📚 Understanding SI Units | 理解国际单位制（SI）
The International System of Units (SI) is the foundation of all physical measurement. In CIE A-Level Physics, you must be able to name base units, express derived units in terms of base units, use metric prefixes, and check whether equations are dimensionally consistent.
国际单位制 (SI) 是一切物理测量的基础。在 CIE A-Level 物理中，你必须能够说出基本单位、用基本单位表示导出单位、使用公制前缀，并检验方程是否量纲一致。
1. What Are SI Base Units? | 什么是SI基本单位？
The Système International d'Unités (SI) defines seven base units from which all other physical units are derived. These base units are chosen by international agreement and are linked to precise physical standards.
国际单位制 (SI) 定义了七个基本单位，所有其他物理单位均由它们导出。这些基本单位经国际协议选定，并与精确的物理标准相关联。
In A-Level Physics, you must know the seven base quantities, their base units, and the standard symbols. Many exam questions test whether you can express a derived unit in terms of base units.
在 A-Level 物理中，你必须知道七个基本量、它们的基本单位及标准符号。许多考题考查你能否用基本单位表示导出单位。
2. The Seven Base Quantities and Units | 七个基本量与单位
The table below lists the seven SI base quantities and their units. You should memorise both the names and the symbols exactly as shown.
下表列出了七个 SI 基本量及其单位。你应当准确记住名称和符号，如下所示。
| Base Quantity 基本量 | Base Unit 基本单位 | Symbol 符号 |
|---|---|---|
| Length 长度 | metre 米 | m |
| Mass 质量 | kilogram 千克 | kg |
| Time 时间 | second 秒 | s |
| Electric current 电流 | ampere 安培 | A |
| Thermodynamic temperature 热力学温度 | kelvin 开尔文 | K |
| Amount of substance 物质的量 | mole 摩尔 | mol |
| Luminous intensity 发光强度 | candela 坎德拉 | cd |
Note that unit symbols are case-sensitive: kg not Kg, s not sec, A not amp. The kilogram is the only base unit that still has a prefix.
注意单位符号区分大小写：kg 不能写成 Kg，s 不能写成 sec，A 不能写成 amp。千克是唯一仍带有前缀的基本单位。
3. Derived SI Units and Special Names | 导出单位与专用名称
Derived units are formed by multiplying or dividing base units. For example, speed is measured in metres per second (m s⁻¹) and acceleration in metres per second squared (m s⁻²).
导出单位由基本单位相乘或相除形成。例如，速度的单位是米每秒 (m s⁻¹)，加速度的单位是米每二次方秒 (m s⁻²)。
Some derived units have special names, such as the newton (N) for force, the joule (J) for energy, and the watt (W) for power. You must be able to express these special units in base units.
一些导出单位有专用名称，如力的单位牛顿 (N)、能量的单位焦耳 (J)、功率的单位瓦特 (W)。你必须能用基本单位表示这些专用单位。
1 N = 1 kg m s⁻²
1 J = 1 kg m² s⁻²
1 W = 1 kg m² s⁻³
4. Writing Units Correctly | 正确书写单位
CIE examiners expect correct unit conventions. Unit symbols are written in upright type, not italics. A space is left between the numerical value and the unit, e.g. 5.0 m not 5.0m. Symbols for units named after people have uppercase first letters when written as symbols: N, J, W, Pa, Hz.
CIE 考官要求正确的单位书写规范。单位符号用正体而非斜体。数值与单位之间留一个空格，例如 5.0 m 而非 5.0m。以人名命名的单位符号首字母大写：N、J、W、Pa、Hz。
When writing compound units, use negative powers instead of slash notation where possible, e.g. m s⁻¹ rather than m/s. Do not write 'sec' for seconds; use s. Do not add 's' to make plurals: 10 m, not 10 ms.
书写复合单位时，尽量使用负指数代替斜线，如 m s⁻¹ 而非 m/s。不要用 'sec' 表示秒，应使用 s。单位符号不加复数 's'：10 m 而非 10 ms。
5. Metric Prefixes and Powers of Ten | 公制前缀与十的幂
Metric prefixes scale base units by powers of ten. Common prefixes in A-Level include pico (p, 10⁻¹²), nano (n, 10⁻⁹), micro (μ, 10⁻⁶), milli (m, 10⁻³), centi (c, 10⁻²), kilo
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