📚 Year 7 AQA Physics: Case Study Practice – Real-World Applications | Year 7 AQA 物理：案例分析实战演练
Physics explains everything around us, from riding a bike to hearing sounds underwater. In this case study practice, we will look at real-life situations and apply key ideas from the Year 7 AQA Physics course – forces, energy, electricity, particles and waves. Each case includes a clear physical explanation so you can see how textbook concepts solve everyday puzzles.
物理解释了我们周围的一切，从骑自行车到在水下听到声音。在这个案例分析练习中，我们将观察真实情景，并运用 Year 7 AQA 物理课程的关键概念——力、能量、电、粒子和波。每个案例都包含清晰的物理解释，让你看到课本知识如何解决日常谜题。
1. Bicycle Brakes and Friction | 自行车刹车与摩擦力
You ride your bike to a friend' y and cover the distance in 30 s. Your average speed v is found using v = distance ÷ time.
你骑自行车去 180 米远的朋友家，用时 30 秒。平均速度 v 用 v = 路程 ÷ 时间计算。
v = d ÷ t = 180 m ÷ 30 s = 6 m/s
When you need to stop, you squeeze the brake lever. The brake pads press against the wheel rim, creating friction that opposes the motion. This friction converts kinetic energy into thermal energy, heating the pads and rim, and brings the bike to a halt. The force of friction depends on how hard the pads press and on the roughness of the surfaces. Wet or smooth tyres reduce friction, increasing the stopping distance.
当你需要停车时，捏紧刹车手柄。刹车片压紧轮圈，产生阻碍运动的摩擦力。这种摩擦力将动能转化为热能，加热刹车片和轮圈，使自行车停下。摩擦力的大小取决于刹车片压紧的力度和表面的粗糙程度。潮湿或光滑的轮胎会减小摩擦力，从而增加刹车距离。
2. Cooling Hot Chocolate | 热巧克力的冷却
A mug of steaming hot chocolate at 80 °C sits on a kitchen table. Over time, its temperature drops to match the room temperature, 20 °C. Thermal energy moves from the hot drink to the cooler surroundings by conduction through the mug, convection in the air above, and infrared radiation. A metal mug conducts heat away quickly because metals are good conductors. A ceramic mug slows heat loss, and a flask with a vacuum layer almost stops conduction and convection entirely.
一杯热气腾腾的 80 °C 热巧克力放在餐桌上。慢慢地，它的温度会下降到与室温 20 °C 相同。热能从热饮传递到较冷的周围环境，方式有：通过杯子的传导、上方空气的对流以及红外辐射。金属杯子导热快，因为金属是良导热体；陶瓷杯能减缓热量散失；而带真空层的保温瓶几乎完全阻止传导和对流。
| Material | Thermal conductivity | Time to cool from 80 °C to 40 °C (approx.) |
|---|---|---|
| Steel | High | 10 minutes |
| Glass | Medium | 25 minutes |
| Vacuum flask | Very low | Several hours |
Insulation works by trapping air, which is a poor conductor, or by creating a vacuum so there are no particles to carry heat. Shiny silvered surfaces also reflect infrared radiation back into the drink, keeping it warmer for longer.
保温的原理是困住不善于导热的空气，或是制造真空使没有粒子来传递热量。光亮的镀银表面还能将红外辐射反射回饮料中，从而更久地保温。
3. Why a Simple Circuit Won't Light | 简单电路为何不亮
A pupil builds a series circuit with a cell, a switch and one lamp, but the lamp stays dark. In year 7 AQA physics, circuit faults can be diagnosed step by step. First, check that the cell is not flat and that it is placed the correct way around in the holder. Next, look at all wires: a loose connection or a broken wire will stop the current. Finally, examine the lamp – its filament may be broken. When any part of a series circuit is broken, current cannot flow anywhere in the loop.
一个学生用一节电池、一个开关和一个小灯泡搭建串联电路，但灯泡就是不亮。在 Year 7 AQA 物理中，电路故障可以逐步排查。首先，检查电池是不是没电了，以及电池是否装反。接着，看看所有导线：松动的连接或断开的导线会停止电流。最后，检查灯泡——灯丝可能烧断了。串联电路中任何一处断开，整个回路都不会有电流。
Current is the flow of electric charge, and it needs a complete conducting path. If everything is connected correctly, the cell pushes charge through the lamp, and the filament heats up until it glows. A simple way to test components is to swap them one by one with known working parts.
电流是电荷的流动，它需要一个完整的导电路径。如果一切连接正确，电池会推动电荷流过灯泡，灯丝加热至发光。测试元件的一个简单方法是逐个用已知能正常工作的部件进行替换。
4. Perfume and the Particle Model | 香水与粒子模型
A bottle of perfume is opened in a corner of a room. Within minutes, the scent reaches the opposite side. The particle model explains this: all matter is made of tiny particles in constant, random motion. In a gas, particles move quickly and spread out to fill available space. This process is called diffusion. Perfume particles collide with air particles and gradually mix, moving from a region of high concentration to one of low concentration.
房间里打开一瓶香水，几分钟后对面的角落就能闻到香味。粒子模型解释了这个现象：所有物质都由微小的粒子组成，它们持续做无规则运动。气体中粒子运动速度快，会扩散并充满可用空间。这个过程叫扩散。香水粒子与空气粒子碰撞，逐渐混合，从高浓度区域向低浓度区域移动。
Diffusion is faster in warmer rooms because particles have more kinetic energy and move faster. In solids, diffusion is extremely slow because particles are held tightly in fixed positions and only vibrate.
房间温度较高时扩散更快，因为粒子具有更多动能，运动更快。在固体中，扩散极为缓慢，因为粒子被紧固在固定位置上，只能振动。
5. Bouncing on a Trampoline | 蹦床跳跃
When a student stands on a trampoline, the springs and mat stretch, storing elastic potential energy. As they jump, this stored energy is transferred into kinetic energy and gravitational potential energy. At the highest point, gravitational potential energy is at a maximum and kinetic energy is momentarily zero. On the way down, gravitational force pulls the jumper downward, converting gravitational potential energy back into kinetic energy, and the trampoline stretches again.
当一个学生站在蹦床上，弹簧和床面会被拉伸，储存弹性势能。跳起时，这份储存的能量转化为动能和重力势能。在最高点，重力势能达到最大，动能为零。下落时，重力将跳跃者向下拉，重力势能再度转化为动能，蹦床重新被拉伸。
The energy transfers continue until air resistance and internal heating in the springs gradually remove energy from the bouncing system. Without any energy inputs, the bouncing height decreases over time. This is a perfect example of how energy is never destroyed but can be transferred into less useful forms, like thermal energy, that disperse into the surroundings.
这种能量转换会持续下去，直到空气阻力和弹簧内部发热逐渐将能量从弹跳系统中带走。没有能量输入时，弹跳高度会随时间降低。这完美地说明了能量既不会凭空消失，只会转化为一些不太有用的形式，如热能，并散逸到周围环境中。
6. Hearing Sounds Underwater | 水下听到声音
A diver submerges and hears a boat engine's hum clearly, even though it sounds muffled above water. Sound is a wave produced by vibrating objects. It needs a medium – solid, liquid or gas – to travel. Water is denser than air, so its particles are closer together. This allows sound waves to travel faster in water (about 1500 m/s) than in air (about 340 m/s). In solids, vibrations travel even more swiftly for the same reason.
一位潜水者潜入水中，清晰地听到了船的引擎声，尽管在水上这声音听起来很模糊。声音是由振动物体产生的波，需要介质——固体、液体或气体——才能传播。水的密度比空气大，粒子排列得更紧密，这使得声波在水中的传播速度（约 1500 m/s）比在空气中（约 340 m/s）更快。由于同样原因，声音在固体中传播得更快。
The diver hears the sound because water particles vibrate and pass the vibrations to the eardrum. Sound cannot travel in a vacuum, as there are no particles to carry the wave. This is why space is silent.
潜水者听到声音，因为水的粒子振动并将振动传递给鼓膜。声音无法在真空中传播，因为那里没有粒子来传递波动。这就是太空中寂静无声的原因。
7. Maglev Trains and Non-Contact Forces | 磁悬浮列车与非接触力
A maglev (magnetic levitation) train floats above the track without touching it. Powerful electromagnets on the train and the track repel each other, creating an upward magnetic force that balances the train's weight. Because there is no physical contact, friction between the train and the track is almost eliminated, allowing very high speeds.
磁悬浮列车悬浮在轨道上方，不与轨道接触。列车和轨道上的强电磁铁相互排斥，产生向上的磁力，平衡列车的重量。由于没有物理接触，列车与轨道之间的摩擦力几乎被消除，从而能实现极高的速度。
Magnetic force is a non-contact force, like gravity and electrostatic force. It can act over a distance, and its strength depends on the distance between magnets – the closer together, the stronger the repulsion or attraction.
磁力是一种非接触力，类似于重力和静电力。它可以在一定距离外起作用，其强度取决于磁铁之间的距离——靠得越近，排斥或吸引力越强。
8. Designing a Travel Mug | 旅行保温杯的设计
A travel mug keeps coffee hot for hours. Its design reduces all three methods of heat transfer:
旅行保温杯能让咖啡保持热度数小时。它的设计减少了所有三种传热方式：
A double-wall with a vacuum between the layers stops conduction and convection because there are almost no particles to transfer heat. A shiny silver coating inside the vacuum layer reflects infrared radiation back towards the drink. The tight-fitting plastic lid prevents hot air from rising out (convection) and stops cold air entering. The outer case is often made of thin stainless steel, which is a poor conductor in thin layers, and it has a small contact patch with the cup interior to further limit conduction.
双层杯壁之间的真空层阻止了传导和对流，因为几乎没有粒子可以传递热量。真空层内的镀银光亮涂层将红外辐射反射回饮料。紧密的塑料盖子防止热空气逸出（对流）并阻挡冷空气进入。外壳通常使用薄不锈钢，薄层不锈钢导热性差，且与内胆的接触面很小，进一步限制了传导。
Conduction is reduced by vacuum and minimal contact.
真空层和最小接触面积减少了传导。
Convection is stopped because fluid movement is blocked by the vacuum and the lid.
真空和盖子阻止了流体的移动，从而防止对流。
Radiation is reflected by the mirrored surface.
镜面反射了热辐射。
9. Flying a Kite – Balanced and Unbalanced Forces | 放风筝——平衡力与不平衡力
A kite stays in the sky when four forces balance: weight pulling downwards, lift from the air pushing upwards, tension in the string pulling partly downwards and backwards, and thrust from the wind pushing forward. When you pull on the string, tension increases, and if the wind is steady, the kite will climb or dive. Unbalanced forces cause the kite's velocity to change – it speeds up, slows down or changes direction.
风筝在空中停留时，四种力达到平衡：重力向下拉，升力由空气向上推，线的张力向后下方拉，风的推力向前。当你拉动风筝线时，张力增大，如果风稳定，风筝就会上升或俯冲。不平衡的力会改变风筝的速度——它可能加速、减速或改变方向。
If the string snaps, tension is removed, and the unbalanced weight and drag soon bring the kite down. Whenever forces on an object are not balanced, the object accelerates in the direction of the resultant force, which is a key concept in Year 7 forces.
如果线断了，张力消失，不平衡的重力和阻力很快使风筝坠落。只要作用在物体上的力不平衡，物体就会沿合力的方向加速，这是 Year 7 力学中一个关键概念。
10. Light Bulb vs. LED – Energy Efficiency | 灯泡与 LED 灯——能量效率
A traditional filament light bulb glows when an electric current passes through a thin tungsten wire. The wire heats up to around 2500 °C and emits visible light. However, only about 10 % of the electrical energy is transferred usefully as light; the rest is wasted as thermal energy spreading into the room. An LED (light-emitting diode) works differently – it passes current through a semiconductor, which emits light without needing extreme heat. An LED converts roughly 80 % of the electrical energy into light, making it far more efficient.
传统的灯丝灯泡在电流通过细钨丝时会发光。钨丝被加热到约 2500 °C 并发出可见光。然而，只有大约 10 % 的电能被有效地转化为光，其余都以热能的形式散失到房间里。LED（发光二极管）的工作方式不同——电流通过半导体，无需极高温度就能发光。LED 将大约 80 % 的电能转化为光，效率高得多。
| Bulb type | Input power | Useful light output | Efficiency (approx.) |
|---|---|---|---|
| Filament | 60 W | 6 W as light | 10 % |
| LED | 10 W | 8 W as light | 80 % |
Choosing more efficient appliances reduces wasted energy and saves money. This case study ties together energy transfer, power and the importance of thinking about useful versus wasted energy pathways.
选择更高效的电器能减少能量浪费，节省开支。这个案例将能量转化、功率以及考虑有效能量与浪费能量路径的重要性联系起来。
Published by TutorHao | Physics Revision Series | aleveler.com
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