Edexcel IGCSE Science: Units and Measurement | Edexcel IGCSE科学：单位与测量

📚 Edexcel IGCSE Science: Units and Measurement | Edexcel IGCSE科学：单位与测量

In any scientific investigation, measurements are the bridge between observation and understanding. Whether you are measuring the temperature of a solution, the mass of a precipitate, or the distance a trolley travels, the units you attach to those numbers determine whether your results are meaningful. This guide covers everything you need to know about units and measurement for the Edexcel IGCSE Science exams, including the SI system, unit conversion, reading instruments, estimating uncertainty, and presenting data.

在任何科学探究中，测量都是连接观察与理解的桥梁。无论你是在测量溶液的温度、沉淀物的质量，还是小车运动的距离，你所附加的单位决定了结果是否有意义。本指南涵盖Edexcel IGCSE科学考试中关于单位与测量的全部核心知识，包括国际单位制、单位换算、仪器读数、不确定度估算以及数据呈现。

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1. Why Units Matter in Science | 为什么单位在科学中至关重要

A measurement such as “30” is completely ambiguous without a unit. Is it thirty metres, thirty seconds, or thirty degrees Celsius? In science, every physical quantity must be expressed as a number multiplied by a unit. For example, the speed of a car could be 30 m/s or 30 km/h; these represent very different physical situations.

没有单位的测量值“30”是完全没有意义的。它是三十米、三十秒，还是三十摄氏度？在科学中，每个物理量都必须表示为数字乘以单位。例如，一辆汽车的速度可以是30 m/s或30 km/h；这代表完全不同的物理情境。

Units allow scientists around the world to share data unambiguously. They also help you check whether your calculations are reasonable. If you calculate a mass and get an answer in grams, but you expected kilograms, a simple conversion can reveal the error.

单位使世界各地的科学家能够毫无歧义地共享数据。它们还能帮助你检查计算是否合理。如果你计算一个质量，得到以克为单位的结果，但预期是千克，简单的换算就能揭示错误。

In Edexcel IGCSE Science, candidates are expected to use the correct SI unit for every quantity, and to convert between prefixes such as milli-, centi-, kilo-, and micro-.

在Edexcel IGCSE科学考试中，考生需要为每个物理量使用正确的国际单位，并能在毫、厘、千、微等词头之间进行换算。

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2. The International System of Units (SI) | 国际单位制（SI）

The SI system defines seven base units, each corresponding to a fundamental physical quantity. You do not need to memorise the modern definitions, but you must know the quantity, the unit, and its symbol.

国际单位制定义了七个基本单位，每个对应一个基本物理量。你不需要记住现代定义，但必须知道物理量、单位名称和符号。

Quantity	Unit	Symbol	物理量	单位	符号
Length	metre	m	长度	米	m
Mass	kilogram	kg	质量	千克	kg
Time	second	s	时间	秒	s
Electric current	ampere	A	电流	安培	A
Temperature	kelvin	K	温度	开尔文	K
Amount of substance	mole	mol	物质的量	摩尔	mol
Luminous intensity	candela	cd	发光强度	坎德拉	cd

For IGCSE Chemistry and Biology, you will mainly use m, kg, s, A, K and mol. The candela is rarely needed in the core syllabus.

对于IGCSE化学和生物学，你主要使用m、kg、s、A、K和mol。坎德拉在核心大纲中很少需要。

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3. Derived Units | 导出单位

Many quantities are combinations of base units. For example, speed is distance ÷ time, so its unit is metre per second (m/s). Force, energy, and power are all derived units with special names.

许多物理量是基本单位的组合。例如，速度等于距离÷时间，所以其单位是米每秒（m/s）。力、能量和功率都是具有特殊名称的导出单位。

• Force (牛顿) – unit: newton (N) = kg·m/s²

  力 – 单位：牛顿（N）= kg·m/s²

• Energy (能量) – unit: joule (J) = kg·m²/s²

  能量 – 单位：焦耳（J）= kg·m²/s²

• Power (功率) – unit: watt (W) = J/s = kg·m²/s³

  功率 – 单位：瓦特（W）= J/s = kg·m²/s³

• Pressure (压强) – unit: pascal (Pa) = N/m² = kg/(m·s²)

  压强 – 单位：帕斯卡（Pa）= N/m² = kg/(m·s²)

You do not need to recall these breakdowns under exam pressure, but understanding them helps you decide whether a formula produces a sensible unit.

在考试压力下你不需要记住这些分解式，但理解它们有助于你判断一个公式是否产生了合理的单位。

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

Scientific measurements often involve very large or very small numbers. Prefixes replace powers of ten, making values easier to read. For example, 0.001 m is written as 1 mm.

科学测量常常涉及非常大或非常小的数字。词头可以替代十的幂，使数值更容易阅读。例如，0.001 m写作1 mm。

Prefix	Symbol	Power of ten	词头	符号	十的幂
kilo-	k	10³	千	k	10³
centi-	c	10⁻²	厘	c	10⁻²
milli-	m	10⁻³	毫	m	10⁻³
micro-	μ	10⁻⁶	微	μ	10⁻⁶
nano-	n	10⁻⁹	纳	n	10⁻⁹

You should also know mega- (M, 10⁶) for quantities like megawatts, and giga- (G, 10⁹) for electrical energy in Joules.

你还应该知道兆（M, 10⁶），例如兆瓦；以及吉（G, 10⁹），用于以焦耳为单位的电能。

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5. Converting Units | 单位换算

Unit conversion is a skill you will use in almost every calculation. The safest method is to replace the prefix with its power of ten, then simplify.

单位换算几乎在每次计算中都会用到。最安全的方法是将词头替换为十的幂，然后化简。

For example, convert 776 g to kilograms. Since 1 g = 10⁻³ kg, we multiply:

例如，将776克换算为千克。因为1 g = 10⁻³ kg，所以乘以：

776 g = 776 × 10⁻³ kg = 0.776 kg

Similarly, convert 25 cm to metres: 25 cm = 25 × 10⁻² m = 0.25 m.

同样，将25厘米换算为米：25 cm = 25 × 10⁻² m = 0.25 m。

For squared and cubed units, remember that the conversion factor also squares or cubes. For example, 1 cm² = (10⁻² m)² = 10⁻⁴ m², and 1 cm³ = (10⁻² m)³ = 10⁻⁶ m³.

对于平方和立方单位，记住换算因子也需要平方或立方。例如，1 cm² = (10⁻² m)² = 10⁻⁴ m²，而1 cm³ = (10⁻² m)³ = 10⁻⁶ m³。

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6. Measuring Length and Volume | 测量长度与体积

Length is usually measured with a ruler, tape measure, or vernier calipers. For a ruler, the smallest division is often 1 mm, so you can estimate to the nearest 0.5 mm or even 0.1 mm depending on the instrument.

长度通常用直尺、卷尺或游标卡尺测量。直尺的最小刻度通常是1毫米，因此根据仪器你可以估读到最接近0.5毫米甚至0.1毫米。

Volume of a liquid is read from a measuring cylinder. To avoid parallax error, place your eye at the same level as the bottom of the meniscus. The meniscus of water curves down at the edges, so read the central lowest point.

液体体积从量筒中读取。为避免视差误差，视线应与弯月面底部在同一水平线上。水的弯月面在边缘处下弯，因此应读取中央最低点。

For irregular solids, use the displacement method: measure the volume of water in a measuring cylinder, then add the solid and record the new volume. The difference is the volume of the solid.

对于不规则固体，使用排水法：先量取量筒中水的体积，然后加入固体并记录新体积，差值即为固体的体积。

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7. Measuring Mass and Time | 测量质量与时间

Mass is measured with a balance. The most common in schools is a digital balance, which gives a direct reading to 0.1 g or 0.01 g. For best results, ensure the balance is zeroed before use, and place the object on the centre of the pan.

质量用天平测量。学校中最常见的是电子天平，直接读数可精确到0.1克或0.01克。为了获得最佳结果，使用前确保天平归零，并将物体放在秤盘中央。

Time is measured with a stopwatch or an electronic timer. Human reaction time when starting and stopping a stopwatch is typically around 0.2–0.5 s, so for faster reactions, use a light gate or data logger to improve accuracy.

时间用秒表或电子计时器测量。人为按下秒表开始和停止时的反应时间通常约为0.2–0.5秒，因此对于快速反应，使用光门或数据记录器以提高准确性。

When repeating experiments, always record the first measurement before reporting an average, and discard any obvious anomalous results.

重复实验时，一定要先记录原始测量值再计算平均值，并舍弃任何明显的异常结果。

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8. Temperature and Its Measurement | 温度及其测量

In IGCSE Science, temperature is often measured in degrees Celsius (°C), but the SI base unit is the kelvin (K). A temperature difference of 1 °C is equal to a difference of 1 K. This means you can use °C values in calculations that involve a change in temperature without converting to kelvin first.

在IGCSE科学中，温度通常以摄氏度（°C）测量，但国际单位制的基本单位是开尔文（K）。1°C的温度差等于1 K的温度差。这意味着在涉及温度变化的计算中，你可以直接使用°C值，无需先转换为开尔文。

To convert from Celsius to kelvin, add 273.15:

将摄氏度转换为开尔文，加上273.15：

T(K) = T(°C) + 273.15

A thermometer must be read with the bulb fully immersed in the liquid, and you should wait for the reading to stabilise. In chemistry experiments such as measuring the temperature change during a neutralisation reaction, the maximum or minimum reading is often what you need.

使用温度计时，液泡必须完全浸入液体中，并等待读数稳定。在化学实验中，例如测量中和反应过程中的温度变化，你需要记录最高或最低读数。

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9. Significant Figures and Decimal Places | 有效数字与小数位数

You must report final answers to the appropriate number of significant figures (s.f.). A rule of thumb is to use the same number of s.f. as the least precise value in the question. For example, if a measurement has 2 s.f., your answer should also have 2 s.f.

最终答案必须保留适当的有效数字位数。经验法则是与题目中精度最低的数值保持一致。例如，如果某个测量值有2位有效数字，你的答案也应有2位有效数字。

Remember: zeros inside a number count, but leading zeros do not. For example, 0.00456 has 3 s.f., 4.56 × 10⁻³ also has 3 s.f., and 4.560 has 4 s.f.

记住：数字中间的零算有效数字，但前导零不算。例如，0.00456有3位有效数字，4.56 × 10⁻³也有3位有效数字，而4.560有4位有效数字。

When rounding, look at the next digit. If it is 5 or greater, round up. If it is less than 5, round down.

四舍五入时，看下一位数字。如果大于或等于5，则进位；如果小于5，则舍去。

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10. Estimating and Reducing Uncertainty | 估算与减小不确定度

Every measurement has uncertainty. For a digital balance reading 776 g, the uncertainty is often ±0.1 g if it displays to one decimal place, or ±1 g if it displays to the nearest gram.

每个测量都存在不确定度。对于显示776 g的电子天平，如果显示到一位小数，不确定度通常为±0.1 g；如果显示到最接近的克，则为±1 g。

Repeating measurements and calculating the mean reduces the effect of random errors, but it does not remove systematic errors. A systematic error affects all readings equally, such as a thermometer that reads 2 °C too high.

重复测量并计算平均值可以减少随机误差的影响，但不能消除系统误差。系统误差对全部读数产生相同影响，例如温度计的读数总是偏高2°C。

To calculate the range of a set of readings, subtract the smallest value from the largest. Half the range is an estimate of the absolute uncertainty of a single reading.

计算一组读数的极差时，用最大值减去最小值。极差的一半是单次读数绝对不确定度的估计值。

When plotting graphs, error bars can be used to show uncertainty, but in IGCSE you are more often asked to draw a line of best fit through the plotted points.

在绘图时，误差线可以用来表示不确定度，但在IGCSE考试中，更常要求你穿过数据点画一条最佳拟合线。

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11. Presenting Data in Tables and Graphs | 用表格和图表呈现数据

Tables must have clear headings with units in the header row, for example “Time (s)” not just “Time”. All values should be recorded with the same precision, and units should not be repeated in every cell.

表格必须有清晰的表头，并在表头中注明单位，例如“时间（s）”，而不能只写“时间”。所有数值应以相同精度记录，且单位不应在每个单元格中重复。

When drawing a line graph, put the independent variable on the x-axis and the dependent variable on the y-axis. Choose a scale that uses at least half of the graph paper, label both axes with quantity and unit, and circle or mark the plotted points neatly.

画线形图时，将自变量放在x轴，因变量放在y轴。选择至少使用一半坐标纸的刻度，用“物理量/单位”标注两个坐标轴，并整齐地标记数据点。

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