📚 Year 13 CAIE Physics: Common Misconceptions and Correction Methods | Year 13 CAIE 物理：常见误区与纠正方法
Many A-level Physics students struggle with subtle concepts that appear straightforward but hide deep misunderstandings. This article targets the most persistent misconceptions in Year 13 CAIE Physics, providing clear corrections to help you avoid losing marks in exams. Each section addresses a specific error and explains the correct physical principle.
许多 A-level 物理学生在一些看似简单但隐藏深层误解的概念上屡屡出错。本文瞄准 Year 13 CAIE 物理中最顽固的常见误区，提供清晰的纠正方法，帮助你避免在考试中丢分。每个小节针对一个具体错误，并解释正确的物理原理。
1. Centripetal Force Is Not an Extra Force | 向心力并不是多出来的一种力
A very common mistake is to treat centripetal force as a separate, additional force acting on an object moving in a circle. Students often draw a free-body diagram and add an arrow labelled 'centripetal force' independent of tension, gravity or normal reaction. In reality, centripetal force is simply the resultant force directed towards the centre of the circle, provided by one or more real forces. For a car turning a corner, it is the friction; for a planet orbiting a star, it is the gravitational pull. The correct approach is: identify all real forces, resolve towards the centre, and set the net inward force equal to mv²/r or mω²r. Never invent a new force.
一个非常常见的错误是把向心力当成一个独立的、额外的力作用在做圆周运动的物体上。学生画受力图时，常常在拉力、重力或支持力之外单独添加一个标为“向心力”的箭头。实际上，向心力只是指向圆心的合力，由一个或多个真实的力来提供。汽车转弯时它是摩擦力；行星绕恒星运行时它是万有引力。正确的方法是：找出所有真实力，向圆心方向分解，令向内的净力等于 mv²/r 或 mω²r。绝对不要凭空添加新力。
2. Gravitational Potential Is Negative and Its Sign Matters | 引力势为负值，符号很重要
mass to bring a test mass from infinity to that point. Since gravity is attractive, work is done by the field, so potential is negative. When calculating potential energy or escape velocity, omitting the negative sign leads to nonsense. For instance, the total energy of a satellite is E = ½ mv² - GMm/r, not plus. If you get a positive total energy, the orbit is unbound. Also, potential becomes less negative with increasing r, meaning the potential increases as you move away from the mass, which confuses some students.
许多学生记住了 φ = -GM/r，但随意对待负号，忘记了引力势的定义是把单位质量从无限远处移到该点外力做的功。由于引力是吸引力，引力场做正功，所以势能为负。在计算势能或逃逸速度时，忽略负号会得出荒谬结果。例如卫星的总能量应为 E = ½ mv² - GMm/r，而不是加号。如果算出的总能量为正，轨道就是非束缚的。此外，随着 r 增大，势的绝对值减小，即势增大（变得更“正”），这往往让学生困惑。
3. In SHM, Maximum Speed Occurs at Equilibrium, Not at Maximum Displacement | 简谐运动中，最大速度在平衡位置，而非最大位移处
n simple harmonic motion, energy transforms between kinetic and potential. At the amplitude positions x = ±A, the velocity is momentarily zero, and all energy is potential. At the equilibrium position x = 0, potential energy is zero and kinetic energy, hence speed, is maximum. The velocity equation v = ± ω√A² - x² . Remember that acceleration is maximum at x = ±A and zero at x = 0, opposite to velocity.
一个顽固的误区是认为位移最大时速度也最大，或许是受到势能最大的影响。在简谐运动中，能量在动能和势能之间转换。在振幅位置 x = ±A，速度瞬间为零，全部能量为势能。在平衡位置 x = 0，势能为零，动能最大，因此速度最大。速度公式 v = ± ω√A² - x² 证实 x = 0 时 v 最大。要记住，加速度在 x = ±A 处最大，在 x = 0 处为零，与速度正好相反。
4. Internal Energy Is Not Just Temperature | 内能不止是温度
Students often equate high temperature with high internal energy, ignoring the role of number of molecules and intermolecular potential energy. Internal energy of an ideal gas depends only on temperature (kinetic energy), but for real gases, liquids and solids, it includes both random kinetic energy and the potential energy due to intermolecular forces. During a phase change, temperature stays constant while internal energy increases (melting/boiling) because potential energy increases as bonds are broken. Saying 'heat is energy' is also vague; heat is energy transferred due to a temperature difference, not a substance contained in an object.
学生常把高温等同于高内能，却忽略了分子数量和分子间势能的作用。理想气体的内能只取决于温度（动能），但对于真实气体、液体和固体，内能包括分子随机动能和由于分子间作用力而具有的势能。在相变过程中，温度保持不变但内能增加（熔化/沸腾），因为破坏键合需要增加势能。把“热量”简单等同于“能量”也是模糊的；热量是由于温差而传递的能量，不是物体内含有的某种物质。
5. Electric Potential Is Not Electric Potential Energy | 电势不是电势能
energy U of a charge q at that point is U = qV. Many errors arise from confusing the two. For a point charge, V = kQ/r, which is a scalar. The potential can be positive or negative depending on the sign of Q. When moving a positive charge from a region of higher potential to lower potential, the electric field does positive work, decreasing potential energy. But for a negative charge, it is the opposite. Always check the sign of q when linking U and V.
某点的电势 V 是把单位正电荷从无限远处移到该点外力做的功。电荷 q 在该点的电势能 U 是 U = qV。很多错误源于混淆两者。对于点电荷，V = kQ/r，是个标量。电势可以为正或负，取决于 Q 的符号。把正电荷从高电势移向低电势时，电场做正功，电势能减小。但如果是负电荷，情况正好相反。在联系 U 和 V 时，永远要检查 q 的符号。
6. Time Constant of a Capacitor Does Not Mean Full Charge/Discharge | 电容器的时间常数不意味着完全充放电
capacitor is fully charged or discharged. In fact, during charging, the voltage reaches 63% of the supply voltage after τ, and about 99% after 5τ. Similarly, during discharge, voltage drops to 37% after τ. The equations v = V₀ e-t/RC and v = V₀ (1 - e-t/RC is shorter than τ. Always check whether the question refers to half-life or time constant.
有些学生认为经过一个时间常数 τ = RC 后，电容器就完全充电或放电了。实际上，充电时过 τ 后电压达到电源电压的 63%，到 5τ 时才达到约 99%。放电时，经过 τ 电压降至原来的 37%。方程 v = V₀ e-t/RC 和 v = V₀ (1 - e-t/RC) 描述的是指数变化，不是线性变化。误解 τ 会导致在计算半衰期 t₁/₂ = RC ln 2 时出错，半衰期其实比 τ 短。务必看清题目问的是半衰期还是时间常数。
7. Magnetic Flux and Flux Linkage Are Different | 磁通量和磁链是不同的
number of turns N and flux through each turn. Students often use the wrong one in Faraday' ° = 1), not minimum. Another error: thinking that a constant magnetic flux can induce emf.
Published by TutorHao | Year 13 Physics Revision Series | aleveler.com
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