📚 IGCSE OCR Physics: Common Mistakes Analysis | IGCSE OCR 物理：易错题精讲
Many IGCSE OCR Physics candidates lose marks not because they don’t know the content, but because they repeatedly fall into the same traps. In this article, we break down some of the most common errors seen in past papers and show you how to avoid them.
许多 IGCSE OCR 物理考生丢分并非因为不会，而是反复掉进相同的陷阱。本文整理了历年真题中最典型的错误，并给出了避坑方法。
1. Current in Series and Parallel Circuits | 串并联电路中的电流
A classic mistake is to think that the current is ‘used up’ by components in a series circuit. Students often state that the current before a lamp is larger than the current after it. In reality, the current is the same at every point in a series circuit. In a parallel circuit, the current from the source splits at a junction, but the sum of the branch currents equals the source current.
一个经典错误是认为串联电路中的电流会被元件”消耗”。许多学生会说灯泡前面的电流比后面大。实际上，串联电路中各处的电流相等。在并联电路中，电源电流在节点处分配，但各支路电流之和等于电源电流。
- Common mistake: ‘Current decreases as it passes through a resistor.’
- 常见错误：”电流经过电阻后变小。”
- Correction: Ammeters placed anywhere in a series loop read the same value. Charge is conserved.
- 纠正：串联回路中任何位置的电流表读数相同。电荷守恒。
2. Newton’s Third Law Pairs | 牛顿第三定律的作用力与反作用力
Students regularly pair forces incorrectly when identifying Newton’s third law pairs. A force that the Earth exerts on a book is often paired with the normal force from the table on the book. This is wrong: third law pairs act on different objects and are of the same type. The correct partner for the Earth’s gravitational pull on the book is the gravitational pull of the book on the Earth.
学生在识别牛顿第三定律的力对时经常配错。地球对书本的引力常被与桌面对书本的支持力配对。这是错误的：第三定律力对作用在不同物体上且属于同种类型。地球对书本引力的正确搭档是书本对地球的引力。
- Error: Action = weight, reaction = normal force.
- 错误：作用力=重力，反作用力=支持力。
- Correct: Action: Earth pulls book (gravitational). Reaction: Book pulls Earth (gravitational). Both are gravitational forces.
- 正确：作用力：地球拉书（引力）。反作用力：书拉地球（引力）。两者都是引力。
3. Specific Heat Capacity and Temperature Change | 比热容与温度变化
A very common slip occurs when students use the equation E = m c Δθ. They often confuse the mass of the substance with the mass of the heater or forget to convert grams to kilograms. More tricky is forgetting that Δθ is the change in temperature, not the final temperature. For a substance cooling down, Δθ is still positive (initial minus final).
学生使用方程 E = m c Δθ 时经常出现疏漏。他们常混淆物质的质量和加热器的质量，或者忘记将克换算成千克。更隐蔽的是忘记 Δθ 是温度变化量，而不是最终温度。对于冷却过程，Δθ 仍然是正值（初温减末温）。
E = m c Δθ
Example: 500 g of water cools from 35 °C to 20 °C. c = 4200 J/(kg °C). E = 0.5 × 4200 × (35 − 20) = 31 500 J. Using 20 °C as Δθ would give a completely wrong answer.
示例：500 克水从 35°C 冷却到 20°C，c = 4200 J/(kg·°C)。E = 0.5 × 4200 × (35 − 20) = 31 500 J。如果误用 20°C 作为 Δθ 会得到完全错误的答案。
4. Half-life Interpretation | 半衰期的解读
Many candidates think that after one half-life, all the remaining radioactive atoms will decay in the next half-life. This leads them to say that after two half-lives the activity is zero. The correct understanding is that the activity halves every half-life, approaching but never reaching zero (for a pure sample).
许多考生以为经过一个半衰期后，剩余的放射性原子会在下一个半衰期内全部衰变。于是他们说两个半衰期后活度为零。正确的理解是活度每过一个半衰期减半，趋近于零但永远不会变成零（对纯样品而言）。
Typical graph reading error: From a decay curve, a student reads the time for the count rate to fall from 800 to 400 as 2 minutes, and then claims the half-life is 2 minutes. That is correct. But when asked for the time to fall from 400 to 200, they sometimes add another 2 minutes to get 4 minutes total, but may incorrectly state the second half-life is 4 minutes. The half-life remains constant (2 min). After 4 minutes (two half-lives), the count rate reaches 200.
典型的读图错误：从衰变曲线上，学生读出计数率从 800 降到 400 需要 2 分钟，于是说半衰期为 2 分钟，这没错。但当问到从 400 降到 200 需要多久时，他们有时会再加 2 分钟得到总共 4 分钟，却可能错误地说第二个半衰期是 4 分钟。半衰期保持不变（2 分钟）。4 分钟（两个半衰期）后，计数率达到 200。
5. Wave Properties: Frequency, Amplitude and Wavelength | 波动性质：频率、振幅与波长
Students often mix up which changes affect the pitch and which affect the loudness of a sound wave on an oscilloscope display. Increasing frequency (more waves across the screen) increases pitch. Increasing amplitude (taller waves) increases loudness. A misconception is that increasing frequency also increases amplitude automatically, which is not true.
学生经常混淆示波器上哪种变化会影响音调，哪种会影响响度。频率增大（屏幕上波数增多）会提高音调。振幅增大（波形变高）会增大响度。一个误解是认为频率增大会自动导致振幅增大，这是不对的。
Wave speed is determined by the medium, not by frequency or amplitude separately. For water waves, a common wrong statement: ‘If the frequency increases, the wave travels faster.’ The speed stays constant in the same medium; instead, the wavelength changes (v = f λ).
波速由介质决定，与频率或振幅无关。对于水波，常见错误说法：”频率增加，波速变快。” 同一介质中波速恒定，变化的是波长 (v = f λ)。
v = f λ
6. Efficiency and Energy Transfer Chains | 效率与能量传递链条
In efficiency calculations, learners sometimes use the wrong output or input. A heater transfers 500 J of electrical energy and gives 350 J of useful thermal energy to the water. The efficiency is 350/500 = 0.7 or 70%. A common mistake is to use 500/350 or to think efficiency can exceed 100%. Also, when drawing Sankey diagrams, the useful output arrow must be shown to scale, and wasted energy should be labelled.
在效率计算中，学习者有时用错输出能量或输入能量。一个加热器传输 500 J 电能并给水提供 350 J 有用热能。效率为 350/500 = 0.7（70%）。常见错误是使用 500/350，或者认为效率可以超过 100%。此外，画 Sankey 图时，有用输出箭头需按比例画出，浪费的能量应标示清楚。
Efficiency = Useful energy output / Total energy input
Energy is always conserved, so the total input equals useful + wasted energy. Many forget that waste energy can be in several forms (heat, sound, light not needed).
能量总是守恒的，因此总输入等于有用能量加浪费能量。很多人忘记浪费能量可有多种形式（热量、声音、不需要的光）。
7. Magnetic Effect of a Current and Fleming’s Left‑Hand Rule | 电流的磁效应与弗莱明左手定则
Students frequently misalign their fingers for Fleming’s left‑hand rule. The rule is used for the motor effect: thumb = motion (force), first finger = field (N to S), second finger = current (conventional, + to -). A common mistake is reversing current direction or using electron flow. Another error is thinking that a wire parallel to the magnetic field experiences a force; force is maximum when the wire is perpendicular to the field.
学生使用弗莱明左手定则时经常摆错手指。该定则用于电动机效应：拇指 = 运动（力），食指 = 磁场（N 到 S），中指 = 电流（传统方向，正到负）。常见错误是颠倒电流方向或使用电子流方向。另一个错误是认为平行于磁场的导线会受到力；当导线垂直于磁场时力最大。
Also, when a current-carrying coil is in a magnetic field, the forces on opposite sides create a turning effect. Many candidates cannot identify which way the coil rotates unless they systematically apply the left‑hand rule to each side.
另外，当载流线圈处在磁场中时，对边上的力形成转动效应。许多考生若不系统地对每边应用左手定则，就无法确定线圈的旋转方向。
8. Density and the Concept of Upthrust | 密度与浮力概念
Density = mass / volume is straightforward, but errors occur with unit conversions (cm³ to m³) and with irregular solids. Upthrust (buoyancy) equals the weight of fluid displaced. A heavy object sinks because its weight is greater than the upthrust; an object floats when upthrust equals weight. Students often claim that a dense object always sinks, but a ship made of steel floats because its shape gives a large displaced volume, creating enough upthrust.
密度 = 质量 / 体积 很简单，但单位换算（cm³ 到 m³）和不规则固体会带来错误。浮力（浮力）等于排开流体的重量。重物下沉是因为它的重力大于浮力；当浮力等于重力时物体漂浮。学生常声称密度大的物体总会下沉，但钢制轮船能浮在水面，因为它的形状使得排开体积大，产生足够的浮力。
ρ = m / V
Never forget that volume in density calculations must be in m³ for SI units. 1 cm³ = 1 × 10⁻⁶ m³, and 1 litre = 0.001 m³.
永远不要忘记密度计算中的体积必须用立方米。1 cm³ = 1 × 10⁻⁶ m³，1 升 = 0.001 m³。
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