Alternating Current: Key Points for CCEA IGCSE Physics | IGCSE CCEA 物理：交流电 考点精讲

📚 Alternating Current: Key Points for CCEA IGCSE Physics | IGCSE CCEA 物理：交流电 考点精讲

Alternating current (AC) is a core topic in CCEA IGCSE Physics, linking electricity generation, transformers and the national grid. Mastering AC concepts will help you explain how homes receive power and why high-voltage transmission is used. This revision guide covers the essential points you need for your exam, from AC waveforms to rectification.

交流电是 CCEA IGCSE 物理的核心主题，连接了发电、变压器和国家电网。掌握交流电概念能帮助你解释家庭如何获得电力以及为什么使用高压输电。本复习指南涵盖了你考试所需的基本要点，从交流电波形到整流。

1. What is Alternating Current? | 什么是交流电？

Alternating current is an electric current that periodically reverses direction. Unlike direct current (DC), which flows steadily in one direction, AC voltage varies with time and typically follows a smooth sine wave. In the UK mains supply, the frequency is 50 Hz, meaning the current completes 50 full cycles each second and changes direction 100 times per second.

交流电是一种周期性地反转方向的电流。与稳定单向流动的直流电（DC）不同，交流电的电压随时间变化，通常遵循平滑的正弦波形。在英国市电中，频率为 50 Hz，意味着电流每秒完成 50 个完整周期并改变方向 100 次。

The magnitude of an alternating voltage oscillates between a positive peak and a negative peak. A graph of voltage against time for AC is a sine wave, while for DC it is a horizontal straight line.

交流电压的大小在正峰值和负峰值之间振荡。交流电的电压-时间图是正弦波，而直流电的是一条水平直线。

AC is used in mains electricity because it can easily be transformed to higher or lower voltages, making long-distance transmission more efficient.

市电使用交流电是因为它很容易被变换为更高或更低的电压，使远距离输电更高效。

2. AC Generator (Alternator) | 交流发电机

An AC generator, or alternator, converts kinetic energy into electrical energy through electromagnetic induction. It has a coil of wire that rotates in a magnetic field, plus two slip rings and carbon brushes to connect the coil to the external circuit without twisting the wires.

交流发电机（也称同步发电机）通过电磁感应将动能转化为电能。它有一个在磁场中旋转的线圈，以及两个滑环和碳刷，用于连接线圈与外电路而不扭绞导线。

As the coil rotates, it cuts through magnetic field lines, inducing an alternating emf. The slip rings allow the induced current to flow out in the same alternating pattern. When the coil is parallel to the field lines, the rate of flux cutting is greatest, giving the peak voltage Vₘ. When the coil is perpendicular, the induced voltage is momentarily zero. This produces a sinusoidal output voltage.

当线圈旋转时，它切割磁感线，感应出交变电动势。滑环使感应电流以相同的交变模式流出。当线圈平面平行于磁场时，磁通量切割速率最大，产生峰值电压 Vₘ。当线圈平面垂直时，感应电压瞬时为零。这就产生了正弦输出电压。

The frequency of the AC output depends on the rotation speed of the coil and the number of magnetic pole pairs. A simple two-pole generator turning at 3000 rpm produces 50 Hz AC.

交流电输出的频率取决于线圈的转速和磁极对数。一台简单的两极发电机以 3000 rpm 转动会产生 50 Hz 的交流电。

3. AC Waveform on an Oscilloscope | 示波器上的交流电波形

An oscilloscope displays a voltage-time graph. For an AC supply, the trace is a smooth, repeating sine wave. The vertical axis represents voltage (calibrated in volts/div), and the horizontal axis represents time (controlled by the time-base setting in seconds/div).

示波器显示电压-时间图。对于交流电源，迹线是一条平滑、重复的正弦波。垂直轴表示电压（校准为伏特/格），水平轴表示时间（由时基设置控制，单位为秒/格）。

Key features on the waveform include the peak voltage Vₘ, which is the maximum displacement from the centre line, and the peak-to-peak voltage Vpp, which is the total voltage swing from the positive peak to the negative peak. The time period T is the time taken for one complete cycle.

波形上的关键特征包括峰值电压 Vₘ

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