📚 High-Frequency Topics and Common Mistakes in Year 7 CAIE Physics | Year 7 CAIE 物理：高频考点与易错题分析
Welcome to your guide on high-frequency topics and common pitfalls in Year 7 CAIE Physics. Mastering these areas will boost your confidence and exam performance. We will explore the most tested concepts, highlight typical mistakes, and show you how to avoid them.
欢迎来到 Year 7 CAIE 物理高频考点与易错题分析指南。掌握这些领域能增强你的信心和考试表现。我们将探讨最常考的概念，指出典型错误，并教你如何避免。
1. Mass vs Weight | 质量与重量
Mass is the quantity of matter in an object, measured in kilograms (kg). It is a scalar quantity and remains constant everywhere in the universe. Weight is the gravitational force acting on that mass, measured in newtons (N). It depends on the local gravitational field strength (g). On Earth, g ≈ 10 N/kg, so weight (W) = mass (m) × g.
质量是物体所含物质的多少，单位是千克(kg)。它是标量，在宇宙任何地方保持不变。重量是作用在质量上的重力，单位是牛顿(N)。它取决于当地引力场强度(g)。在地球上，g ≈ 10 N/kg，因此重量(W) = 质量(m) × g。
- Common mistake: “My weight is 50 kg.” Correction: “My mass is 50 kg, so my weight is 500 N.”
- 常见错误：”我的重量是50千克。” 纠正：”我的质量是50 kg，因此我的重量是500 N。”
- Mistake: thinking mass changes on the Moon. Correction: mass stays the same, but weight decreases because g is lower.
- 错误：认为在月球上质量会变。纠正：质量不变，但重量减小，因为g较小。
2. Speed, Velocity and Acceleration | 速率、速度与加速度
Speed is the distance travelled per unit time (scalar). Velocity includes direction (vector). Acceleration is the rate of change of velocity. The equation is a = (v − u) / t, where v is final velocity, u is initial velocity, t is time taken. The unit is m/s².
速率是单位时间经过的距离(标量)。速度包含方向(矢量)。加速度是速度变化的快慢。公式为 a = (v − u) / t，其中 v 是末速度，u 是初速度，t 是时间。单位是 m/s²。
- Mistake: Using total distance and total time to find average speed but ignoring direction. Correct: average speed = total distance / total time.
- 错误：用总路程和总时间求平均速率，但忽略方向。正确：平均速率 = 总路程 / 总时间。
- Mistake: Saying acceleration only means speeding up. Deceleration (negative acceleration) is also acceleration.
- 错误：认为加速度只表示加速。减速(负加速度)也是加速度。
- Many students forget to convert units, e.g., minutes to seconds before using the formula.
- 许多同学忘记换算单位，如在使用公式前将分钟转换为秒。
3. Distance-Time Graphs | 距离-时间图
A distance-time graph shows how distance changes with time. The gradient (slope) represents speed. A straight horizontal line means the object is stationary. A straight sloping line indicates constant speed. A curved line shows acceleration or deceleration.
距离-时间图表示距离随时间的变化。斜率代表速率。水平直线表示物体静止。倾斜直线表示匀速。曲线表示加速或减速。
- Common error: Confusing a flat line on a distance-time graph with ‘moving slowly’. It actually means ‘at rest’.
- 常见错误：将距离-时间图中的水平线误认为是“缓慢移动”。实际上它表示“静止”。
- Error: Calculating speed from a curved graph by using the whole time without drawing a tangent (but in Year 7, students may just describe the motion).
- 错误：在曲线图中直接用总时间计算速度，而没有画切线（在Year 7，学生只需要描述运动）。
- When interpreting, always check axes: distance on y-axis, time on x-axis.
- 解读时，务必检查坐标轴：距离在y轴，时间在x轴。
4. Forces and Friction | 力与摩擦力
Forces are pushes or pulls, measured in newtons (N). Friction is a force that opposes motion between two surfaces in contact. It can be useful (e.g., walking) or a nuisance (e.g., heat in engines). Air resistance and water resistance are types of friction.
力是推或拉，单位是牛顿(N)。摩擦力是阻碍两个接触面相对运动的力。它可能是有用的（例如行走），也可能造成麻烦（例如引擎发热）。空气阻力和水阻力都是摩擦力的类型。
- Mistake: Thinking that friction always acts opposite to an object’s motion. It can also act in the direction of motion when driving a wheel, preventing slipping.
- 错误：认为摩擦力总是与物体运动方向相反。在汽车驱动轮时，摩擦力可以和运动方向相同，防止打滑。
- Common confusion: ‘No friction would make everything easier.’ In reality, without friction, we could not walk or hold objects.
- 常见困惑：“没有摩擦力一切都会更简单。”实际上，没有摩擦力我们无法行走或握住物体。
- Balanced forces vs. unbalanced forces: When forces are balanced, an object moves at constant speed or stays at rest. Unbalanced forces cause acceleration.
- 平衡力与不平衡力：当力平衡时，物体匀速运动或保持静止。不平衡力导致加速度。
5. Energy Transfer and Conservation | 能量转移与守恒
Energy cannot be created or destroyed, only transferred from one store to another. Common energy stores: kinetic, thermal (heat), chemical, gravitational potential, elastic, nuclear. Energy transfer pathways: heating, waves, electric current, and mechanical work.
能量不会凭空产生或消失，只会从一个储存库转移到另一个。常见的能量储存：动能、热能（热）、化学能、重力势能、弹性势能、核能。能量转移途径：加热、波、电流和机械做功。
- Mistake: Saying ‘energy is used up’. Correct: ‘Energy is transferred and may become less useful, often dissipated as heat.’
- 错误：说“能量被用完了”。正确：“能量被转移，可能变得不太有用，通常以热的形式散失。”
- Common error in Sankey diagrams: drawing the input arrow too wide compared with useful output arrows.
- 桑基图常见错误：输入箭头的宽度与有用输出不成比例。
- Calculating efficiency: Efficiency = (useful energy output / total energy input) × 100%. Many forget the × 100% or use incorrect numbers.
- 计算效率：效率 = (有用的能量输出 / 总能量输入) × 100%。许多同学忘记乘100%或代入错误数据。
6. Density and Particle Theory | 密度与粒子理论
Density (ρ) is mass per unit volume: ρ = m / V. Units: kg/m³ or g/cm³. 1 g/cm³ = 1000 kg/m³. The particle model explains properties of solids, liquids and gases. Particles are always moving.
密度(ρ)是单位体积的质量：ρ = m / V。单位：kg/m³ 或 g/cm³。1 g/cm³ = 1000 kg/m³。粒子模型解释固体、液体和气体的性质。粒子总是在运动。
- Mistake: Confusing density with ‘heaviness’. A large sponge might be heavier than a small rock but less dense.
- 错误：混淆密度和“轻重”。一块大海绵可能比一块小石头重，但密度更小。
- Error in density calculations: using g for mass and cm for volume but writing unit as kg/m³ without conversion.
- 密度计算错误：质量用克，体积用厘米³，单位却写kg/m³而没有换算。
- Particle misconception: “Particles in a solid do not move.” Correction: they vibrate about fixed positions.
- 粒子误解：“固体中的粒子不运动。”纠正：它们在固定位置振动。
- During expansion, the mass of a substance does not change; the space between particles increases.
- 膨胀时，物质的质量不变；粒子间的间距增大。
7. Series and Parallel Circuits | 串联与并联电路
In a series circuit, components are connected one after another. The current (I) is the same at all points, but the voltage (V) is shared. In a parallel circuit, each branch has the same voltage as the source, and the total current is the sum of branch currents.
在串联电路中，元件依次连接。电流(I)处处相等，但电压(V)被分配。在并联电路中，各支路电压与电源电压相同，总电流等于各支路电流之和。
- Mistake: ‘Adding more bulbs in series makes them brighter.’ Actually, resistance increases, current drops, bulbs become dimmer.
- 错误：“串联更多灯泡会使它们更亮。”实际上，电阻增大，电流减小，灯泡变暗。
- Mistake: Thinking the switch position in a series circuit affects which components turn off; a single switch controls the whole circuit.
- 错误：认为串联电路中开关的位置会影响哪些元件关闭；单个开关控制整个电路。
- In parallel: if one bulb blows, the others remain on. Many students think all go out.
- 并联中：如果一个灯泡坏了，其他灯泡仍然亮。很多学生认为全都熄灭。
- Measuring current and voltage: ammeter in series, voltmeter in parallel. Mixing them up causes short circuits.
- 测量电流和电压：电流表串联，电压表并联。混用会造成短路。
8. Light: Reflection and Refraction | 光的反射与折射
Law of reflection: angle of incidence (i) = angle of reflection (r). Both are measured from the normal. Refraction is the bending of light when it passes from one medium to another. When entering a denser medium (e.g., air to glass), light slows down and bends towards the normal.
反射定律：入射角(i) = 反射角(r)。两者都从法线量起。折射是光从一种介质进入另一种介质时发生弯曲。光进入更密介质（如空气到玻璃）时，速度减慢，向法线偏折。
- Mistake: drawing the reflected ray on the same side of the normal as the incident ray. They must be on opposite sides.
- 错误：将反射光线画在入射光线的同一侧。它们必须在法线两侧。
- Refraction error: when light enters a less dense medium (e.g., glass to air), it bends away from the normal, not towards.
- 折射错误：光进入更疏介质（如玻璃到空气）时，应远离法线偏折，而不是靠近。
- Virtual image in a plane mirror: located as far behind the mirror as the object is in front. It is laterally inverted, not rotated.
- 平面镜中的虚像：像在镜后的距离等于物在镜前的距离。虚像是左右颠倒的，不是旋转。
9. Heat Transfer: Conduction, Convection, Radiation | 热传递：传导、对流、辐射
Conduction is heat transfer through solids, especially metals, due to particle collisions and free electrons. Convection occurs in liquids and gases, where warm fluid rises and cold fluid sinks. Radiation is the transfer of heat as infrared waves; it does not require a medium.
传导通过固体传热，特别是金属，依靠粒子碰撞和自由电子。对流发生在液体和气体中，热的流体上升，冷的流体下降。辐射以红外波形式传热，不需要介质。
- Mistake: Thinking conduction happens in vacuum. It requires particles. Radiation is the only way heat can travel through vacuum.
- 错误：认为传导可以在真空中发生。传导需要粒子。辐射是唯一能在真空中传热的方式。
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