📚 Why use a.c. for electricity supply? | 为什么使用交流电供电？
Nearly all electricity grids around the world deliver power as alternating current (a.c.) rather than direct current (d.c.). This choice is not arbitrary — it emerges from the physics of generation, the mathematics of power loss, and the unique ability of transformers to change voltage efficiently. In this CIE A-Level Physics revision article, we examine the key reasons why a.c. dominates electricity supply, exploring the core concepts of electromagnetic induction, I²R losses, and the economics of power distribution.
全球几乎所有的电网都采用交流电而非直流电供电。这个选择并非偶然，它源于发电的物理原理、功率损耗的数学规律，以及变压器高效变换电压的独特能力。在这篇 CIE A-Level 物理复习文章中，我们逐一剖析交流电主导电力供应的关键原因，深入理解电磁感应、I²R 损耗以及电力输送的经济学。
1. Historical Context: The War of Currents | 历史背景：电流之战
In the late 19th century, a fierce commercial and technical battle — the 'War of Currents' — pitted Thomas Edison's d.c. system against Nikola Tesla and George Westinghouse's a.c. system. Edison's low-voltage d.c. required power stations to be built every few kilometres because voltage could not be raised, resulting in enormous copper costs and energy wasted as heat. Tesla's a.c., combined with his polyphase system and transformers, allowed high-voltage transmission over hundreds of kilometres. The eventual success of a.c. at the 1893 Chicago World's Fair and the Niagara Falls power project sealed its global adoption.
19 世纪末，一场激烈的商业与技术之争——“电流之战”——在托马斯·爱迪生的直流系统与尼古拉·特斯拉及乔治·威斯汀豪斯的交流系统之间展开。爱迪生的低压直流电要求每隔几公里就建一个发电站，因为电压无法升高，导致铜材成本巨大，且大量能量以热的形式损耗。特斯拉的交流电结合其多相系统和变压器，能够实现数百公里的高压输电。随着 1893 年芝加哥世博会和尼亚加拉瀑布电力项目中交流电的成功，交流电的全球应用已成定局。
2. AC Generation is Natural and Simple | 交流发电的自然性与简易性
An a.c. generator (alternator) produces an alternating electromotive force simply by rotating a coil in a magnetic field. According to Faraday's law of electromagnetic induction, a changing magnetic flux linkage induces an e.m.f. that reverses direction every half turn, giving a sinusoidal output. The frequency is directly proportional to the rotation speed, with no need for the split-ring commutators found in d.c. generators. Commutators in d.c. machines cause friction, sparking and require frequent maintenance, whereas a.c. slip rings are much simpler and more durable.
交流发电机仅通过线圈在磁场中旋转就能产生交变电动势。根据法拉第电磁感应定律，变化的磁通链会感应出每半圈方向反转的电势，输出正弦波形。频率与转速成正比，完全无需直流发电机中的裂环换向器。直流电机中的换向器会造成摩擦、火花并需要频繁维护，而交流电的滑环结构简单得多，也更耐用。
3. The Transformer: Core Reason for AC | 变压器：采用交流的核心原因
The single most compelling reason for using a.c. is the transformer. A transformer cannot operate on steady d.c. because a constant current produces a constant magnetic flux, and Faraday's law requires a changing flux to induce an e.m.f. in the secondary coil. With a.c., the alternating current creates a continuously changing flux, enabling efficient energy transfer between coils with practically no moving parts. The ideal transformer equations summarise its behaviour beautifully.
使用交流电最根本的原因是变压器。变压器无法在恒稳直流电下工作，因为恒定的电流产生恒定的磁通量，而法拉第定律要求变化的磁通才能在次级线圈中感应出电动势。对于交流电，交替变化的电流产生持续变化的磁通，从而能够以几乎没有运动部件的方式在线圈之间高效传输能量。理想变压器方程极其简洁地概括了它的行为。
Vₛ / Vₚ = Nₛ / Nₚ = Iₚ / Iₛ
equals power out (Vₚ Iₚ = Vₛ Iₛ). This relationship means that a.c. voltages can be stepped up for transmission or stepped down for domestic use with negligible energy loss — a feat impossible with d.c. without expensive power electronics.
式中 Vₚ 和 Vₛ 为初、次级电压，Nₚ、Nₛ 为匝数，Iₚ 和 Iₛ 为电流。对理想变压器，输入功率等于输出功率（Vₚ Iₚ = Vₛ Iₛ）。这意味着交流电压可以几乎无能量损耗地升高以用于输电，或降低以供家用——这在不借助昂贵电力电子装置的情况下，对直流电而言是无法实现的。
4. Step-Up Transmission: Reducing I²R Losses | 升压输电：降低 I²R 损耗
When electrical power travels through cables, the resistive heating loss is governed by Joule's law. The power wasted as heat in a transmission line of resistance R is given by the square of the current times the resistance. This is the infamous I²R loss.
当电能在电缆中传输时，电阻性热损耗由焦耳定律决定。在电阻为 R 的输电线路中，以热量形式损耗的功率等于电流的平方乘以电阻，这就是著名的 I²R 损耗。
Power loss = I²R
e factors for simplicity), for a fixed amount of power, increasing the voltage V forces the current I to decrease proportionally. Doubling the voltage halves the current and reduces the I²R loss to one-quarter. A.c. makes it trivial to step generation voltage up to 132 kV, 275 kV or even 400 kV using transformers, slashing the current and greatly minimising thermal waste.
由于必须输送给用户的功率为 P = V I（为简化忽略相位因数），在功率固定的情况下，升高电压 V 将迫使电流 I 成比例减小。电压加倍则电流减半，I²R 损耗降为原来的四分之一。交流电利用变压器可以轻松将发电机电压升至 132 kV、275 kV 甚至 400 kV，大幅削减电流，从而显著降低热损耗。
5. High Voltage, High Efficiency | 高电压，高效率
A typical modern a.c. transmission system operates with an end-to-end efficiency above 95%. For instance, a 400 kV line delivering 1000 MW of power requires only about 2500 A. If the same power were transmitted at 33 kV, the current would surge to over 30 000 A — causing such intense heating that the cables would melt unless they were made impossibly thick. High voltage also permits the use of thinner, lighter conductors, which reduces the mechanical load on pylons and the overall cost of infrastructure.
典型的现代交流输电系统端到端效率超过 95%。例如，一条 400 kV 线路输送 1000 MW 功率时，所需电流仅为约 2500 A。如果同样的功率在 33 kV 下输送，电流将飙升至 30 000 A 以上，导致极其剧烈的发热，除非使用不切实际的粗电缆，否则导线将熔断。高电压还允许使用更细、更轻的导体，从而减轻铁塔的机械负荷并降低整体基础设施成本。
6. Step-Down for Safe Domestic Use | 降压以供安全家用
At substations near populated areas, transformers step the high transmission voltage down through several stages — typically from 400 kV to 132 kV, then to 33 kV or 11 kV, and finally to the 230 V (UK) or 120 V supply entering homes. Each step-down stage is accomplished with simple, passive transformers that require almost no maintenance. This graduated voltage reduction would be enormously complex and inefficient if attempted with d.c., where one would need high-power electronic choppers and inverters at every stage.
在靠近居民区的变电站中，变压器通过多个阶段将高输电电压逐步降低——通常从 400 kV 降至 132 kV，再到 33 kV 或 11 kV，最终变为进入家庭的 230 V（英国）或 120 V 供电。每一次降压都利用简单、无源的变压器完成，几乎不需要维护。这种分级的电压降低若用直流电来实现，将在每一级都需要大功率的电子斩波器和逆变器，极其复杂且低效。
7. AC Motors: Rugged and Widespread | 交流电动机：坚固且广泛应用
More than 60% of industrial electricity consumption powers motors, and the majority are a.c. induction motors. These motors have no brushes or commutators, and their rotors are simply laminated iron cores with conducting bars. The a.c. supply naturally creates a rotating magnetic field in the stator, inducing currents in the rotor and producing smooth torque. For fixed-speed applications — from factory conveyors to household fans and washing machines — a.c. motors are cheaper, more robust and require far less maintenance than brushed d.c. motors.
工业用电中 60% 以上用于驱动电动机，其中绝大多数是交流感应电动机。这类电机没有电刷或换向器，转子仅为叠片铁芯加上导电条。交流供电在定子中自然产生旋转磁场，在转子中感生出电流并产生平稳的转矩。对于从工厂传送带到家用风扇和洗衣机的定速应用场合，交流电动机比有刷直流电机更便宜、更牢固且所需维护少得多。
8. Rectification: AC to DC When Needed | 整流：需要时交流变直流
Modern electronic devices — smartphones, LED lighting, computers — all operate internally on d.c. This does not weaken the case for a.c. distribution. A.c. can be converted cleanly and efficiently to any desired d.c. voltage using rectifiers (diodes) and smoothing capacitors or more advanced switched-mode power supplies. The reverse process — converting low-voltage d.c. to high-voltage a.c. for grid transmission — requires inverters, which are heavier, more
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