Mains Electricity and Electrical Safety | 市电电路与用电安全

📚 Mains Electricity and Electrical Safety | 市电电路与用电安全

In this revision article, we will explore mains electricity and the safety devices that protect us from electric shock. This is a key topic in the Edexcel IGCSE Physics specification.

本复习文章将带你探讨市电以及保护我们免受触电的安全装置。这是 Edexcel IGCSE 物理考纲中的关键知识点。

1. What is Mains Electricity? | 什么是市电？

Mains electricity is the alternating current (a.c.) supply that is delivered to homes, schools and businesses through the national grid. It is generated at power stations and stepped down to a safe voltage for domestic use.

市电是通过国家电网输送到家庭、学校和商业场所的交流电（a.c.）。它在发电站产生，随后通过变压器降到适合家庭使用的安全电压。

In the UK, the domestic mains supply is 230 V and has a frequency of 50 Hz. This means the alternating voltage completes 50 full cycles every second.

在英国，家用市电电压为 230 V，频率为 50 Hz。这意味着交流电压每秒钟完成 50 个完整周期。

Mains electricity is one of the most important applications of electromagnetism; it is efficiently transmitted at high voltage and then reduced for consumers.

市电是电磁学最重要的应用之一；它先在高压下高效传输，再降低电压供用户使用。

2. Alternating Current vs Direct Current | 交流电与直流电

An alternating current (a.c.) repeatedly reverses its direction. The voltage and current vary sinusoidally with time.

交流电（a.c.）会反复改变方向。电压和电流随时间按正弦规律变化。

A direct current (d.c.) flows in one direction only, such as the current from a battery or a cell.

直流电（d.c.）只沿一个方向流动，例如电池或干电池提供的电流。

Mains electricity is a.c. because it can be easily transformed to very high voltages for efficient transmission over long distances and then stepped down for safe use.

市电采用交流电，是因为交流电可以方便地用变压器升到很高的电压来进行远距离高效输电，然后再降为安全电压使用。

3. The UK Mains Supply Specifications | 英国市电规格

The standard UK domestic mains supply has the following specifications:

英国家庭市电标准规格如下：

• Voltage: 230 V (effective value)

  电压：230 V（有效值）

• Frequency: 50 Hz

  频率：50 Hz

• Supply type: alternating current (a.c.)

  供电类型：交流电（a.c.）

Because the voltage is alternating, the live wire can be at positive or negative potential relative to the neutral wire. At the peak of each cycle, the live wire reaches a potential of about ±325 V.

由于电压是交变的，火线相对于零线的电位可正可负。在每个周期的峰值处，火线的电位约为 ±325 V。

However, the 230 V rating is the root-mean-square (rms) value, which is the equivalent steady d.c. voltage that would produce the same heating effect.

不过，230 V 这个标称值是均方根（rms）值，即产生相同热效应所需的等效稳定直流电压。

4. Wiring in a Three-Pin Plug | 三脚插头的接线

A correctly wired three-pin plug connects an appliance to the mains. The three wires are colour-coded to avoid dangerous mistakes.

正确接线的三脚插头可将电器连接到市电。三根导线都有颜色标识，以避免危险的接线错误。