📚 IGCSE WJEC Physics: Common Mistake Questions Explained | IGCSE WJEC 物理：易错题精讲
Mistakes in physics often come from applying everyday intuition instead of scientific principles. This article walks through some of the most frequently misunderstood WJEC IGCSE Physics questions, clarifying the correct reasoning for each. By studying these examples, you can avoid losing marks on trickier topics such as forces, energy, circuits, waves, and radioactivity.
物理中的很多错误源于用日常直觉取代了科学原理。本文梳理了一些 WJEC IGCSE 物理中高频易错题目，逐一讲解正确思路。通过学习这些例子，你可以避免在力、能量、电路、波、放射性等难度较大的知识点上丢分。
1. Newton’s Third Law Pairs | 牛顿第三定律的作用力对
A classic mistake is confusing balanced forces with action–reaction pairs. A book resting on a table experiences a downward weight and an upward normal reaction. These are balanced, not a Newton’s third law pair. The third law pair to the weight is the gravitational pull of the book on the Earth. The third law pair to the normal force on the book is the normal force exerted by the book on the table.
一个经典错误是把平衡力和作用力–反作用力对混为一谈。放在桌子上的书受到向下的重力和向上的支持力，它们是一对平衡力，而不是牛顿第三定律的力对。重力的反作用力是书对地球的引力。支持力的反作用力是书对桌面的压力。
Common error: stating that the weight and the normal force are action–reaction pairs. They act on the same object, whereas action–reaction pairs must act on two different objects.
常见错误：认为重力与支持力是作用力–反作用力对。它们作用在同一物体上，而作用力–反作用力必须作用在两个不同的物体上。
To identify a third law pair, check: same type of force, same magnitude, opposite direction, and acting on different bodies.
识别第三定律力对的要点：同种力、大小相等、方向相反、且作用在不同物体上。
2. Resultant Force and Motion | 合力与运动
Many students think that a moving object must have a resultant force in the direction of motion. This idea goes back to Aristotle. In Newtonian mechanics, a constant velocity means zero resultant force. If an object is moving at a steady speed in a straight line, all forces balance. A forward force is only needed to overcome friction; if there is no friction, no force is required to keep the object moving.
许多学生认为运动的物体一定受到一个与运动方向一致的合力。这个想法可以追溯到亚里士多德。在牛顿力学中，匀速直线运动意味着合力为零。如果物体匀速直线运动，所有力平衡。向前的力只需要用来克服摩擦力；如果没有摩擦力，保持物体运动不需要力。
Typical exam trap: “A car is travelling at a constant speed. The driving force is 2000 N. What is the resistive force?” The answer is 2000 N backwards, not something smaller. Learners often subtract to get a net force, forgetting constant speed means equilibrium.
典型考试陷阱：“一辆汽车匀速行驶，驱动力为 2000 N。阻力多大？”答案是向后的 2000 N，而不是更小的值。学生常常相减得到净力，忘记了匀速意味着平衡。
For a cyclist freewheeling downhill at constant speed, the component of weight down the slope equals the total resistive forces (air resistance + friction). No net force, so speed remains constant.
自行车沿斜坡匀速滑行时，重力沿斜面的分量等于总阻力（空气阻力 + 摩擦力）。没有净力，因此速度保持不变。
3. Energy Transfers and Conservation | 能量转化与守恒
When an object falls at terminal velocity, its gravitational potential energy (GPE) is being converted into kinetic energy (KE) – this is a common wrong answer. At terminal velocity, speed is constant, so KE is constant. The GPE lost is transferred to the surroundings as thermal energy due to air resistance, not to KE.
当物体以终极速度下落时，其重力势能转化为动能——这是一个常见错误答案。在终极速度下，速度恒定，所以动能不变。损失的重力势能通过空气阻力以热能形式传递给环境，而不是转化为动能。
Another tricky point is the roller coaster or pendulum: at the lowest point, speed is maximum, but the forces are not balanced; there is a net centripetal force towards the centre of the circle. Students often assume equilibrium at maximum speed.
另一个易错点是过山车或摆锤：在最低点速度最大，但合力并不平衡；存在指向圆心的净向心力。学生经常误以为最大速度时处于平衡状态。
Work done = force × distance moved in the direction of the force. When holding a heavy bag still, no work is done on the bag in the physics sense because the distance moved is zero, even though muscles feel tired.
做功 = 力 × 在力的方向上移动的距离。静止提着重物时，从物理学角度对物体做功为零，因为移动距离为零，尽管肌肉感到疲劳。
4. Series and Parallel Circuits | 串联与并联电路
Lamps in series: if one lamp breaks, all go out because the circuit is broken. Lamps in parallel allow independent operation. A frequent mistake is thinking that the current in a series circuit is “used up” by the components. Current is the same at all points in a single loop.
串联灯泡：如果一个灯泡坏了，所有灯泡都熄灭，因为电路断开了。并联灯泡可以独立工作。一个常见错误是认为串联电路中的电流被元件“消耗”了。在单环回路中，各点电流处处相等。
When an extra resistor is added in parallel, the total resistance decreases, and the total current from the battery increases. Students often incorrectly add the resistance directly as if in series.
当额外并联一个电阻时，总电阻减小，电池总电流增大。学生经常错误地像串联那样直接相加电阻值。
Potential divider questions also cause errors. For a variable resistor in series with a fixed resistor, changing the variable resistor alters the p.d. across both. If one component’s resistance increases, its share of the supply voltage increases.
分压器问题同样容易出错。在一个可变电阻与固定电阻串联的电路中，改变可变电阻会改变两端的电压分配。如果一个元件的电阻增加，它分得的电源电压就增加。
5. Specific Heat Capacity and Latent Heat | 比热容与潜热
When heating water, its temperature rises as energy is supplied, according to Q = mcΔθ. However, at the boiling point, the temperature stays at 100°C (at standard pressure) while water vaporises. Energy continues to be supplied, but it is used to break intermolecular bonds, not to raise temperature. Many students mix up latent heat with a temperature rise and think it gets hotter while boiling.
加热水时，根据 Q = mcΔθ，供给能量会使温度上升。然而，在沸点时，温度保持在 100°C（标准气压下），水继续吸热汽化。能量用来破坏分子间作用力，而不是提高温度。许多学生混淆了潜热与升温，误以为沸腾过程中温度继续升高。
In an exam question: “Explain why the temperature of the ice‑water mixture stays at 0°C until all the ice has melted, even though heating continues.” Correct: Energy is used to overcome the attractive forces between water molecules in the solid lattice, increasing the potential energy of particles, not their average kinetic energy, so temperature remains constant.
考试题：“解释为什么冰水混合物在冰完全融化前温度保持在 0°C，尽管持续加热。”正确答案：能量用来克服固态晶格中水分子间的吸引力，增加粒子的势能，而不是平均动能，因此温度保持不变。
6. Momentum and Impulse | 动量与冲量
In a collision, momentum is always conserved provided no external forces act. Many students forget that momentum is a vector: direction matters. When two objects stick together after a collision and move in one direction, the sign of velocities must be taken into account when using the conservation equation.
在没有外力作用的情况下，碰撞中动量总是守恒的。许多学生忘记动量是矢量：方向非常重要。当两个物体碰撞后粘在一起朝一个方向运动时，使用守恒方程必须考虑速度的正负号。
Impulse (force × time) equals the change in momentum. Catching an egg with a “give” (moving hands backward) increases the time over which the egg stops, reducing the average force. Students often think that the cushioning reduces momentum change. It does not; the change in momentum is the same (from initial velocity to zero), but extending the time reduces the force according to FΔt = Δp.
冲量（力 × 时间）等于动量的变化量。接鸡蛋时“随退”（手向后移动）增加了鸡蛋停止过程的时间，从而减小了平均受力。学生经常误以为缓冲减小了动量的变化。其实没有，动量的变化量是相同的（从初速度到零），但延长时间根据 FΔt = Δp 减小了受力。
7. Waves: Reflection, Refraction and Superposition | 波：反射、折射与叠加
When a wave enters a denser medium, its speed decreases, wavelength decreases, and the wave bends towards the normal. Frequency remains the same because it is determined by the source. Many errors arise from thinking that frequency changes at a boundary.
当波进入更密的介质时，波速减小，波长减小，波向法线偏折。频率保持不变，因为它由波源决定。许多错误源于认为频率会在界面处改变。
For total internal reflection (TIR), two conditions must be met: light must travel from a denser to a less dense medium, and the angle of incidence must exceed the critical angle. Simply having a large angle is not enough; the direction of travel is crucial.
全内反射需要满足两个条件：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。仅仅角度很大是不够的；传播方向至关重要。
Superposition and interference: when two waves meet, displacements add. A peak plus a peak gives a larger peak (constructive interference). A peak plus a trough of equal amplitude cancels (destructive). Students sometimes think waves reflect off each other or change speed.
叠加与干涉：当两列波相遇时，位移相加。波峰加波峰给出更大的波峰（相长干涉）。等振幅的波峰加波谷则抵消（相消干涉）。学生有时会误以为波会互相反射或速度改变。
8. Radioactive Decay and Half-life | 放射性衰变与半衰期
A common misconception is that half-life is the time for half the atoms to disappear. Radioactive atoms decay into other atoms; they do not vanish. Half-life is the time taken for half of the unstable nuclei in a sample to decay, or for the count rate to halve (after subtracting background). The mass of the original radioactive isotope halves, but the total mass of the sample does not simply halve because daughter nuclei remain.
一个常见误解是半衰期是半数原子消失的时间。放射性原子衰变成其他原子，它们并非消失。半衰期是指样品中半数不稳定原子核发生衰变所需的时间，或计数率（扣除本底后）减半的时间。原来放射性同位素的质量减半，但样品的总质量并不简单减半，因为子核留在了样品中。
Activity vs. count rate: activity is the number of decays per second from the source; count rate is what the detector records. Due to background radiation, the detected count rate is higher. Students often forget to subtract background, leading to incorrect half‑life values.
活度与计数率：活度是放射源每秒钟衰变的数量；计数率是探测器记录到的数值。由于本底辐射的存在，探测到的计数率偏高。学生经常忘记扣除本底，导致半衰期计算错误。
9. Electromagnetic Induction and Motors | 电磁感应与电动机
For a coil rotating in a magnetic field, the induced e.m.f. is maximum when the coil is cutting field lines at the greatest rate – when the plane of the coil is parallel to the field lines. Many students think maximum e.m.f. occurs when the coil is perpendicular to the field. The induced e.m.f. depends on the rate of change of flux linkage, not the amount of flux.
对于在磁场中转动的线圈，当线圈切割磁力线的速率最大时——即线圈平面与磁力线平行时，感生电动势最大。许多学生认为线圈垂直于磁场时电动势最大。感生电动势取决于磁链的变化率，而不是磁通量的大小。
In a d.c. motor, a split‑ring commutator reverses the current direction every half‑turn to keep the coil rotating in the same direction. Students often confuse the split ring with slip rings used in an a.c. generator (alternator).
在直流电动机中，换向器每半圈改变电流方向，确保线圈持续朝一个方向转动。学生经常将换向器与交流发电机（交流发电机）使用的滑环混淆。
10. Gas Pressure and Particle Motion | 气体压强与粒子运动
Increasing the temperature of a fixed volume of gas raises the average kinetic energy and speed of the particles. They collide more frequently and with greater force on the walls, increasing pressure. A frequent error is to say that increasing temperature makes the particles expand. The particles themselves do not expand; their average separation may increase if the container can expand, but at constant volume the particles simply move faster.
升高固定体积气体的温度，会提高粒子的平均动能和速率。它们与器壁的碰撞更频繁且更有力，从而增大压强。一个常见错误是说升温使粒子膨胀。粒子本身不膨胀；如果容器可以膨胀，粒子的平均间距可能会增大，但在体积不变时粒子只是运动更快。
In a closed container, when a piston is pushed in (compression), the pressure increases because particles hit the walls more often due to reduced volume, and collisions become more frequent. Temperature does not necessarily increase unless the compression is adiabatic, but at IGCSE level we usually assume constant temperature for slow changes. Clarify the assumptions made.
在密闭容器中，当活塞被推入（压缩）时，压强因体积减小、粒子更频繁撞击器壁而增大。温度不一定会升高，除非是绝热压缩，但在 IGCSE 阶段对于缓慢变化我们通常假设恒温。要明确所做的假设。
Published by TutorHao | Physics Revision Series | aleveler.com
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