📚 IGCSE Edexcel Physics: Common Mistakes Explained | IGCSE Edexcel 物理：易错题精讲
Many IGCSE Edexcel Physics students lose marks not because they don’t understand the concepts, but because they fall into predictable traps. This article highlights common mistake questions and clarifies the correct reasoning, helping you avoid these pitfalls in the exam.
许多 IGCSE Edexcel 物理考生丢分并非因为不理解概念，而是掉入了可预见的陷阱。本文精选常见易错题，并阐明正确思路，助你考试中避开这些雷区。
1. Acceleration and Sign Errors | 加速度与符号错误
A typical error occurs when students calculate acceleration without properly defining a positive direction. For example, a car moving east at 15 m/s slows to 5 m/s east in 4 seconds. If east is positive, the acceleration is a = (5 – 15) / 4 = -2.5 m/s². Many students will simply subtract the smaller number from the larger and get +2.5 m/s², ignoring the direction. The negative sign indicates acceleration is towards the west, opposite to the velocity. Always set a sign convention: the sign of acceleration tells you its direction relative to the chosen frame, not just ‘speeding up’ or ‘slowing down’.
典型错误是学生在未规定正方向的情况下计算加速度。例如，一辆车以 15 m/s 向东行驶，4 秒后减速至 5 m/s 向东。若定义向东为正，则加速度 a = (5 – 15) / 4 = -2.5 m/s²。很多学生会直接用大数减小数，得出 +2.5 m/s²，忽略了方向。负号表示加速度方向向西，与速度方向相反。务必先设定正方向：加速度的正负表示其相对于所选方向的方向，而不简单代表 ‘加速’ 或 ‘减速’。
A related mistake: in a vertical throw, when a ball is thrown upwards, students often assign positive acceleration during ascent and negative during descent, or assume acceleration is zero at the top. In fact, acceleration due to gravity g is constant at 9.8 m/s² downwards. If upwards is positive, then a = -9.8 m/s² throughout the motion, even at the highest point where velocity is momentarily zero.
相关错误：在竖直上抛运动中，学生常以为上升时加速度为正，下降时为负，或者以为最高点加速度为零。事实上，重力加速度 g 向下恒为 9.8 m/s²。若取向上为正，则整个过程中 a = -9.8 m/s²，即使在最高点瞬时速度为零时也是如此。
2. Resultant Force and Equilibrium | 合力与平衡
A persistent misconception is that a moving object must have a resultant force acting on it. According to Newton’s first law, an object will continue at constant velocity in a straight line if the resultant force is zero. A book sliding across a table with constant speed does have forces acting on it (friction, applied force), but they are balanced. Students often draw the applied force arrow larger than friction, implying a net force, which is wrong for constant velocity.
一个根深蒂固的误解是：运动的物体一定受到合外力作用。根据牛顿第一定律，若合外力为零，物体将保持匀速直线运动。一本书在桌面上匀速滑动时，的确受到力（摩擦力、推力）的作用，但这些力相互平衡。学生常将推力箭头画得比摩擦力大，暗示存在净力，这对于匀速运动来说是错误的。
Another typical error: confusing mass and weight. Weight is the gravitational force (W = mg) and measured in newtons; mass is measured in kilograms. In free-body diagrams, students sometimes label the downward arrow as ‘mass’ instead of ‘weight’. Always use correct terminology and units.
另一个常见错误：混淆质量与重量。重量是重力（W = mg），单位为牛顿；质量单位为千克。在受力分析图中，学生有时把向下的箭头标为 ‘质量’ 而非 ‘重量’。务必使用正确的术语和单位。
3. Energy Efficiency Calculations | 能量效率计算
Efficiency = (useful output energy / total input energy) × 100%. A frequent slip is forgetting to multiply by 100, leaving the answer as a decimal, or using the wrong total. For instance, a motor lifts a mass using 500 J of electrical energy but 150 J is dissipated as heat and sound; useful work done is 350 J. Efficiency = (350 / 500) × 100% = 70%. Some students mistakenly use 150 J as the useful output or reverse the fraction.
效率 = （有用输出能量 / 总输入能量）× 100%。常见错误是忘记乘以 100，答案给成小数，或者用错了总输入。例如，一台电动机消耗 500 J 电能提升了重物，其中 150 J 作为热和声散失；有用功为 350 J。效率 = (350 / 500) × 100% = 70%。有些学生误把 150 J 当作有用输出，或颠倒了分子分母。
Also, be careful with power efficiency: it’s (useful power out / total power in) × 100%. A question may give power in kW and time, but you usually need energy (power × time) for the calculation. Always check the units and whether you are asked for a decimal or percentage.
同时，注意功率效率：它是（有用输出功率 / 总输入功率）× 100%。题目也许给出以 kW 为单位的功率和时间，但通常计算需要能量（功率 × 时间）。要检查单位，以及题目是要求用小数还是百分比作答。
4. Series and Parallel Circuits | 串联与并联电路
This is a minefield of common errors. In a series circuit, current is the same at all points, but the voltage is shared. In a parallel circuit, voltage across each branch is the same, but current divides. Many students reverse these rules. For example, they might think that if two resistors are in parallel, the one with larger resistance gets more current – actually it gets less (I = V/R).
这是一片充满常见错误的雷区。在串联电路中，电流处处相等，但电压被分配。在并联电路中，各支路两端电压相等，但电流分流。很多学生颠倒了这些规则。例如，他们可能以为两个电阻并联时，阻值较大的电阻分得更多电流——实际上它分得的电流更小（I = V/R）。
A classic mistake is to treat a circuit with both series and parallel sections as entirely series or entirely parallel. Always simplify step by step: combine parallel branches first to find effective resistance, then add series resistances. Another problem: misunderstanding voltmeter and ammeter placement. An ammeter must be in series (same current), a voltmeter in parallel (same voltage). Connecting a voltmeter in series can drastically alter the circuit and give meaningless readings.
一个经典错误是把既有串联又有并联的电路当作纯串联或纯并联来处理。务必逐步简化：先合并并联支路求等效电阻，再与串联电阻相加。另一个问题是电压表和电流表的连接方式。电流表必须串联（测量同一电流），电压表必须并联（测量同一电压）。把电压表串联在电路中会大大改变电路状态，得出无意义的读数。
5. Wave Speed, Frequency and Wavelength | 波速、频率与波长
The wave equation v = f λ is simple, but mark loss often comes from unit conversion. Frequency must be in hertz (Hz, or s⁻¹) and wavelength in metres (m) to get speed in m/s. If a question gives frequency in kHz or wavelength in cm, convert first. For example, a wave with f = 500 kHz and λ = 0.6 cm: convert to 500 000 Hz and 0.006 m, then v = 500 000 × 0.006 = 3000 m/s. Using 500 and 0.6 gives 300, which is wrong by a factor of 10.
波速方程 v = f λ 很简单，但丢分往往来自单位换算。频率必须以赫兹（Hz，即 s⁻¹）为单位，波长以米（m）为单位，才能得到以 m/s 为单位的波速。如果题目给出的频率是 kHz 或波长是 cm，要先换算。例如，一波的频率 f = 500 kHz，波长 λ = 0.6 cm：换算为 500 000 Hz 和 0.006 m，则 v = 500 000 × 0.006 = 3000 m/s。若直接使用 500 和 0.6 会得到 300，差了 10 倍。
Another subtle error: confusing the time period T and frequency. T = 1/f. Students sometimes use the period directly as frequency. For instance, if the time for one complete wave is 0.02 s, f = 50 Hz, but a student may incorrectly write f = 0.02 Hz. Always check the relationship.
另一个不易察觉的错误：混淆周期 T 和频率。T = 1/f。学生有时直接把周期当作频率使用。比如，若完成一个完整波所需时间为 0.02 s，则 f = 50 Hz，但学生可能错误地写成 f = 0.02 Hz。务必核对两者的关系。
6. Half-Life Calculations | 半衰期计算
IGCSE students often struggle with half-life problems involving non-integer numbers of half-lives. The remaining mass or activity after n half-lives is given by initial amount × (1/2)ⁿ. If a sample has a half-life of 3 days and an initial mass of 160 g, after 9 days (3 half-lives), the remaining mass is 160 × (1/2)³ = 20 g. A common mistake is to divide 160 by 9 or subtract incorrectly. Another error: using the half-life time as the number of half-lives (e.g., thinking after 3 days only 1/3 remains).
IGCSE 学生在处理非整数个半衰期的问题时常遇困难。经过 n 个半衰期后，剩余质量或活度 = 初始量 × (1/2)ⁿ。若某样品半衰期为 3 天，初始质量 160 g，经过 9 天（3 个半衰期），剩余质量为 160 × (1/2)³ = 20 g。常见错误是把 160 除以 9，或者用错了减法。另一个错误是把半衰期时间误作半衰期个数（例如以为 3 天后只剩下 1/3）。
Be careful when reading graphs: the x-axis may show time in hours or years. Always determine how many half-lives fit into the given time. If the half-life is 8 days and the graph shows 24 days, that’s 3 half-lives. Sometimes the question asks for the time taken for the activity to fall to a certain fraction, e.g., 1/8 of the original. Since 1/8 = (1/2)³, it corresponds to 3 half-lives.
读图时要小心：横轴可能以小时或年为单位。一定要先算出给定时间内包含多少个半衰期。若半衰期为 8 天，图上显示 24 天，那就是 3 个半衰期。有时题目问活度下降到初始的几分之一所需的时间，例如 1/8。由于 1/8 = (1/2)³，这对应 3 个半衰期。
7. Density and Pressure Pitfalls | 密度与压强陷阱
Density ρ = mass / volume. Units matter: g/cm³ and kg/m³. A typical question: a block has mass 500 g and volume 200 cm³; density = 2.5 g/cm³. If you need kg/m³, multiply by 1000: 2500 kg/m³. Students often forget that 1 g/cm³ = 1000 kg/m³ and get the conversion wrong by a factor of 1000.
密度 ρ = 质量 / 体积。单位很关键：g/cm³ 和 kg/m³。常见题目：一个物块质量 500 g，体积 200 cm³；密度 = 2.5 g/cm³。如果需要以 kg/m³ 表示，乘以 1000 得到 2500 kg/m³。学生常常忘记 1 g/cm³ = 1000 kg/m³，换算时出现 1000 倍的错误。
Pressure P = F / A. The force must be perpendicular to the area and in newtons; area in m² gives pascals (Pa). If area is given in cm², convert to m² first. For example, a force of 50 N acts on an area of 25 cm²: area = 25 / 10000 = 0.0025 m², so P = 50 / 0.0025 = 20000 Pa. Students often use 25 directly and get 2 Pa. When dealing with liquid pressure, P = ρ g h, where h is depth in metres, ρ in kg/m³. Don’t mix this with solid pressure.
压强 P = F / A。力必须垂直于面积且以牛顿为单位，面积以 m² 为单位得到帕斯卡（Pa）。如果面积以 cm² 给出，先转换为 m²。例如，50 N 的力作用在 25 cm² 的面积上：面积 = 25 / 10000 = 0.0025 m²，所以 P = 50 / 0.0025 = 20000 Pa。学生常直接用 25 计算，得到 2 Pa。处理液体压强时，用 P = ρ g h，其中 h 为深度（米），ρ 单位 kg/m³。切勿将固体压强与液体压强公式混淆。
8. Refraction and Critical Angle | 折射与临界角
A frequent conceptual error is thinking that light always bends towards the normal when entering a denser medium, but away from the normal when leaving it – which is correct – yet then applying it backwards. When light goes from glass to air, it speeds up and bends away from the normal. The angle of refraction is larger than the angle of incidence. Some students draw the refracted ray bending towards the normal even when exiting glass.
一个常见概念错误是知道光进入光密介质时向法线靠拢，离开时远离法线——这本身没错——但画图时却反过来。当光从玻璃进入空气时，速度加快并偏离法线，折射角大于入射角。一些学生在光离开玻璃时仍将折射光线画得靠近法线。
For total internal reflection, there are two conditions: light must travel from a denser to a less dense medium (e.g., glass to air), and the angle of incidence must be greater than the critical angle c. The critical angle is given by sin c = 1/n, where n is the refractive index of the denser medium. A mistake is to use n = sin i / sin r from previous data and then incorrectly solve for c. Remember: sin c = 1/n, not n = 1/sin c. If n = 1.5, c = sin⁻¹(1/1.5) ≈ 41.8°.
对于全反射，有两个条件：光必须从光密介质射向光疏介质（例如玻璃到空气），且入射角必须大于临界角 c。临界角由 sin c = 1/n 给出，其中 n 为光密介质的折射率。一个错误是用之前数据里的 n = sin i / sin r 然后错误求解 c。记住：sin c = 1/n，而不是 n = 1/sin c。若 n = 1.5，c = sin⁻¹(1/1.5) ≈ 41.8°。
9. Electromagnetic Induction and Lenz’s Law | 电磁感应与楞次定律
Students often confuse the direction of induced current when a magnet moves relative to a coil. Lenz’s law states the induced current flows in a direction that opposes the change causing it. If a north pole moves into a coil, the induced current creates a north pole at the near end to repel it. If the magnet is pulled out, the induced current creates a south pole to attract it. Many remember ‘oppose the motion’ but then predict the direction incorrectly because they forget the polarity of the induced magnet.
学生常弄混磁铁与线圈相对运动时感应电流的方向。楞次定律指出，感应电流的方向总是阻碍引起它的变化。若将 N 极插入线圈，感应电流会在近端产生 N 极以排斥它；若拔出磁铁，感应电流则产生 S 极以吸引。许多学生记得 ‘阻碍运动’，但由于忘记感应磁极的极性而预测错误方向。
Another common mistake: thinking a simple generator always produces direct current (d.c.). In reality, a rotating coil in a magnetic field induces an alternating e.m.f. due to the continual change of flux direction. To get d.c., a split-ring commutator is needed, which rectifies the output. Also, the magnitude of the induced e.m.f. depends on the rate of change of magnetic flux, not the magnetic field strength alone. A faster-moving magnet induces a larger e.m.f.
另一个常见错误：以为简单的发电机直接产生直流电。实际上，旋转线圈在磁场中因磁通方向不断变化而产生交变电动势。要得到直流电，需要换向器对输出进行整流。此外，感应电动势的大小取决于磁通量的变化率，而不仅仅是磁场强度。磁铁运动越快，感应电动势越大。
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