📚 Year 7 CAIE Science: Key Formulas and Principles Quick Reference Handbook | 七年级CAIE科学：公式定理速查手册
This handbook brings together the essential formulas and scientific principles you need in Year 7 CAIE Science. It covers key equations from physics, chemistry and biology, helping you revise efficiently and solve problems with confidence. Each section presents a core idea, its calculation if applicable, and the units involved – all in plain language with Chinese translation for bilingual learners.
这本手册汇集了七年级CAIE科学课程中的核心公式和定理。内容涵盖物理、化学和生物的关键方程式，帮助你高效复习，轻松解题。每一小节以简明语言介绍一个核心思想及其计算方法，并配有中文释义，方便双语学习者理解和记忆。
1. Speed, Distance and Time | 速度、距离和时间
Speed tells us how fast an object is moving. It is a measure of the distance travelled in a certain amount of time. The relationship between speed, distance and time is given by the formula below.
速度告诉我们物体运动的快慢，是物体在一段时间内移动的距离的量度。速度、距离和时间的关系用以下公式表示。
Speed = Distance ÷ Time
In symbols, we often write v = d / t, where v stands for speed, d for distance and t for time. The standard unit of speed is metres per second (m/s), but you may also see kilometres per hour (km/h) or miles per hour (mph).
通常用符号表示为 v = d / t，其中 v 表示速度，d 表示距离，t 表示时间。速度的标准单位是米每秒（m/s），但也常见千米每小时（km/h）或英里每小时（mph）。
To find distance when you know speed and time, rearrange the formula: Distance = Speed × Time. Similarly, to find time taken: Time = Distance ÷ Speed. Always make sure your units match before calculating.
已知速度和时间求距离时，可变形为：距离 = 速度 × 时间。求时间则用：时间 = 距离 ÷ 速度。计算前务必确保单位统一。
2. Density | 密度
Density is a property that tells us how much mass is packed into a certain volume of a substance. A dense material, like lead, has more mass in the same volume compared to a less dense material, like cork.
密度是表示物质单位体积内所含质量的一种性质。密度大的材料（如铅）在相同体积下比密度小的材料（如软木）具有更大的质量。
Density (ρ) = Mass (m) ÷ Volume (V)
Here ρ (the Greek letter rho) stands for density, m for mass and V for volume. Density can be measured in grams per cubic centimetre (g/cm³) for solids and liquids, or kilograms per cubic metre (kg/m³) in SI units. 1 g/cm³ equals 1000 kg/m³.
式中 ρ（希腊字母 rho）表示密度，m 代表质量，V 代表体积。密度常用的单位有克每立方厘米 (g/cm³) 或国际单位制的千克每立方米 (kg/m³)。1 g/cm³ 等于 1000 kg/m³。
You can use this formula to identify substances or to predict whether an object will float. An object floats in a liquid if its density is less than the liquid’s density.
你可以用这个公式鉴别物质或判断物体是否浮起。当物体的密度小于液体的密度时，物体会浮在液面上。
3. Weight and Mass | 重量与质量
Many people use the words ‘weight’ and ‘mass’ as if they mean the same thing, but in science they are different. Mass is the amount of matter in an object and is measured in kilograms (kg). Weight is the force of gravity acting on that mass.
很多人把“重量”和“质量”混为一谈，但在科学中两者不同。质量是物体所含物质的多少，单位是千克 (kg)。重量是重力作用在该质量上的力。
Weight (W) = Mass (m) × Gravitational field strength (g)
On Earth, the gravitational field strength (g) is approximately 10 N/kg. So a 1 kg mass has a weight of 10 newtons (N). Weight is measured in newtons using a spring balance or a force meter, while mass is measured with a balance scale.
地球表面附近的重力场强度 (g) 约为 10 N/kg。因此，1 千克质量的物体重量为 10 牛顿 (N)。重量用弹簧秤或测力计测量，单位是牛顿；质量则用天平测量。
Remember: your mass stays the same wherever you go in the universe, but your weight would change on the Moon because the Moon’s gravity is weaker.
记住：无论你到宇宙何处，你的质量保持不变；但你的重量在月球上会改变，因为月球表面的引力场强度较小。
4. Pressure | 压强
Pressure tells us how concentrated a force is over an area. A large force spread over a small area gives high pressure – think of a sharp knife cutting easily. The same force over a large area gives low pressure, like snowshoes stopping you from sinking into the snow.
压强表示力在面积上的集中程度。较大的力作用在很小的面积上会产生高压强——想想锋利的刀子很容易切割。同样的力作用在较大的面积上则产生低压强，就像雪鞋防止你陷入雪中。
Pressure (P) = Force (F) ÷ Area (A)
Force is measured in newtons (N) and area in square metres (m²). The unit of pressure is the pascal (Pa), where 1 Pa = 1 N/m². High-pressure applications include knives, nails and heels; low-pressure applications include wide tyres and building foundations.
力的单位是牛顿 (N)，面积的单位是平方米 (m²)。压强的单位是帕斯卡 (Pa)，1 Pa = 1 N/m²。高压强常见的应用有刀具、钉子和高跟鞋；低压强的应用包括宽轮胎和建筑物地基。
When solving problems, make sure you convert area to m² if it is given in cm² or mm². For example, 1 cm² = 0.0001 m².
解题时，若面积以 cm² 或 mm² 给出，需要转换成 m²。例如 1 cm² = 0.0001 m²。
5. Work Done | 做功
In physics, ‘work’ has a precise meaning: work is done when a force moves an object in the direction of the force. If there is no movement, no work is done, even if you feel tired holding a heavy bag.
在物理学中，“功”有精确的含义：当力使物体沿力的方向移动时，力就做了功。如果没有移动，即使你提着重物感到疲劳，也没有做功。
Work Done (W) = Force (F) × Distance moved in the direction of the force (d)
Work is measured in joules (J). One joule is the work done when a force of 1 newton moves an object 1 metre in the direction of the force. So 1 J = 1 N m.
功的单位是焦耳 (J)。1 焦耳相当于 1 牛顿的力使物体沿力的方向移动 1 米所做的功。即 1 J = 1 N m。
Energy is transferred when work is done. The amount of work done equals the amount of energy transferred. This is the link between the chapters on forces and energy.
做功的过程伴随着能量的转移。做功的多少等于转移的能量多少。这便将“力”和“能量”两个章节联系起来。
6. Energy Efficiency | 能量效率
Whenever energy is transferred or transformed, some of it is always transferred to less useful forms, often as heat. Efficiency tells us what fraction of the total input energy is turned into useful output energy.
无论何时进行能量转移或转化，总有一部分能量转化为不太有用的形式（通常为热能）。效率告诉我们在总输入能量中，有多少比例转化为有用的输出能量。
Efficiency = (Useful energy output ÷ Total energy input) × 100%
Efficiency can also be calculated using power: Efficiency = (Useful power output ÷ Total power input) × 100%. Efficiency has no units; it is expressed as a percentage between 0% and 100% (or a decimal between 0 and 1). A perfect machine, which is impossible, would have an efficiency of 100%.
效率也可以使用功率计算：效率 = (有用输出功率 ÷ 总输入功率) × 100%。效率没有单位，用 0% 到 100% 的百分数表示（或 0 到 1 之间的小数）。一台（不可能存在的）完美机器的效率是 100%。
Real machines like car engines have efficiencies well below 50% because of friction and heat loss. LED light bulbs are much more efficient than filament bulbs.
真实机器（如汽车发动机）由于摩擦和热损失，效率远低于 50%。LED 灯泡比白炽灯泡效率高得多。
7. Conservation of Energy | 能量守恒定律
The law of conservation of energy is one of the most important ideas in science. It states that energy cannot be created or destroyed; it can only be transferred from one store to another or transformed from one form to another.
能量守恒定律是科学中最重要的概念之一。它指出：能量既不会凭空产生，也不会凭空消失；它只能从一个储存转移到另一个储存，或从一种形式转化为另一种形式。
This means the total energy in a closed system always remains constant. For example, when a swinging pendulum slows down, its kinetic energy is not lost but is gradually transferred to thermal energy in the air and the pendulum’s support because of friction.
这意味着在封闭系统中，总能量保持不变。例如，摆锤的速度减慢时，它的动能并没有消失，而是因摩擦逐渐转移到周围空气和支架中，变成了热能。
In energy transfer diagrams (Sankey diagrams), the width of the arrows represents the amount of energy. The total width going in equals the total width coming out, illustrating the conservation principle.
在能量转移图（桑基图）中，箭头的宽度表示能量的多少。输入的总宽度等于输出的总宽度，直观地体现了能量守恒原理。
8. Electric Current and Ohm’s Law | 电流与欧姆定律
Electric current is the rate of flow of electric charge. Charges move through a circuit when a voltage pushes them. The size of the current depends on the voltage and the resistance in the circuit.
电流是电荷流动的速率。当电压推动电荷时，电荷在电路中流动。电流的大小取决于电压和电路中的电阻。
Current (I) = Charge (Q) ÷ Time (t)
Current is measured in amperes (A), charge in coulombs (C) and time in seconds (s). This is often written as I = Q / t. In a metal wire, current is a flow of free electrons.
电流的单位是安培 (A)，电荷的单位是库仑 (C)，时间的单位是秒 (s)。常写为 I = Q / t。在金属导线中，电流是自由电子的定向移动。
Voltage (V) = Current (I) × Resistance (R)
This relationship is called Ohm’s Law. Voltage (potential difference) is measured in volts (V), current in amps (A) and resistance in ohms (Ω). For a fixed resistor, if you double the voltage, the current doubles as long as temperature remains constant.
这一关系称为欧姆定律。电压（电位差）的单位是伏特 (V)，电流单位是安培 (A)，电阻单位是欧姆 (Ω)。对一个固定电阻，在温度不变的条件下，电压加倍，电流也加倍。
9. Power | 功率
Power is the rate at which energy is transferred or work is done. A powerful machine does the same amount of work in a shorter time, or more work in the same time.
功率是能量转移或做功的快慢程度。一台大功率机器能在更短时间内完成等量的功，或在相同时间内做更多的功。
Power (P) = Energy transferred (E) ÷ Time (t)
Since work done and energy transferred are both measured in joules (J), power is measured in joules per second, which we call watts (W). 1 W = 1 J/s. A 60 W bulb transfers 60 J of electrical energy every second into light and heat.
做功和能量转移的单位都是焦耳 (J)，因此功率的单位是焦耳每秒，称为瓦特 (W)。1 W = 1 J/s。一只 60 W 的灯泡每秒钟把 60 J 的电能转化为光和热。
Power can also be calculated using current and voltage: P = I × V. This is useful when analysing electrical appliances.
功率也可以用电流和电压计算：P = I × V。这在分析电器时非常有用。
10. Law of Conservation of Mass | 质量守恒定律
In any chemical reaction, atoms are not created or destroyed; they are simply rearranged. This leads to the law of conservation of mass: the total mass of the products is always equal to the total mass of the reactants.
在任何化学反应中，原子既不会凭空产生也不会消失，只是进行了重新组合。这就引出了质量守恒定律：生成物的总质量总是等于反应物的总质量。
For example, when magnesium burns in oxygen to form magnesium oxide, if you could collect all the product (including any gases), its mass would equal the mass of the magnesium plus the oxygen that reacted. Experiments with open containers may appear to show a mass change because a gas enters or escapes.
例如，镁在氧气中燃烧生成氧化镁。如果你能收集全部生成物（包括任何气体），其质量将等于参加反应的镁和氧气的总质量。在敞口容器中的实验可能会显示质量改变，这是因为有气体进入或逸出。
This law is the foundation for balancing chemical equations. When you write an equation, you must have the same number of each type of atom on both sides.
这一定律是配平化学方程式的基础。写方程式时，等式两边每种原子的数量必须相同。
11. Cell Theory | 细胞学说
Cell theory is the cornerstone of biology. It describes the basic properties of all living things and consists of three main statements:
细胞学说是生物学的基石。它描述了所有生命的基本特性，包含三个主要论点：
1. All living organisms are composed of one or more cells.
2. The cell is the most basic unit of structure and function in living things.
3. All cells arise from pre-existing cells through cell division.
1. 所有生物体都是由一个或多个细胞组成的。
2. 细胞是生物体结构和功能的基本单位。
3. 所有细胞都通过细胞分裂由原有细胞产生。
This theory was developed over many years by scientists like Robert Hooke, who first observed cells in cork, and later by Schwann and Virchow. Today, we know there are two main types of cells: prokaryotic (no nucleus) and eukaryotic (with a nucleus).
这一理论经过多年发展，由罗伯特·胡克（首次在软木中观察到细胞）、施旺、魏尔肖等科学家共同建立。今天我们知道细胞主要分为两大类：原核细胞（无细胞核）和真核细胞（有细胞核）。
12. Binomial Nomenclature and Classification | 双名法与生物分类
To avoid confusion caused by common names, scientists use a universal system called binomial nomenclature to name every species. This system was introduced by Carl Linnaeus and uses Latin or latinised words.
为了避免俗名带来的混淆，科学家使用一种名为双名法的通用系统为每个物种命名。这个系统由卡尔·林奈提出，使用拉丁语或拉丁化词汇。
Scientific name = Genus + species
For example, the scientific name for humans is Homo sapiens. Homo is the genus (always written with a capital letter) and sapiens is the species (lowercase). Both are written in italics when typed, or underlined when handwritten.
例如，人类的学名是 Homo sapiens。Homo 是属名（首字母大写），sapiens 是种加词（小写）。印刷时应以斜体表示，手写时以下划线表示。
The full hierarchical classification groups organisms into: Kingdom, Phylum, Class, Order, Family, Genus and Species. A handy way to remember the order is: ‘King Philip Came Over For Good Soup’.
完整的生物分类阶元包括：界、门、纲、目、科、属、种。一个有趣的记忆口诀是“界门纲目科属种”。
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