📚 Essential Physics for Cambridge IGCSE: Core Concepts Explained | 剑桥IGCSE物理核心概念解析
Welcome to this in-depth breakdown of the essential physics concepts for Cambridge IGCSE Physics (0625). This article covers the core topics every student must master, from kinematics and forces to electricity and radioactivity. Each section pairs clear English explanations with their Chinese equivalents, helping you build a bilingual understanding and solid exam readiness.
欢迎阅读这篇针对剑桥IGCSE物理（0625）核心概念的深度解析。本文涵盖了每一位学生必须掌握的关键主题，从运动学和力到电学和放射性。每个部分都将清晰的英文解释与对应的中文说明配对呈现，助你建立双语理解，扎实备考。
1. Measurement and Motion | 测量与运动
Physical quantities are categorised as scalars or vectors. Scalars possess magnitude only, examples being speed, distance, mass and time. Vectors have both magnitude and direction, such as velocity, displacement, force and acceleration. Distinguishing between these is fundamental to correctly setting up equations and interpreting graphs.
物理量分为标量和矢量。标量仅有大小，例如速率、路程、质量和时间。矢量既有大小又有方向，如速度、位移、力和加速度。区分这两者是正确建立方程和解读图像的基础。
Speed is the rate of change of distance, calculated as total distance divided by total time. Average speed = total distance / total time. The SI unit is metres per second (m/s). Velocity, on the other hand, specifies speed in a given direction.
速率是距离变化的快慢，等于总路程除以总时间。平均速率 = 总路程 / 总时间，国际单位是米/秒（m/s）。而速度则是给定方向上的速率。
v = s / t
Acceleration defines the rate of change of velocity. It is a vector and can be positive (speeding up) or negative (slowing down). Uniform acceleration gives rise to the classic equations of motion.
加速度描述了速度变化的快慢。它是一个矢量，可为正（加速）或负（减速）。匀加速运动下，可以导出经典的运动学方程。
a = (v – u) / t
v² = u² + 2 a s
Distance–time graphs provide a visual way to analyse motion. A straight sloping line indicates constant speed; a horizontal line means the object is at rest. The gradient of a speed–time graph yields acceleration, while the area under the graph gives the distance travelled.
距离–时间图可直观展现运动状态。一条倾斜的直线表示匀速运动；水平线表示物体静止。速度–时间图的斜率表示加速度，而图线下的面积代表所通过的距离。
2. Forces and Momentum | 力与动量
Forces can change the shape, speed or direction of an object. Newton’s first law states that an object remains at rest or in uniform motion unless acted upon by a resultant force. Newton’s second law quantifies this as F = ma, where force is measured in newtons (N).
力可以改变物体的形状、速度或方向。牛顿第一定律指出：除非受到合外力作用，物体将保持静止或匀速直线运动状态。牛顿第二定律将其量化为F = ma，力的单位是牛顿（N）。
F = m a
Weight is the gravitational force acting on a mass: W = mg, with g being the gravitational field strength (≈ 10 N/kg on Earth). Mass remains constant everywhere, whereas weight varies with location.
重量是作用在物体上的引力：W = mg，其中g为引力场强度（地球上约为10 N/kg）。质量处处不变，而重量随位置变化。
Momentum p is defined as the product of mass and velocity: p = mv. It is a vector. The principle of conservation of momentum states that, in the absence of external forces, the total momentum before a collision equals the total momentum after. Impulse equals change in momentum, and is given by force × time.
动量p定义为质量与速度的乘积：p = mv，它是一个矢量。动量守恒定律指出，在没有外力作用时，碰撞前的总动量等于碰撞后的总动量。冲量等于动量的变化，也等于力 × 时间。
p = m v
Impulse = F t = Δp
3. Energy, Work and Power | 能量、功和功率
Energy exists in various forms – kinetic, potential (gravitational and elastic), thermal, chemical, nuclear, and more. The law of conservation of energy states that energy can be transformed from one form to another, but cannot be created or destroyed.
能量以多种形式存在——动能、势能（重力势能和弹性势能）、热能、化学能、核能等。能量守恒定律指出，能量可以从一种形式转化为另一种形式，但不能被创造或消灭。
Work is done when a force moves an object in the direction of the force. Work = force × distance moved in the direction of the force: W = F d. The unit is the joule (J). Power is the rate of doing work: P = W / t, measured in watts (W).
当力使物体沿力的方向移动时，力做功。功 = 力 × 沿力方向移动的距离：W = F d，单位是焦耳（J）。功率是做功的快慢：P = W / t，单位为瓦特（W）。
W = F d
P = W / t
Kinetic energy is given by Eₖ = ½ m v², and gravitational potential energy by Eₚ = m g h. Efficiency is the ratio of useful output energy (or power) to total input energy, often expressed as a percentage.
动能公式为Eₖ = ½ m v²，重力势能为Eₚ = m g h。效率是有用输出能量（或功率）与总输入能量之比，通常以百分数表示。
Eₖ = ½ m v²
Eₚ = m g h
Efficiency = (useful output / total input) × 100%
4. Pressure and Density | 压强与密度
Density ρ (rho) measures how much mass is packed into a given volume. It is calculated as mass per unit volume: ρ = m / V. Solids and liquids usually have high density compared to gases because their particles are more tightly packed.
密度ρ衡量单位体积内所含质量的多少，计算式为ρ = m / V。由于固体和液体的粒子排列更紧密，其密度通常比气体大。
ρ = m / V
Pressure P is the force acting perpendicularly per unit area: P = F / A. In fluids, pressure increases with depth because of the weight of the fluid above: P = ρ g h. Atmospheric pressure at sea level is about 101 kPa and is often measured with a barometer or manometer.
压强P是垂直作用在单位面积上的力：P = F / A。在流体中，压强随深度增加，因为上方液体的重量：P = ρ g h。海平面的大气压强约为101 kPa，常用气压计或压强计测量。
P = F / A
Liquid pressure: P = ρ g h
5. Thermal Physics | 热物理学
The kinetic particle model explains the three states of matter: solids have fixed shape and volume with particles vibrating in fixed positions; liquids have fixed volume but take the shape of their container; gases have neither fixed shape nor volume and particles move rapidly in random directions.
分子动理论模型解释了物质的三态：固体有固定的形状和体积，粒子在固定位置振动；液体有固定体积但随容器改变形状；气体既无固定形状也无固定体积，粒子快速向各个方向随机运动。
Temperature reflects the average kinetic energy of particles. Two common temperature scales are Celsius (°C) and Kelvin (K); 0 K is absolute zero. Heat can be transferred by conduction, convection and radiation. Conduction occurs mainly in solids, convection in fluids, and radiation does not require a medium.
温度反映了粒子的平均动能。两种常用的温标是摄氏度（°C）和开尔文（K）；0 K为绝对零度。热量可以通过传导、对流和辐射传递。传导主要发生在固体中，对流在流体中，而辐射不需要介质。
Specific heat capacity c is the energy required to raise 1 kg of a substance by 1 °C. The formula is Q = m c Δθ. Latent heat L is the energy needed to change the state of 1 kg of a substance without temperature change: Q = m L.
比热容c是1 kg物质温度升高1 °C所需的能量，公式为Q = m c Δθ。潜热L是1 kg物质在温度不变时发生状态变化所需的能量：Q = m L。
Q = m c Δθ
Q = m L
6. Waves | 波
Waves transfer energy without transferring matter. Transverse waves (e.g. light, water ripples) have vibrations perpendicular to the direction of energy transfer, while longitudinal waves (e.g. sound) have vibrations parallel. Key wave properties include amplitude, wavelength λ, frequency f and period T.
波传递能量但不传递物质。横波（如光、水波）的振动方向与能量传播方向垂直，而纵波（如声音）的振动方向平行。波的主要特性包括振幅、波长λ、频率f和周期T。
The wave equation links speed v, frequency f and wavelength: v = f λ. The period is the reciprocal of frequency: T = 1/f. Reflection, refraction and diffraction are wave behaviours observed in both light and sound.
波速公式建立了波速v、频率f和波长λ的关系：v = f λ。周期是频率的倒数：T = 1/f。光与声均会表现出反射、折射和衍射等波动行为。
v = f λ
The electromagnetic spectrum, in order of increasing wavelength (or decreasing frequency), includes gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves. All travel at the speed of light in a vacuum (3.0 × 10⁸ m/s).
按波长递增（或频率递减）排序，电磁波谱包含γ射线、X射线、紫外线、可见光、红外线、微波和无线电波。所有电磁波在真空中均以光速（3.0 × 10⁸ m/s）传播。
7. Electricity | 电学
Electric current I is the rate of flow of charge: I = Q / t, measured in amperes (A). Potential difference V (voltage) is the work done per unit charge: V = W / Q. Resistance R opposes the flow of current and is defined by R = V / I (Ohm’s law for ohmic conductors at constant temperature).
电流I是电荷流动的快慢：I = Q / t，单位为安培（A）。电压V是每单位电荷所做的功：V = W / Q。电阻R阻碍电流流动，由R = V / I定义（在恒温下，欧姆导体遵循欧姆定律）。
I = Q / t
V = W / Q
R = V / I
In series circuits, current is the same everywhere and total resistance is the sum of individual resistances. In parallel circuits, voltage is the same across each branch and total resistance decreases as more paths are added. Electrical power is given by P = I V, and energy transferred by E = I V t.
在串联电路中，电流处处相等，总电阻等于各电阻之和。在并联电路中，各支路两端电压相等，增加支路会使总电阻减小。电功率由P = I V计算，电能为E = I V t。
P = I V
E = I V t
Mains electricity usually supplies alternating current (a.c.) at 230 V. Safety features include fuses, circuit breakers and double insulation. Live, neutral and earth wires are colour-coded (brown, blue, green/yellow).
家庭用电通常为交流电（a.c.），电压230 V。安全装置包括保险丝、断路器和双重绝缘。火线、零线和地线分别用棕色、蓝色和黄绿色标识。
8. Magnetism and Electromagnetism | 磁学与电磁学
Magnets have north and south poles; like poles repel and unlike poles attract. A magnetic field is a region where a magnetic material experiences a force. Its direction is from north to south, represented by field lines.
磁铁有北极和南极；同名磁极相斥，异名相吸。磁场是对磁性材料施加力的区域，方向由北极指向南极，可用磁感线表示。
An electric current produces a magnetic field – this is the basis of electromagnets. A coil (solenoid) with a soft iron core creates a strong, controllable magnet. The motor effect describes the force on a current-carrying conductor in a magnetic field, used in d.c. motors.
电流产生磁场，这是电磁铁的基础。绕有软铁芯的线圈（螺线管）可产生强而可控的磁铁。电动机效应描述了磁场对载流导体的作用力，应用于直流电动机。
Electromagnetic induction occurs when a wire cuts magnetic field lines, inducing an e.m.f. This principle is used in generators to produce electricity. Transformers change a.c. voltages using the ratio of turns: Vp / Vs = Np / Ns (where p = primary, s = secondary).
当导线切割磁感线时，会产生电磁感应，从而感生出电动势。这一原理用于发电机发电。变压器利用匝数比改变交流电压：Vp / Vs = Np / Ns（p为原边，s为副边）。
Vp / Vs = Np / Ns
An ideal transformer assumes 100% efficiency, so input power equals output power: Vp Ip = Vs Is.
理想变压器假设效率为100%，因此输入功率等于输出功率：Vp Ip = Vs Is。
9. Atomic Physics and Radioactivity | 原子物理与放射性
Atoms contain a tiny, dense nucleus of protons and neutrons, surrounded by electrons in energy levels. The proton number (atomic number) defines the element; the nucleon number (mass number) is the total of protons and neutrons. Isotopes have the same proton number but different nucleon numbers.
原子包含微小而致密的由质子和中子组成的原子核，以及分布在能级上的电子。质子数（原子序数）决定了元素种类；核子数（质量数）是质子数与中子数之和。同位素具有相同的质子数但不同的核子数。
Unstable nuclei emit radiation to become more stable. Alpha particles (α) are helium nuclei, strongly ionising but easily absorbed. Beta particles (β) are fast electrons, moderately ionising. Gamma rays (γ) are electromagnetic waves, weakly ionising but highly penetrating. Decay processes are random, and the half-life is the time for half the nuclei in a sample to decay.
不稳定的原子核通过发出辐射变得更稳定。α粒子是氦原子核，电离能力强但易被吸收；β粒子是高速电子，电离能力中等；γ射线是电磁波，电离能力弱但穿透力强。衰变过程是随机的，半衰期是样品中一半原子核发生衰变所需的时间。
Background radiation comes from natural sources (rocks, cosmic rays) and artificial sources. Radioactive materials must be handled with care, using tongs, shielding and minimising exposure time. Applications include medical imaging, cancer therapy and carbon dating.
背景辐射来自天然（岩石、宇宙射线）和人为来源。放射性物质必须小心处理，使用镊子、屏蔽防护并缩短接触时间。其应用包括医学成像、癌症治疗和碳定年法。
10. Space Physics | 空间物理学
The Earth rotates on its axis once every 24 hours, causing day and night, and orbits the Sun once a year. The Solar System includes planets, moons, asteroids and comets. Orbital motion is maintained by gravitational attraction; the orbital speed of a planet depends on its distance from the Sun.
地球每24小时绕地轴自转一次，形成昼夜；每年绕太阳公转一周。太阳系包含行星、卫星、小行星和彗星。引力维持轨道运动；行星的轨道速度取决于其与太阳的距离。
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