Common Misconceptions in IGCSE CIE Physics | IGCSE CIE 物理常见误区

📚 Common Misconceptions in IGCSE CIE Physics | IGCSE CIE 物理常见误区

IGCSE Physics can be tricky because some ideas feel intuitive but are scientifically wrong. In this article, we will tackle the most common misconceptions that students hold, helping you avoid lost marks in exams. Each point is explained clearly so you can replace faulty reasoning with solid understanding.

IGCSE 物理中有很多直觉上似乎正确、其实却错误的观念，它们常常导致考试中不必要的失分。本文将梳理学生最常见的认知误区，并逐一澄清，帮助你把错误的理解替换成准确的物理概念。

1. Force and Motion: Constant Force Means Constant Speed | 力与运动：受恒力作用就做匀速运动

Many students believe that an object needs a constant force to keep moving at a steady speed. In reality, a constant resultant force causes constant acceleration, not constant velocity. If an object moves at constant velocity, the resultant force on it must be zero (Newton’s first law).

很多同学误以为物体要保持匀速运动就必须持续受到一个恒定的力。事实上，恒定的合力产生的是恒定的加速度，而不是恒定的速度。如果物体做匀速直线运动，它受到的合力一定是零（牛顿第一定律）。

On a frictionless surface, a moving object continues at constant velocity without any forward force. In everyday life, friction usually hides this truth: you need to push a box at constant speed to balance friction, making the resultant force zero — not to ‘maintain the motion’ itself.

在无摩擦的表面上，运动的物体会保持匀速，根本不需要向前的力。生活中的摩擦力掩盖了这一事实：你需要用恒力推箱子，是为了平衡摩擦力，让合力为零，而不是为了“维持运动”。

Resultant force F = 0 ⇌ constant velocity (or rest)

合力 F = 0 ⇌ 速度恒定（或静止）

2. Mass vs. Weight: They Are the Same Thing | 质量与重量：它们是一回事

A very frequent error is using ‘mass’ and ‘weight’ interchangeably. Mass is the amount of matter in an object, measured in kilograms (kg). Weight is the gravitational force acting on that mass, measured in newtons (N). Your mass stays the same on the Moon, but your weight is only about one-sixth of that on Earth.

最常见的错误之一就是混淆质量和重量。质量是物体所含物质的多少，单位是千克 (kg)；重量是作用在该质量上的引力，单位是牛顿 (N)。你在月球上的质量不变，但重量只有地球上的约六分之一。

In IGCSE exams, forgetting to convert mass into weight (multiplying by g) when using W = mg loses marks. Always check whether a question asks for mass or weight, and remember that weight is a force with both magnitude and direction.

在 IGCSE 考试中，忘记用 W = mg 将质量转化成重量会直接丢分。一定要看清题目问的是质量还是重量，并且记住重量是一种力，既有大小也有方向。

Weight (N) = mass (kg) × gravitational field strength (N/kg)

重量 (N) = 质量 (kg) × 重力场强度 (N/kg)

3. Understanding Newton’s Third Law: Action–Reaction Pairs Cancel Out | 理解牛顿第三定律：作用力与反作用力互相抵消

When you sit on a chair, your weight pulls down and the chair pushes up on you — these two forces are equal and opposite, so you stay still. But these are not an action–reaction pair! They cancel because they act on the same object (you). An action–reaction pair must act on two different objects.

你坐在椅子上，重力向下拉你，椅子向上推你——这两个力大小相等、方向相反，所以你保持静止。但这对力并不是作用力与反作用力！它们之所以能抵消，是因为它们作用在同一个物体（你）上。而作用力与反作用力必须作用在不同物体上。

The true action–reaction pair here is: Earth pulls you down (action), you pull Earth up (reaction) — acting on different objects. Another pair: you push down on the chair, the chair pushes up on you. Understanding this avoids confusion in equilibrium and motion problems.

这里真正的作用力与反作用力是：地球向下拉你（作用力），你向上拉地球（反作用力）——它们分别作用在不同物体上。另外一对：你向下压椅子，椅子向上推你。搞清这一点，在平衡和运动问题中就不会糊涂。

4. Speed and Acceleration: Zero Velocity Means Zero Acceleration | 速度与加速度：速度为零加速度也为零

A ball thrown vertically upwards has zero velocity at its highest point, yet its acceleration is still g = 9.8 m/s² downwards. Velocity and acceleration do not have to be zero simultaneously. Acceleration is the rate of change of velocity, not the velocity itself.

竖直上抛的小球在最高点瞬时速度为零，但它的加速度仍然是向下的 9.8 m/s²。速度和加速度并不需要同时为零。加速度是速度的变化率，而不是速度本身。

Similarly, a car moving at high but constant speed has zero acceleration. Many students wrongly link high speed with large acceleration. The key is whether the velocity is changing, not how big it is.

同样，一辆高速匀速行驶的汽车加速度为零。不少学生错误地将高速与大加速度联系在一起。关键是看速度是否在变化，而不是速度本身的大小。

a = (v − u) / t

加速度 a = (末速度 − 初速度) / 时间

5. Electric Current ‘Consumption’ Myth | 电流“消耗”误区

Many learners think current gets ‘used up’ as it passes through a bulb or resistor. In a simple series circuit, the current is the same at every point. Charge cannot be destroyed — what gets transferred is energy, not current.

很多同学认为电流在经过灯泡或电阻时会被“用掉”。在简单的串联电路中，各处的电流大小是完全相同的。电荷不会被消灭——被转移的是能量，而不是电流本身。

The battery gives energy to the charges, which then deliver this energy to the components (as light or heat). The charges themselves complete the loop. Therefore, ammeter readings before and after a lamp are identical.

电池给电荷提供能量，电荷再把能量传递给用电器（变成光或热）。电荷自身只是在回路中循环。因此灯泡前后的电流表读数完全相同。

Current is the flow of charge, measured in amperes (A), same everywhere in a series loop.
电流是电荷的流动，单位是安培 (A)，串联回路中各处相等。
Energy is transferred to components, causing a drop in electrical potential (voltage), not current.
能量被传递给电器，导致电势降低（电压降），而不是电流减小。

6. Voltage and Current in Series and Parallel | 串联与并联中的电压和电流

Confusing the rules for series and parallel circuits is a regular pitfall. In series: current is the same, voltage splits. In parallel: voltage across each branch is the same, current splits. Many students reverse these or think both voltage and current stay the same in parallel.

混淆串联和并联的规律是一个常见大坑。串联电路中：电流处处相等，电压被分配；并联电路中：各支路电压相等，电流被分配。不少学生把这两条记反，或者认为并联时电压和电流都保持不变。

For identical resistors in parallel, total resistance is halved (if two resistors). The circuit draws more total current from the battery, but each branch carries less than the total. A good exam tip: always label current direction and check junction rules.

对于两个相同的电阻并联，总电阻减半，电源提供的总电流会增大，但每一支路中的电流小于总电流。一个实用的考试技巧：始终标出电流方向，并用节点电流规律检验。

Series / 串联	Parallel / 并联
Current same: I₁ = I₂ = I₃	Voltage same: V₁ = V₂ = V₃
Voltage divides: V_total = V₁ + V₂	Current divides: I_total = I₁ + I₂
R_total = R₁ + R₂ + …	1/R_total = 1/R₁ + 1/R₂

7. Heat vs. Temperature: Adding Heat Always Raises Temperature | 热量与温度：加热一定会升温

Heating a substance does not always raise its temperature. During a state change (melting, boiling), the added energy goes into breaking intermolecular bonds, not increasing kinetic energy. The temperature stays constant — the flat portions on a heating curve.

加热一种物质并不一定能让它的温度升高。在物态变化时（熔化、沸腾），吸收的能量用于打破分子间的作用力，而不是增加分子动能，因此温度保持不变——这在加热曲线上就是水平段。

Students often mistake ‘heat’ for a substance contained in an object. Heat is energy in transit from a hotter object to a colder one. A hot object does not ‘contain heat’; it has internal energy. Temperature measures the average kinetic energy of particles.

学生常将“热量”误解为物体内部含有的一种物质。热量其实是从高温物体传向低温物体的能量。高温物体并不“含有热量”，它拥有的是内能。温度则衡量粒子平均动能的高低。

8. Direction of Electric Current: Conventional vs. Electron Flow | 电流方向：传统方向与电子流动

In metallic conductors, current is the flow of negatively charged electrons from the negative terminal to the positive terminal. However, conventional current is defined as the direction positive charges would move — from positive to negative. This historical convention still appears in circuit diagrams.

在金属导体中，电流实际上是带负电的电子从负极向正极的流动。但“传统电流方向”被定义为正电荷移动的方向，也就是从正极流向负极。这个历史习惯依然出现在电路图中。

A common exam error is drawing electron flow arrows opposite to the conventional current arrows but then confusing the deflection of an ammeter or the diode’s forward bias. In IGCSE, unless specifically asked for electron flow, use conventional current.

考试中常见错误是：把电子流动的方向与传统电流方向画反，但随后又在判断电流表偏转或二极管正偏时搞混。在 IGCSE 里，除非题目明确要求画电子流动方向，否则一律使用传统电流方向。

9. Wave Propagation: Medium Travels with the Wave | 波的传播：介质随波一起移动

When a water wave moves across a pond, a floating cork bobs up and down but is not carried forward. Waves transfer energy without transferring matter. In a sound wave, air molecules vibrate back and forth but do not travel from the source to the ear.

当水波掠过池塘，水面上的软木塞做上下振动，并不会随波前进。波传递的是能量，而不是物质。声波中，空气分子在原地前后振动，并不会从声源跑到你的耳朵里。

This misconception often surfaces in questions about transverse and longitudinal waves. In both cases, the medium moves only about a mean position. The amplitude is the maximum displacement from the rest position, not the distance the particle travels.

这种误解常常出现在有关横波与纵波的题目中。无论是哪种波，介质都只在其平衡位置附近运动。振幅是离开平衡位置的最大位移，而不是粒子“旅行”的距离。

10. Electromagnetic Induction: A Magnetic Field Always Induces Voltage | 电磁感应：只要有磁场就能产生感应电压

Simply placing a coil in a magnetic field does not induce an e.m.f. A voltage is induced only when there is a change in the magnetic flux linkage — by moving the magnet, moving the coil, or changing the current in an adjacent coil. A stationary magnet and stationary coil produce nothing.

仅仅把线圈放在磁场里并不会产生感应电动势。只有当穿过线圈的磁通量发生变化时才会感应出电压——可以通过移动磁铁、移动线圈或改变相邻线圈中的电流来实现。静止的磁铁和静止的线圈不会产生任何感应。

Similarly, a d.c. current in a coil can induce a voltage in another coil only at the moment of switch-on or switch-off, because the magnetic field changes. A steady d.c. does not cause continuous induction. Transformers work only with alternating current (a.c.) for this reason.

同样，线圈中的直流电只能在接通或断开的瞬间在另一线圈中感应出电压，因为那时磁场在变化。恒定的直流无法产生连续的感应。正是这个原因，变压器必须使用交流电 (a.c.)。

11. Radioactive Decay and Half-life | 放射性衰变与半衰期

A common mistake is thinking that after one half-life, exactly half the atoms have decayed, and after two half-lives, all atoms are gone. Radioactive decay is a random process. The half-life is the time for the activity (or number of radioactive nuclei) to halve on average, not a deterministic countdown.

一个常见错误是认为经过一个半衰期，一定恰好有一半原子发生衰变；经过两个半衰期，所有原子都衰变完了。实际上，放射性衰变是一个随机过程。半衰期是指放射性活度（或放射性核的数目）平均减半所需的时间，而不是一个确定的倒计时。

Another misconception: the radiation emitted is not a ‘gas’ or ‘poison’ that seeps out continuously. Alpha, beta, and gamma radiation are particles or electromagnetic waves emitted from unstable nuclei. Background radiation comes from natural sources all around us.

另一个误区是：把放出的射线当作某种会不断渗出的“毒气”或“毒物”。α、β 和 γ 射线是从不稳定的原子核中发射出的粒子或电磁波。背景辐射则来自我们周围的天然放射源。

12. Reflection and Refraction: The Image Is Always on the Mirror Surface | 反射与折射：像总在镜面上

In a plane mirror, the image is not on the mirror surface — it is formed behind the mirror, at the same perpendicular distance as the object is in front. Many students draw rays stopping at the mirror and locate the image incorrectly.

平面镜成像时，像并不在镜面上——它形成在镜子后面，且与物体到镜面的垂直距离相等。许多同学画光路图时让光线停在镜面，导致像的位置标错。

For refraction, a frequent error is to say light always bends towards the normal when entering a denser medium. This is correct. But students often forget that the frequency remains constant, and only speed and wavelength change. Also, the angle of incidence is always measured from the normal, not the surface.

在折射中，一个常见错误是忘记光的频率不变，变化的是速度和波长。同时，入射角总是从法线量起，而不是从界面。学生虽然知道光进入光密介质时会靠近法线，却常常画错角度或搞混正弦值。

n = sin i / sin r = speed in medium 1 / speed in medium 2

折射率 n = sin i / sin r = 介质 1 中的速度 / 介质 2 中的速度

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Common Misconceptions in IGCSE CIE Physics | IGCSE CIE 物理常见误区