📚 Core Concepts in Cambridge IGCSE® O Level Physics (Fourth Edition) | 剑桥IGCSE® O Level物理核心概念（第四版）
The Cambridge IGCSE® O Level Complete Physics Student Book Fourth Edition provides a thorough, concept‑focused journey through the entire syllabus. It builds understanding from basic principles to real‑world applications, making physics accessible and engaging. This article distils the key concepts that underpin the course, offering clear explanations to support revision and deeper learning.
剑桥 IGCSE® O Level 完整物理学生书第四版引领学生经历一趟以概念为核心的完整学习之旅，从基本原理延伸到实际应用。本文提炼了支撑整个课程的核心概念，提供清晰的解析，以辅助复习和深度学习。
1. Scalars, Vectors and Motion | 标量、矢量与运动
A scalar quantity has magnitude only, for example mass (kg), time (s) or speed (m/s). A vector has both magnitude and direction, such as velocity, force or acceleration.
标量只有大小，例如质量（千克）、时间（秒）或速率（米/秒）。矢量既有大小又有方向，例如速度、力或加速度。
Distance is the total path length travelled and is a scalar. Displacement is the straight‑line distance in a specific direction; it is a vector.
距离是走过的路径总长度，是标量。位移是某一方向上的直线距离，是矢量。
Speed is the rate of change of distance: v = d / t. Velocity is displacement per unit time: v = Δs / Δt. Acceleration is the rate of change of velocity: a = (v - u) / t, where u is initial velocity and v is final velocity.
速率是距离的变化率：v = d / t。速度是单位时间的位移：v = Δs / Δt。加速度是速度的变化率：a = (v - u) / t，其中 u 为初速度，v 为末速度。
2. Forces and Newton's Laws | 力与牛顿定律
A force is a push or pull that can change an object's speed, direction or shape. The unit is the newton (N). Forces are vectors.
力是一种推或拉，能改变物体的速度、方向或形状。单位是牛顿（N）。力是矢量。
Newton's First Law states that an object remains at rest or in uniform motion unless acted on by a resultant force. This property is inertia.
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。这种性质就是惯性。
Newton's Second Law: resultant force = mass × acceleration, F = ma. The acceleration is directly proportional to force and inversely proportional to mass.
牛顿第二定律：合外力 = 质量 × 加速度，F = ma。加速度与力成正比，与质量成反比。
Newton's Third Law: for every action force there is an equal and opposite reaction force. These forces act on different bodies.
牛顿第三定律：每一个作用力都有一个大小相等、方向相反的反作用力，且它们作用在不同物体上。
3. Energy, Work and Power | 能量、功与功率
Energy is the capacity to do work. The SI unit is the joule (J). Energy can exist in kinetic, potential (gravitational, elastic), thermal, chemical and nuclear forms.
能量是做功的本领，国际单位是焦耳（J）。能量可以以动能、势能（重力势能、弹性势能）、热能、化学能和核能等形式存在。
Kinetic energy: KE = ½ mv². Gravitational potential energy: GPE = mgh, where h is the height above a reference level.
动能：KE = ½ mv²。重力势能：GPE = mgh，其中 h 是相对参考水平的高度。
Work is done when a force moves its point of application: W = Fd (when force and displacement are parallel). Work transfers energy.
当力使其作用点移动时做了功：W = Fd（力与位移平行时）。功转移能量。
Power is the rate of doing work or transferring energy: P = W / t = ΔE / t. The unit is the watt (W).
功率是做功或转移能量的快慢：P = W / t = ΔE / t。单位是瓦特（W）。
4. Momentum and Collisions | 动量与碰撞
Momentum is the product of mass and velocity: p = mv. It is a vector with unit kg m/s. A resultant force changes momentum: F = Δp / Δt.
动量是质量与速度的乘积：p = mv。它是矢量，单位为千克·米/秒。合外力改变动量：F = Δp / Δt。
In a closed system, the total momentum before a collision or explosion equals the total momentum after. This is the principle of conservation of momentum.
在封闭系统中，碰撞或爆炸前的总动量等于碰撞或爆炸后的总动量。这就是动量守恒定律。
Collisions can be elastic (kinetic energy conserved) or inelastic (kinetic energy is not conserved, often converted to thermal or sound energy).
碰撞可分为弹性碰撞（动能守恒）和非弹性碰撞（动能不守恒，通常转化为热能或声能）。
5. Thermal Physics: Temperature, Heat and Gas Laws | 热物理：温度、热与气体定律
Temperature measures the average kinetic energy of particles. The absolute scale is kelvin (K), where 0 K is absolute zero. Heat is thermal energy in transit.
温度量度粒子平均动能。绝对温标是开尔文（K），0 K 即绝对零度。热是传递中的热能。
Specific heat capacity c is the energy needed to raise 1 kg of a substance by 1 °C (or 1 K): Q = mcΔθ.
比热容 c 是使 1 kg 物质升高 1 °C（或 1 K）所需的能量：Q = mcΔθ。
Specific latent heat L is the energy absorbed or released during a change of state at constant temperature: Q = mL.
比潜热 L 是在温度不变时，物态变化过程中吸收或释放的能量：Q = mL。
For a fixed mass of ideal gas at constant temperature, pressure and volume obey Boyle's law: pV = constant. At constant pressure, volume is proportional to absolute temperature: V/T = constant.
对于一定质量的理想气体，温度不变时压强与体积遵循玻意耳定律：pV = 常数。压强不变时，体积与热力学温度成正比：V/T = 常数。
6. Waves: Properties and Types | 波：特性与类型
Waves transfer energy without transferring matter. In transverse waves (e.g. light, water ripples) the oscillations are perpendicular to energy flow. In longitudinal waves (e.g. sound) they are parallel.
波传递能量而不传递物质。横波（如光、水波）的振动方向与能量传播方向垂直。纵波（如声波）的振动方向与能量传播方向平行。
wavelength λ, amplitude, frequency f, period T = 1/f, and wave speed v. The wave equation: v = f λ.
关键波参数：波长 λ、振幅、频率 f、周期 T = 1/f 和波速 v。波动方程：v = f λ。
Diffraction is the spreading of waves around obstacles or through gaps. Refraction is the change in wave speed and direction when entering a new medium.
衍射是波绕过障碍物或穿过缝隙扩散的现象。折射是波进入新介质时波速和方向发生改变的现象。
7. Light and Optics | 光与光学
Light travels in straight lines. The law of reflection states that the angle of incidence equals the angle of reflection, measured from the normal.
光沿直线传播。反射定律指出，入射角等于反射角，均从法线量度。
When light enters a denser medium it slows down and bends towards the normal. This is described by Snell's law: n₁ sin θ₁ = n₂ sin θ₂, where n is refractive index.
光进入光密介质时会变慢并向法线偏折。这由斯涅尔定律描述：n₁ sin θ₁ = n₂ sin θ₂，其中 n 为折射率。
Total internal reflection occurs when light travels from a denser to a less dense medium at an angle greater than the critical angle c: sin c = 1/n (for light entering air).
当光从光密介质射向光疏介质且入射角大于临界角 c 时，发生全内反射：sin c = 1/n（光射向空气时）。
8. Electricity: Circuits, Resistance and Power | 电：电路、电阻与电功率
Electric current I is the rate of flow of charge: I = Q / t. The unit is the ampere (A). In a series circuit, current is the same at all points.
电流 I 是电荷流动的速率：I = Q / t。单位是安培（A）。串联电路中各处电流相等。
Voltage (potential difference) V is the energy transferred per unit charge: V = W / Q. In a parallel circuit, the voltage across each branch is the same.
电压（电势差）V 是单位电荷转移的能量：V = W / Q。并联电路中各支路两端电压相等。
Resistance R is opposition to current: R = V / I (Ohm's law for constant temperature). Resistors in series: Rtotal = R₁ + R₂. In parallel: 1/Rtotal = 1/R₁ + 1/R₂.
电阻 R 是对电流的阻碍：R = V / I（恒温下欧姆定律）。串联总电阻：R总 = R₁ + R₂。并联：1/R总 = 1/R₁ + 1/R₂。
Electrical power P = IV = I²R = V²/R. Energy E = P t = IV t.
电功率 P = IV = I²R = V²/R。电能 E = P t = IV t。
9. Magnetism and Electromagnetism | 磁与电磁
A magnet has north and south poles; like poles repel, unlike poles attract. Magnetic field lines go from north to south and show the direction of force on a north pole.
磁体有 N 极和 S 极；同极相斥，异极相吸。磁感线从 N 极指向 S 极，表示 N 极的受力方向。
A current‑carrying wire produces a circular magnetic field. The right‑hand grip rule gives the direction. A solenoid concentrates the field, creating an electromagnet.
载流导线周围产生环形磁场。右手螺旋定则给出方向。螺线管使磁场集中，形成电磁铁。
When a current‑carrying conductor is in a magnetic field, a force acts on it (motor effect). The force is maximum when current and field are perpendicular: F = BIl sin θ.
当通电导线处于磁场中时会受力（电动机效应）。当电流与磁场垂直时力最大：F = BIl sin θ。
Electromagnetic induction occurs when a conductor cuts magnetic field lines, inducing an e.m.f. Transformers change alternating voltages according to the turns ratio: Vs/Vp = Ns/Np.
当导体切割磁感线时发生电磁感应，产生感应电动势。变压器按匝数比改变交变电压：Vs/Vp = Ns/Np。
10. Nuclear Physics: Radioactivity and Atomic Structure | 核物理：放射性与原子结构
Atoms consist of a nucleus containing protons (positive) and neutrons (neutral), surrounded by electrons. The proton number Z defines the element.
原子由包含质子（正电）和中子（中性）的原子核以及绕核电子构成。质子数 Z 决定元素种类。
Some nuclei are unstable and emit radiation. Alpha particles (α) are helium nuclei, helium‑4: ⁴₂He, highly ionising but weakly penetrating. Beta particles (β⁻) are fast electrons, moderately penetrating. Gamma rays (γ) are electromagnetic waves, very penetrating.
某些原子核不稳定，会放出辐射。α 粒子是氦核（⁴₂He），电离能力强但穿透力弱。β⁻ 粒子是高速电子，穿透力中等。γ 射线是电磁波，穿透力极强。
Radioactive decay is random. Half‑ ime for half the nuclei in a sample to decay. The activity A = ΔN/Δt decays exponentially.
放射性衰变是随机的。半衰期是样本中一半原子核发生衰变所需的时间。活度 A = ΔN/Δt 呈指数衰减。
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