IGCSE Physics: Alternating Current – Key Exam Points | IGCSE 物理：交流电 考点精讲

📚 IGCSE Physics: Alternating Current – Key Exam Points | IGCSE 物理：交流电 考点精讲

Alternating current (AC) plays a fundamental role in modern electrical power systems, from household sockets to massive transmission networks. Understanding AC is essential for IGCSE Physics, as it underpins concepts such as electromagnetic induction, transformers, and efficient power distribution. This revision guide provides a comprehensive breakdown of key AC topics, including the differences between AC and DC, generator operation, oscilloscope traces, RMS values, transformer theory, and the rationale behind high-voltage transmission. Mastering these points will not only prepare you for exam questions but also deepen your appreciation for the technology that powers our world.

交流电（AC）在现代电力系统中扮演着基础角色，从家用插座到庞大的输电网络都离不开它。理解交流电对于IGCSE 物理至关重要，因为它支撑着电磁感应、变压器以及高效配电等概念。本复习指南全面解析交流电的关键考点，包括交流与直流的区别、发电机运作、示波器波形、有效值（RMS）、变压器原理以及高压输电的合理性。掌握这些要点不仅能帮助你应对考试，还能加深你对支撑世界运转的这项技术的理解。

1. AC vs DC: Definitions and Differences | 交流电与直流电：定义与区别

Direct current (DC) flows in one constant direction, typically from the positive to negative terminal of a battery. All batteries and solar cells produce DC. In contrast, alternating current (AC) periodically reverses direction, causing the current and voltage to change polarity with time. This constant switching makes AC ideal for transformers and long-distance transmission. The mains supply in most countries operates at a frequency of 50 Hz or 60 Hz, meaning the current changes direction 100 or 120 times per second.

直流电（DC）沿一个固定方向流动，通常从电池的正极流向负极。所有电池和太阳能电池都产生直流电。相比之下，交流电（AC）周期性地改变方向，导致电流和电压随时间改变极性。这种持续变化的特性使得交流电非常适合变压器和远距离传输。大多数国家的市电频率为 50 Hz 或 60 Hz，意味着电流每秒改变方向 100 次或 120 次。

A key exam requirement is to be able to identify AC and DC from current–time graphs. A horizontal straight line (positive or negative) represents DC, while a sinusoidal wave, typically starting at zero and rising to a peak, crossing zero to a negative peak, and repeating, indicates AC. Remember that AC is not just any varying current; it must reverse direction. Thus, a pulsating DC, which varies in magnitude but never goes negative, is not AC.

一个重要的考试要求是能够从电流–时间图中识别交流和直流。水平直线（正或负）代表直流，而正弦波形（通常从零开始升到正峰值，穿过零到负峰值并重复）表示交流。请记住，交流不仅仅是变化电流；它

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