📚 Common Misconceptions in IGCSE Physics | IGCSE 物理常见误区
Many IGCSE Physics students lose marks not because they lack knowledge, but because they hold onto common misconceptions. These misunderstandings often arise from everyday language or simplified early models and can persist into the exam. This article highlights the most frequent errors and provides clear, examiner-friendly explanations to help you avoid them. Mastering the correct concepts will strengthen your understanding and boost your confidence in Paper 2, Paper 4, and the practical paper.
许多 IGCSE 物理学生丢分并不是因为缺乏知识，而是因为他们固守着一些常见的误区。这些误解往往源于日常语言或早期简化模型，并可能一直延续到考试中。本文重点剖析最常见的错误，并提供清晰、符合考官视角的解释，帮助你避开这些坑。掌握正确的概念将加深你的理解，并增强你在 Paper 2、Paper 4 和实验考试中的信心。
1. Mass and Weight Are Not the Same | 质量和重量不是一回事
Students often use ‘mass’ and ‘weight’ as if they mean the same thing. In physics, mass is a scalar quantity that measures the amount of matter in an object; it is measured in kilograms (kg) and does not change with location. Weight is a force, specifically the gravitational pull on an object, and is measured in newtons (N). Confusing the two can lead to errors in calculations involving gravitational field strength g.
学生经常把“质量”和“重量”混为一谈。在物理学中，质量是标量，衡量物体所含物质的多少，单位是千克（kg），且不随位置改变。重量是一种力，确切地说是作用在物体上的引力，单位是牛顿（N）。混淆二者会导致在涉及引力场强度 g 的计算中出现错误。
The relationship is given by the equation:
W = m × g
On Earth, g ≈ 10 N/kg, so an object with a mass of 60 kg has a weight of 600 N. If you take the same object to the Moon, where g ≈ 1.6 N/kg, its weight becomes 96 N, but its mass stays exactly 60 kg. A common exam mistake is to say the mass ‘got lighter’ on the Moon—never do that.
关系式如下：
W = m × g
在地球上，g ≈ 10 N/kg，所以一个质量为 60 kg 的物体重量为 600 N。如果你把同一个物体带到月球上，那里 g ≈ 1.6 N/kg，它的重量变为 96 N，但质量仍然是 60 kg。一个常见的考试错误是声称物体在月球上“质量变轻了”——千万不要这么说。
Remember: weight is a vector and always points towards the centre of the planet; mass is just a number representing how much ‘stuff’ there is. When answering questions about astronauts floating in space, clarify that they are weightless due to free fall, not because mass disappears.
记住：重量是矢量，总是指向地心；而质量只是一个表示物质“有多少”的数字。在回答有关太空宇航员漂浮的问题时，要说明他们是因为自由落体而处于失重状态，并不是质量消失了。
2. Speed and Velocity Are Different | 速率和速度是不同的
Many learners treat speed and velocity as interchangeable. Speed is a scalar—it tells you how fast something moves but not in which direction. Velocity is a vector—it includes both magnitude and direction. In IGCSE questions, a car going round a roundabout at constant speed is accelerating because its direction, and hence its velocity, changes continuously.
许多学习者把速率和速度当作可以互换的概念。速率是标量——它告诉你要移动得多快，但不涉及方向。速度是矢量——它同时包含大小和方向。在 IGCSE 题目中，一辆汽车以恒定速率绕行环岛，其实是在加速，因为它的方向（从而速度）在持续变化。
Acceleration is defined as the rate of change of velocity, not speed. If velocity is constant, acceleration is zero. But if speed is constant while direction changes, there is acceleration towards the centre of the circle—centripetal acceleration. Students often wrongly think that ‘constant speed means no acceleration’, which loses marks on circular motion and satellite orbit topics.
加速度被定义为速度的变化率，而不是速率的变化率。如果速度恒定，加速度为零。但如果速率不变而方向改变，就会存在指向圆心的加速度——向心加速度。学生常常错误地认为“速率不变就没有加速度”，这会在圆周运动和卫星轨道题目中丢分。
For straight-line motion, speed and the magnitude of velocity are the same. The formula v = s / t works for both if direction is ignored. But always check whether the question asks for speed or velocity—if it asks for velocity, you must state the direction as well (e.g. 5 m/s north).
对于直线运动，速率和速度的大小是一样的。如果忽略方向，公式 v = s / t 对两者都适用。但一定要看清楚题目问的是速率还是速度——如果问的是速度，你必须同时给出方向（例如 5 m/s 向北）。
3. A Constant Force Does Not Give Constant Velocity | 恒定的力不会产生恒定的速度
This is perhaps the most deeply rooted misconception, dating back to Aristotle. Everyday experience with friction tricks us into believing that a constant push is needed to keep something moving at constant speed. Newton’s first law states that if resultant force is zero, a body remains at rest or moves with constant velocity. Constant velocity requires zero net force, not a constant force.
这可能是最根深蒂固的误区，可以追溯到亚里士多德时代。日常生活中的摩擦效应让我们误以为需要持续推着东西才能保持匀速。牛顿第一定律指出：如果合力为零，物体将保持静止或匀速直线运动。匀速运动需要的是零合力，而不是恒定的力。
When a constant resultant force is applied, Newton’s second law applies:
F = m × a
A constant net force produces constant acceleration, meaning the velocity changes by the same amount every second. For example, if you push a shopping trolley on a frictionless surface with a steady force, it will speed up continuously, not settle at a fixed speed.
当施加一个恒定的合力时，牛顿第二定律适用：
F = m × a
恒定的合力产生恒定的加速度，这意味着速度每秒变化相同的量。例如，如果你在无摩擦的表面用稳定的力推一辆购物车，它会持续加速，而不会维持在一个固定的速度上。
In real life, as speed increases, air resistance builds up until it balances the driving force, resulting in zero net force and thus constant (terminal) velocity. Students should distinguish between the driving force and the resultant force: a constant driving force can eventually lead to constant velocity only if opposing forces increase with speed to cancel it out.
在现实生活中，随着速度的增加，空气阻力会逐渐增大，直到与驱动力平衡，从而实现零合力，进而达到恒定（终极）速度。学生应当区分驱动力和合力：只有当阻碍力随速度增加而增大，最终与驱动力抵消时，恒定的驱动力才可能最终导致匀速。
4. Current Is Not Used Up in a Circuit | 电流在电路中不会被消耗
A widespread mistake is believing that electric current is ‘consumed’ by components like bulbs or resistors. Students often draw diagrams where the current leaving a bulb is smaller than the current entering it. In reality, charge is conserved. In a series circuit, the current is the same at every point; an ammeter placed anywhere in the loop will read the same value.
一个普遍的错误是认为电流会被灯泡或电阻等元件“消耗”。学生在画图时常常画出流出一个灯泡的电流比流入的要小。实际上，电荷是守恒的。在串联电路中，各处电流都相等；无论把电流表放在回路中哪个位置，读数都相同。
What gets used up or transferred is energy, not current. The bulb converts electrical energy into light and heat, making the potential difference drop across it. The current carries the energy, but the number of charges per second (the current) stays constant. Think of water in a pipe: the flow rate (current) does not decrease after passing a water wheel, even though energy is extracted.
真正被消耗或转化的是能量，而不是电流本身。灯泡把电能转化为光能和热能，导致其两端的电势差下降。电流携带能量，但每秒流过的电荷数（即电流）保持不变。想想水管中的水流：水流过水轮后流速并不会减小，尽管能量已经被提取了。
In parallel circuits, the current splits at junctions, but the total current entering a junction equals the total current leaving it (Kirchhoff’s first law). A common exam error is to say ‘current is shared equally’—this is only true if all parallel branches have identical resistance.
在并联电路中，电流在节点处会分流，但流入节点的总电流等于流出节点的总电流（基尔霍夫第一定律）。一个常见的考试错误是说“电流被均分”——只有在所有并联支路电阻完全相等时才对。
5. Heat and Temperature Are Not the Same | 热量和温度不是一回事
In everyday language we say ‘a hot object contains a lot of heat’, but this is physically incorrect. Temperature is a measure of the average kinetic energy of particles; it is not energy itself. Heat, in physics, refers to the transfer of thermal energy from a hotter body to a colder one. An object does not ‘contain’ heat—it contains internal energy.
在日常用语中我们说“热物体含有很多热量”，但这在物理学上是错误的。温度是粒子平均动能的量度，它本身不是能量。在物理学中，热量指的是热能从较热物体向较冷物体的转移。一个物体并不“含有”热量——它含有的是内能。
Two objects at the same temperature can have very different amounts of internal energy depending on their mass and material. For example, a swimming pool at 25 °C has far more internal energy than a cup of coffee at 80 °C, but the coffee is at a higher temperature. Saying the pool has ‘more heat’ is misleading.
两个温度相同的物体，其内能的多少可能因质量和材料的不同而差异巨大。例如，一个 25 °C 的游泳池所含的内能远比一杯 80 °C 的咖啡要多，但咖啡的温度更高。笼统地说游泳池有“更多的热量”会带来误导。
During phase changes (e.g., melting or boiling), the temperature remains constant even though energy is being supplied. The supplied energy breaks bonds rather than raising kinetic energy. Students often forget this and incorrectly state that temperature must rise while ice melts.
在相变过程中（例如熔化或沸腾），尽管在持续供给能量，温度却保持不变。供给的能量用于打破键合而不是增加动能。学生常常忘记这一点，错误地断言冰在熔化时温度一定会升高。
6. Energy Cannot Be Destroyed, Only Transferred or Transformed | 能量不能被消灭，只能被转移或转化
Phrases like ‘energy is used up’ are common in conversation, but they contradict the principle of conservation of energy. Energy is never created or destroyed; it changes from one form to another. When a battery runs flat, its chemical energy has not vanished—it has been transferred into light, heat and kinetic energy in the circuit and surroundings.
像“能量用光了”这样的说法在日常对话中很常见，但这与能量守恒原理相矛盾。能量既不能被创造也不能被消灭；它从一种形式转变为另一种形式。当电池没电时，它的化学能并没有消失——而是被转移为电路和周围环境中的光能、热能和动能。
The total energy in a closed system remains constant. In efficiency calculations, the ‘lost’ energy is often dissipated as thermal energy, raising the temperature of the surroundings. The phrase ‘energy is lost’ should be avoided in exam answers unless you specify it means transferred to the thermal store of the environment.
在封闭系统中，总能量保持不变。在效率计算中，“损失”的能量常常以热能的形式耗散，使周围环境温度升高。考试答题时应避免使用“能量损失”这种表述，除非你明确说明它是指转移到了环境的热能储存中。
When a ball bounces, it does not reach its original height because some kinetic energy is converted to thermal energy during the impact. Students may think energy ‘disappears’, but it merely spreads out and becomes less useful. This relates to the idea of energy dissipation and resource efficiency.
球弹跳时无法回到原来的高度，因为一部分动能在碰撞过程中转化为热能。学生可能以为能量“消失”了，但它其实只是耗散开来，变得不那么有用而已。这涉及到能量耗散和资源效率的概念。
7. Action and Reaction Forces Act on Different Bodies | 作用力与反作用力作用在不同物体上
Newton’s third law states that if object A exerts a force on object B, then object B exerts an equal and opposite force on object A. The critical detail that many students miss is that these two forces act on different objects. A common exam trap is to ask whether the weight of a book and the normal force from the table are an action-reaction pair.
牛顿第三定律说：如果物体 A 对物体 B 施加一个力，那么物体 B 会同时对物体 A 施加一个大小相等、方向相反的力。许多学生遗漏的关键细节是：这两个力作用在不同的物体上。一个常见的考试陷阱就是问：书的重力和桌子的支持力是不是一对作用力与反作用力。
The weight of a book is the gravitational pull by Earth on the book. The reaction to this weight is the gravitational pull by the book on Earth—not the normal force from the table. The normal force and the weight that act on the same book are a pair of balanced forces if the book is stationary, but they are not an action-reaction pair.
书的重力是地球对书的引力。这个重力的反作用力是书对地球的引力——而不是桌子的支持力。支持力与重力都作用在同一本书上，如果书本静止，它们是一对平衡力，而不是一对作用力与反作用力。
To identify action-reaction pairs, use the phrase: ‘A exerts a force on B, B exerts a force on A’. Check that the forces are of the same type (e.g. both gravitational, both contact) and act on different bodies. A rocket moving forward is a classic example: the rocket pushes exhaust gases backwards, and the gases push the rocket forwards.
识别作用力与反作用力时，可以用这样的表述：“A 对 B 施加一个力，B 对 A 施加一个力”。检查这两个力是否属于同一类型（例如都是引力，或者都是接触力），并且作用在不同物体上。向前飞行的火箭就是一个经典例子：火箭向后推出燃气，燃气则向前推动火箭。
8. Refraction Is Caused by a Change in Speed | 折射是由速度变化引起的
When light enters a denser medium, it slows down. If it hits the boundary at an angle, this change in speed causes the light to change direction—this is refraction. Many students mistakenly believe that light bends towards the normal simply because the medium is ‘thicker’ or that the ray ‘prefers’ the normal. The correct reasoning connects bending directly to the change in wave speed.
当光进入更光密的介质时，速度会减慢。如果它以一定角度射入界面，速度的变化会导致光改变方向——这就是折射。许多学生错误地认为光向法线偏折只是因为介质“更厚”，或者光线“偏爱”法线。正确的逻辑是将偏折与波速变化直接联系起来。
A useful memory rule: if the speed decreases (entering denser medium), the ray bends towards the normal; if speed increases (entering less dense medium), it bends away from the normal. When light enters glass from air, its speed drops from 3.0 × 10⁸ m/s to about 2.0 × 10⁸ m/s, and the ray bends towards the normal.
一个有用的记忆规则：如果速度减小（进入光密介质），光线向法线偏折；如果速度增大（进入光疏介质），光线远离法线偏折。当光从空气进入玻璃时，速度从 3.0 × 10⁸ m/s 降低到约 2.0 × 10⁸ m/s，光线便向法线偏折。
Refraction explains apparent depth illusions: a swimming pool looks shallower than it really is because light from the bottom speeds up as it leaves the water, bending away from the normal. Students sometimes incorrectly attribute this to reflection or scattering. Always draw a clear ray diagram with arrows and label the normal.
折射可以解释视深错觉：游泳池看起来比实际浅，因为池底发出的光离开水面时速度加快，从而远离法线偏折。学生有时会错误地将此归因于反射或散射。答题时务必画出清晰的带箭头的光路图，并标出法线。
9. Floating and Sinking Depend on Density, Not Just Weight | 浮沉取决于密度，而不只是重量
A heavy ship floats while a small pebble sinks—this contradicts the intuition that ‘heavier things sink’. Whether an object floats or sinks depends on its average density compared to the fluid. If the object’s density is less than that of the liquid, it floats; if greater, it sinks. The large steel ship floats because its overall volume includes a lot of air, making the average density less than water.
沉重的轮船能浮在水上，而一颗小石子却会下沉——这与“重的东西会下沉”的直觉相悖。一个物体浮还是沉，取决于它的平均密度与流体密度的比较。如果物体的密度小于液体的密度，它就上浮；反之则下沉。巨大的钢船之所以能浮，是因为它的总体积包含大量空气，使得平均密度小于水的密度。
Archimedes’ principle states that the upthrust on an object equals the weight of fluid displaced. When an object is fully submerged, the upthrust depends only on the volume of displaced fluid and the fluid’s density. Whether the object then sinks or rises depends on whether this upthrust is less than or greater than the object’s weight.
阿基米德原理指出，物体受到的浮力等于它排开的流体的重量。当物体完全浸没时，浮力只取决于排开流体的体积和流体的密度。物体随后是下沉还是上浮，则取决于这个浮力是小于还是大于物体自身重量。
A common exam answer to ‘what makes an object float?’ is simply ‘upthrust equals weight’. That is true only when the object is floating (equilibrium). For a submerged object that is rising, upthrust is greater than weight. Avoid mixing up the conditions for floating versus rising.
对于“什么使物体上浮”这个问题，一个常见的考试答案是“浮力等于重力”。这只有在物体漂浮（平衡）时才成立。对于一个正在上浮的浸没物体来说，浮力是大于重力的。不要混淆了漂浮与上浮的条件。
10. Series and Parallel Circuit Misconceptions | 串联和并联电路误区
When a cell is connected to bulbs in series, many students expect the bulb nearest the positive terminal to be brighter because ‘it gets the current first’. In a series circuit, the rate of charge flow is established almost instantaneously everywhere, and the current is identical through all components. Brightness differences in series are due to resistance differences, not position.
当电池与串联灯泡连接时，许多学生会认为最靠近正极的灯泡会更亮，因为“它最先得到电流”。在串联电路中，各处电荷流动的速率几乎瞬间建立，并且所有元件中的电流都相同。串联中灯泡的亮度差异是由于电阻不同，而不是位置。
Adding more resistors in series increases the total resistance, which reduces the current from the cell. In parallel, adding an extra branch provides an additional path for current, and the total resistance decreases, so the current drawn from the cell increases. Students often reverse these effects, thinking that parallel circuits always have larger resistance.
在串联电路中增加更多电阻会增大总电阻，从而减小电池输出的电流。在并联电路中，增加一条支路会为电流提供额外通路，总电阻减小，因而从电池获取的电流增大。学生常常把这两种效应搞反，误以为并联电路总是有更大的电阻。
Voltage behaves differently: in a series circuit, the supply voltage is shared between components (potential divider); in a parallel circuit, each branch receives the full supply voltage. A classic error is to say that ‘voltage is the same in all series components’—that is only true for current. Always use V, I, R relationships carefully.
电压的表现则不同：在串联电路中，电源电压被各元件分担（分压器）；在并联电路中，每一条支路都获得完整的电源电压。一个典型错误是声称“串联电路中各元件电压相等”——这只有对电流才成立。务必仔细使用 V、I、R 的关系。
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