📚 Formula & Theorem Quick Reference | 公式定理速查手册
Welcome to your Year 8 Cambridge Science quick reference guide. This article brings together the most important formulas, principles, and theorems you will encounter in physics, chemistry, and biology. Use it to check your knowledge, reinforce key ideas, and prepare confidently for assessments.
欢迎使用 Year 8 剑桥科学公式定理速查手册。这篇文章汇集了物理、化学和生物中最重要的公式、原理和定理。你可以用它查漏补缺、巩固重点，并为各种测评做好充分准备。
1. Speed and Acceleration | 速度与加速度
Speed describes how fast an object moves. It is calculated by dividing the distance travelled by the time taken. The standard unit is metres per second (m/s).
速度描述物体运动的快慢。它等于物体移动的距离除以所用的时间，标准单位是米每秒（m/s）。
Average Speed = Distance ÷ Time
平均速度 = 距离 ÷ 时间
When an object's speed changes, we talk about acceleration. Acceleration is the rate at which speed increases. It can be positive (speeding up) or negative (slowing down, sometimes called deceleration).
当物体的速度发生变化时，我们就引入了加速度。加速度是速度增加的快慢程度，可以是正（加速）或负（减速，有时叫减速度）。
Acceleration = Change in Speed ÷ Time Taken
加速度 = 速度变化量 ÷ 所用时间
On a distance–time graph, the gradient represents speed. On a speed–time graph, the gradient gives the acceleration and the area under the graph gives the distance travelled.
在距离-时间图像中，斜率代表速度。在速度-时间图像中，斜率给出加速度，图像下方的面积表示移动的距离。
2. Forces and Resultant Forces | 力与合力
A force is a push or a pull. Force is measured in newtons (N). When several forces act on an object, they can be combined into a single resultant force.
力是推或拉的作用，单位是牛顿（N）。当多个力作用在物体上时，它们可以合成为一个合力。
If forces are balanced, the resultant force is zero and the object remains at rest or moves at a constant speed in a straight line. If forces are unbalanced, the object accelerates in the direction of the resultant force.
如果力平衡，合力为零，物体将保持静止或匀速直线运动。如果力不平衡，物体将沿合力方向加速运动。
Friction is a force that opposes motion. Air resistance and water resistance are types of friction. These forces can be reduced by streamlining.
摩擦力是一种阻碍运动的力。空气阻力和水阻力是摩擦力的两种类型，通过流线型设计可以减小它们。
3. Mass, Weight and Gravity | 质量、重量与重力
Mass is the amount of matter in an object and is measured in kilograms (kg). It does not change with location. Weight is the force of gravity acting on a mass and is measured in newtons (N).
质量是物体所含物质的多少，单位是千克（kg），不随位置改变。重量是重力对质量的作用力，单位是牛顿（N）。
Weight = Mass × Gravitational Field Strength
重量 = 质量 × 重力场强度
On Earth, the gravitational field strength is approximately 10 N/kg. This means every 1 kg of mass experiences a weight of about 10 N. On the Moon, gravity is much weaker (about 1.6 N/kg), so your weight would be less, but your mass stays the same.
地球表面的重力场强度约为 10 N/kg，即每 1 kg 质量受到大约 10 N 的重力。月球上的重力场强度小得多（约 1.6 N/kg），所以你的重量会变小，但质量保持不变。
4. Pressure in Solids and Liquids | 固体与液体中的压强
Pressure is a measure of how concentrated a force is. It is calculated by dividing the force by the area over which it acts.
压强衡量力的集中程度。它等于作用力除以受力面积。
Pressure = Force ÷ Area
压强 = 力 ÷ 面积
uces pressure (e.g. snowshoes), while a smaller area increases pressure (e.g. a sharp knife blade).
压强单位是帕斯卡（Pa），1 Pa = 1 N/m²。面积越大压强越小（如雪地鞋），面积越小压强越大（如锋利的刀刃）。
In liquids, pressure increases with depth and acts equally in all directions at a given depth. This explains why dams are thicker at the bottom and why submarines have depth limits.
在液体中，压强随深度增加而增大，并且在相同深度处朝各个方向大小相等。这就解释了为什么大坝底部更厚，以及潜艇有下潜深度限制。
5. Density and the Particle Model | 密度与粒子模型
Density tells us how much mass is packed into a given volume. It is a property that helps identify substances.
密度表示单位体积内所含的质量，是帮助识别物质的一种属性。
Density = Mass ÷ Volume
密度 = 质量 ÷ 体积
Density is often given in g/cm³ or kg/m³. Solids usually have higher densities than liquids and gases because their particles are packed more closely together. According to the particle model, the arrangement and movement of particles determine the state of matter.
密度常用单位是 g/cm³ 或 kg/m³。固体的密度通常比液体和气体大，因为它们的粒子排列更紧密。根据粒子模型，粒子的排列和运动方式决定了物质的状态。
6. Energy Transfers and Conservation | 能量转移与守恒
Energy cannot be created or destroyed; it can only be transferred from one form to another. This is the principle of conservation of energy.
能量既不会创生也不会消灭，只能从一种形式转化为另一种形式，这就是能量守恒定律。
Common energy stores include kinetic, thermal, chemical, gravitational potential, elastic potential, and nuclear. Energy is transferred by heating, by doing work (mechanical), and by waves (light and sound).
常见的能量储存包括动能、热能、化学能、重力势能、弹性势能和核能。能量可以通过加热、做功（机械做功）和波（光和声）进行转移。
In any energy transfer, some energy is almost always dissipated (spread out) as thermal energy, making the process less than 100% efficient. Efficiency can be calculated as useful output energy divided by total input energy.
在任何能量转移过程中，总有一部分能量会以热能的形式耗散，使得过程效率低于 100%。效率可以用有用输出能量除以总输入能量来计算。
7. Atoms, Elements and the Periodic Table | 原子、元素与周期表
All substances are made of atoms. An element contains only one type of atom. The Periodic Table lists all known elements, arranged in order of increasing atomic number (the number of protons).
所有物质都由原子构成。元素只含有一种原子。周期表按原子序数（质子数）递增的顺序列出了所有已知元素。
Atoms have a tiny central nucleus containing protons (positive) and neutrons (no charge), surrounded by electrons (negative) in shells. In a neutral atom, the number of electrons equals the number of protons.
原子有一个微小的原子核，里面包含质子（带正电）和中子（不带电），核外是分层排布的电子（带负电）。在中性原子中，电子数等于质子数。
Groups are vertical columns in the table; elements in the same group have similar chemical properties because they have the same number of outer electrons. Periods are horizontal rows, showing a repeating pattern of properties.
族是周期表中的纵列，同一族的元素化学性质相似，因为它们最外层电子数相同。周期是横排，显示出性质的重现规律。
8. Chemical Reactions and Conservation of Mass | 化学反应与质量守恒
During a chemical reaction, atoms are rearranged to form new substances. No atoms are created or destroyed, so the total mass remains unchanged. This is the law of conservation of mass.
在化学反应中，原子重新组合生成新物质。原子既不会创生也不会消灭，所以总质量保持不变，这就是质量守恒定律。
ium oxide. Symbol equations use chemical symbols and should be balanced with coefficients.
文字方程式使用反应物和生成物的名称。例如：镁 + 氧气 → 氧化镁。符号方程式使用化学符号，并应配上系数使其配平。
ion (breaking down a compound with heat), and neutralisation (acid + base → salt + water).
常见的反应类型有：燃烧（在氧气中点燃，放出热和光）、热分解（加热使化合物分解）和中和反应（酸 + 碱 → 盐 + 水）。
9. Acids, Alkalis and pH Scale | 酸、碱和pH标度
Acids are substances with a pH less than 7; they release hydrogen ions (H⁺) in water. Alkalis are soluble bases with a pH greater than 7; they release hydroxide ions (OH⁻) in water. A pH of 7 is neutral.
酸是 pH 小于 7 的物质，它们在水溶液中释放氢离子（H⁺）。碱是可溶性的碱，pH 大于 7，在水溶液中释放氢氧根离子（OH⁻）。pH 等于 7 时为中性。
Indicators change colour depending on pH. Universal indicator gives a full colour spectrum from red (strong acid) to violet (strong alkali). Litmus paper turns red in acid and blue in alkali.
指示剂会随 pH 改变颜色。通用指示剂能显示出红（强酸）到紫（强碱）的全色谱。石蕊试纸遇酸变红，遇碱变蓝。
li and the acid used (e.g. hydrochloric acid + sodium hydroxide → sodium chloride + water).
酸与碱反应生成盐和水，这就是中和反应。盐的名称来自碱中的金属和所用的酸（例如：盐酸 + 氢氧化钠 → 氯化钠 + 水）。
10. Electricity and Ohm's Law | 电学与欧姆定律
Electric current is the flow of electric charge. It is measured in amperes (A). Voltage (potential difference) is the energy transferred per unit charge and is measured in volts (V). Resistance, measured in ohms (Ω), measures how difficult it is for current to flow.
电流是电荷的定向流动，单位是安培（A）。电压（电位差）是每单位电荷转移的能量，单位是伏特（V）。电阻衡量电流通过的难易程度，单位是欧姆（Ω）。
Voltage = Current × Resistance
电压 = 电流 × 电阻
This relationship, known as Ohm's Law, holds true for many conductors at constant temperature. In a series circuit, the current is the same everywhere, and the total resistance is the sum of individual resistances. In a parallel circuit, the voltage across each branch is the same, and the total current is the sum of branch currents.
这个关系称为欧姆定律，对温度不变的很多导体都成立。在串联电路中，各处电流相等，总电阻等于各电阻之和。在并联电路中，各支路两端电压相同，总电流等于各支路电流之和。
11. Speed of Waves | 波的传播速度
A wave transfers energy without transferring matter. The wave speed depends on the medium. For water waves on a ripple tank or sound waves in air, the speed can be calculated if the frequency and wavelength are known.
波传递能量而不传递物质。波速依赖于介质。无论是水槽中的水波还是空气中的声波，只要知道频率和波长，就可以计算波速。
Wave Speed = Frequency × Wavelength
波速 = 频率 × 波长
Frequency, measured in hertz (Hz), is the number of complete waves passing a point per second. Wavelength is the distance between two corresponding points on a wave, for example crest to crest. In a transverse wave, the oscillations are perpendicular to the direction of energy transfer; in a longitudinal wave, they are parallel.
频率单位是赫兹（Hz），表示每秒通过某点的完整波数。波长是波上两个对应点（如波峰到波峰）之间的距离。在横波中，振动方向与能量传递方向垂直；在纵波中，振动方向与之平行。
12. Photosynthesis and Respiration | 光合作用与呼吸作用
Photosynthesis is the process by which green plants make their own food using light energy. It takes place in chloroplasts and requires carbon dioxide, water, and light. Glucose and oxygen are produced.
光合作用是绿色植物利用光能制造自身养分的过程。它在叶绿体中进行，需要二氧化碳、水和光，生成葡萄糖和氧气。
Carbon Dioxide + Water → Glucose + Oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
Aerobic respiration is the process cells use to release energy from glucose in the presence of oxygen. It occurs in mitochondria and is summarised as:
有氧呼吸是细胞在有氧条件下从葡萄糖中释放能量的过程。它发生在线粒体中，概括起来就是：
Glucose + Oxygen → Carbon Dioxide + Water (+ Energy)
葡萄糖 + 氧气 → 二氧化碳 + 水（+ 能量）
These two processes are complementary – the products of photosynthesis become the reactants of respiration, and vice versa, forming a cycle of energy and matter in ecosystems.
这两个过程互为补充——光合作用的产物变成了呼吸作用的反应物，反过来也是如此，从而在生态系统中形成了能量与物质的循环。
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