📚 Common Misconceptions in IB Physics | IB物理常见误区
In IB Physics, a deep conceptual understanding is essential for tackling exam questions and internal assessments. However, many students hold persistent misconceptions that can hinder their ability to apply principles correctly. This article addresses some of the most common misunderstandings across key topics, clarifying the correct physics behind each one.
在IB物理中，深刻的概念理解对于应对考试题目和内部评估至关重要。然而，许多学生抱有根深蒂固的误解，这会妨碍他们正确应用物理原理。本文针对各核心主题中最常见的一些误解，逐一阐明背后的正确物理概念。
1. Velocity and Acceleration Confusion | 速度与加速度的混淆
Many students assume that a fast-moving object must have a large acceleration, and that zero velocity implies zero acceleration. In reality, velocity and acceleration are independent kinematic quantities. Velocity describes the rate of change of displacement, while acceleration describes the rate of change of velocity.
许多学生认为快速运动的物体必定有很大的加速度，而速度为零时加速度也一定为零。实际上，速度和加速度是独立的运动学量。速度描述位移的变化率，加速度则描述速度的变化率。
A car cruising on a motorway at a constant 130 km/h has zero acceleration, because its velocity is not changing. Conversely, a ball thrown vertically upwards has an instantaneous velocity of zero at its highest point, yet its acceleration is still 9.8 m/s² downwards due to gravity.
一辆在高速公路上以130公里/小时匀速行驶的汽车，加速度为零，因为其速度没有变化。相反，垂直上抛的小球在最高点瞬时速度为零，但由于重力，其向下的加速度仍为9.8米/秒²。
a = Δv / Δt
2. Newton’s First Law: “Force for Motion” | 牛顿第一定律：“运动需要力”
A deeply embedded misconception is that a continuous net force is required to keep an object moving. Aristotelian intuition tells us that to maintain motion, you must keep pushing. Newton’s first law states the opposite: an object will remain at rest, or move with constant velocity, unless acted upon by a net external force.
一个根深蒂固的误解是，要保持物体运动就需要持续施加净力。亚里士多德的直觉告诉我们，要维持运动，就得不断推动。牛顿第一定律则相反：一切物体在不受净外力作用时，总保持静止状态或匀速直线运动状态。
In everyday experience, friction complicates this. A book pushed across a table slows down because of friction, not because a force is needed to sustain motion. In deep space, a probe with its engines off will travel indefinitely at constant speed without any net force acting on it.
在日常生活中，摩擦力使问题复杂化。一本书在桌上滑动会慢下来，是因为摩擦力，而不是因为需要力来维持运动。在深空中，关闭发动机的探测器在没有净力作用的情况下，将以恒定速度永远飞行下去。
3. Centrifugal Force as a Real Interaction | 离心力是真实的力吗？
When a car rounds a corner, passengers feel pushed outward and often refer to a “centrifugal force”. In the inertial reference frame taught at IB, centrifugal force is not a real force arising from an interaction; it is a fictitious force experienced only in a rotating frame. The actual force causing circular motion is the centripetal force, directed towards the centre of the circle.
当汽车转弯时，乘客感觉被向外推，并常提到“离心力”。在IB所教授的惯性参考系中，离心力并非由相互作用产生的真实力，它只是一个在旋转参考系中才能感受到的虚拟力。导致圆周运动的真实力是向心力，指向圆心。
For a car turning, the centripetal force is the friction between tyres and road. If this force suddenly disappears (e.g., on black ice), the car will not fly radially outward; it will continue moving in a straight line tangential to the curve, in line with Newton’s first law.
对于转弯的汽车，向心力是轮胎与路面之间的摩擦力。如果这一力突然消失（如遇到黑冰），汽车不会沿径向向外飞出，而是会沿着曲线的切线方向作匀速直线运动，这符合牛顿第一定律。
4. Newton’s Third Law Pairs Misidentified | 牛顿第三定律力对的错误识别
Students frequently misidentify action–reaction pairs by pairing forces that act on the same object, such as the weight of a book and the normal reaction from a table. These are not a Third Law pair; they act on the same book and can balance each other. A true Newton’s Third Law pair always acts on two different bodies and is of the same type.
学生常将作用在同一物体上的力错误地配成作用力与反作用力，例如一本书的重力和桌面对书的支持力。它们并不构成第三定律力对，因为它们作用在同一本书上，可以相互平衡。真正的牛顿第三定律力对总是作用在两个不同的物体上，且属于同一种力。
The correct pair for the book’s weight is the gravitational force that the book exerts on the Earth. The pair for the normal force on the book is the contact force the book exerts downward on the table.
书所受重力的正确反作用力是书对地球的引力。书所受支持力的反作用力是书对桌面向下的接触力。
5. Friction Always Opposes Motion | 摩擦力总是与运动方向相反
It is commonly believed that friction always acts in the direction opposite to an object’s motion. In reality, static friction often acts in the direction of motion to enable movement. When you walk, your foot pushes backward against the ground; the ground exerts a static friction force forward on your foot, accelerating you forward. Without this forward friction, walking would be impossible.
人们通常认为摩擦力总是与物体运动方向相反。实际上，静摩擦力常常沿运动方向作用，从而使人得以移动。当你走路时，脚向后蹬地，地面对你的脚施加向前的静摩擦力，使你向前加速。如果没有这个向前的摩擦力，走路是不可能的。
Friction opposes relative motion or the tendency for relative motion between surfaces. A rolling tyre’s friction with the road can point forward (driving wheel) or backward (idling wheel), depending on the slipping tendency.
摩擦力阻碍的是接触面之间的相对运动或相对运动趋势。滚动的轮胎与路面之间的摩擦力可能指向前方（驱动轮）或后方（从动轮），取决于打滑的趋势。
6. Electric Current Gets Used Up | 电流被消耗
A classic circuits misconception is that current is “used up” as it passes through bulbs or resistors, so less current returns to the battery than leaves it. In a series circuit, charge is conserved; the current – the rate of flow of charge – is exactly the same at every point. Energy is transferred, not current.
一个经典的电路误解是，电流流过灯泡或电阻时会被“消耗”，因此返回电池的电流比流出电池的少。在串联电路中，电荷是守恒的；电流——电荷流动的速率——在每一点都完全相同。被传递的是能量，而不是电流。
What changes across a resistor is the electrical potential energy per unit charge, measured as potential difference (voltage). The number of charge carriers passing any cross-section per second remains constant, ensuring current is the same throughout a single loop.
电阻两端变化的是单位电荷的电势能，用电压来衡量。每秒通过任意截面的载流子数目保持不变，因此在单一回路中电流处处相等。
7. Voltage and Current in Parallel vs Series | 串并联电路中的电压与电流误区
Many IB students confuse the rules for potential difference (p.d.) and current in series and parallel arrangements. In a parallel circuit, the p.d. across each branch is equal to the supply p.d., while the current divides. In a series circuit, the current is constant, while the supply p.d. is shared among components.
许多IB学生混淆了串联和并联电路中电势差（电压）和电流的规律。在并联电路中，各支路两端的电压等于电源电压，而电流会分流。在串联电路中，电流处处相等，而电源电压被各元件分担。
A common error is to assume that adding a resistor in parallel increases the overall resistance of a circuit. In fact, adding a resistor in parallel always provides an additional path for current, decreasing the total resistance.
一个常见错误是认为并联一个电阻会增加电路的总电阻。实际上，并联电阻总是为电流提供额外的路径，从而降低总电阻。
8. Waves Carry Matter | 波动传递物质
Observing ocean waves might suggest that water travels great distances towards the shore, but waves transfer energy without globally transporting matter. In both transverse and longitudinal waves, particles of the medium oscillate about fixed equilibrium positions; the energy and information move through the medium, but the medium itself does not travel with the wave.
观察海浪时，我们或许会以为水远距离地传向岸边，但波动传递的是能量而非物质。在横波和纵波中，介质中的质点围绕固定的平衡位置振动；能量和信息通过介质传播，而介质本身并不随波前进。
A floating seagull on the water merely bobs up and down as a wave passes, rather than surfing forward. Similarly, sound waves compress and rarefy air locally, but air molecules do not stream from the speaker to your ear.
一只漂浮的海鸥在波浪经过时只是上下浮动，而不是向前冲浪。同样，声波使空气局部压缩和稀疏，但空气分子并不会从喇叭流到你的耳朵。
9. Photoelectric Effect Intensity Misconception | 光电效应强度误解
When studying the photoelectric effect, students often think that increasing the intensity of incident light will increase the maximum kinetic energy of emitted photoelectrons. According to Einstein’s photon model, the maximum kinetic energy depends only on the frequency of the light and the work function of the metal, not on intensity.
在学习光电效应时，学生常以为增加入射光的强度就能增大出射光电子的最大动能。根据爱因斯坦的光子模型，最大动能只取决于光的频率和金属的功函数，与光强无关。
Ek,max = hf – Φ
Higher intensity means more photons per second, so more electrons are emitted per unit time (greater photocurrent), provided the frequency is above the threshold. Increasing intensity cannot supply energy beyond the photon energy per electron, so the kinetic energy per electron remains unchanged.
更高的光强意味着每秒有更多的光子，只要频率高于截止频率，单位时间内发射的电子数就会增多（光电流增大）。但增加光强不能为单个电子提供超出光子能量的能量，因此单个电子的动能保持不变。
10. Half-life: Mass Always Halves | 半衰期：质量总是减半
Radioactive half-life is defined as the time taken for half the radioactive nuclei in a sample to decay. A common misconception is that the total mass of the sample halves every half-life. In reality, the parent nuclei transmute into daughter nuclei, which remain in the sample. The total mass stays roughly constant, though a tiny fraction may escape as radiation or particles, and some net mass change occurs via binding energy differences.
放射性半衰期定义为样品中一半放射性原子核发生衰变所需的时间。一个常见误解是，每经过一个半衰期，样品的总质量就减半。实际上，母核会衰变成子核，子核仍留在样品中。总质量基本保持不变，尽管极小部分会以辐射或粒子形式逸出，且因结合能差异有微小的净质量变化。
The number of undecayed parent nuclei halves, but the sample still contains the daughter nuclei along with the undecayed parents. Therefore, a sample does not “vanish” to half its mass. In IB, focus on the exponential decay of the number of radioactive nuclei N = N₀e⁻ᵗ, noting that it is the count of unstable atoms, not mass, that halves.
未衰变的母核数目确实减半，但样品中除了未衰变的母核，还包含了子核。因此，样品的质量并不会真的减半。在IB物理中，应关注放射性核数目N = N₀e⁻⁽⁾的指数衰减，明确减半的是不稳定原子的数量，而非总质量。
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