📚 IB Physics Core Concepts and Exam Strategy | IB物理核心概念与备考策略
The IB Diploma Programme physics course asks you to move beyond memorising formulas and to build coherent models of the physical world. It combines conceptual understanding, mathematical reasoning, experimental design and data analysis across topics ranging from mechanics to quantum physics, and every topic is examined both as content and as a way of thinking.
IB文凭项目物理课程要求你超越记忆公式，建立连贯的物理世界模型。它融合了概念理解、数学推理、实验设计和数据分析，涵盖从力学到量子物理的多个主题；每一个主题既作为知识内容被考核，也作为一种思维方式被考核。
This guide walks through the core concepts you must master, the equations you must be able to use fluently, and the examination strategies that convert understanding into marks. Each section pairs the physics with the wording that examiners reward.
本文梳理你必须掌握的核心概念、必须能熟练运用的方程，以及把理解转化为分数的考试策略。每一节都把物理内容与考官给分的表述方式配对讲解。
1. Measurement and Uncertainties | 测量与不确定度
Every measurement in IB physics carries an uncertainty, and the course treats uncertainty analysis as a transferable skill. You need to distinguish between random errors, which can be reduced by averaging, and systematic errors, which shift all readings in the same direction and cannot be reduced by repetition.
IB物理中的每一次测量都带有不确定度，课程将不确定度分析视为可迁移技能。你需要区分随机误差和系统误差：随机误差可通过取平均值减小，而系统误差使所有读数朝同一方向偏移，不能通过重复测量减小。
calibration or a consistent reading technique such as parallax. A reading on an analogue scale is usually quoted to ± half the smallest division, while a digital display is quoted to ± the smallest division.
随机误差影响精密度，表现为数据围绕平均值散布。系统误差影响准确度，来自零点误差、校准错误或视差等一贯的读数方式。模拟量具的读数通常记为 ± 最小分度的一半，数字显示的读数通常记为 ± 最小分度。
When you combine measurements, the propagation rules depend on the operation. For addition and subtraction, add absolute uncertainties. For multiplication and division, add percentage or fractional uncertainties.
当你合并测量值时，传递规则取决于运算类型。对于加减法，应相加绝对不确定度；对于乘除法，应相加百分比不确定度或相对不确定度。
If a = b + c, then Δa = Δb + Δc
If y = ab / c, then %Δy = %Δa + %Δb + %Δc
If y = aⁿ, then %Δy = n × %Δa
ou should also compare percentage uncertainties when evaluating whether an experimental result supports a theoretical prediction.
报告结果时应写出最佳估计值、绝对不确定度和单位，例如 (5.24 ± 0.05) m。在评估实验结果是否支持理论预测时，还应比较百分比不确定度。
- Percentage uncertainty of (5.24 ± 0.05) m = 0.05 ÷ 5.24 × 100% ≈ 0.95%
- Graph work: plot error bars and a line of best fit, then draw maximum and minimum gradient lines to estimate the uncertainty in the gradient
- Quote final answers to the same number of significant figures as the least precise data, usually 2 or 3
2. Mechanics: Kinematics and Forces | 力学：运动学与力
Mechanics is the language of motion and force. In uniformly accelerated motion, four equations connect initial velocity u, final velocity v, acceleration a, displacement s and time t. They are valid only for constant acceleration in a straight line, so the first step in any problem is to decide whether that condition holds.
力学是运动和力的语言。在匀加速运动中，四个方程将初速度 u、末速度 v、加速度 a、位移 s 和时间 t 联系起来。它们仅适用于直线上的匀加速运动，因此解题第一步是判断该条件是否成立。
v = u + at
s = ut + ½at²
v² = u² + 2as
s = ½(u + v)t
Forces are represented as vectors on a free-body diagram, and Newton's laws turn those diagrams into predictions. The first law states that a body remains at rest or moves with constant velocity unless a resultant force acts. The second law links resultant force, mass and acceleration. The third law states that forces occur in equal and opposite pairs acting on different bodies.
力在受力分析图上以矢量表示，牛顿定律把这些图转化为预测。第一定律指出，除非有合力作用，物体保持静止或匀速直线运动。第二定律把合力、质量与加速度联系起来。第三定律指出，力成对出现，大小相等、方向相反，且作用在不同物体上。
Fnet = ma F_friction ≤ μN
inal velocity occurs when the resultant force becomes zero, for example when drag equals weight.
在倾角为 θ 的斜面上，重力分解为沿斜面的 mg sin θ 和垂直斜面的 mg cos θ，因此法向力 N = mg cos θ。当合力为零时达到收尾速度，例如阻力等于重力时。
3. Work, Energy and Momentum | 功、能量与动量
Work is done when a force moves its point of application along its own direction, so only the component of force parallel to the displacement transfers energy. Energy is conserved in every closed system, and the work-energy principle connects the net work done to the change in kinetic energy.
当力的作用点沿力的方向发生位移时，力做了功，因此只有与位移平行的力分量才传递能量。在任何封闭系统中能量守恒，动能定理把合外力做的功与动能变化联系起来。
W = Fs cos θ Ek = ½mv² Ep = mgh E_elastic = ½kx²
P = W / t = Fv η = useful output ÷ total input × 100%
Momentum is a vector quantity defined as the product of mass and velocity, and Newton's second law in its general form states that the resultant force equals the rate of change of momentum. The impulse-momentum theorem follows directly and explains why airbags and crumple zones reduce injury by extending the collision time.
动量是矢量，定义为质量与速度的乘积；牛顿第二定律的一般形式指出，合力等于动量的变化率。冲量-动量定理由此直接得出，并解释了安全气囊和溃缩区如何通过延长碰撞时间来减轻伤害。
p = mv FΔt = Δp = mv − mu
In a collision or explosion, total momentum is conserved provided the net external force is negligible. Kinetic energy is conserved only in elastic collisions; in inelastic collisions some kinetic energy becomes thermal energy, sound or deformation.
在碰撞或爆炸中，只要合外力可忽略，总动量守恒。只有弹性碰撞中动能守恒；在非弹性碰撞中，一部分动能转化为热能、声能或形变能。
4. Circular Motion and Gravitation | 圆周运动与万有引力
An object moving in a circle at constant speed is still accelerating because its direction changes continuously. That centripetal acceleration points towards the centre, and it must be provided by a real force such as tension, friction, gravity or the normal force from a banked surface.
以恒定速率做圆周运动的物体仍在加速，因为其速度方向不断改变。该向心加速度指向圆心，必须由真实的力提供，例如张力、摩擦力、重力或斜面支持力。
ω = 2π / T = 2πf v = ωr a = v² / r = ω²r
Newton' uare of their separation. Gravitational field strength at a point is the force per unit mass, which reduces to g = GM / r² near a spherical body.
牛顿万有引力定律指出，两个质点之间的引力与质量乘积成正比，与距离平方成反比。某点的引力场强度是单位质量所受的力，在球形天体附近化简为 g = GM / r²。
F = GMm / r² g = GM / r² T² = (4π² / GM) r³
The last relation is Kepler's third law. It explains why distant planets orbit more slowly, and it lets you deduce the mass of a central body from the orbital period and radius of a satellite. Geostationary orbits must have a period of one sidereal day and lie above the equator.
最后一个关系式即开普勒第三定律。它解释了为何遥远的行星公转更慢，并让你由卫星的周期和轨道半径推断中心天体的质量。地球同步轨道必须具有一个恒星日的周期，且位于赤道正上方。
5. Thermal Physics and Gases | 热学与气体
Thermal physics links the macroscopic variables of pressure, volume and temperature to the microscopic behaviour of particles. The kinetic model of an ideal gas assumes random motion, negligible molecular volume, elastic collisions and no intermolecular forces except during collisions.
热学把压强、体积和温度这些宏观量与粒子的微观行为联系起来。理想气体的动力学模型假设分子随机运动、分子体积可忽略、碰撞为弹性碰撞，且除碰撞外无分子间作用力。
pV = nRT
Temperature must be expressed in kelvin in every gas calculation, because the Kelvin scale has its zero at absolute zero where molecular translational kinetic energy is minimal. Internal energy of an ideal gas depends only on temperature.
在所有气体计算中，温度都必须用开尔文表示，因为开尔文温标的零点在绝对零度，此时分子的平动动能最小。理想气体的内能只取决于温度。
Q = mcΔT Q = mL
Specific heat capacity describes the energy needed to raise the temperature of unit mass by one kelvin, while specific latent heat describes the energy needed to change phase at constant temperature. During a phase change the temperature stays constant even though energy is being supplied.
比热容描述使单位质量升高一开尔文所需能量，比潜热描述在恒定温度下发生相变所需能量。在相变过程中，尽管持续供热，温度保持不变。
- Conduction: energy transferred by molecular collisions and free electrons, faster in metals
- Convection: energy transferred by bulk movement of a fluid with different densities
- Radiation: energy transferred by electromagnetic waves, requiring no medium
6. Waves, Sound and Light | 波动、声与光
A wave transfers energy without transferring matter. Transverse waves oscillate perpendicular to the direction of travel, while longitudinal waves oscillate along it. Frequency is set by the source, and speed is set by the medium, so wavelength changes when a wave enters a new medium.
波传递能量而不传递物质。横波的振动方向与传播方向垂直，纵波的振动方向与传播方向一致。频率由波源决定，波速由介质决定，因此波进入新介质时波长改变。
fλ n = c / v n₁ sin θ₁ = n₂ sin θ₂ sin θ_c = 1 / n
Superposition explains interference: when two waves meet, the resultant displacement is the vector sum of the individual displacements. Constructive interference occurs for path differences of nλ, destructive interference for path differences of (n + ½)λ, giving the patterns seen in the double-slit and diffraction grating experiments.
叠加原理解释了干涉：两列波相遇时，合位移等于各分位移的矢量和。光程差为 nλ 时发生相长干涉，为 (n + ½)λ 时发生相消干涉，这给出双缝和衍射光栅实验中的条纹图样。
d sin θ = nλ
Total internal reflection occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle; it underpins optical fibres and prisms. The Doppler effect shifts observed frequency when source and observer have a relative velocity along the line joining them.
当光从光密介质射向光疏介质且入射角大于临界角时发生全反射，它是光纤与棱镜的工作原理。当波源与观察者沿连线方向有相对速度时，多普勒效应使观测频率发生偏移。
7. Electricity and Circuits | 电学与电路
Electric current is the rate of flow of charge, and potential difference is the energy transferred per unit charge between two points. Ohm's law applies only to ohmic conductors at constant temperature, so always check whether a component is ohmic before using it.
电流是电荷的流动速率，电势差是两点间单位电荷转移的能量。欧姆定律只适用于温度恒定的欧姆导体，因此使用前必须判断元件是否为欧姆元件。
I = Δq / Δt V = IR R = ρL / A
ε = I(R + r) P = VI = I²R = V² / R
In series circuits the current is the same through every component and the potential differences add. In parallel circuits the potential difference is the same across each branch and the currents add. Kirchhoff's laws are simply conservation of charge and conservation of energy applied to circuits.
串联电路中各元件电流相同，电势差相加。并联电路各支路电势差相同，电流相加。基尔霍夫定律本质上是电荷守恒与能量守恒在电路中的应用。
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