📚 IGCSE Physics Common Mistakes: Detailed Explanations and Strategies | IGCSE 物理易错题精讲：详细解析与策略
Welcome to the TutorHao IGCSE Physics revision guide focusing on the most common mistakes students make in past papers. We will walk through typical errors, clarify misconceptions, and provide you with clear, exam-ready corrections for each topic. Use these detailed explanations to boost your confidence and avoid losing marks on tricky questions.
欢迎来到 TutorHao 的 IGCSE 物理复习指南，我们将聚焦历年真题中学生最容易失分的典型错误。我们会逐一分析常见的错误作答，澄清误解，并为每个知识点提供清晰、适合考试使用的正确解释。利用这些详细解析，你将能有效增强信心，避开难题中的失分陷阱。
1. Confusing Speed with Velocity | 混淆速率和速度
A very common error is using ‘speed’ and ‘velocity’ interchangeably. In IGCSE Physics, speed is a scalar quantity that only measures how fast an object moves, while velocity is a vector that specifies both speed and direction. A car moving around a roundabout at constant speed still has a changing velocity because its direction changes continuously. When calculating average velocity, students often forget to consider displacement rather than total distance travelled, leading to an incorrect vector result. Always check whether the question asks for a vector or a scalar.
一个很常见的错误是将 ‘速率’ 和 ‘速度’ 混用。在 IGCSE 物理中，速率是标量，只衡量物体运动的多快；而速度是矢量，既包含快慢也包含方向。一辆汽车以恒定速率绕环岛行驶时，其速度方向不断改变，因此速度是变化的。学生在计算平均速度时，常忘记应当使用位移而不是总路程，从而得到错误的矢量结果。务必先审清题目要求的是矢量还是标量。
2. Misidentifying Newton’s Third Law Pairs | 误判牛顿第三定律的力对
Many students incorrectly pair the normal reaction force from a table with the weight of a book resting on it, thinking they form a Third Law pair. In reality, the two forces in a Newton’s Third Law pair must act on different bodies and be of the same type. The weight of the book is the gravitational pull of the Earth on the book; its Third Law pair is the gravitational pull of the book on the Earth. The normal force from the table on the book has its pair as the normal force from the book on the table. Always identify the two objects involved in each force.
许多学生会错误地把桌面对书本的支持力和书本所受的重力配成一对作用力与反作用力，认为它们满足牛顿第三定律。事实上，第三定律的力对必须作用在不同的物体上，并且是同种性质的力。书本的重力是地球对书本的引力，它的反作用力是书本对地球的引力。桌面对书本的支持力，其反作用力则是书本对桌面的压力。请务必分别找出每一个力涉及的两个物体。
3. Current and Voltage Misunderstandings in Circuits | 电路中对电流和电压的误解
A typical mistake is assuming that current is ‘used up’ as it passes through components. In a series circuit, the current remains the same everywhere. What changes across a component is the potential difference (voltage). Students also mistakenly apply series rules to parallel circuits, for example thinking that the total current is the same in every branch. In a parallel circuit, the current splits, while the voltage across each branch remains equal to the source voltage. Using a table to compare series and parallel properties helps avoid confusion, as shown below.
一个典型的错误是认为电流在经过用电器时会被’消耗掉’。在串联电路中，电流处处相等。发生变化的是元件两端的电势差（电压）。学生还常将串联规律错误地套用到并联电路中，比如误以为各支路的电流都相同。在并联电路中，电流是分流的，而各支路两端的电压相等，都等于电源电压。用表格对比串联与并联的特性有助于理清概念，如下表所示。
| Property | 性质 | Series Circuit | 串联电路 | Parallel Circuit | 并联电路 |
|---|---|---|
| Current (I) | 电流 | Same everywhere | 处处相等 | Splits; I_total = I₁ + I₂ | 分流 |
| Voltage (V) | 电压 | Divides; V_total = V₁ + V₂ | 分压 | Same across each branch | 各支路相等 |
| Resistance (R) | 电阻 | R_total = R₁ + R₂ | 直接相加 | 1/R_total = 1/R₁ + 1/R₂ | 倒数相加 |
4. Energy Transfers and Sankey Diagram Errors | 能量转化与桑基图的错误
Students frequently misidentify the main energy stores and pathways in open systems. For instance, when a ball falls, gravitational potential energy decreases while kinetic energy and thermal energy (due to air resistance) increase. A common mistake is neglecting energy dissipated as heat to the surroundings. In Sankey diagrams, the width of each arrow must be proportional to the amount of energy, and the total input width must equal the sum of all output widths. Overlooking the ‘wasted’ energy path or drawing it proportionally incorrectly can cost marks.
学生经常错误识别开放系统中的主要能量储存和转移途径。例如，当球下落时，重力势能减少，动能和内能（因空气阻力生热）增加。常见的错误是忽略了向周围环境以热能形式耗散的能量。在画桑基图时，每个箭头的宽度必须与它所表示的能量数值成正比，且输入端的箭头总宽度必须等于所有输出端箭头宽度之和。遗漏’浪费’能量的路径，或未能按比例正确绘制，都会导致失分。
5. Specific Heat Capacity: Confusing Temperature and Heat | 比热容：混淆温度和热量
Many IGCSE students think that adding the same amount of heat to different materials always produces the same temperature rise. The temperature change depends on the specific heat capacity (SHC) of the material. Another frequent error is applying the SHC formula ΔQ = m × c × Δθ incorrectly by using the wrong mass or mixing up units. The mass must be in kilograms, the energy in joules, and the temperature change in degrees Celsius or Kelvin. Always convert grams to kilograms and check that you are using the correct mass of the substance being heated, not the total mass of the container.
许多 IGCSE 学生认为，对不同物质加入相同的热量总会引起相同的温度升高。实际上，温度的变化取决于该物质的比热容。另一个常见错误是在使用比热容公式 ΔQ = m × c × Δθ 时用错质量或混淆单位。质量必须以千克为单位，能量以焦耳为单位，温度变化用摄氏度或开尔文均可。务必先将克转换为千克，并确认你所用的是被加热物质本身的质量，而不是容器的总质量。
6. Terminal Velocity: Misreading Force Diagrams | 终端速度：误读受力图
When a skydiver accelerates after jumping, many students claim he never reaches a constant speed. In truth, as speed increases, air resistance builds up until it balances the weight. At terminal velocity, the resultant force becomes zero, and acceleration ceases. A common exam error is marking a terminal velocity graph with a sloping line all the way down, rather than showing the gradient decreasing to zero. Once the parachute opens, a new, lower terminal velocity is reached through a sharp upward resultant force at first. Remember, terminal velocity does not mean zero velocity; it means constant velocity.
当跳伞员跳出后加速下落时，不少学生认为他的速度不会达到恒定。实际上，随着速度增大，空气阻力逐渐增加，直到与重力平衡。达到终端速度时，合力为零，加速度消失。考试中一个典型错误是把终端速度对应的速度–时间图一直画成倾斜线，而没有画出斜率逐渐减小至零的过程。降落伞打开后，跳伞员会经历短暂的向上合力的减速，最终达到一个新的、较低的终端速度。请记住，终端速度并不意味着速度为零，而是指速度大小保持不变。
7. Generator Effect and Electromagnetic Induction | 发电机效应与电磁感应
Students often mistakenly believe that any stationary magnet and coil will induce an e.m.f. For an e.m.f. to be induced, the magnetic field lines linking the coil must change. This means there must be relative motion between the magnet and the coil, or the current in an electromagnet must be changing. A common mistake in explaining the a.c. generator is to forget that the induced e.m.f. is zero when the coil is perpendicular to the magnetic field (maximum flux, zero rate of change), and maximum when the coil is parallel to the field lines (zero flux, maximum rate of cutting field lines). Understanding ‘rate of change’ is key.
学生常常误以为，只要把磁铁和线圈静止地放在一起就能感应出电动势。要产生感应电动势，穿过线圈的磁感线必须发生变化。也就是说，磁铁和线圈之间必须有相对运动，或者电磁铁的电流必须变化。在解释交流发电机时，一个普遍错误是忘记当线圈平面垂直于磁场时，感应电动势为零（磁通量最大，但变化率为零）；而当线圈平行于磁场时，感应电动势最大（磁通量为零，但切割磁感线的速率最大）。理解’变化率’是解题的关键。
Induced e.m.f. ∝ rate of change of magnetic flux linkage | 感应电动势 ∝ 磁链变化率
8. Force-Extension Graphs and Hooke’s Law | 力–伸长图线与胡克定律
A common error is extending Hooke’s law beyond the limit of proportionality. Hooke’s law states that force is directly proportional to extension only within the elastic limit. Once the limit of proportionality is passed, the graph curves and force is no longer proportional to extension. Students also mix up elastic deformation and plastic deformation. If an object returns to its original shape after the force is removed, it has undergone elastic deformation; if not, it is plastic. On a force-extension graph, the area under the straight-line portion represents the elastic potential energy stored in the material.
一个常见的错误是将胡克定律用于比例极限之外。胡克定律指出，只有在弹性限度内，力和伸长量才成正比。一旦超出比例极限，图线就会弯曲，力不再与伸长量成正比。学生还经常混淆弹性形变与塑性形变。如果撤去外力后物体能恢复原状，这就是弹性形变；如果不能恢复，则是塑性形变。在力–伸长图线上，直线段下方的面积表示储存在材料中的弹性势能。
9. Refraction and Total Internal Reflection (TIR) | 折射与全内反射
Many students incorrectly predict the direction of bending when light enters a different medium. Remember, when light enters a denser medium (higher refractive index), it bends towards the normal; when it enters a less dense medium, it bends away from the normal. Mistakes also arise in TIR questions. The two conditions for TIR are: light must travel from a denser to a less dense medium, and the angle of incidence must be greater than the critical angle. If you forget the first condition, you may incorrectly claim that TIR can occur when light travels from air to glass.
很多学生在预测光从一种介质进入另一种介质时的偏折方向时会出错。请记住，当光进入光密介质（折射率较大）时，会向法线偏折；进入光疏介质时，则偏离法线。在全内反射题目中也容易出现错误。发生全内反射的两个条件是：光必须从光密介质射向光疏介质，且入射角必须大于临界角。如果忘记了第一个条件，就可能错误地认为光从空气射入玻璃时也能发生全内反射。
10. Half-Life and Radioactive Decay Calculations | 半衰期与放射性衰变计算
A typical error is mistaking activity for half-life or thinking that after one half-life, all the remaining nuclei decay in the next half-life. The activity halves every half-life, so after two half-lives, only one quarter of the original number of active nuclei remains. When calculating half-life from a decay graph, students sometimes read the time for the count rate to drop to zero, rather than measuring the time taken for the count rate to halve. Always draw a clear construction line from half the initial activity on the y-axis across to the curve and down to the x-axis to read off the half-life accurately.
一个典型错误是将活度与半衰期混为一谈，或者误以为经过一个半衰期后，剩下的一半原子核会在下一个半衰期内全部衰变。活度（或未衰变核数）每经过一个半衰期就减半，因此经过两个半衰期后只剩下原始数量的四分之一。在根据衰变曲线计算半衰期时，学生有时会去读计数率降到零的时间，而不是去测量计数率减半所需的时间。请一定从 y 轴上初始活度一半的位置画一条清晰的水平辅助线与曲线相交，再垂直向下到 x 轴，从而准确读出半衰期。
Published by TutorHao | Physics Revision Series | aleveler.com
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