📚 IB WJEC Physics: Common Misconceptions | IB WJEC 物理常见误区
Physics requires a precise understanding of its core principles, yet many students develop intuitive ideas that interfere with learning. This article addresses the most persistent misconceptions encountered in IB and WJEC Physics courses, helping you replace faulty mental models with accurate scientific understanding. Each section contrasts incorrect assumptions with correct explanations, ensuring you can tackle exam questions with confidence.
物理学要求对其核心原理有精确的理解，但许多学生会产生与学习相冲突的直觉性想法。本文针对 IB 和 WJEC 物理课程中最顽固的常见误区，帮助你将错误的心智模型替换为正确的科学理解。每节对比错误假设和正确解释，确保你能自信地应对考试题目。
1. Mass and Weight | 质量与重量
Many learners believe mass and weight are the same quantity because they use ‘weight’ in everyday language to mean ‘how heavy something is’. In physics, mass is a measure of the amount of matter in an object and is measured in kilograms (kg). It does not change with location. Weight is the gravitational force acting on that mass and is measured in newtons (N). An astronaut has the same mass on the Moon as on Earth, but their weight is about one-sixth.
许多学习者认为质量和重量是同一个量，因为日常用语中常用“重量”表示“有多重”。在物理学中，质量是物体所含物质的量度，单位为千克(kg)，不随位置改变。重量则是作用在该质量上的引力，单位为牛顿(N)。航天员在月球上的质量与地球相同，但其重量约为地球的六分之一。
The relationship is given by W = mg, where g is the gravitational field strength. On Earth, g ≈ 9.8 N kg⁻¹. This means a 1.0 kg mass has a weight of 9.8 N. Always distinguish between the scalar mass and the vector weight.
关系式为 W = mg，其中 g 是引力场强度。在地球上，g ≈ 9.8 N kg⁻¹。这意味着 1.0 kg 的质量具有 9.8 N 的重量。务必区分标量质量和矢量重量。
2. Speed and Velocity | 速率与速度
A widespread mistake is treating speed and velocity as interchangeable. Speed is a scalar quantity – it only has magnitude, such as 30 m s⁻¹. Velocity is a vector – it has both magnitude and direction, such as 30 m s⁻¹ due north. An object moving in a circle at constant speed has a changing velocity because its direction continuously changes, resulting in centripetal acceleration.
一个普遍的错误是将速率和速度互换使用。速率是标量——只有大小，如 30 m s⁻¹。速度是矢量——既有大小又有方向，如 30 m s⁻¹ 朝正北。一个物体以恒定速率作圆周运动时，其速度在不断变化，因为方向持续改变，由此产生向心加速度。
When solving kinematic problems, always check if the quantity demands the vector form (displacement, velocity, acceleration) or the scalar form (distance, speed). Using the wrong one leads to errors in sign and direction, especially in projectile motion.
在解运动学问题时，始终检查该量是需要矢量形式（位移、速度、加速度）还是标量形式（路程、速率）。用错形式会导致符号和方向错误，尤其在抛体运动中。
3. Acceleration, Deceleration, and Negative Acceleration | 加速度、减速与负加速度
Students often assume that negative acceleration always means slowing down. In physics, negative acceleration (with respect to a chosen positive direction) means the acceleration vector points opposite to the positive axis. If the velocity is also negative, negative acceleration can mean the object is speeding up in the negative direction. For example, a ball thrown upwards has positive velocity and negative acceleration (g = -9.8 m s⁻²) until it reaches the top; on the way down, both velocity and acceleration are negative, so its speed increases.
学生常假设负加速度总意味着减速。在物理中，负加速度（相对于选定的正方向）是指加速度矢量与正方向相反。如果速度也为负，则负加速度可能意味着物体朝负方向加速。例如，向上抛出的球在到达最高点前具有正速度和负加速度（g = -9.8 m s⁻²）；下落时速度和加速度均为负，因此速率增加。
The term ‘deceleration’ simply means that the acceleration opposes the instantaneous velocity, causing the speed to decrease. Depending on the reference frame, this could be positive acceleration if taken in the opposite sense. Always refer to the vector relationship: if a and v have opposite signs, the object is slowing down.
“减速”一词仅表示加速度与瞬时速度方向相反，导致速率减小。根据参考系，若取相反方向，这也可能是正加速度。始终参照矢量关系：若 a 与 v 符号相反，物体在减速。
4. Force Implies Motion – The Aristotelian Fallacy | 力意味着运动——亚里士多德谬误
A deep-rooted misconception is that a constant force is needed to maintain constant velocity. Newton’s first law states that an object remains at rest or in uniform motion in a straight line unless acted upon by a net external force. If an ice skater is gliding at constant speed on smooth ice, the net force is zero; no forward force is required to keep moving. The apparent ‘need for force’ in daily life arises from friction and air resistance that must be balanced.
一种根深蒂固的误区是认为需要恒定的力来维持恒定速度。牛顿第一定律指出，除非受到净外力作用，否则物体保持静止或匀速直线运动状态。如果冰上滑行者以恒定速度滑行，净力为零；不需要前向力来保持运动。日常生活中“需要力”的感觉来自于必须平衡的摩擦和空气阻力。
This misconception often appears in questions about terminal velocity: a skydiver reaches terminal speed when weight equals air resistance, net force becomes zero, and they continue at constant speed – not because the forces ‘disappear’.
该误区常出现在关于终端速度的问题中：跳伞者在重量等于空气阻力时达到终端速度，净力为零，他们以恒定速度继续运动——并不是因为力“消失了”。
| Misconception | Correct Physics |
| A moving object has a force in the direction of motion. | Motion does not imply force; objects continue due to inertia. |
| If an object is moving, a net force must be acting. | If velocity is constant, net force is zero. |
5. Action-Reaction Pairs and ‘Cancelling’ Forces | 作用力与反作用力及“抵消”力
Newton’s third law states that forces come in pairs: if body A exerts a force on body B, body B exerts an equal and opposite force on body A of the same type. A common error is to say that these forces cancel each other out. They act on different bodies, so they cannot cancel. When you push a wall, the wall pushes back on you; these are equal and opposite, but the wall does not accelerate because its net force includes also other forces from its foundations.
牛顿第三定律指出力以成对形式出现：若物体 A 对物体 B 施加一个力，则物体 B 对物体 A 施加一个大小相等、方向相反且同类型的力。常见错误是认为这些力相互抵消。它们作用在不同物体上，因此无法抵消。当你推墙时，墙也对你施加推力；这两个力大小相等方向相反，但墙不加速是因为它受到的净力还包括来自地基的其他力。
When drawing free-body diagrams, always include only forces acting on the body under consideration. The reaction to a force does not appear on the same body, which is why you never add it to the net force on that body.
绘制受力分析图时，只包含作用在所考虑物体上的力。一个力的反作用力并不作用在同一物体上，因此绝不要将其加在该物体的净力中。
6. Work Done and Potential Energy Confusion | 做功与势能混淆
Students often think that lifting an object slowly requires less work than lifting it quickly. The work done against gravity, mgh, depends only on the vertical displacement, not the speed or path taken. Even if you lift a book extremely slowly at constant velocity, the work done by the applied force equals mgh, assuming the kinetic energy change is negligible.
学生常认为缓慢提起物体所需的功比快速提起少。克服重力所做的功 mgh 只取决于垂直位移，与速度或路径无关。即使你非常缓慢地以恒定速度提起一本书，施加的力所做的功仍等于 mgh，假定动能变化可忽略不计。
Another misconception arises when gravitational potential energy is linked to a single object. Gravitational potential energy belongs to the system of objects interacting via gravity – typically the object and the Earth. Lifting the object increases the energy stored in the system, not just in the object itself.
另一个误区是将重力势能与单个物体关联。重力势能属于通过引力相互作用的物体系统——通常是物体和地球。提升物体增加了存储在该系统中的能量，而不仅仅在物体本身。
7. Direction of Electric Current vs Electron Flow | 电流方向与电子流动方向
In metallic conductors, mobile charge carriers are electrons moving from the negative terminal to the positive terminal. However, conventional current is defined as the flow of positive charge – from positive to negative. This historical convention remains standard in circuit analysis. Many students mistakenly label the direction of current as the direction of electron movement, leading to confusion in diodes, transistors, and Hall effect problems.
在金属导体中，可移动的载流子是电子，从负极流向正极。然而，常规电流被定义为正电荷的流动——从正极到负极。这一历史惯例在电路分析中保持为标准。许多学生错误地将电流方向标为电子运动方向，这在二极管、晶体管和霍尔效应问题中会造成混淆。
When a conductor moves in a magnetic field, we use Fleming’s left-hand rule (for motors) or right-hand rule (for generators) with conventional current. Substituting electron flow without adjusting the sign will give the wrong force direction.
当导体在磁场中运动时，我们使用弗莱明左手定则（电动机）或右手定则（发电机）时采用常规电流。若直接代入电子流动方向而不调整符号，会得到错误受力方向。
8. Ohm’s Law and Constant Resistance | 欧姆定律与恒定电阻
Ohm’s law, V = IR, is often treated as a universal statement that resistance is constant. In reality, the law holds only for ohmic conductors at constant temperature. The resistance R is defined as the ratio V/I, which can vary with voltage for non-ohmic devices such as filament lamps and diodes. As the filament lamp gets hotter, its resistance increases because the lattice vibrations impede electron flow more intensely. So the V-I graph is not a straight line.
欧姆定律 V = IR 常被当作电阻恒定的普遍陈述。实际上，该定律仅适用于恒温下的欧姆导体。电阻 R 定义为 V/I 的比值，对非欧姆器件（如白炽灯和二极管）可能随电压变化。白炽灯变热后，由于晶格振动更剧烈地阻碍电子流动，其电阻增加。因此 V-I 图不是直线。
Students must recognise that ‘resistance’ is a property that can change, and ‘Ohm’s law’ is a specific proportionality, not a definition. For a fixed resistor, doubling voltage doubles current, but this does not always apply.
学生必须认识到“电阻”是一个可变的属性，而“欧姆定律”是一种特定的比例关系，并非定义。对于固定电阻器，电压翻倍则电流翻倍，但这并非普遍适用。
9. Series and Parallel Circuits: Voltage and Current Distribution | 串联与并联电路：电压和电流的分配
A classic mistake is believing that current gets ‘used up’ as it passes through components. In a series circuit, current is the same at every point because charge is conserved. The total current leaving the battery equals the total current returning to it. The voltage (potential difference) across each resistor, on the other hand, depends on its resistance and adds up to the total source voltage.
一个典型错误是认为电流在经过元件时被“消耗掉”。在串联电路中，各处电流相等，因为电荷守恒。流出电池的总电流等于返回电池的总电流。而每个电阻两端的电压（电势差）取决于其阻值，且总和等于总电源电压。
In parallel circuits, the potential difference across each branch is the same as the source voltage, but the current splits. Paths with lower resistance carry larger currents. A common misunderstanding is that current ‘prefers’ the path of least resistance absolutely, ignoring that all branches with finite resistance share current according to I = V/R.
在并联电路中，各支路两端的电势差与电源电压相同，但电流会分流。电阻较低的支路承载较大电流。一个常见误解是电流“绝对”选择最小电阻路径，忽略了所有有限电阻支路都依 I = V/R 分配电流。
10. Momentum Conservation and Isolated Systems | 动量守恒与孤立系统
The principle of conservation of momentum states that the total momentum of an isolated system remains constant. Students frequently apply it to situations where external forces act, such as a ball bouncing off a wall. The ball’s momentum reverses, so momentum is not conserved for the ball alone; the Earth-wall system gains an equal and opposite momentum, but it is undetectable due to huge mass. Only systems with zero net external force can have unchanged total momentum.
动量守恒原理指出孤立系统的总动量保持不变。学生常将其应用于有外力作用的情境，如球从墙上弹回。球的动量反向，所以球本身的动量不守恒；地球–墙系统获得大小相等方向相反的动量，但因质量巨大而难以察觉。只有净外力为零的系统，总动量才不变。
In collisions, ptotal before = ptotal after, but kinetic energy may not be conserved (inelastic collisions). The misconception that momentum and kinetic energy behave identically leads to errors in calculating final speeds.
在碰撞中，碰撞前 p总 = 碰撞后 p总，但动能可能不守恒（非弹性碰撞）。误认为动量和动能的守恒方式相同，会导致计算末速度时出错。
11. Waves: Speed Depends on the Medium, Not Frequency | 波：波速取决于介质，而非频率
Many learners think that increasing the frequency of a wave increases its speed. The speed of a mechanical wave (sound, water, seismic) is determined by the properties of the medium – elasticity and density. For a given medium, wave speed v is roughly constant, so changing the frequency f forces the wavelength λ to adjust according to v = fλ. Shouting at a higher pitch does not make your voice travel faster through air; it reduces the wavelength.
许多学习者认为提高波的频率会增大波速。机械波（声波、水波、地震波）的波速由介质的性质决定——弹性和密度。对于给定介质，波速 v 大致恒定，因此改变频率 f 会迫使波长 λ 按 v = fλ 调整。用更高的音调喊叫不会使声音在空气中传播更快；它只是减小了波长。
For electromagnetic waves in a vacuum, all frequencies travel at the same speed c. In a material, different frequencies may have slightly different speeds, causing dispersion, but the relationship remains medium-dependent, not frequency-driven.
对于真空中的电磁波，所有频率均以相同速度 c 传播。在材料中，不同频率可能具有略微不同的速度，导致色散，但这种关系仍取决于介质，而非由频率驱动。
12. Photons, Energy, and Momentum of Light | 光子、能量与光的动量
A persistent error is to think that photons have mass because they have momentum. The relativistic energy-momentum relation E² = (mc²)² + (pc)² reveals that a particle can have zero rest mass and still carry momentum if it has energy. Photons have zero rest mass, and their momentum p is given by p = h/λ = E/c. This momentum is responsible for radiation pressure and the Compton effect, but it does not imply any mass.
一个持续的误区是认为光子因具有动量而拥有质量。相对论能量–动量关系 E² = (mc²)² + (pc)² 表明，粒子可以具有零静止质量，但只要拥有能量，仍可携带动量。光子的静止质量为零，其动量 p 由 p = h/λ = E/c 给出。这种动量导致辐射压力和康普顿效应，但并不意味着任何质量。
In photoelectric effect questions, students often confuse the photon’s energy with its momentum or think the kinetic energy of ejected electrons depends on light intensity. The maximum kinetic energy depends solely on the photon energy (hf) and the work function Φ: KEmax = hf − Φ. Intensity affects the number of photons, thus the photocurrent, but not the max KE per electron, provided the frequency is above the threshold.
在光电效应题目中，学生常将光子的能量与动量混淆，或认为逸出电子的动能取决于光强。最大动能仅取决于光子能量 (hf) 和功函数 Φ：KEmax = hf − Φ。光强影响光子数量，进而影响光电流，但不影响单个电子的最大动能，前提是频率高于截止频率。
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