Case Study Practice Exercises in Year 8 CIE Physics | 八年级CIE物理案例分析实战演练

📚 Case Study Practice Exercises in Year 8 CIE Physics | 八年级CIE物理案例分析实战演练

This article presents ten real-life case studies for Year 8 CIE Physics students. Each case is designed to reinforce core concepts from forces, energy, waves, electricity, heat, light, magnetism, and motion. By working through the scenarios, you can sharpen your understanding of how physics explains everyday observations and design practical solutions. Read each case carefully, think about the questions, and study the explanations provided.

本文为八年级CIE物理学习者准备了十个贴近生活的案例分析。每个案例旨在巩固力、能量、波、电、热、光、磁和运动等核心概念。通过分析这些场景，你可以加深对物理如何解释日常现象与设计实用方案的理解。请认真阅读每个案例，思考问题并学习给出的解释。

1. Case Study 1: The Bicycle Descent – Energy and Forces | 案例一：自行车下坡 – 能量与力

Scenario: A student rides a bicycle down a gentle slope without pedalling. At the top of the slope, the bicycle is stationary. As it moves downhill, its speed increases until it reaches a steady value on a flatter section, then she applies the brakes to stop completely.

场景：一名学生骑着自行车，不踩踏板，沿一段缓坡下行。坡顶处自行车静止。下行过程中速度不断增加，直到较平路段达到一个稳定值，最后捏刹车完全停下。

Energy analysis: At the top, the bicycle has gravitational potential energy relative to the bottom. As it descends, this energy is converted into kinetic energy (movement). Some energy is transferred as thermal energy due to friction in the wheel bearings and air resistance. The conversion continues until she brakes, when the kinetic energy transforms into thermal energy in the brake pads, stopping the bicycle. The total energy of the system (bicycle + surroundings) stays constant – this is the principle of conservation of energy.

能量分析：在坡顶，自行车相对于坡底具有重力势能。下行时这部分能量转化为动能（运动）。部分能量因车轮轴承摩擦和空气阻力转化为热能。这种转化一直持续到刹车，此时动能转化为刹车片的热能，使自行车停下。系统（自行车+环境）的总能量保持不变——这就是能量守恒定律。

Forces and steady speed: Why does the bicycle reach a steady speed before braking? On the slope, a component of the gravitational force pulls her forward. Air resistance and friction oppose this motion. At first the forward force is greater, so the bicycle accelerates. As speed increases, air resistance grows. Eventually the backward resistive forces balance the forward pull from gravity. The net force becomes zero, and according to Newton’s first law, the bicycle moves at constant velocity.

力与稳定速度：为什么自行车在刹车前会达到稳定速度？下坡时，重力沿斜面的分力向前拉，而空气阻力和摩擦力向后阻碍运动。起初向前力更大，自行车加速。随着速度增加，空气阻力变大，最终向后的阻力与向前的重力分力平衡，净力为零，根据牛顿第一定律，自行车做匀速直线运动。

Gravitational potential energy → Kinetic energy + Thermal energy

重力势能 → 动能 + 热能

Practical thinking: Braking transfers kinetic energy by friction. Hard braking makes the pads hotter but wears them down. Engineers design brake materials with high heat tolerance.

实际思考：刹车通过摩擦转移动能。急刹车会让刹车片更热但也会磨损。工程师设计刹车材料需要耐高温。

2. Case Study 2: Investigating Sound Waves with a Tuning Fork | 案例二：用音叉探究声波

Scenario: A student strikes a tuning fork and holds it near a table tennis ball suspended by a thread. The ball starts to vibrate. She then dips the vibrating fork into a beaker of water and sees tiny splashes. Next she places the fork on a wooden table and hears a loud sound that can be heard across the room.

场景：一名学生敲击音叉并靠近用细线悬挂的乒乓球，球开始振动。接着她把振动的音叉浸入烧杯水中，看到细小水花飞溅。然后她把音叉放在木桌上，整个房间都能听到响亮的声音。

Interpretation: The vibrating tuning fork produces sound by making the surrounding air particles vibrate back and forth. These vibrations are longitudinal waves. When the fork is near the ping-pong ball, the air particle vibrations hit the ball and make it move, demonstrating that sound carries energy. The water splash also shows that the fork is vibrating vigorously – sound is produced by mechanical vibrations. The loudness on the table increases because the table surface itself vibrates over a larger area, pushing more air particles and amplifying the sound. This is forced vibration of a larger object.

解读：振动的音叉通过迫使周围空气粒子来回振动而产生声音。这些振动是纵波。当音叉靠近乒乓球，空气粒子的振动撞击球并使其运动，证明声音携带能量。水花飞溅也表明音叉在剧烈振动——声音由机械振动产生。放在木桌上声音更响，因为桌面本身在一个更大面积上振动，推动更多空气粒子，从而放大声音。这是较大物体的受迫振动。

Sound travel: Sound needs a medium (solid, liquid, or gas) to travel. In air, sound travels at about 340 m/s. In water, it is faster (about 1500 m/s) because particles are closer together and pass vibrations more quickly. In a solid like wood, sound is even faster. However, sound cannot travel through a vacuum because there are no particles to vibrate.

声的传播：声音需要介质（固体、液体或气体）传播。在空气中声速约340 m/s。在水中更快（约1500 m/s），因为粒子间距更近，振动传递更快。在木头等固体中声速更快。但声音无法在真空中传播，因为没有粒子振动。

Pitch and frequency: The tuning fork is marked ‘440 Hz’ – it vibrates 440 times each second. The frequency determines the pitch. A higher frequency gives a higher-pitched sound. Loudness depends on the amplitude of vibration: harder strike gives larger amplitude and louder sound.

音调与频率：音叉标有’440 Hz’——每秒振动440次。频率决定音调高低，频率越高声音越尖锐。响度取决于振幅：敲击越用力，振幅越大，声音越响。

3. Case Study 3: A Faulty Circuit – Electrical Troubleshooting | 案例三：故障电路 – 电路故障排查

Scenario: A student builds a circuit with a cell, a switch, a lamp, and two identical resistors in series. When she closes the switch, the lamp does not light. She checks the cell with a voltmeter and it reads 3.0 V. She then replaces the lamp with a working one – still no light. She measures the voltage across one resistor and gets 0 V, and across the other resistor also 0 V. A continuity tester shows that both resistors are conducting. Where is the fault?

场景：一名学生用一节电池、一个开关、一盏灯和两个相同电阻串联搭建电路。闭合开关后灯不亮。她用电压表检查电池，读数为3.0 V。然后换了一个完好的灯泡，仍然不亮。她测量其中一个电阻两端电压为0 V，另一个电阻也是0 V。用通断测试器检查两个电阻都导通。故障在哪里？

Deduction: In a series circuit, current must flow through all components for the lamp to light. The cell has voltage, the resistors conduct, and the lamp is new – so the fault is not an open circuit in the resistors or lamp. However, if the switch is faulty (high resistance when closed) or a wire is broken, no current flows. The key clue: voltage across both resistors is 0 V even though the cell is 3.0 V. This means no current is flowing at all, so there must be a break somewhere else, possibly in the wires, a loose connection, or the switch itself not closing properly. A continuity check of the wires and switch would reveal the break.

推理：在串联电路中，电流必须流过所有元件灯才会亮。电池有电压，电阻导通，灯泡是新的，所以故障不是电阻或灯泡断路。但是，如果开关故障（闭合时电阻很大）或某根导线断开，就没有电流。关键线索：两个电阻两端电压都是0 V，尽管电池有3.0 V。这意味着完全没有电流存在，因此电路中某处一定有断开，可能在导线中、连接松动或开关本身未正常闭合。对导线和开关进行通断测试即可发现断点。

Troubleshooting steps: 1. Check cell voltage – OK. 2. Check lamp – OK. 3. Measure voltage across individual components – if all read 0 V, the circuit is open somewhere before these points. 4. Use a continuity tester (or buzzer) across each wire and across the switch when closed – the faulty element will show no continuity. In this case, the switch might appear closed but its contacts are dirty or broken.

排查步骤：1. 检查电池电压——正常。2. 检查灯泡——正常。3. 逐个测量元件电压——如果都读0 V，说明在这些测量点之前的某处断路。4. 使用通断测试器（或蜂鸣器）测量每根导线和闭合时开关的两端——故障元件会显示不导通。本例中，开关外观可能处于闭合状态，但其触点可能脏污或断裂。

Symptom	Possible cause
Lamp off, cell voltage normal	Open circuit (broken wire, bad switch contact)
Lamp off but voltage across switch closed is 3.0 V	Switch has high resistance (bad contact)

以上表格整理了常见故障现象与可能原因。

4. Case Study 4: The Cooling Cup of Tea – Heat Transfer | 案例四：一杯茶变凉 – 热传递

Scenario: A student pours hot tea at 85 °C into three different cups: a ceramic mug, a stainless steel cup, and a polystyrene foam cup. She records the temperature every minute for 10 minutes. The ceramic mug cools fastest, the steel cup slightly slower, and the polystyrene cup stays hot longest. Room temperature is 22 °C.

场景：一名学生将85 °C的热茶分别倒入陶瓷杯、不锈钢杯和聚苯乙烯泡沫杯。她每隔一分钟记录温度，持续10分钟。陶瓷杯降温最快，不锈钢杯稍慢，聚苯乙烯杯保温最久。室温为22 °C。

Heat transfer mechanisms: The tea loses heat through conduction (through cup walls), convection (air currents carrying warm air away), and radiation (infrared from the surface). Metals like steel are good conductors – they allow heat to travel quickly through their walls to the outside, so the steel cup cools nearly as fast as ceramic, but ceramic is also a decent conductor and typically has more surface area or thinner walls. Polystyrene foam contains trapped air pockets; air is a poor conductor (an insulator), greatly reducing conduction. This is why the foam cup keeps the tea hot longer. Convection and radiation still occur from the top surface of the tea in all cups, which is why none can stay hot indefinitely.

热传递机制：茶通过传导（经杯壁）、对流（气流带走热空气）和辐射（液体表面红外线）散热。金属如不锈钢是良导体，热量可快速通过杯壁传至外部，因此钢杯降温几乎和陶瓷杯一样快，但陶瓷也是不错的导体，而且通常表面积更大或杯壁更薄。聚苯乙烯泡沫含有密封的空气泡；空气是热的不良导体（绝缘体），显著降低传导。这就是泡沫杯保温更久的原因。所有杯中茶的上表面仍会发生对流和辐射，所以没有任何杯子可以无限保温。

Practical design: Insulating flasks use a double-wall vacuum to stop conduction and convection; silvered surfaces reduce radiation. At home, putting a lid on a cup reduces evaporation and convection, keeping the drink hotter for longer. This case highlights that the rate of heat transfer depends on the material’s thermal conductivity, surface area, temperature difference, and insulation.

实际设计：保温瓶使用双层真空内胆来杜绝传导和对流；镀银表面减少辐射。在家里，盖上杯盖可以减少蒸发和对流，让热饮保持更久。这个案例说明热量传递速率取决于材料的导热性、表面积、温差和隔热措施。

5. Case Study 5: The Convex Lens and the Burning Paper – Light and Optics | 案例五：凸透镜与烧纸实验 – 光的折射

Scenario: On a sunny day, a student holds a convex lens between a sheet of paper and the sun. She adjusts the distance until a tiny, bright point appears on the paper. After a few seconds, the paper begins to smoke and burn. She measures the distance from the lens to the paper as 8 cm.

场景：晴天，一名学生把一张纸放在阳光下，手持凸透镜置于纸与太阳之间。她调整距离，直到纸上出现一个极小的明亮光点。几秒后纸开始冒烟并燃烧。她测得透镜到纸的距离为8 cm。

Optical principle: A convex lens converges parallel light rays from the sun to a single point called the focal point. The distance from the centre of the lens to the focal point is the focal length – here 8 cm. The intense concentration of light energy at the focal point raises the temperature of the paper above its ignition point, causing it to burn. This is a simple way to estimate the focal length of a convex lens. The lens bends light by refraction; the curvature of the lens surfaces determines the focal length.

光学原理：凸透镜将来自太阳的平行光线会聚于一点，即焦点。透镜中心到焦点的距离就是焦距——此处为8 cm。焦点处光能高度集中，使纸张温度升至燃点以上，导致燃烧。这是估算凸透镜焦距的简易方法。透镜通过折射使光线弯曲；透镜表面的曲率决定焦距。

Image formation: At the focal plane, you do not see a clear image of the sun; you see a tiny bright disk (in fact a real image of the sun). A convex lens can form real or virtual images depending on object distance. When an object is placed beyond the focal length, a real, inverted image appears on the opposite side. This principle is used in cameras and the human eye.

成像规律：在焦平面上，你看到的不是太阳的清晰图像，而是一个微小亮斑（实际是太阳的实像）。凸透镜可根据物距生成实像或虚像。物体置于焦距之外时，透镜另一侧会出现倒立的实像。这一原理应用于相机和人眼。

6. Case Study 6: Floating and Sinking – Density and Buoyancy | 案例六：浮与沉 – 密度与浮力

Scenario: A student places a wooden block, a metal cube of the same volume, and a large block of plastic in a tank of water. The wooden block floats, the metal cube sinks, and the plastic block floats but lower in the water. She then puts a lump of plasticine into the water – it sinks. She reshapes the same plasticine into a boat shape, and it floats.

场景：学生将一块木块、一个同体积的金属立方体和一大块塑料放入水箱中。木块漂浮，金属块下沉，塑料块漂浮但吃水较深。然后她把一块橡皮泥放入水中——它沉了。她又把同一块橡皮泥捏成船形，它浮起来了。

Explanation: Whether an object floats or sinks depends on its average density compared to the density of water (1 g/cm³). Wood has density less than water, so it floats. Metal (e.g., iron, density ≈ 7.8 g/cm³) sinks. Plastic may have density slightly less than water, so it floats but with most of its volume submerged. The plasticine initially sinks because its material density is greater than water and it is a solid lump, displacing a small weight of water; the upthrust (buoyant force) equals the weight of water displaced. When reshaped into a boat, the overall volume is much larger and the effective average density (mass/total volume including air inside) becomes less than water, so it floats. The boat displaces more water, causing a larger upthrust equal to its weight.

解释：物体浮沉取决于其平均密度与水的密度（1 g/cm³）的比较。木材密度小于水，因此漂浮。金属（如铁，密度约7.8 g/cm³）下沉。塑料密度可能略小于水，所以漂浮但大部分体积浸在水中。橡皮泥一开始下沉，因为它材料的密度大于水，且为实心块，排开的水量小；浮力等于排开水重。捏成船形后，整体体积变大，平均密度（质量/包含空气的总体积）变得小于水，于是漂浮。船排开更多水，产生更大的等于其重量的浮力。

Density ρ = Mass m ÷ Volume V ; Upthrust = Weight of displaced fluid

密度 ρ = 质量 m ÷ 体积 V ; 浮力 = 排开流体的重量

Applications: Ships are made of steel but are hollow, so their average density is less than water, allowing them to carry heavy cargo. Submarines adjust their average density by taking in or releasing water ballast to dive or surface.

应用：轮船用钢制造但中空，平均密度小于水，因此能装载重货。潜水艇通过吸入或排出压载水调节平均密度，实现下潜或上浮。

7. Case Study 7: Measuring Speed on a School Run – Motion Graphs | 案例七：上学路上测速度 – 运动图像

Scenario: A student walks from home to school, 1200 m away, in 20 minutes. She records her journey with a fitness tracker. She starts from rest, speeds up to a brisk walk, then stops for 2 minutes at a traffic light, continues at the same speed, and finally slows down as she arrives. Her distance–time data is plotted.

场景：一名学生从家步行到1200米外的学校，用时20分钟。她用手环记录行程。她从静止出发，加速至快走，然后在红绿灯处停下两分钟，接着以同样速度继续，最后到达时慢下来。她的路程–时间数据被绘成图表。

Graph interpretation: On a distance–time graph, a straight sloping line represents constant speed. The steeper the slope, the greater the speed. A horizontal line indicates stopping. Acceleration and deceleration appear as curves. Her journey shows an initial curve upwards (acceleration), then a steep straight line (constant brisk speed, e.g. 100 m/min), a flat section for the stop, another steep line, and a final downward curve (slowing down). The average speed for the whole journey is total distance ÷ total time = 1200 m ÷ 20 min = 60 m/min (or 1 m/s).

图像解读：在路程–时间图中，倾斜直线代表匀速运动。斜率越大，速度越快。水平线表示停止。加速和减速表示为曲线。她的行程显示：开始为向上的曲线（加速），随后是一条陡直直线（恒定快走速度，例如100 m/min），一段水平线（停止），又一条直线，最终是一条趋于平缓的曲线（减速）。全程平均速度 = 总路程 ÷ 总时间 = 1200 m ÷ 20 min = 60 m/min（或1 m/s）。

Speed calculations: For the brisk walking segment, speed = slope = change in distance / change in time. If the graph shows that she covers 600 m in 6 minutes in that segment, speed = 600 ÷ 6 = 100 m/min. Her speed while crossing the road is zero. Plotting a speed–time graph would show a rise to 100 m/min, a constant line, a drop to zero, then repeat.

速度计算：对于快走阶段，速度 = 斜率 = 距离变化量 / 时间变化量。如果她在该阶段用6分钟走完600米，速度 = 600 ÷ 6 = 100 m/min。过马路时速度为零。绘制速度–时间图会看到升至100 m/min的上升段、水平段、降为零等。

8. Case Study 8: Magnetic Attraction and the Steel Paperclip – Magnetism | 案例八：磁铁吸引回形针 – 磁性

Scenario: A student hangs a steel paperclip from a cotton thread. She brings a bar magnet close to the paperclip without touching it. The paperclip swings towards the magnet and sticks to it. She then touches the paperclip with the magnet; now the paperclip itself can attract another paperclip.

场景：学生用棉线悬挂一枚钢制回形针。她将条形磁铁靠近回形针但不接触。回形针摆向磁铁并吸附上。然后她用磁铁接触回形针；现在回形针本身也能吸引另一枚回形针。

Magnetic induction: Steel is a ferromagnetic material (contains iron). Before the magnet is near, the magnetic domains in the paperclip point randomly, cancelling each other. When a magnet approaches, the magnetic field aligns some domains in the paperclip, inducing temporary magnetism – the paperclip becomes an induced magnet. The opposite pole of the induced magnet faces the real magnet, so they attract. Once the magnet is removed, the domains in a hard steel paperclip may stay partially aligned, retaining some magnetism – this is how permanent magnets are made. A soft iron nail would lose its magnetism quickly because its domains readily return to random alignment.

磁感应：钢是铁磁性材料（含铁）。磁铁未靠近时，回形针内部的磁畴指向混乱，相互抵消。当磁铁靠近，磁场使回形针内一部分磁畴排列整齐，产生感应磁性——回形针成为被诱导的磁体。被诱导磁体的异极对着磁铁，因此相吸。一旦移开磁铁，硬钢制成的回形针可能保留部分有序磁畴，从而留有残磁——这就是永磁体的制作原理。软铁钉则会很快失去磁性，因为其磁畴容易恢复无序。

Everyday magnets: Refrigerator magnets, magnetic strips on credit cards, and compass needles all rely on permanent magnetism. A compass works because the Earth itself is a giant magnet, and a free-moving magnet aligns with Earth’s magnetic field, pointing roughly north–south.

日常磁体：冰箱贴、信用卡磁条和指南针都依赖永磁性。指南针之所以工作，是因为地球本身就是一个巨大的磁体，可自由旋转的小磁针会与地磁场对齐，指向南北。

9. Case Study 9: Insulating the Hot Water Tank – Energy Saving | 案例九：热水箱的保温 – 节能设计

Scenario: A household hot water tank is a metal cylinder. It loses heat rapidly when first installed, so the family adds a 50 mm thick fibre wool jacket around it. The temperature of the hot water dropped from 70 °C to 65 °C in 2 hours without the jacket, but only dropped to 68 °C in the same time with the jacket. The family saves on their energy bill.

场景：家中的热水箱是一个金属圆筒。刚安装时散热很快，于是加装了一层50毫米厚的纤维棉保温套。未加保温套时，热水在2小时内从70 °C降至65 °C；加装后同等时间内仅降至68 °C。家庭能源账单因此节省。

Heat loss reduction: The metal tank is a good conductor. Heat conducts through the metal to the outside air, and then convection and radiation carry the heat away. The fibre wool jacket traps air in tiny pockets; air is a very poor conductor (low thermal conductivity), so the conduction path is broken. Convection within the wool is also reduced because air is held in place. The rate of heat transfer depends on the temperature difference between the water and the surroundings, the surface area, and the thickness and conductivity of the insulation. Adding the wool increases thermal resistance, slowing heat loss. The heat lost is given by Q = m c Δθ (where m is mass, c specific heat capacity, Δθ temperature change).

减少热损失：金属水箱是良导体。热量通过金属传导至外部空气，再由对流和辐射带走。纤维棉保温套将空气储存在微小孔隙中；空气是极差的导体（导热系数低），因此传导路径被阻断。棉套内的对流传热也因空气被锁定而减少。热量传递速率取决于水温与环境的温差、表面积以及隔热层的厚度与导热性。加装纤维棉提高了热阻，减缓热损失。损失的热量可用 Q = m c Δθ 计算（m 为质量，c 比热容，Δθ 温度变化）。

Energy saving and the environment: Slowing heat loss means less energy is needed to reheat the water, reducing fuel use and carbon emissions. Building regulations mandate insulation for hot water tanks and pipes. Double-glazed windows use a similar principle – trapped air between two glass panes cuts heat transfer through windows.

节能与环保：减缓热损失意味着重新加热水所需的能量减少，从而降低燃料消耗和碳排放。建筑规范强制要求给热水箱和管道加保温层。双层玻璃窗利用类似原理——两片玻璃之间的密闭空气层减少了通过窗户的热传递。

10. Case Study 10: Why We See Lightning Before Thunder – Speed of Light and Sound | 案例十：先见闪电后闻雷声 – 光速与声速比较

Scenario: During a thunderstorm, a student observes a bright flash of lightning. Ten seconds later, she hears the rumble of thunder. She knows that lightning and thunder happen at the same time and from the same distance. Using this data, she estimates how far away the lightning struck.

场景：雷雨时，学生看到一次明亮的闪电，10秒后听到雷声隆隆。她知道闪电和雷声同时同地发生。利用这个数据，她估算出闪电发生地有多远。

Speed difference: Light travels at about 300,000,000 m/s (3 × 10₈ m/s), while sound in air travels at roughly 340 m/s. The light from the lightning reaches her eyes almost instantly (time is negligible). The sound travels much slower, so the delay (10 s) is essentially the time the thunder takes to reach her. Distance = speed of sound × time = 340 m/s × 10 s = 3400 m (about 3.4 km). If the delay is shorter, the storm is closer. Every 3 seconds delay corresponds to roughly 1 km. This case shows the dramatic difference in speed between light and sound.

速度差：光速约 300,000,000 m/s (3×10₈ m/s)，而空气中声速约340 m/s。闪电的光几乎瞬间到达眼睛（时间可忽略）。声音慢得多，所以延迟的10秒几乎是雷声传播的时间。距离 = 声速 × 时间 = 340 m/s × 10 s = 3400米（约3.4公里）。延时越短，雷电越近。大约每3秒延迟对应1公里。此案例展示了光与声速度的巨大差异。

Distance d = Speed of sound v × Time t

距离 d = 声速 v × 时间 t

Further applications: This principle is used in echolocation by bats and in sonar by ships. The time for a sound pulse to travel to an object and back is measured, and distance is calculated as d = (v × t) ÷ 2. Light’s great speed also

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Case Study Practice Exercises in Year 8 CIE Physics | 八年级CIE物理案例分析实战演练