📚 IB & OCR Physics: Common Mistakes Explained | IB与OCR物理易错题精讲
Many high-achieving physics students lose marks not because they lack knowledge, but because they fall into subtle traps set by examiners. Whether you are sitting IB Physics or OCR A Level Physics, certain errors appear year after year. This article walks through some of the most persistent misconceptions, shows you exactly where students go wrong, and gives you clear, correct reasoning. Read each section carefully – the difference between a grade 6 and a grade 7, or a B and an A, often lies in avoiding these pitfalls.
许多成绩优秀的物理学生丢分并不是因为知识欠缺，而是掉进了考官精心设计的思维陷阱。无论你参加的是IB物理还是OCR A Level物理考试，某些错误每年都会反复出现。本文带你梳理那些最顽固的误解，明确指出学生错在哪里，并给出清晰正确的推理过程。请仔细阅读每一节——6分和7分之间、B和A之间的差距，往往就在于能否避开这些易错点。
1. Projectile Motion: Independence of Components | 抛体运动：速度分量的独立性
A student is asked to find the time of flight for a ball kicked horizontally from a cliff. They calculate the initial speed, then try to use the horizontal distance – but the horizontal motion does not affect the time to hit the ground. The common mistake is mixing horizontal and vertical variables in kinematic equations.
学生被要求计算从悬崖上水平踢出的小球的飞行时间。他们算出初速度，然后试图利用水平距离——但水平运动并不影响落地时间。常见错误就是将水平变量和竖直变量混在运动学方程中。
The correct approach: For vertical motion, initial vertical velocity uy = 0, acceleration ay = g (downwards). Use s = uyt + ½gt². Time is determined solely by vertical displacement and g. The horizontal velocity stays constant (ignoring air resistance) and is only used to find range after you know the time.
正确的思路：对于竖直运动，初始竖直速度 uy = 0，加速度 ay = g（向下）。使用 s = uyt + ½gt²。时间完全由竖直位移和重力加速度决定。水平速度保持不变（忽略空气阻力），且只在知道时间后才用来求水平射程。
2. Newton's Third Law: Action-Reaction Pairs | 牛顿第三定律：作用力与反作用力
Many learners think that the normal force and weight acting on a book on a table are an action-reaction pair. That is wrong. The normal force is exerted by the table on the book; its third-law partner is the force the book exerts on the table. The weight is the gravitational pull of Earth on the book; its partner is the gravitational pull of the book on Earth. Because the normal force and weight act on the same object (the book), they can cancel each other in equilibrium, but they are not a Newton's third law pair.
许多学生认为放在桌上的书本受到的支持力和重力是一对作用力与反作用力。这是错误的。支持力是桌子施加给书的力；它的第三定律搭档是书施加给桌子的力。重力是地球对书的引力；它的搭档是书对地球的引力。由于支持力和重力作用在同一个物体（书）上，它们在平衡时可以相互抵消，但它们并不是牛顿第三定律中的作用力-反作用力对。
3. Centripetal Force: Not an Extra Force | 向心力：不是一个额外的力
A classic exam trap: drawing a free-body diagram for an object moving in a horizontal circle and adding an arrow labelled 'centripetal force'. In reality, centripetal force is the name we give to the resultant force directed towards the centre. It is provided by existing forces such as tension, friction, or the horizontal component of lift. Never add it as an independent force.
典型的考试陷阱：为一个做水平圆周运动的物体画受力分析图时，加上一个标为“向心力”的箭头。实际上，向心力是我们给予指向圆心的合力的名称。它由已有的力提供，比如绳子的拉力、摩擦力或升力的水平分量。绝对不要将它当作一个独立的力加上去。
For example, a car turning on a flat road: the centripetal force is the friction between tyres and road. No separate 'centripetal' arrow appears. Similarly, a conical pendulum has tension and weight; the horizontal component of tension provides the centripetal force.
例如，汽车在水平路面上转弯：向心力是轮胎与路面之间的摩擦力。没有单独的“向心”箭头。同样，圆锥摆受到拉力和重力；拉力的水平分量提供向心力。
4. Work Done and Potential Energy | 做功与势能
st gravity as ΔGPE = mgΔh without considering the direction. The work done by gravity is Wg tive work against gravity, and the gravitational potential energy increases. But gravity itself does negative work. Confusing these signs leads to errors in energy conservation calculations.
学生常常错误地将克服重力做功写成 ΔGPE = mgΔh 而不考虑方向。重力做的功是 Wg = -ΔGPE。当你把一本书提起时，你克服重力做正功，重力势能增加。但重力本身做负功。混淆这些符号会导致能量守恒计算的错误。
, you risk double-counting. Use either work done + change in kinetic energy + change in potential energy = 0, or net work = ΔKE.
还要记住：功能定理指的是所有力做的总功。如果你既计入施加力做的功又计入重力势能的变化，就可能重复计算。应该使用“外力做功 + 动能变化 + 势能变化 = 0”或者“总功 = ΔKE”。
5. Internal Resistance and Terminal Voltage | 内阻与路端电压
o emf. Actually, terminal voltage V = ε - Ir. When the circuit draws current, the lost volts (Ir) increase, and V drops. A common question: 'Why does the voltmeter reading decrease when a second resistor is added in parallel?' – rcuit resistance decreases, current increases, so Ir increases, reducing terminal voltage.
一个电动势为 ε、内阻为 r 的电池连接到负载电阻 R。许多学生错误地认为路端电压总是等于电动势。实际上，路端电压 V = ε - Ir。当电路中有电流通过时，内阻分掉的电压 Ir 增大，路端电压下降。常见考题：“为什么并联一个电阻后电压表读数减小？”——因为总电阻减小，电流增大，因此 Ir 增大，路端电压降低。
This is heavily tested in OCR practical skills and IB paper 3. Always draw the circuit with emf and internal resistance in series. The internal resistance is inside the battery, so you measure V across the external terminals.
这是OCR实验技能和IB试卷三的重点考查内容。始终要把电池画成电动势和内阻串联。内阻在电池内部，因此你测量的是外部接线柱之间的电压 V。
6. Kirchhoff's Laws: Sign Errors in Loops | 基尔霍夫定律：回路中的符号错误
When applying Kirchhoff's voltage law (loop rule), the most frequent slip is assigning inconsistent signs to potential differences. Choose a loop direction. For a resistor, if the loop direction matches the assumed current direction, the potential drops, so use -IR. If you travel through a cell from negative to positive, the potential increases, so add +emf. Doing this haphazardly leads to incorrect simultaneous equations.
在应用基尔霍夫电压定律（回路定则）时，最常见的错误是对电势差赋予不一致的符号。选定一个回路方向。对于电阻，如果回路方向与假设的电流方向一致，电势下降，用 -IR。如果经过电池时是从负极到正极，电势升高，加 +emf。随意地处理符号会得到错误的联立方程。
the same sense. This method reduces sign confusion. Always draw current directions and stick to them, even if you suspect the direction is wrong – the algebra will just give a negative value for I.
更稳妥的技巧：写成 Σε = ΣIR。根据极性对回路中的电动势求和，并在同一取向下对 I×R 求和。这种方法可以减少符号混淆。始终先标出电流方向并坚持使用，即使你怀疑方向是错的——代数运算只会给出一个负的电流值。
7. Magnetic Flux and Faraday's Law | 磁通量与法拉第定律
A magnet is pushed into a coil connected to a galvanometer. Students often say, 'There is an induced emf because there is a magnetic flux in the coil.' ). The faster the magnet moves, the larger the rate of change, the larger the induced emf.
将一块磁铁推入与电流计相连的线圈，学生经常说：“线圈中有磁通量，所以产生了感应电动势。”这是错误的。大的静态磁通量不会感应出电动势。只有磁通链的变化才能感应出电动势（ε = -N ΔΦ/Δt）。磁铁运动得越快，变化率越大，感应电动势越大。
eld and normal to area. When the coil is perpendicular to the field, flux is maximum, but rate of change of flux can be zero if it' ximum emf occurs when the coil is parallel to the field (rate of flux change is greatest).
另一常见疏漏：混淆磁通量 Φ（单位 Wb）和磁通密度 B（单位 T）。Φ = BA cosθ，其中 θ 是磁场与面积法线的夹角。当线圈与磁场垂直时磁通量最大，但如果不转动的话磁通量的变化率可能为零。在发电机中，当线圈与磁场平行时感应的电动势最大（此时磁通量变化率最大）。
8. Photoelectric Effect: Threshold & Intensity | 光电效应：截止频率与光强
A very common mistake: 'If I increase the intensity of light, the kinetic energy of the emitted electrons increases.' According to the photon model, kinetic energy Kmax he number of photons (and thus the photocurrent), not the energy per photon. To increase Kmax, you must increase frequency f (i.e. use bluer light), not intensity.
极其常见的错误：“增大光强，发射出的光电子动能就会增加。”根据光子模型，最大动能 Kmax = hf - Φ。光强只增加光子数量（因此增加光电流），而不增加单个光子能量。要想增加 Kmax，必须增大频率 f（即使用更蓝的光），而不是增大光强。
Also, many students forget that if the frequency is below the threshold frequency f0 l, no matter how intense the light. This comes up frequently in IB multiple choice and OCR 'explain why' questions.
此外，许多学生忘记如果频率低于截止频率 f0 = Φ/h，无论光有多强都不会有电子发射出来。这在IB选择题和OCR“解释原因”题中频繁出现。
9. Wave Phase and Path Difference | 波的相位与波程差
fference = (2π/λ) × path difference. A path difference of λ gives a phase difference of 2π (constructive interference). A path difference of λ/2 gives π (destructive). But when waves reflect at a boundary, an additional phase change of π may occur, which must be added.
在双缝干涉中，学生常算出波程差并直接将其等同于相位差。他们必须记住：相位差 = (2π/λ) × 波程差。波程差为 λ 时，相位差为 2π（相长干涉）。波程差为 λ/2 时，相位差为 π（相消干涉）。但当波在边界反射时，可能会产生额外的 π 相变，这个也必须加上。
In thin film interference, forgetting the phase reversal at the higher refractive index interface is a classic error. Soap bubble colours arise from interference between light reflected from the front and back surfaces; one reflection may undergo a π shift, altering the conditions for bright and dark fringes.
在薄膜干涉中，忘记在高折射率界面处的相位反转是一个经典错误。肥皂泡的颜色来源于前后表面反射光之间的干涉；其中一个反射可能会经历 π 相移，从而改变亮纹和暗纹的条件。
10. First Law of Thermodynamics: Sign of Work Done | 热力学第一定律：做功的符号
(compression: W positive). OCR sometimes uses ΔU = Q - W where W is work done BY the gas (expansion: W positive). Mixing up these conventions is disastrous. Always check the data booklet or question statement.
热力学第一定律常写作 ΔU = Q + W，但 W 的符号约定取决于教材。IB 一般用 ΔU = Q + W，其中 W 是对气体做的功（压缩时 W 为正）。OCR 则有时用 ΔU = Q - W，其中 W 是气体对外做的功（膨胀时 W 为正）。搞混这些约定会带来灾难。一定要先看公式手册或题目说明。
. This provides a neat way to test understanding. Common error: thinking that if a gas expands, it always cools down. Not true if heat is supplied to keep temperature constant.
对于一定质量的理想气体，还要牢记内能 U 只与温度有关。在等温膨胀中，ΔU = 0，因此根据一种约定有 Q = -W。这提供了一个巧妙的检验理解的方式。常见错误：认为气体膨胀就一定会降温。如果有热量供给使温度保持不变，那就不一定了。
11. Error in Circular Motion: Centripetal Acceleration Formula | 圆周运动易错点：向心加速度公式
ω²r without checking which variable is constant. If two objects have the same angular velocity ω, then a ∝ r. If they have the same linear speed v, then a ∝ ellites at different orbital radii leads to incorrect conclusions about acceleration or force.
学生有时在使用 a = v²/r 和 a = ω²r 时不检查哪个变量是恒定的。如果两个物体有相同的角速度 ω，那么 a ∝ r。如果它们有相同的线速度 v，那么 a ∝ 1/r。在比较不同轨道半径的卫星时搞混这些，会导致对加速度或力的错误结论。
For orbits, v is not constant; orbital speed v = √GM/r itational field strength at radius r. This underlines the importance of not just memorising equations but knowing their dependencies.
对于轨道运动，v 不是常数；轨道速率 v = √GM/r。所以直接使用 a = v²/r 可能是危险的。更稳妥的是用 a = GM/r²，即半径为 r 处的引力场强。这突显了不能仅死记公式，还要知道公式的依赖关系。
12. Standing Waves: Nodes and Antinodes Misunderstanding | 驻波：波节与波腹的误解
tube closed at one end, students often say the first harmonic has a wavelength of 4L. That' ect but they then apply the same to a tube open at both ends, which has a node at each end? at the ends, giving λ = 2L. Missing this leads to wrong frequencies for wind instruments.
在一端封闭的共振管中，学生常说基频的波长为 4L。这没错，但他们之后将同样的规则用于两端开口的管，认为两端都是波节？不——开口端是波腹。因此开口管的基频在中间有一个波节，两端是波腹，得出 λ = 2L。错过这一点会导致管乐器频率的计算错误。
Also note that pressure nodes correspond to displacement antinodes. Many exam questions ask about the pressure variation along a standing wave tube. If you confuse displacement and pressure node positions, the whole graph is inverted. Always check what the question is measuring.
还要注意，压强波节对应着位移波腹。许多考题会问及驻波管内压强的变化。如果你混淆了位移和压强波节的位置，整个图像都会反过来。一定要看清题目在测量什么物理量。
Published by TutorHao | Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图