A-Level Physics: Why Does the National Grid Use Alternating Current? | A-Level 物理：电网为何采用交流输电

📚 A-Level Physics: Why Does the National Grid Use Alternating Current? | A-Level 物理：电网为何采用交流输电

Electricity is the lifeblood of modern society. From homes to industries, the demand for electrical energy is met by a vast network of transmission lines that carry power over long distances. The vast majority of these networks, known as the National Grid, use alternating current (AC) rather than direct current (DC). This article explores the fundamental physics that makes AC the preferred choice for power transmission.

电力是现代社会的命脉。从家庭到工业，电能的需求由跨越长距离的庞大输电网来满足。这些被称为“国家电网”的绝大多数网络采用的是交流电（AC），而非直流电（DC）。本文将探究使交流电成为电力传输首选的基本物理原理。

1. Generation of Alternating Current | 交流电的产生

In a power station, the prime mover (turbine) rotates a coil within a strong magnetic field. According to Faraday’s law of electromagnetic induction, the induced emf is equal to the rate of change of magnetic flux linkage. Because the angle between the coil and the magnetic field changes continuously, the flux linkage changes sinusoidally, producing a sinusoidal alternating emf.

在发电站中，原动机（涡轮机）在一强磁场内旋转线圈。根据法拉第电磁感应定律，感应电动势等于磁通匝链数的变化率。由于线圈与磁场之间的夹角连续变化，磁通匝链数呈正弦变化，从而产生正弦交变电动势。

The design of an AC generator is relatively simple: it uses slip rings to collect the alternating current, with no commutator needed. In contrast, a DC generator requires a commutator to mechanically reverse the current direction, adding complexity and maintenance. Thus, generating AC at the source is more robust and cost-effective.

交流发电机的设计相对简单：它使用滑环来引出交变电流，不需要换向器。相比之下，直流发电机需要换向器来机械地切换电流方向，增加了复杂性和维护负担。因此，源头产生交流电更可靠、更经济。

2. The Transformer: AC’s Key Advantage | 变压器：交流电的关键优势

A transformer exploits mutual induction between two coils coupled by a soft iron core. An alternating current in the primary coil creates a time-varying magnetic flux in the core. This flux induces an alternating emf in the secondary coil, whose magnitude is proportional to the number of turns: Vₛ/Vₚ = Nₛ/Nₚ. For an ideal transformer with no losses, the input power equals the output power, so Vₚ Iₚ = Vₛ Iₛ.

变压器利用两个线圈通过软铁芯耦合的互感现象。初级线圈中的交变电流在铁芯中产生随时间变化的磁通量。该磁通在次级线圈中感应出交变电动势，其大小与匝数成正比：Vₛ/Vₚ = Nₛ/Nₚ。对于无损耗的理想变压器，输入功率等于输出功率，因此 Vₚ Iₚ = Vₛ Iₛ。

Because the flux must be changing to induce an emf, a constant DC current in the primary produces no induced emf in the secondary. Consequently, DC cannot be used with step-up or step-down transformers. This is the fundamental reason why the grid operates on AC.

因为只有变化的磁通量才能感应出电动势，初级线圈中的恒定直流电在次级线圈中不会产生感应电动势。因此，直流电无法用于升压或降压变压器。这正是电网采用交流电的根本原因。

3. High-Voltage Transmission Reduces Power Loss | 高压输电减少能量损失

The distribution of electrical energy over long distances involves significant resistive losses in transmission lines. For a cable of resistance R, the power loss is Pₗₒₛₛ = I²R. To deliver a given power P at a load, the transmission line carries a current I = P/V. Therefore, higher voltages reduce current, greatly reducing loss.

长距离输送电能时，输电线中的电阻损耗非常显著。对于电阻为R的电缆，损耗功率为 Pₗₒₛₛ = I²R。要输送指定功率P，输电线上电流为 I = P/V。因此，更高的电压能减小电流，从而大幅降低损耗。

Example: Suppose 100 MW is transmitted at 400 kV, I = 250 A. If the line resistance is 10 Ω, loss = 250² × 10 = 625 kW, about 0.6% of transmitted power. If transmitted at 100 kV, current is 1000 A and loss = 10 MW (10%). AC allows voltage to be raised cheaply using transformers to realize these savings.

例如：以400 kV输送100 MW，电流为250 A。若线路电阻为10 Ω，损耗 = 250² × 10 = 625 kW，约占输送功率的0.6%。若以100 kV输送，电流为1000 A，损耗为10 MW（10%）。交流电可通过变压器廉价地升高电压，从而实现这些节省。

4. Why Not High-Voltage DC? | 为何不用高压直流电？

Historically, in the “War of Currents,” Edison promoted DC while Westinghouse and Tesla promoted AC. DC power stations were limited to a few kilometres because low voltage meant high current and enormous losses, while high voltage DC could not be easily stepped down. The lack of practical DC-DC converters made DC unsuitable for a nationwide grid.

历史上，在“电流之战”中，爱迪生倡导直流，而西屋和特斯拉倡导交流。直流发电站只能覆盖几公里范围，因为低电压意味着大电流和巨大损耗，而高压直流电又难以降压。由于缺乏实用的直流-直流变换装置，直流电不适合全国性电网。

Modern semiconductor technology enables HVDC, but it requires expensive conversion equipment. In contrast, AC transformers are simple, efficient, and reliable, making AC the economically superior choice for mainstream transmission.

现代半导体技术使高压直流成为可能，但需要昂贵的换流设备。相比之下，交流变压器简单、高效、可靠，使交流成为主流输电经济上更优的选择。

5. Root Mean Square (RMS) Values | 有效值（RMS）

Since AC voltage and current are sinusoidal, their average value is zero. For power calculations, we use root mean square (RMS) values. For a sine wave, V_rms = V_peak / √2 and I_rms = I_peak / √2. The power dissipated in a resistor is P = I_rms²R = V_rms I_rms = V_rms² / R.

由于交流电压和电流是正弦的，其平均值为零。在功率计算中，我们使用方均根（RMS）值。对于正弦波，V_有效 = V_峰值 / √2，I_有效 = I_峰值 / √2。电阻中耗散的功率为 P = I_有效²R = V_有效 I_有效 = V

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