The International System of Units (SI) for AQA A-Level Physics | AQA A-Level 物理中的国际单位制(SI)

📚 The International System of Units (SI) for AQA A-Level Physics | AQA A-Level 物理中的国际单位制(SI)

The International System of Units, abbreviated SI from the French "Système International d'Unités", is the foundation of all quantitative work in AQA A-Level Physics. It defines the base quantities, base units, derived units and prefixes that you will use in every topic, from mechanics and materials to fields and nuclear physics. Mastering SI units is not just about memorising symbols; it is a tool for checking equations, converting data and avoiding errors in practical work and exams.

国际单位制（SI，源自法语 "Système International d'Unités"）是 AQA A-Level 物理所有定量工作的基础。它定义了基本量、基本单位、导出单位和词头，从力学、材料到场与核物理的每个主题都会用到。掌握 SI 单位不只是记住符号；它是检查方程、转换数据以及避免实验和考试错误的工具。

1. What Is the SI? | 什么是国际单位制？

The SI is a coherent system of units agreed internationally so that scientists and engineers can share measurements without ambiguity. A coherent system means that derived units are formed by multiplying or dividing base units without numerical conversion factors. For example, the newton is exactly 1 kg m s⁻². In AQA papers, you are expected to use SI units unless a question expressly asks for another unit such as eV, kWh or °C.

SI 是国际公认的一致单位制，使科学家和工程师可以无歧义地共享测量结果。“一致”意味着导出单位由基本单位相乘或相除得到，无需数值换算因子。例如，牛顿恰好等于 1 kg m s⁻²。在 AQA 试卷中，除非题目明确要求使用其他单位（如 eV、kWh 或 °C），否则都应使用 SI 单位。

The SI system is not static. Since May 2019, all seven base units are defined in terms of fixed fundamental constants such as the speed of light c, the Planck constant h and the elementary charge e. For AQA Physics, you do not need to memorise the formal definitions in detail, but you should understand that these definitions make the units stable and universally reproducible.

SI 单位制并非一成不变。自 2019 年 5 月起，七个基本单位均根据固定的基本常数来定义，如光速 c、普朗克常数 h 和基本电荷 e。对于 AQA 物理，你不需详细记住正式定义，但应理解这些定义使单位稳定且可在全球复现。

2. The Seven SI Base Units | 七个 SI 基本单位

AQA requires you to know the seven SI base quantities, their base units and symbols. These are the independent quantities from which all other units are derived. A common exam question asks you to state the base units of a quantity such as force or energy, so you should know them securely.

AQA 要求掌握七个 SI 基本量、基本单位及其符号。它们是所有其他单位所导出的独立量。考试中常见的问题是写出力或能量等物理量的基本单位，因此必须熟练掌握。

Notice that kilogram is the only base unit with a prefix (kilo) in its name. This is a historical exception: mass is still measured relative to the kilogram, not the gram, so always write kg when working in SI. Also, temperature in formulas must be in kelvin, not degrees Celsius, because only kelvin is an SI base unit.

注意，千克是唯一名称中含词头（千）的基本单位。这是一个历史例外：质量仍以千克为基准，而不是克，因此在 SI 中始终写成 kg。此外，公式中的温度必须使用开尔文，而不是摄氏度，因为只有开尔文是 SI 基本单位。

3. Derived Units and Special Names | 导出单位与专用名称

² A⁻¹). In AQA exams you may be asked to express a named unit in base units, so practise writing these expansions.

导出单位是基本单位的乘积或商。有些具有专用名称和符号，如牛顿 (N = kg m s⁻²)、焦耳 (J = kg m² s⁻²)、瓦特 (W = J s⁻¹ = kg m² s⁻³)、库仑 (C = A s)、伏特 (V = J C⁻¹ = kg m² s⁻³ A⁻¹)、欧姆 (Ω = V A⁻¹ = kg m² s⁻³ A⁻²) 和特斯拉 (T = kg s⁻² A⁻¹)。AQA 考试可能要求将某个命名单位用基本单位表示，因此要练习这些展开式。

m⁻¹ s⁻². Similarly, the hertz is s⁻¹ because frequency is the number of cycles per second.

将命名单位用基本单位表示时，要使用定义方程。例如，帕斯卡由压强 = 力 ÷ 面积定义，因此 Pa = N m⁻²。由于 N = kg m s⁻²，帕斯卡变为 kg m⁻¹ s⁻²。类似地，赫兹是 s⁻¹，因为频率是每秒的循环次数。

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4. Prefixes and Powers of Ten | 词头与十的幂

mple 5.0 pF = 5.0 × 10⁻¹² F, and to use them in calculations without losing powers of ten.

词头用于简洁表示非常大或非常小的数值。AQA 要求掌握从皮可 (10⁻¹²) 到太拉 (10¹²) 的常用词头。你必须能够在词头之间转换，例如 5.0 pF = 5.0 × 10⁻¹² F，并在计算中使用它们而不丢失十的幂。

Always check case sensitivity with prefixes. A lowercase m means milli (10⁻³), but an uppercase M means mega (10⁶). A level calculation can go badly wrong if you write MW when you mean mW.

始终注意词头的大小写。小写 m 表示毫 (10⁻³

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