
尽管现代高压直流输电（HVDC）已应用于部分长距离线路，但交流电因变压器的简单高效仍占据主导地位。
11. Summary and Key Equations | 总结与关键公式
- AC current and voltage alternate in direction; displayed as a sine wave.
交流电的电流和电压方向交替变化，波形为正弦波。 - AC generator uses slip rings to maintain alternating output.
交流发电机使用滑环来保持交流输出。 - Period T and frequency f: f = 1 / T.
周期 T 与频率 f 的关系：f = 1 / T。 - Peak voltage V₀; peak‑to‑peak = 2 V₀.
峰值电压 V₀；峰‑峰值 = 2 V₀。 - Mains supply: 230 V (rms), 50 Hz.
市电：230 V（有效值），50 Hz。 - Transformer equation: V₁/V₂ = N₁/N₂ (for an ideal transformer).
变压器方程：V₁/V₂ = N₁/N₂（理想变压器）。 - Transformers require AC; they do not work with DC.
变压器需要交流电，无法使用直流电。 - Oscilloscope can display both AC and DC waveforms.
示波器可以显示交流和直流波形。
Published by TutorHao | Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply