📚 Common Misconceptions in IGCSE WJEC Physics | IGCSE WJEC 物理常见误区
Many learners arrive at their IGCSE Physics revision with ideas that feel right but contradict the scientific model. These persistent misconceptions can cost valuable marks in the WJEC examination. This article identifies the most widespread pitfalls, states the correct physics clearly, and shows how to apply the right thinking in typical exam contexts.
许多学生在复习 IGCSE 物理时，脑海中常常有“感觉正确”却与科学模型相悖的想法。这些根深蒂固的误区会在 WJEC 考试中白白丢分。本文梳理最常见的认知陷阱，用清晰的表述给出正确的物理图景，并示范如何在典型考题中运用正确的思维方式。
1. Force & Motion: A Force Keeps Things Moving? | 力与运动：力是维持运动的原因吗？
Misconception: A constant force is needed to keep an object moving at a steady speed. If the force is removed, the object will stop.
常见误区：物体要保持匀速运动，必须有力一直推着它。一旦力消失，物体就会停下来。
Correct physics: An object continues at constant velocity (including at rest) unless a resultant force acts on it (Newton I). A force produces acceleration, a change in velocity. In the absence of a resultant force (e.g. in deep space or on a frictionless surface), an object moves at constant speed in a straight line.
正确概念：根据牛顿第一定律，除非有合力作用，物体将保持静止或匀速直线运动。力产生的是加速度，是速度的变化。在没有合力的情况下（如深空或完全无摩擦的表面），物体会保持恒定的速度沿直线运动。
Exam clue: When a WJEC question describes a cyclist moving at constant velocity, the driving force equals the resistive forces; the resultant force is zero. That is why the speed stays constant, not because the forces “disappear”.
解题提示：WJEC 试题中描述自行车手匀速骑行时，驱动力等于阻力，合力为零。正是合力为零才使得速度保持不变，而不是因为力“消失”了。
2. Mass vs. Weight: They Are the Same Thing | 质量与重量：它们是一回事
Misconception: Mass and weight are identical; you can use the terms interchangeably. An object’s weight in kg does not change.
常见误区：质量和重量没有差别，两个词可以混用。一个物体的“重量”是固定的，单位就是千克。
Correct physics: Mass (scalar, unit: kg) measures the amount of matter and is the same everywhere. Weight (vector, unit: N) is the gravitational force on that mass. Weight W = m g. Because g varies (≈ 9.8 N/kg on Earth, ≈ 1.6 N/kg on the Moon), the same mass has different weights in different gravitational fields.
正确概念：质量（标量，单位 kg）表示物质的多少，处处相同。重力（矢量，单位 N）是该质量所受的地心引力。重力 W = m g。由于 g 值不同（地球上约为 9.8 N/kg，月面约为 1.6 N/kg），同一质量在不同引力场中重力不同。
In the exam, if a question gives mass in grams, convert to kg before using W = m g. Avoid saying “the weight is 5 kg”; always state weight in newtons.
考试中，若题目给出的质量是克，务必先化成千克再用 W = m g。切忌说“重量是 5 kg”；重力必须用牛顿表示。
3. Velocity & Acceleration: v = 0 Means a = 0 | 速度与加速度：速度为零加速度也为零
Misconception: When an object’s velocity is zero, its acceleration must also be zero. This misinterpretation often appears in problems involving vertical projection or a pendulum bob at the extremes.
常见误区：物体速度为零时，加速度也一定是零。这种误解常出现在竖直上抛问题或摆球到达端点时的分析中。
Correct physics: Velocity and acceleration are independent. At the highest point of a vertically thrown ball, velocity is momentarily zero but acceleration is still g = 9.8 m/s² downwards. The ball is changing its state of motion (from up to down) at that instant, exactly because a resultant force (gravity) acts.
正确概念：速度和加速度是相互独立的。竖直上抛的球到达最高点时，速度瞬时为零，但加速度仍为向下的 g ≈ 9.8 m/s²。此时球正在改变运动状态——由向上转为向下，正是因为有重力这个合力在作用。
Use v = u + a t; at the top v = 0, so 0 = u – g t ⇒ t = u/g. The acceleration never vanishes during free flight.
利用公式 v = u + a t，在最高点 v = 0，可得 0 = u – g t，从而 t = u/g。整个飞行过程中加速度始终存在。
4. Newton’s Third Law: Action–Reaction Acts on the Same Object | 牛顿第三定律：作用力与反作用力作用在同一物体上
Misconception: The normal contact force from a table on a book and the weight of the book are an action–reaction pair.
常见误区：桌面对书的支持力与书的重力是一对作用力与反作用力。
Correct physics: Action–reaction pairs always act on different bodies. The book’s weight is the gravitational pull of the Earth on the book; its counterpart is the pull by the book on the Earth. The contact pair is the push of the table on the book and the push of the book on the table. The two forces on the book (weight and normal) happen to be balanced when the book is at rest, but they are not a Newton III pair.
正确概念：作用力与反作用力总是作用在不同的物体上。书的重力是地球对书的引力，其反作用力是书对地球的引力。桌面–书本的接触力对是桌子对书的向上推力和书对桌子的向下压力。书静止时，重力和支持力大小相等、方向相反，但它们是一对平衡力，而不是作用与反作用对。
When identifying pairs, check: are they the same type of force? Do they act on two different bodies? This simple test eliminates the confusion.
判断作用反作用力对时，问两个问题：它们是同种性质的力吗？是否分别作用在两个物体上？这个简便方法能立刻消除混淆。
5. Electric Current: Current Is Used Up in a Circuit | 电流：电流在电路中被消耗
Misconception: The current flowing out of a bulb is smaller than the current going in because some current is “consumed” to produce light.
常见误区：流过灯泡之后的电流变小了，因为一部分电流被用来发光了。
Correct physics: In a series circuit, charge is conserved. The rate of flow of charge (current) is the same at all points. For a single-loop series circuit, the ammeter reading is identical before and after a lamp. The energy is transferred to the bulb—not the current—and the electrical potential energy per charge (voltage) drops across the component.
正确概念：串联电路中电荷守恒，电流（电荷流动的速率）处处相等。在单回路串联电路中，灯泡前后的电流表读数完全相同。是能量传递给了灯泡，而不是电流本身被消耗；单位电荷的电势能（电压）在元件两端降落。
Think of cycling charges like a chain; the whole chain moves at the same speed around the loop. Energy is delivered to the lamp via the electric field, but the number of charges per second stays constant.
可以把电荷的流动想象成一条传动链；整条链以相同速率绕回路运动。能量通过电场传递给灯泡，但每秒经过的电荷数目保持不变。
6. Energy: Energy Can Be Destroyed | 能量：能量可以被消灭
Misconception: When a moving object comes to rest, the kinetic energy is lost and disappears.
常见误区：运动物体停止后，它的动能消失不见了。
Correct physics: Energy is conserved; it cannot be created or destroyed, only transferred into different forms or stores. Kinetic energy dissipates as internal (thermal) energy of the brakes, tyres and surroundings. The total energy remains constant, even though it may spread out and become less useful.
正确概念：能量守恒，只能从一种形式转化为另一种形式，或从一个系统转移到另一个系统，不会凭空消失。动能通过摩擦转化为刹车、轮胎和周围环境的热能（内能）。总能量始终不变，尽管它会分散开来，变得不那么“有用”。
In WJEC energy questions, always describe the pathway: e.g. “chemical energy store → kinetic energy store → thermal energy store of surroundings”. Never say “energy is lost”.
在 WJEC 的能量题目中，一定要说明能量转移路径：比如“化学能→动能→内能（周围环境）”。绝不要说“能量消失了”。
7. Heat & Temperature: They Are Interchangeable | 热与温度：可以互换使用
Misconception: A cup of boiling water at 100 °C contains more heat than a swimming pool at 30 °C.
常见误区：一杯 100 °C 的开水所含的“热量”远比 30 °C 的游泳池水多。
Correct physics: Temperature reflects the average kinetic energy of particles. Internal energy is the total kinetic and potential energy of all particles. The swimming pool, although cooler, contains a far larger mass of water, so its internal energy can be greater than that of the boiling cup. ‘Heat’ refers to energy transferred because of a temperature difference, not something an object stores.
正确概念：温度反映的是粒子平均动能。内能是所有粒子的动能与势能之和。游泳池水温虽低，但水量巨大，因而它的内能可能远大于那杯开水。“热量”指的是因温差而传递的能量，而不是物体自身储存的东西。
When explaining heating, say “energy is transferred from the hotter body to the cooler body until they reach thermal equilibrium”. Avoid saying “heat flows” as if it is a fluid.
解释热传递时，应说“能量从高温物体传递到低温物体，直至两者达到热平衡”。避免使用“热量流动”这类仿佛是一种流体的说法。
8. Waves: All Waves Require a Medium | 波：所有波都需要介质
Misconception: Since sound waves need air (or other material) to travel, light must also need a medium.
常见误区：声波需要空气（或其他物质）才能传播，因此光也一定需要介质。
Correct physics: Electromagnetic waves (radio, microwaves, infrared, visible light, UV, X‑rays, gamma) are transverse waves that can travel through a vacuum. They do not rely on particle vibrations. Sound waves are longitudinal and require a material medium. Space is silent, but light from the Sun reaches Earth through the vacuum of space.
正确概念：电磁波（无线电波、微波、红外线、可见光、紫外线、X 射线、γ 射线）是横波，可以在真空中传播，不依赖粒子振动。声波是纵波，必须通过介质传播。太空中是寂静的，但太阳光却能穿过真空到达地球。
WJEC often asks about the speed of light in a vacuum (3.0 × 10⁸ m/s) and compares it with sound. Remember, the speed of an electromagnetic wave does not depend on the motion of a medium.
WJEC 考试常问真空中的光速 (3.0 × 10⁸ m/s) 并与声速做对比。记住，电磁波的速度不依赖于介质的运动。
9. Radioactivity: Half-Life Means Half the Mass Disappears | 放射性：半衰期意味着质量减半
Misconception: After one half‑life, half of the original radioactive sample has vanished, so its mass is halved.
常见误区：经过一个半衰期，原来的放射性样品消失了一半，质量也减半了。
Correct physics: The term half‑life describes the time taken for the number of unstable nuclei in a sample to fall by half. The sample still contains the daughter nuclides (and the remaining parent nuclei), so the total mass changes very little. For instance, when carbon‑14 decays, it becomes nitrogen‑14; the mass remains nearly the same because the emitted beta particle and antineutrino have very small mass.
正确概念：半衰期是指样品中不稳定原子核的数量减少一半所需的时间。样品中仍然存在衰变产生的子核（以及剩余的母核），因此总质量变化极小。例如碳‑14 衰变为氮‑14，质量几乎不变，因为放出的 β 粒子和反中微子质量极小。
In graphs, the y‑axis might be “number of undecayed nuclei” or “activity”, not “mass”. Do not confuse these.
图表类题目中，纵坐标通常是“未衰变原子核数量”或“活度”，而不是“质量”。切勿混淆。
10. Atomic Structure: Electrons Orbit Like Planets | 原子结构：电子像行星一样绕核旋转
Misconception: Electrons travel around the nucleus in fixed, well‑defined circular orbits, like planets around the Sun.
常见误区：电子沿着确定的圆形轨道绕核运行，就像行星绕太阳运转。
Correct physics: The planetary model is a historic simplification. Modern physics describes electrons as existing in orbitals—regions where there is a high probability of finding an electron. These are energy levels, not precise orbits. Electron behaviour is governed by quantum mechanics; an electron can gain or lose specific amounts of energy and jump between levels, but its path is not a fixed circle.
正确概念：行星模型是历史上的简化。现代物理用轨道（orbitals）来描述——即电子出现的概率较高的区域，它们对应的是能级，而不是精确的轨道。电子的行为由量子力学支配；电子只能吸收或放出特定能量在能级间跃迁，但它的路径并不是一个固定的圆。
At IGCSE, you learn about electron shells (2,8,8…), which are energy levels. Avoid drawing tiny electron balls on circular paths; use dots in spherical shells.
在 IGCSE 阶段，我们学习电子层（2,8,8…），它们就是能级。避免画成小圆球在圆形轨道上奔跑；应该用分布在球形壳层上的圆点表示。
Published by TutorHao | Physics Revision Series | aleveler.com
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