📚 Year 9 Edexcel Science: Quick Reference Formula & Theorem Handbook | Edexcel 九年级科学：公式定理速查手册
This handbook is designed as a rapid revision tool for Year 9 students studying the Edexcel science curriculum. It brings together the essential formulas, equations, and key theorems from physics, chemistry, and biology. Use it to test yourself, practise calculations, and build the confidence needed for exams and practical work.
本手册是为学习 Edexcel 科学课程的九年级学生设计的快速复习工具。它汇集了物理、化学和生物学科中最基本的公式、方程式和重要定理。用来自我测试、练习计算，并建立应对考试和实验课所需的信心。
1. Speed, Distance, and Time | 速度、距离与时间
The basic speed formula links distance and time: speed (m/s) = distance (m) ÷ time (s), often written as v = d / t. It tells you how far an object travels in one second. You can rearrange this to find distance = speed × time, or time = distance ÷ speed.
基本速度公式将距离与时间联系起来：速度（米/秒）= 距离（米）÷ 时间（秒），通常写作 v = d / t。它表示物体每秒移动的距离。你可以变形为 距离 = 速度 × 时间，或 时间 = 距离 ÷ 速度。
When speed is not constant, the average speed is more useful: average speed = total distance ÷ total time. This smooths out any changes in pace and is the only speed you can calculate for a journey with stops or acceleration.
当速度不恒定时，平均速度更为有用：平均速度 = 总距离 ÷ 总时间。它抹平了速度的变化，是处理有停顿或加速的行程时唯一能计算的速度。
2. Acceleration | 加速度
Acceleration measures how quickly velocity changes: acceleration (m/s²) = (final velocity − initial velocity) ÷ time, or a = (v − u) / t. A positive value means speeding up; a negative value (deceleration) means slowing down.
加速度衡量速度变化的快慢：加速度（米/秒²）= (末速度 − 初速度) ÷ 时间，即 a = (v − u) / t。正值表示加速，负值（减速度）表示减速。
Always use compatible units: velocity in m/s, time in s. The result is in m/s². This equation assumes constant acceleration, which is often a simplification in real‑world motion.
务必使用一致的单位：速度用 m/s，时间用 s。结果单位为 m/s²。该方程假设加速度恒定，这在实际运动中常是一种简化。
3. Force, Mass, and Weight | 力、质量与重量
Newton’s Second Law is the foundation: force (N) = mass (kg) × acceleration (m/s²), or F = m a. It shows that a greater force is needed to accelerate a larger mass, or to produce a larger acceleration.
牛顿第二定律是基石：力（牛顿）= 质量（千克）× 加速度（米/秒²），即 F = m a。它表明质量越大或加速度越大，所需力就越大。
Weight is a specific force caused by gravity: weight (N) = mass (kg) × gravitational field strength (N/kg), or W = m g. On Earth, g ≈ 9.8 N/kg, but for many calculations 10 N/kg is used.
重量是由重力引起的特殊力：重量（牛）= 质量（千克）× 引力场强度（牛/千克），即 W = m g。地球上的 g 约为 9.8 N/kg，但在许多计算中使用 10 N/kg。
4. Work, Energy, and Power | 功、能量与功率
Work done transfers energy: work (J) = force (N) × distance moved in direction of force (m), W = F d. If the force and motion are not in the same line, only the parallel component counts.
做功转移能量：功（焦耳）= 力（牛）× 沿力方向移动的距离（米），W = F d。如果力与运动方向不一致，只有平行分量参与计算。
Kinetic energy (KE) depends on mass and speed: KE = ½ × m × v², where m is mass in kg and v is speed in m/s. A small increase in speed produces a much larger increase in KE because velocity is squared.
动能（KE）取决于质量和速度：KE = ½ × m × v²，其中 m 为质量（kg），v 为速度（m/s）。速度的微小增加会使动能大幅上升，因为速度被平方。
Gravitational potential energy (GPE) is stored when an object is raised: GPE = m × g × h, where h is the height change in metres. Raising the same object to twice the height stores twice the GPE.
重力势能（GPE）在物体升高时储存：GPE = m × g × h，其中 h 为高度变化（米）。将同一物体提升至两倍高度，储存的重力势能也加倍。
Power is the rate of energy transfer: power (W) = work done (J) ÷ time (s), or P = E / t. A more powerful device does the same work in less time, or more work in the same time.
功率是能量转移的速率：功率（瓦特）= 做功（焦）÷ 时间（秒），即 P = E / t。功率更大的设备，相同时间内能做更多功，或完成同样功用时更短。
5. Electricity: Ohm’s Law and Electrical Quantities | 电学：欧姆定律与电量
Ohm’s Law relates voltage, current, and resistance: voltage (V) = current (A) × resistance (Ω), or V = I R. For a fixed resistor, doubling the voltage doubles the current.
欧姆定律联系电压、电流和电阻：电压（伏）= 电流（安）× 电阻（欧姆），即 V = I R。对定值电阻，电压加倍则电流也加倍。
Electric charge transferred is given by: charge (C) = current (A) × time (s), Q = I t. This tells you how much charge flows through a point in a circuit during a certain period.
转移的电荷量公式：电荷（库仑）= 电流（安）× 时间（秒），Q = I t。它表明一段时间内流过电路中某点的电荷总量。
Energy transferred in a circuit can be found from: energy (J) = charge (C) × voltage (V), E = Q V. Combining with Q = I t gives E = I V t, and power P = I V.
电路中转移的能量可由下式计算：能量（焦）= 电荷（库）× 电压（伏），E = Q V。再结合 Q = I t 可得 E = I V t，并且 功率 P = I V。
6. Waves: Speed, Frequency, and Wavelength | 波：波速、频率与波长
The wave equation links how fast a wave travels to its frequency and wavelength: wave speed (m/s) = frequency (Hz) × wavelength (m), v = f λ. This works for sound, light, water waves, and all other wave types.
波动方程将波速与频率、波长关联：波速（米/秒）= 频率（赫兹）× 波长（米），v = f λ。该方程适用于声波、光波、水波等所有波的类型。
The period of a wave is the time for one complete cycle: period (s) = 1 ÷ frequency (Hz), T = 1 / f. High‑frequency waves have very short periods.
波的周期是完成一个完整循环所需的时间：周期（秒）= 1 ÷ 频率（赫兹），T = 1 / f。高频波的周期非常短。
7. Density | 密度
Density describes how much mass is packed into a volume: density (kg/m³) = mass (kg) ÷ volume (m³), ρ = m / V. Materials with tightly packed particles, like lead, have high density; gases have very low density.
密度描述单位体积内所含质量的多少：密度（千克/立方米）= 质量（千克）÷ 体积（立方米），ρ = m / V。像铅这样粒子排列紧密的材料密度高；气体的密度则很低。
You can also express density in g/cm³ for smaller objects. Water has a density of 1 g/cm³ (or 1000 kg/m³). An object floats if its density is less than the fluid’s density.
对于较小物体，密度也可用 g/cm³ 表示。水的密度为 1 g/cm³（或 1000 kg/m³）。物体密度小于所在液体密度时会浮起来。
8. Chemical Calculations: Moles and Relative Formula Mass | 化学计算：摩尔与相对式量
The relative formula mass (Mr) is the sum of relative atomic masses (Ar) of all atoms in a formula. For H₂O: (2×1) + 16 = 18. No units are used for Ar and Mr, but they allow conversion to moles.
相对式量（Mr）是化学式中所有原子相对原子质量（Ar）的总和。例如 H₂O： (2×1) + 16 = 18。Ar 和 Mr 没有单位，但借助它们可转换为摩尔。
The mole links chemical amounts to mass: number of moles = mass (g) ÷ molar mass (g/mol), n = m / M. One mole of any substance contains 6.02 × 10²³ particles (Avogadro’s constant).
摩尔建立了化学量与质量之间的联系：摩尔数 = 质量（克）÷ 摩尔质量（克/摩尔），n = m / M。任何物质的 1 摩尔均含有 6.02 × 10²³ 个粒子（阿伏加德罗常数）。
For solutions, concentration is given by: concentration (mol/dm³) = number of moles ÷ volume (dm³), c = n / V. A 1 mol/dm³ solution contains 1 mole of solute in every 1 dm³ of solution.
对于溶液，浓度的计算公式为：浓度（摩尔/立方分米）= 摩尔数 ÷ 体积（立方分米），c = n / V。1 mol/dm³ 的溶液指每 1 dm³ 溶液中含有 1 摩尔溶质。
9. Magnification in Biology | 生物学中的放大倍率
Microscopes allow us to see cells and tiny structures. The magnification formula is: magnification = image size ÷ actual size. Both measurements must be in the same unit, usually millimetres or micrometres.
显微镜让我们看到细胞和微小结构。放大倍率公式为：放大倍率 = 图像尺寸 ÷ 实际尺寸。两个测量值必须使用相同单位，通常为毫米或微米。
You can rearrange to find actual size = image size ÷ magnification. A scale bar on a diagram is often used to calculate the magnification directly.
你可以变形求出 实际尺寸 = 图像尺寸 ÷ 放大倍率。图中的比例尺常用来直接计算放大倍率。
10. Genetics: Punnett Squares and Mendel’s Laws | 遗传学：旁氏表与孟德尔定律
Mendel’s Law of Segregation states that each organism carries two alleles for a trait, and these separate during gamete formation. A dominant allele masks a recessive allele in a heterozygous individual.
孟德尔分离定律指出：每个生物体携带一对控制某一性状的等位基因，它们在配子形成时彼此分离。在杂合个体中，显性等位基因会掩盖隐性等位基因。
A Punnett square predicts the genotypes of offspring. For a cross between two heterozygous parents (Aa × Aa), the expected genotype ratio is 1 AA : 2 Aa : 1 aa, and the phenotype ratio is 3 dominant : 1 recessive.
旁氏表用于预测后代的基因型。两个杂合亲本杂交（Aa × Aa），预期的基因型比例为 1 AA : 2 Aa : 1 aa，表现型比例为 3 显性 : 1 隐性。
Mendel’s Law of Independent Assortment adds that genes for different traits are inherited independently of each other, leading to greater variation in dihybrid crosses (e.g., 9:3:3:1 ratio).
孟德尔自由组合定律补充指出：不同性状的基因相互独立地遗传，从而在双因子杂交中产生更大的变异（例如 9:3:3:1 比例）。
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