📚 Year 8 CIE Science: Formula & Theorem Quick Reference | Year 8 CIE 科学：公式定理速查手册
This handbook brings together the essential formulas, laws, and word equations you will encounter in Year 8 CIE Science. From calculating speed and density to understanding Ohm's Law and photosynthesis, each entry is presented clearly with both the English explanation and the Chinese translation. Use it as a quick revision tool before tests or while completing homework. The content covers physics, chemistry, and biology topics aligned with the Cambridge Lower Secondary Stage 8 syllabus.
这本手册汇集了 Year 8 CIE 科学课程中最重要的公式、定律和文字方程式。从计算速度和密度，到理解欧姆定律和光合作用，每个条目都清晰地配有英文说明和中文翻译。你可以将此手册作为考试前的快速复习工具，或在完成作业时查阅。内容涵盖物理、化学和生物，与剑桥初中 Stage 8 课程大纲完全对应。
1. Speed Formula | 速度公式
Speed describes how fast an object is moving. It is a scalar quantity, which means it only has magnitude, not direction. The average speed of an object can be found by dividing the total distance travelled by the total time taken. The standard unit for speed is metres per second (m/s), but you may also see kilometres per hour (km/h). When using the formula, make sure the units of distance and time are consistent.
v = s / t
速度描述物体运动的快慢。它是一个标量，只有大小没有方向。物体的平均速度可以通过总路程除以总时间来计算。速度的标准单位是米每秒（m/s），但你也会看到千米每小时（km/h）。使用此公式时，请确保路程和时间的单位一致。
2. Density Formula | 密度公式
Density is a measure of how much mass is packed into a given volume. It is a characteristic property of a material and does not depend on the size of the sample. To calculate density, divide the mass of an object by its volume. Common units are grams per cubic centimetre (g/cm³) for solids and liquids, and kilograms per cubic metre (kg/m³) for gases. For example, the density of water is approximately 1 g/cm³ or 1000 kg/m³.
ρ = m / V
密度表示在一定体积内含有多少质量。它是材料的一种特性，不随样品大小改变。要计算密度，用物体的质量除以其体积。常用的单位有克每立方厘米（g/cm³）用于固体和液体，以及千克每立方米（kg/m³）用于气体。例如，水的密度大约为 1 g/cm³ 或 1000 kg/m³。
3. Pressure Formula | 压强公式
Pressure is the force applied per unit area. When a force acts over a small area, the pressure is high; when the same force is spread over a larger area, the pressure is lower. This explains why sharp knives cut more easily than blunt ones. Pressure is measured in pascals (Pa), where 1 Pa equals 1 newton per square metre (N/m²). Remember to use the force in newtons and the area in square metres.
P = F / A
压强是单位面积上所受的力。当力作用在较小面积上时，压强较大；同样的力分散在较大面积上时，压强较小。这解释了为什么锋利的刀刃比钝刀更容易切割。压强的单位是帕斯卡（Pa），1 Pa = 1 牛每平方米（N/m²）。记得要以牛顿为单位计算力，以平方米为单位计算面积。
4. Moment of a Force | 力矩公式
A moment is the turning effect of a force around a pivot. It depends on two factors: the size of the force and the perpendicular distance from the pivot to the line of action of the force. The moment increases if the force is larger or if the distance is greater. The unit of a moment is the newton metre (Nm). In equilibrium, the total clockwise moments equal the total anticlockwise moments – this is the principle of moments.
M = F × d
力矩是力绕支点产生的转动效果。它取决于两个因素：力的大小以及从支点到力作用线的垂直距离。力越大或距离越大，力矩就越大。力矩的单位是牛顿米（Nm）。在平衡状态下，顺时针力矩的总和等于逆时针力矩的总和——这就是力矩原理。
5. Work Done Formula | 做功公式
Work is done when a force moves an object in the direction of the force. Energy is transferred from one store to another. The amount of work done is calculated by multiplying the force by the distance moved along the line of the force. Work and energy are both measured in joules (J). If the force is in newtons and the distance in metres, the work done is in joules. For instance, lifting a book applies a force over a vertical distance, doing work against gravity.
W = F × d
当力使物体沿力的方向移动时，就做了功。能量从一个储存库转移到另一个储存库。做功的多少等于力乘以物体沿力方向移动的距离。功和能量都以焦耳（J）为单位。如果力以牛顿为单位，距离以米为单位，那么所做的功就以焦耳计算。例如，提起一本书需要施加一个力并通过竖直距离，这就为了克服重力而做了功。
6. Ohm's Law | 欧姆定律
Ohm' voltage, current, and resistance in an electrical circuit. For a component that obeys Ohm' the opposition to the flow of electric current. The formula can be rearranged to find any of the three quantities: V = I × R, I = V / R, or R = V / I. Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω).
V = I × R
欧姆定律描述了电路中电压、电流和电阻之间的关系。对于遵守欧姆定律的元件，当温度恒定时，流过它的电流与它两端的电压成正比。电阻是对电流流动的阻碍。该公式可以变形以求出三个量中的任何一个：V = I × R，I = V / R，或 R = V / I。电压的单位是伏特（V），电流的单位是安培（A），电阻的单位是欧姆（Ω）。
7. Law of Reflection | 反射定律
The Law of Reflection governs how light behaves when it bounces off a smooth surface. It states that the angle of incidence is always equal to the angle of reflection. These angles are measured between the rays and an imaginary line called the normal, which is perpendicular to the surface at the point of incidence. The incident ray, the reflected ray, and the normal all lie in the same plane. This law applies to all types of waves, including light and sound, when they reflect.
∠i = ∠r
反射定律描述了光在光滑表面反射时的行为。它指出入射角始终等于反射角。这些角是在光线与一条称为法线的假想线之间测量的，法线在入射点处垂直于表面。入射光线、反射光线和法线都位于同一平面内。该定律适用于所有类型的波反射，包括光波和声波。
8. Conservation of Mass | 质量守恒定律
The Law of Conservation of Mass is a fundamental principle in chemistry. It states that in a chemical reaction, the total mass of the reactants is always equal to the total mass of the products. No atoms are lost or gained; they are simply rearranged to form new substances. This is why chemical equations must be balanced. For example, when magnesium burns in oxygen, the mass of magnesium oxide produced equals the combined masses of magnesium and oxygen that reacted.
mreactants = mproducts
质量守恒定律是化学中的一项基本原理。它指出，在化学反应中，反应物的总质量总是等于生成物的总质量。原子没有损失或增加，它们只是重新排列形成新的物质。这就是化学方程式必须配平的原因。例如，当镁在氧气中燃烧时，生成的氧化镁的质量等于参加反应的镁和氧气的质量之和。
9. Photosynthesis Word Equation | 光合作用文字方程式
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. The raw materials are carbon dioxide (from the air) and water (from the soil). The process requires light energy, which is absorbed by the green pigment chlorophyll found in chloroplasts. Oxygen is released as a by-product. The word equation summarises the inputs and outputs of this vital reaction.
carbon dioxide + water → glucose + oxygen
(in the presence of light and chlorophyll)
光合作用是绿色植物、藻类和一些细菌将光能转化为储存在葡萄糖中的化学能的过程。原料是二氧化碳（来自空气）和水（来自土壤）。这个过程需要光能，光能被叶绿体中存在的绿色色素叶绿素吸收。氧气作为副产品释放出来。文字方程式概括了这一重要反应的反应物和生成物。（在光和叶绿体的作用下）
10. Aerobic Respiration Word Equation | 有氧呼吸文字方程式
Aerobic respiration is the chemical reaction that occurs in the cells of living organisms to release energy from glucose. It requires oxygen and produces carbon dioxide and water as waste products. The energy released is used for life processes such as growth, movement, and keeping warm. This process is essentially the reverse of photosynthesis in terms of the chemicals involved. The word equation is a simple way to represent this energy-releasing reaction.
glucose + oxygen → carbon dioxide + water + energy
有氧呼吸是发生在生物体细胞中，从葡萄糖中释放能量的化学反应。它需要氧气，并产生二氧化碳和水作为废物。释放的能量用于生长、运动和保持体温等生命活动。就涉及的化学物质而言，这一过程基本上是光合作用的逆过程。文字方程式是表示这种放能反应的简单方式。
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