📚 KS3 Cambridge Physics: Core Knowledge Summary | KS3 剑桥物理：核心知识点梳理
This revision guide covers the essential topics in Key Stage 3 Cambridge Physics. It is designed to help you master forces, energy, waves, electricity, magnetism and space science through clear bilingual explanations. Each section pairs English descriptions with matching Chinese translations to strengthen understanding.
这份复习指南涵盖 KS3 剑桥物理的核心主题。它通过清晰的双语解释，帮助你掌握力、能量、波、电、磁和空间科学。每个小节都将英文描述与对应的中文翻译配对，以加深理解。
1. Forces and Motion | 力与运动
A force is a push or a pull, measured in newtons (N). Forces can change an object's speed, direction or shape.
力是推或拉，以牛顿（N）为单位。力可以改变物体的速度、方向或形状。
When forces on an object are balanced, it stays at rest or moves at constant speed. Unbalanced forces cause acceleration or deceleration.
当物体所受的力平衡时，它会保持静止或匀速直线运动。不平衡的力会导致加速或减速。
Speed is the rate of change of distance. The formula is:
速度是距离的变化率。公式为：
v = s / t
where v = speed, s = distance, t = time. Average speed = total distance / total time.
其中 v 表示速度，s 表示距离，t 表示时间。平均速度 = 总距离 / 总时间。
Weight is the gravitational force on a mass and is given by:
重量是作用在质量上的引力，计算公式为：
W = m g
n Earth, g ≈ 10 N/kg. Mass (kg) is the amount of matter and does not change with location.
在地球上，g 约等于 10 N/kg。质量（kg）是物质的多少，不随位置改变。
Friction and air resistance oppose motion. They can cause objects to heat up and eventually reach terminal velocity when falling through a fluid.
摩擦力和空气阻力阻碍运动。它们会使物体升温，当物体在流体中下落时最终会达到终端速度。
Distance–time graphs: a straight line represents constant speed; a flat line means stationary. The gradient equals speed.
距离-时间图：直线表示匀速运动；水平线表示静止。斜率等于速度。
2. Density and Pressure | 密度与压强
Density is mass per unit volume. The equation is:
密度是单位体积的质量。公式为：
ρ = m / V
Units: kg/m³ or g/cm³. An object floats if its density is less than the fluid's density.
单位：千克每立方米（kg/m³）或克每立方厘米（g/cm³）。物体的密度小于流体的密度时，它会浮起。
Pressure is force per unit area:
压强是单位面积上的压力：
p = F / A
Unit: pascal (Pa), equivalent to N/m². A larger area reduces pressure for the same force.
单位：帕斯卡（Pa），等价于 N/m²。同样的力下，面积越大压强越小。
Pressure in liquids increases with depth, because the weight of the liquid above increases. Pressure in gases acts in all directions and can be explained by particle collisions.
液体压强随深度增加而增加，因为上方液体的重量增大。气体压强向各个方向作用，可用粒子碰撞来解释。
3. Energy Forms and Transfers | 能量形式与转移
Energy can exist in many forms: kinetic, gravitational potential, elastic potential, thermal (heat), chemical, nuclear and electromagnetic (light, etc.).
能量有多种形式：动能、重力势能、弹性势能、热能、化学能、核能和电磁能（光等）。
Energy is transferred when a force does work or through heating. The law of conservation of energy states that energy cannot be created or destroyed, only transferred or converted.
能量通过力做功或加热的方式转移。能量守恒定律指出，能量不能被创造或消灭，只能转移或转化。
Work done is the product of force and distance moved in the direction of the force:
做功是力与沿力方向移动距离的乘积：
W = F d
Power is the rate of doing work or transferring energy:
功率是做功或能量转移的速率：
P = E / t
Efficiency = useful energy output / total energy input (often shown in Sankey diagrams).
效率 = 有用能量输出 / 总能量输入（常用桑基图表示）。
Gravitational potential energy (GPE) gained = m g h, where h is height change.
增加的重力势能 = m g h，其中 h 是高度变化。
Kinetic energy = ½ m v². Faster and heavier objects have more kinetic energy.
动能 = ½ m v²。更快、更重的物体动能更大。
4. Energy Resources | 能源
Non-renewable resources: fossil fuels (coal, oil, natural gas) and nuclear fuel (uranium). They are finite and produce carbon dioxide or radioactive waste.
不可再生资源：化石燃料（煤、石油、天然气）和核燃料（铀）。它们储量有限，会排放二氧化碳或产生放射性废物。
Renewable resources: solar, wind, tidal, wave, hydroelectric, geothermal and biomass. They are replenished naturally and usually have lower environmental impact.
可再生资源：太阳能、风能、潮汐能、波浪能、水力发电、地热能和生物质能。它们自然补充，通常对环境的影响较小。
Fossil fuel power stations burn fuel to heat water, producing steam that drives a turbine and generator. Nuclear stations use fission to generate heat.
化石燃料发电站燃烧燃料加热水，产生蒸汽驱动涡轮机和发电机。核电站利用核裂变产生热量。
| Resource | Renewable? | Advantages | Disadvantages |
|---|---|---|---|
| Solar | Yes | No pollution, free energy | Depends on weather, low efficiency |
| Wind | Yes | Low running costs | Noisy, visual impact, unreliable |
| Fossil fuels | No | Reliable, high energy density | CO₂ emissions, finite |
能源对比表（英文略，中文关键点同上）：太阳能可再生无污染但不稳定；风能成本低但有噪音；化石燃料可靠但排放温室气体。
5. Thermal Energy Transfer | 热能传递
Thermal energy always flows from a hotter region to a cooler region. There are three mechanisms: conduction, convection and radiation.
热能总是从高温区域流向低温区域。有三种传递方式：传导、对流和辐射。
Conduction occurs mainly in solids. Particles vibrate and pass on energy. Good conductors (metals) have free electrons; insulators (wood, plastic) conduct poorly.
传导主要发生在固体中。粒子振动并传递能量。良导体（金属）有自由电子；绝缘体（木材、塑料）导热性差。
Convection happens in fluids (liquids and gases). Warmer, less dense fluid rises while cooler, denser fluid sinks, forming a convection current.
对流发生在流体（液体和气体）中。较热、密度较小的流体上升，较冷、密度较大的流体下沉，形成对流循环。
Infrared radiation can travel through a vacuum and does not need particles. Dull black surfaces are good absorbers and emitters; shiny silver surfaces are poor absorbers and emitters.
红外辐射可以在真空中传播，不需要介质。暗黑色表面是良好的吸收体和辐射体；光亮银色表面吸收和辐射能力差。
Practical applications: vacuum flasks reduce all three transfers; home insulation (loft insulation, double glazing) reduces heating losses.
实际应用：保温瓶减少所有三种传递方式；家庭保温（阁楼隔热、双层玻璃）减少热量散失。
6. Sound Waves | 声波
Sound is produced by vibrating objects. It travels as a longitudinal wave and requires a medium (solid, liquid or gas) to propagate; it cannot travel through a vacuum.
声音由振动产生。它作为纵波传播，需要介质（固体、液体或气体）才能传播；真空中不能传播。
The speed of sound in air is about 340 m/s; it is faster in solids and liquids because particles are closer together.
声速在空气中约为 340 m/s；在固体和液体中更快，因为粒子间距更近。
frequency = high pitch). Amplitude determines loudness (larger amplitude = louder sound).
频率决定音调（高频=高音调）。振幅决定响度（振幅大=声音响）。
Sound waves can be displayed on an oscilloscope as waveforms. Echoes are reflections of sound; ultrasound is sound above the human hearing range (greater than 20 000 Hz).
声波可用示波器显示为波形。回声是声音的反射；超声波是超出人类听觉范围的声音（大于 20 000 Hz）。
Sound can be reflected, refracted under certain conditions, and absorbed. Soft materials absorb sound, while hard surfaces reflect it.
声音可以反射，在一定条件下折射，并被吸收。柔软的材料吸收声音，硬表面则反射声音。
7. Light Waves | 光波
Light travels in straight lines until it hits an object. It is a transverse electromagnetic wave and can travel through a vacuum at a speed of about 3 x 10⁸ m/s.
光沿直线传播，直到碰到物体。它是横电磁波，可以在真空中以约 3 x 10⁸ m/s 的速度传播。
es are measured from the normal. In a plane mirror, the image is virtual, upright, laterally inverted and the same distance behind the mirror as the object is in front.
反射定律：入射角 = 反射角。两个角均从法线测量。在平面镜中，像是虚像、正立、左右颠倒，且像在镜后的距离与物在镜前的距离相等。
Refraction occurs when light enters a different medium and changes speed, causing it to bend. For example, light bends towards the normal when entering a denser medium.
当光进入不同介质时速度改变，发生折射，导致光线弯曲。例如，光进入光密介质时向法线弯曲。
White light can be split into a spectrum (dispersion) by a prism. The colours are red, orange, yellow, green, blue, indigo and violet. A coloured filter transmits only its own colour and absorbs others.
白光可被棱镜分解成光谱（色散）。颜色依次为红、橙、黄、绿、蓝、靛、紫。彩色滤光片只允许自身颜色的光透过，吸收其他颜色。
Objects appear a certain colour because they reflect that colour and absorb others. A black object absorbs all colours; a white object reflects all colours.
物体呈现某种颜色是因为它反射该颜色的光而吸收其他颜色。黑色物体吸收所有颜色；白色物体反射所有颜色。
8. Electricity and Circuits | 电与电路
An electric circuit requires a complete loop and a power source (cell or battery). Circuit symbols are used to represent components such as cells, lamps, switches, resistors and ammeters.
电路需要完整的回路和电源（电池或电池组）。用电路符号表示元件，如电池、灯泡、开关、电阻和电流表。
Electric current (I) is the flow of charge, measured in amperes (A). In a single closed loop, current is the same everywhere. Voltage (V) is the energy transferred per unit charge, measured in volts (V).
电流（I）是电荷的流动，单位为安培（A）。在单一闭合回路中，各处电流相等。电压（V）是每单位电荷转移的能量，单位为伏特（V）。
Resistance (R) measures how hard it is for current to flow, unit: ohm (Ω). Ohm's Law links them:
电阻（R）衡量电流流动的难易程度，单位：欧姆（Ω）。欧姆定律将它们联系起来：
V = I R
In a series circuit: current is the same at all points; total voltage is shared across components; total resistance adds up. If one component breaks, the circuit is open.
串联电路中：各处电流相等；总电压由各元件分担；总电阻相加。如果一个元件损坏，电路断路。
In a parallel circuit: voltage across each branch is the same as the source; total current splits between branches; total resistance is less than the smallest individual resistance. A break in one branch does not stop the others.
并联电路中：各支路电压与电源电压相同；总电流在各支路间分配；总电阻小于最小的单个电阻。一条支路断路不会影响其他支路。
Resistance increases with wire length and decreases with greater cross-sectional area. Nichrome wire has higher resistance than copper.
导线越长电阻越大，横截面积越大电阻越小。镍铬合金的电阻大于铜。
9. Magnetism and Electromagnets | 磁与电磁铁
A magnet has a north and a south pole. Like poles repel, unlike poles attract. Magnetic forces are strongest at the poles and act without contact via a magnetic field.
磁铁有北极和南极。同名磁极相互排斥，异名磁极相互吸引。磁力在磁极处最强，通过磁场非接触作用。
A magnetic field line shows the direction a north pole would move. Field lines run from north to south outside a magnet.
磁感线显示北极的移动方向。磁铁外部磁感线从北极指向南极。
An electromagnet consists of a coil of wire (solenoid) often wrapped around an iron core. When current flows, it produces a strong magnetic field; when switched off, the magnetism is lost.
电磁铁由线圈（螺线管）组成，通常绕在铁芯上。当有电流通过时产生强磁场；断电时磁性消失。
The strength of an electromagnet can be increased by: increasing current, adding more turns to the coil, or using a soft iron core.
增强电磁铁强度的方法：增大电流、增加线圈匝数或使用软铁芯。
Uses include electric bells, relays, loudspeakers and scrapyard cranes. A simple motor uses a coil in a magnetic field; a current-carrying wire experiences a force when placed in a magnetic field.
应用包括电铃、继电器、扬声器和废品场起重机。简单的电动机利用磁场中的线圈；通电导线在磁场中会受到力的作用。
10. Earth and Space | 地球与太空
Our Solar System consists of the Sun, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), dwarf planets, moons, asteroids and comets.
太阳系由太阳、八大行星（水星、金星、地球、火星、木星、土星、天王星、海王星）、矮行星、卫星、小行星和彗星组成。
Gravity provides the centripetal force that keeps planets in orbit around the Sun and moons around planets. Stronger gravity and faster orbital speed result in a more stable orbit.
引力提供向心力，使行星绕太阳公转，卫星绕行星公转。引力越强，轨道速度越快，轨道越稳定。
Earth rotates on its axis once every 24 hours, causing day and night. It orbits the Sun in about 365 ¼ days, causing the yearly cycle and, together with its tilted axis, the seasons.
地球每 24 小时自转一周，导致昼夜交替。地球绕太阳公转一周约 365 ¼ 天，产生年周期，并因地轴倾斜导致四季变化。
The phases of the Moon are due to the changing portion of the illuminated half visible from Earth. A solar eclipse occurs when the Moon passes between Earth and the Sun; a lunar eclipse occurs when Earth is between the Sun and the Moon.
月相变化是由于我们从地球看到的月球亮面比例不断变化。日食发生在月球运行到地球和太阳之间时；月食发生在地球位于太阳和月球之间时。
The Sun is a star, one of billions in the Milky Way galaxy. A light-year is the distance light travels in a year, used to measure vast distances in space.
太阳是一颗恒星，是银河系数十亿颗恒星中的一颗。光年是光在一年中传播的距离，用于衡量宇宙中的巨大距离。
Satellites can be natural (moons) or artificial. Artificial satellites are used for communication, weather monitoring and GPS.
卫星可以是天然的（月球）或人造的。人造卫星用于通信、气象监测和全球定位系统（GPS）。
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