📚 Year 8 OCR Science Formula & Theorem Quick Reference Guide | 八年级OCR科学公式定理速查手册
Welcome to your Year 8 OCR Science Quick Reference Handbook! This guide compiles all the essential formulas and key theorems you will encounter across biology, chemistry and physics. Use it for revision, homework and exam preparation. Each formula is explained with clear definitions and units. Let’s dive in!
欢迎使用八年级OCR科学速查手册！本指南汇集了你将在生物、化学和物理课程中遇到的所有重要公式和核心定理。可用于复习、作业和备考。每个公式都配有清晰的定义和单位解释。让我们开始吧！
1. Speed and Acceleration | 速度与加速度
Speed is the distance travelled per unit of time. It tells you how fast an object is moving without regard to direction. The average speed can be calculated using the equation:
速度是单位时间内运动的距离。它表示物体运动快慢，不考虑方向。平均速度可用以下公式计算：
speed = distance ÷ time v = d / t
The standard unit of speed is metres per second (m/s), but we also use kilometres per hour (km/h) or miles per hour (mph) in everyday life. To convert m/s to km/h, multiply by 3.6.
速度的标准单位是米每秒(m/s)，但日常生活中也常用千米每小时(km/h)或英里每小时(mph)。将 m/s 转换为 km/h 需要乘以 3.6。
Acceleration measures how quickly speed changes. It is the rate of change of velocity. Acceleration can be positive (speeding up) or negative (slowing down, often called deceleration).
加速度衡量速度变化的快慢。它是速度的变化率。加速度可以是正值（加速），也可以是负值（减速，常称作负加速度）。
acceleration = (final velocity − initial velocity) ÷ time a = (v − u) / t
Units of acceleration are metres per second squared (m/s²). For example, if a car increases its speed from 10 m/s to 20 m/s in 5 seconds, its acceleration is (20 − 10) ÷ 5 = 2 m/s².
加速度的单位是米每二次方秒(m/s²)。例如，如果一辆车的速度在5秒内从10 m/s增加到20 m/s，其加速度为(20 − 10) ÷ 5 = 2 m/s²。
2. Force and Newton’s Laws | 力与牛顿定律
Force causes objects to move, change direction or change shape. Newton’s Second Law links force, mass and acceleration. When there is an unbalanced resultant force acting on an object, it accelerates in the direction of that force.
力使物体运动、改变方向或形状。牛顿第二定律将力、质量与加速度联系起来。当物体受到不平衡的合力时，它会沿该力的方向加速。
Force = mass × acceleration F = m a
Force is measured in newtons (N). Mass is in kilograms (kg) and acceleration in m/s². One newton is the force needed to accelerate a 1 kg mass by 1 m/s².
力的单位是牛顿(N)。质量单位为千克(kg)，加速度为m/s²。1牛顿就是使1 kg物体产生1 m/s²加速度所需的力。
Weight is a specific force caused by gravity acting on an object’s mass. Do not confuse mass and weight — mass is constant, weight depends on gravitational field strength, g.
重量是由重力作用在物体质量上而产生的一种特殊的力。切勿混淆质量与重量——质量是恒定的，重量取决于引力场强度 g。
Weight = mass × gravitational field strength W = m g
On Earth, g ≈ 10 N/kg (or more precisely 9.8 N/kg). So a 50 kg student has a weight of about 500 N. Weight is a vector; it always acts towards the centre of the Earth.
在地球上 g ≈ 10 N/kg（更精确为9.8 N/kg）。因此，一名50 kg的学生体重约为500 N。重量是矢量，方向总是指向地心。
3. Moments and Pressure | 力矩与压强
A moment is the turning effect of a force. It depends on the size of the force and how far it is applied from the pivot (the perpendicular distance). Moments can make objects rotate clockwise or anticlockwise.
力矩是力的转动效应。它取决于力的大小和力到支点的垂直距离。力矩可以使物体顺时针或逆时针转动。
moment = force × perpendicular distance from pivot M = F d
Moment is measured in newton-metres (N m). If the clockwise moments equal the anticlockwise moments, the object is in equilibrium and will not turn.
力矩的单位是牛·米(N m)。如果顺时针力矩等于逆时针力矩，物体就处于平衡状态，不会转动。
Pressure describes how concentrated a force is over an area. The same force can produce a small pressure if spread out, or a huge pressure if concentrated on a tiny area (like a sharp knife).
压强描述力在面积上的集中程度。同样的力分散时导致小压强，集中在极小面积（如锋利的刀）则产生巨大压强。
pressure = force ÷ area P = F / A
Pressure is measured in pascals (Pa), where 1 Pa = 1 N/m². Large pressures are given in kilopascals (kPa) or megapascals (MPa).
压强的单位是帕斯卡(Pa)，1 Pa = 1 N/m²。较大的压强常用千帕(kPa)或兆帕(MPa)表示。
4. Density | 密度
Density tells you how tightly packed the particles are in a material. Dense materials feel heavy for their size, whereas less dense materials feel light. Density does not depend on the amount of substance — it is a property of the material itself.
密度表示物质中粒子排列的紧密程度。密度大的材料感觉重，密度小的材料感觉轻。密度与物质的多少无关——它是材料本身的属性。
density = mass ÷ volume ρ = m / V
The Greek letter ρ (rho) is the symbol for density. Units are kilograms per cubic metre (kg/m³) or grams per cubic centimetre (g/cm³). Remember that 1 g/cm³ = 1000 kg/m³. For example, water has a density of 1.0 g/cm³, which equals 1000 kg/m³.
希腊字母ρ表示密度。单位是千克每立方米(kg/m³)或克每立方厘米(g/cm³)。记住1 g/cm³ = 1000 kg/m³。例如，水的密度为1.0 g/cm³，即1000 kg/m³。
To find the density of an irregular solid, measure its mass on a balance and its volume by water displacement in a measuring cylinder. Then divide mass by volume.
测量不规则固体的密度时，用天平测质量，用量筒排水法测体积，然后用质量除以体积。
5. Work, Energy and Power | 功、能与功率
In physics, work is done when a force moves an object. Energy is transferred mechanically when work is done. If you push a box and it moves, you have done work on the box.
在物理学中，当力使物体移动时我们就说做了功。做功伴随着能量的机械转移。如果你推箱子并使它移动，你就对箱子做了功。
work done = force × distance moved in the direction of the force W = F d
Work is measured in joules (J). 1 joule is 1 newton-metre. For example, lifting a book weighing 10 N through 1 m does 10 J of work.
功的单位是焦耳(J)。1焦耳等于1牛·米。例如，将一本重10 N的书举高1 m，做功10 J。
Energy comes in many forms. Two important mechanical energy stores are gravitational potential energy (GPE) and kinetic energy (KE).
能量有多种形式。重要的力学能量存储形式是重力势能(GPE)和动能(KE)。
GPE = mass × g × height GPE = m g h
Kinetic energy = ½ × mass × speed² KE = ½ m v²
GPE is measured in joules (J). The higher an object is, the more GPE it stores. Kinetic energy depends on both mass and speed — doubling the speed quadruples the kinetic energy.
GPE的单位是焦耳(J)。物体位置越高，储存的GPE越多。动能取决于质量和速度——速度增大为原来的2倍，动能增大为原来的4倍。
Power is the rate of doing work or transferring energy. The more powerful an appliance, the faster it can transfer energy.
功率是做功或能量传递的速率。设备功率越大，能量传递得越快。
power = work done ÷ time P = W / t
Also power = energy transferred ÷ time P = E / t
Power is measured in watts (W), where 1 W = 1 J/s. A 60 W light bulb transfers 60 joules of electrical energy into light and heat every second.
功率的单位是瓦特(W)，1 W = 1 J/s。一个60 W的灯泡每秒将60焦耳的电能转化为光能和热能。
6. Electrical Circuits: Ohm’s Law | 电路：欧姆定律
Electric current is the flow of charge. Voltage (potential difference) is the ‘push’ that drives the current around a circuit. Resistance opposes the flow of current. These three quantities are linked by Ohm’s Law.
电流是电荷的流动。电压（电势差）是驱动电流在电路中流动的“推力”。电阻阻碍电流的流动。这三个物理量由欧姆定律联系起来。
voltage = current × resistance V = I R
Voltage is measured in volts (V), current in amperes (A) and resistance in ohms (symbol Ω). Ohm’s Law holds for ohmic conductors at constant temperature. If you double the voltage across a resistor, the current also doubles, provided temperature remains constant.
电压单位是伏特(V)，电流单位是安培(A)，电阻单位是欧姆(符号Ω)。在温度恒定的条件下，欧姆定律适用于欧姆导体。如果你将电阻两端的电压加倍，电流也会加倍，前提是温度保持不变。
Components that do not follow Ohm’s Law include diodes and filament lamps (whose resistance changes with temperature).
不遵循欧姆定律的元件包括二极管和白炽灯（其电阻随温度变化）。
7. Electrical Energy and Power | 电能量与电功率
When charge flows through a component, energy is transferred. The rate of this energy transfer is the electrical power. In a circuit, power can be calculated directly from current and voltage.
电荷流经元件时会发生能量转移。这种能量转移的速率就是电功率。在电路中，功率可直接用电流和电压计算。
power = current × voltage P = I V
This equation works for any circuit component. Combined with Ohm’s Law, we get two alternative forms for resistors:
此公式适用于任何电路元件。结合欧姆定律，可以得到两个用于电阻的变形式：
P = I² R and P = V² / R
Energy transferred electrically can be found by multiplying power by time.
电能转移量可以通过功率乘以时间求得。
energy transferred = power × time E = P t
Energy is in joules (J), power in watts (W) and time in seconds (s). For larger amounts of energy, we often use kilowatt-hours (kWh), where 1 kWh = 3.6 million joules.
能量单位是焦耳(J)，功率是瓦特(W)，时间用秒(s)。对于较大的能量值，常用千瓦·时(kWh)，1 kWh = 3600000 J。
8. Chemical Formulae and Relative Atomic Mass | 化学式与相对原子质量
Every element is represented by a chemical symbol, e.g., C for carbon, O for oxygen. When atoms combine, we write chemical formulae to show the type and number of atoms. Subscripts indicate the number of atoms of each element.
每种元素都用一个化学符号表示，如C代表碳，O代表氧。原子结合时，我们用化学式来表示原子的种类和数目。下标数字表示各元素的原子数。
For example, water H₂O contains two hydrogen atoms and one oxygen atom; carbon dioxide CO₂ contains one carbon atom and two oxygen atoms.
例如，水H₂O含有2个氢原子和1个氧原子；二氧化碳CO₂含有1个碳原子和2个氧原子。
Relative atomic mass (Ar) compares the average mass of one atom of an element to 1/12 the mass of a carbon-12 atom. It has no units. For compounds, we use relative formula mass (Mr) — the sum of all Ar values in the formula.
相对原子质量(Ar)是将一种元素一个原子的平均质量与碳-12原子质量的1/12进行比较。它没有单位。对于化合物，我们使用相对分子质量(Mr)，即化学式中所有Ar的总和。
Mr(H₂O) = 2 × Ar(H) + Ar(O) = 2 × 1 + 16 = 18
Similarly, Mr of CO₂ = 12 + 2×16 = 44. These values are used to calculate reacting masses.
同样，CO₂的Mr = 12 + 2×16 = 44。这些数值可用于计算反应中的质量关系。
9. Conservation of Mass and Balancing Equations | 质量守恒与方程配平
In a chemical reaction, atoms are rearranged, but they are never created or destroyed. Therefore the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass.
在化学反应中，原子重新排列，但既不会创生也不会消失。因此生成物的总质量等于反应物的总质量。这就是质量守恒定律。
To represent reactions, we write symbol equations. They must be balanced so the number of each type of atom is the same on both sides. Balancing involves adding coefficients before the formulae.
为了表示反应，我们写出符号方程式。方程式必须配平，使每种原子的数目在两边相等。配平时需要在前加系数。
Example: hydrogen burns in oxygen to form water. The word equation is: hydrogen + oxygen → water. The unbalanced symbol equation is: H₂ + O₂ → H₂O. To balance, put a coefficient 2 in front of H₂O and then 2 in front of H₂: 2H₂ + O₂ → 2H₂O.
例如：氢气在氧气中燃烧生成水。文字方程式为：氢气 + 氧气 → 水。未配平的符号方程式为：H₂ + O₂ → H₂O。配平时，在H₂O前加系数2，然后在H₂前加系数2：2H₂ + O₂ → 2H₂O。
State symbols (s, l, g, aq) are often added to show the physical state: (s) solid, (l) liquid, (g) gas and (aq) aqueous solution. E.g., 2H₂(g) + O₂(g) → 2H₂O(l).
通常要添加状态符号来表示物态：(s)固态，(l)液态，(g)气态，(aq)水溶液。例如2H₂(g) + O₂(g) → 2H₂O(l)。
10. Periodic Table Patterns | 元素周期表规律
The modern periodic table arranges elements by increasing atomic
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