📚 Year 7 CAIE Physics: Key Knowledge Points Overview | Year 7 CAIE 物理：核心知识点梳理
This article provides a structured review of the essential physics topics covered in the Cambridge Lower Secondary Science Stage 7 syllabus. It breaks down core concepts into ten key sections, offering clear explanations in both English and Chinese. Use this guide to consolidate your understanding and prepare effectively for your assessments.
本文系统地梳理了剑桥初中科学Stage 7大纲中的物理核心知识点，分为十个关键章节讲解，每段均配有中英双语解释。这篇指南能帮助你巩固理解，为考试做好充分准备。
1. Physical Quantities and Measurement | 物理量与测量
length is measured in metres (m), mass in kilograms (kg), time in seconds (s), and volume in cubic metres (m³) or litres (L). For smaller or larger values, we use prefixes like centi- (c = 0.01), milli- (m = 0.001), and kilo- (k = 1000).
物理学中，我们用可测量的量来描述世界。最基本的物理量包括长度、质量、时间和体积。在国际单位制中，长度的单位是米(m)，质量是千克(kg)，时间是秒(s)，体积是立方米(m³)或升(L)。遇到更小或更大的数值时，我们会使用词头，如厘(c, 0.01)、毫(m, 0.001)和千(k, 1000)。
Measuring instruments must be used correctly to obtain accurate results. A metre rule can measure length to the nearest millimetre, while a measuring cylinder is used for the volume of liquids. The volume of an irregular solid can be found by the displacement method: immerse the object in water in a measuring cylinder and note the rise in water level.
正确使用测量工具是获得准确结果的关键。米尺可测量长度至毫米级别，量筒用来测量液体体积。不规则固体的体积可以通过排水法求得：将物体浸没在量筒的水中，观察水面上升的刻度即可。
Time is usually measured with a stopwatch or digital timer, and mass is measured using an electronic balance or a beam balance. Always read the scale at eye level to avoid parallax error.
时间通常用秒表或数字计时器测量，质量则用电子天平或杠杆天平测量。读数时眼睛必须与刻度保持水平，以消除视差。
2. Density | 密度
Density tells us how much mass is packed into a given volume. An object that feels heavy for its size has a high density, while a light, fluffy material has a low density. Density is a property of a substance and does not depend on the amount of material present.
密度表示单位体积内所含质量的多少。拿在手里感觉“压手”的物体密度较高，而蓬松轻质材料的密度较低。密度是物质本身的一种属性，与物体的多少无关。
The formula for density is:
ρ = mass ÷ volume
计算的公式为：
密度 = 质量 ÷ 体积
In calculations, mass is in grams (g) or kilograms (kg) and volume in cm³ or m³, so density units are often g/cm³ or kg/m³. For example, water has a density of about 1 g/cm³, whereas iron has a density around 7.9 g/cm³. Objects with a density less than water float; those with a density greater than water sink.
计算中，质量用克(g)或千克(kg)，体积用 cm³ 或 m³，因此密度单位常用 g/cm³ 或 kg/m³。例如，水的密度约为 1 g/cm³，铁的密度约为 7.9 g/cm³。密度小于水的物体会浮起，大于水的物体则下沉。
3. Forces and Motion | 力与运动
A force is a push or a pull that can change the shape, speed, or direction of an object. Forces are measured in newtons (N). Common types include gravitational force (weight), friction, tension, and magnetic force. Forces acting on an object can be represented by arrows; the direction of the arrow shows the direction of the force, and the length shows its magnitude.
力是能改变物体形状、速度或方向的推或拉，其单位是牛顿(N)。常见的力有重力（重量）、摩擦力、张力和磁力等。作用在物体上的力可用箭头表示；箭头的方向代表力的方向，箭头的长度代表力的大小。
When forces on an object are balanced, the object remains at rest or continues to move at a constant speed in a straight line. Unbalanced forces cause a change in speed or direction (acceleration). The relationship between speed, distance, and time is given by the formula:
speed = distance ÷ time
当物体受到的力相互平衡时，物体将保持静止或沿直线匀速运动。如果受力不平衡，物体的速度或方向就会改变（产生加速度）。速度、距离和时间的关系满足：
速度 = 距离 ÷ 时间
Speed can be measured in metres per second (m/s) or kilometres per hour (km/h). A distance–time graph with a straight, sloping line indicates constant speed; a horizontal line means the object is stationary.
速度的单位可以是米每秒(m/s)或公里每小时(km/h)。在路程–时间图中，倾斜的直线表示匀速运动，水平线则表示物体静止。
4. Friction and Air Resistance | 摩擦力与空气阻力
Friction is a force that opposes motion between two surfaces in contact. It acts in the opposite direction to movement and produces heat. Rougher surfaces create more friction; smoother surfaces or lubricants like oil or grease reduce friction.
摩擦力是一种阻碍接触面之间相对运动的力，方向与运动相反，并且会产生热。表面越粗糙，摩擦力越大；光滑表面或油、润滑脂等润滑剂可以减小摩擦力。
Air resistance (or drag) is a type of friction that acts on objects moving through air. It increases with speed and with the surface area facing the direction of motion. Streamlined shapes reduce air resistance, which is why cars, aeroplanes, and cyclists are designed to be aerodynamic.
空气阻力（或称拖曳力）是物体在空气中运动时所受的一种摩擦力，它随速度和迎风面积的增大而增大。流线型设计可以减小空气阻力，因此汽车、飞机和自行车都力求符合空气动力学。
Friction is both useful and problematic. It allows us to walk without slipping, enables car tyres to grip the road, and helps brakes to slow vehicles. However, it also wears out parts and wastes energy as heat. Engineers constantly look for ways to balance the helpful and harmful effects of friction.
摩擦力既有用也有害。它让我们行走时不打滑，使汽车轮胎抓地，还帮助刹车。但它也会造成零件磨损，并以热的形式浪费能量。工程师们一直在寻找平衡摩擦利弊的方法。
5. Energy Forms and Transfers | 能量形式与转化
Energy is the ability to do work. It exists in many forms: kinetic (movement), thermal (heat), light, sound, electrical, chemical (in food and fuels), elastic (stored in stretched springs), and gravitational potential (due to height). The unit of energy is the joule (J).
能量是做功的本领，以多种形式存在：动能（运动）、热能（热）、光能、声能、电能、化学能（储存在食物和燃料中）、弹性势能（拉伸的弹簧）和重力势能（因高度而具有）。能量的单位是焦耳(J)。
Energy can be transferred from one object to another, or transformed from one type to another. For example, a toaster turns electrical energy into heat and light; a falling object transforms gravitational potential energy into kinetic energy. Energy is never created or destroyed, only converted. This is the principle of conservation of energy.
能量可以从一个物体转移到另一个物体，也可以从一种形式转换成另一种形式。例如，烤面包机将电能转化为热能和光能；下落的物体将重力势能转化为动能。能量既不会凭空产生，也不会凭空消失，只会从一种形式转化为另一种，这就是能量守恒定律。
In Year 7, you also learn about renewable and non-renewable energy resources. Fossil fuels (coal, oil, natural gas) are non-renewable and cause pollution. Renewable sources like solar, wind, wave, hydroelectric, and geothermal do not run out and have less impact on the environment.
在七年级阶段，你还要了解可再生和不可再生能源。化石燃料（煤、石油、天然气）属于不可再生能源，且会造成污染。太阳能、风能、波浪能、水力发电和地热能等可再生能源则不会枯竭，对环境的影响也更小。
6. Light | 光
Light travels in straight lines at a very high speed (about 300 000 km/s in a vacuum). When light hits an object, it can be reflected, transmitted, or absorbed. We see objects because light reflects off them and enters our eyes.
光沿直线传播，在真空中的速度非常快（约 300 000 千米/秒）。光照射到物体上时，可能被反射、透射或吸收。我们能看到物体，是因为光从物体表面反射后进入了我们的眼睛。
Reflection follows the law of reflection: the angle of incidence equals the angle of reflection. Both angles are measured from the normal – an imaginary line perpendicular to the surface at the point of incidence. A plane mirror produces a virtual, upright, and laterally inverted image.
反射遵循反射定律：入射角等于反射角。这两个角都是从法线（垂直于入射点切面的假想线）开始测量的。平面镜会产生正立、左右颠倒的虚像。
Refraction is the bending of light when it passes from one transparent medium to another at an angle. It happens because light changes speed in different materials. A ray entering a denser medium (e.g. from air to glass) slows down and bends towards the normal.
折射是光线倾斜穿过不同透明介质时发生的弯曲现象，其原因是光在不同介质中的速度不同。光线进入光密介质（例如从空气射入玻璃）时速度变慢，并朝法线方向偏折。
White light is a mixture of seven colours – red, orange, yellow, green, blue, indigo, and violet. A prism or water droplets can disperse white light into a spectrum (rainbow). The colour of an opaque object is the colour of light it reflects; a black object absorbs all colours.
白光是七种颜色的混合：红、橙、黄、绿、蓝、靛、紫。棱镜或水滴可以把白光分解成光谱（彩虹）。不透明物体的颜色就是它反射的光的颜色；黑色物体吸收所有颜色的光。
7. Sound | 声
Sound is produced by vibrating objects. These vibrations travel through a medium (solid, liquid, or gas) as longitudinal waves, but sound cannot travel through a vacuum. The speed of sound in air is about 340 m/s – much slower than light.
声音由物体的振动产生。振动以纵波的形式通过介质（固体、液体或气体）传播，但声音无法在真空中传播。声音在空气中的速度约为 340 米/秒，比光速慢得多。
The pitch (highness or lowness) of a sound depends on the frequency of vibration: higher frequency gives a higher pitch. Loudness depends on the amplitude of the vibration: larger amplitude produces a louder sound. These concepts can be shown on an oscilloscope trace.
音调的高低取决于振动频率：频率越高，音调越高。响度则取决于振幅：振幅越大，声音越响。这些概念可以在示波器的波形图上直观看到。
Sound can be reflected to produce an echo. Hard, flat surfaces reflect sound well, while soft, porous materials absorb sound and reduce noise. This is the principle behind soundproofing in recording studios.
声音可以被反射形成回声。坚硬平坦的表面反射效果好，而柔软多孔的材料则吸收声音，降低噪音，这就是录音室隔音的基本原理。
8. Electricity | 电学
Electricity is the flow of electric charge, usually electrons, through a conductor. A complete loop called a circuit is needed for the charge to flow. Circuit components include cells (batteries), wires, bulbs, switches, buzzers, and motors. The standard circuit symbols are used to draw circuit diagrams.
电是电荷（通常是电子）在导体中流动的现象。电荷的流动需要一个完整的回路，也就是电路。电路元件包括电池、导线、灯泡、开关、蜂鸣器和电动机等。绘制电路图时使用统一的电路符号。
Current (I) is the rate of flow of charge and is measured in amperes (A) using an ammeter connected in series. Voltage (V), or potential difference, is the push that drives the current around the circuit; it is measured in volts (V) using a voltmeter connected in parallel.
电流(I)是电荷流动的速率，单位是安培(A)，需将安培表串联在电路中测量。电压(V)又称电势差，是推动电荷在电路中流动的“推力”，单位是伏特(V)，使用伏特表并联测量。
Resistance (R) opposes the flow of current. It is measured in ohms (Ω). Components like resistors and long thin wires have higher resistance. Conductors (metals) allow current to flow easily; insulators (plastic, wood, rubber) do not.
电阻(R)是对电流的阻碍，单位是欧姆(Ω)。电阻器等元件以及细长的导线电阻较大。导体（金属）允许电流轻松通过，绝缘体（塑料、木头、橡胶）则不能导电。
In a series circuit, there is only one path for the current, and the current is the same everywhere. If one component breaks, the circuit is broken and everything stops. In a parallel circuit, there are multiple paths, and if one branch breaks, the others can still work.
串联电路中只有一条电流路径，各处电流相等；若一个元件断开，整个电路断开，所有元件停止工作。并联电路中有多条支路，某条支路断开后，其他支路仍能正常工作。
9. Magnetism | 磁学
A magnet has a north pole and a south pole. Like poles repel, opposite poles attract. The region around a magnet where magnetic forces can be felt is called the magnetic field. Field lines show the direction of the force on a north pole; they go from north to south outside the magnet.
磁体有北极和南极，同极相斥，异极相吸。磁体周围能感受到磁力的区域称为磁场。磁感线表示作用在北极上的力的方向；在磁体外部，磁感线从北极指向南极。
Magnetic materials, such as iron, nickel, and cobalt, are attracted to magnets. Non-magnetic materials, like aluminium, wood, and plastic, are not. A temporary magnet, like an electromagnet, can be made by winding a coil of wire around an iron core and passing a current through it.
铁、镍、钴等磁性材料会被磁体吸引，而铝、木材、塑料等非磁性材料则不会。将导线绕在铁芯上并通电，即可制成临时磁体——电磁铁。
The strength of an electromagnet can be increased by using more coils, a larger current, or a soft iron core. Electromagnets are used in scrapyard cranes, electric bells, and relays because their magnetism can be switched on and off.
增加线圈圈数、增大电流或使用软铁芯都能增强电磁铁的磁性。电磁铁广泛应用于废料场起重机、电铃和继电器中，因为它的磁性可以自由开关。
10. States of Matter and the Particle Model | 物质状态与粒子模型
All substances are made of tiny particles. The arrangement and movement of these particles determine whether the substance is a solid, liquid, or gas. In solids, particles are packed closely in a fixed arrangement, vibrating in place. Liquids have particles that are close together but can move around each other. Gas particles are far apart and move rapidly in all directions.
一切物质都由微小的粒子构成。粒子的排列和运动方式决定了物质是固体、液体还是气体。固体中，粒子紧密排列在固定位置，仅做原地振动；液体的粒子彼此接近，但能相对滑动；气体中的粒子则相距很远，朝各个方向快速运动。
Changes of state involve energy transfer. Melting (solid to liquid) and boiling/evaporation (liquid to gas) require heat energy. Freezing (liquid to solid) and condensation (gas to liquid) release energy. The mass stays the same during changes of state because the number of particles does not change.
物态变化伴随着能量转移。熔化（固→液）和沸腾/蒸发（液→气）需要吸收热能，凝固（液→固）和凝结（气→液）则放出能量。物态变化前后质量保持不变，因为粒子的总数没有变化。
Diffusion is the random movement of particles from a region of high concentration to a region of low concentration. This happens in both liquids and gases, and is faster at higher temperatures because the particles have more kinetic energy.
扩散是粒子从高浓度区域向低浓度区域的随机运动，在液体和气体中都会发生。温度越高，扩散越快，因为粒子的动能增大。
Gas pressure is caused by particles colliding with the walls of their container. If the temperature increases, particles move faster and hit the walls harder, increasing pressure. If the volume is reduced, the frequency of collisions increases, so pressure also rises.
气体压强由粒子与容器壁碰撞产生。温度升高时，粒子运动加快，撞击更猛烈，压强增大；体积减小时，碰撞更频繁，压强同样增大。
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