📚 IGCSE AQA Physics: Multiple Choice Killer Techniques | IGCSE AQA 物理：选择题秒杀技巧
In the IGCSE AQA Physics examination, the multiple-choice paper (Paper 1) challenges you with 40 questions to be answered in 45 minutes. While each question is worth only one mark, mastering efficient techniques can boost your speed and accuracy significantly. This guide provides proven strategies to ‘kill’ the multiple-choice questions—helping you eliminate distractors, apply physics reasoning, and avoid common pitfalls.
在 IGCSE AQA 物理考试中，选择题卷（Paper 1）要求你在 45 分钟内完成 40 道题。虽然每题只有 1 分，但掌握高效技巧可以大幅提升你的做题速度与准确率。本指南将为你提供“秒杀”选择题的实战策略——帮你排除干扰项、运用物理逻辑、避开常见陷阱。
1. Unit and Dimensional Analysis | 单位与量纲分析
Always start by checking the units of the given options. If the question asks for a speed, any answer with units of acceleration (m/s²) or force (N) can be instantly eliminated. This quick scan often removes one or two wrong choices.
务必先检查选项的单位。如果题目问的是速度，那么单位是加速度（m/s²）或力（N）的选项可以立刻排除。这样快速一扫，往往能直接去掉一两个错误答案。
Go further by applying dimensional analysis using base SI units. Write the quantity you need in terms of kg, m, s and A. For instance, resistivity has units ohm metre (Ω·m); since resistance R = V/I, and V has units J/C, breaking it down gives kg·m³/(s³·A²). An option that does not simplify to these dimensions is wrong.
更进一步，运用基本国际单位进行量纲分析。将所求物理量用 kg、m、s 和 A 表达。比如电阻率单位是欧姆·米（Ω·m）；由于电阻 R = V/I，而 V 的单位是 J/C，展开后为 kg·m³/(s³·A²)。选项若无法化简为这一量纲，即可排除。
Similarly, if a formula appears in the question, substitute units to see if the result matches the expected quantity. For example, the expression ½ mv² gives kg·(m/s)² = kg·m²/s² = J, which is energy—correct. Use this to validate an unfamiliar formula.
同理，如果题目中出现公式，代入单位检验结果是否符合预期物理量。例如表达式 ½ mv² 的单位为 kg·(m/s)² = kg·m²/s² = J，即能量——正确。用这个方法可以快速验证陌生的公式是否正确。
2. Extreme Case Testing | 极端情况检验
Plug in extreme values such as 0, 90°, or very large masses to see if the equation behaves physically. For an object on a frictionless slope, acceleration down the slope is g sin θ. If an option reads g / cos θ, test with θ = 0: the slope is horizontal, so acceleration must be 0; but g / cos 0 = g, which is clearly impossible.
代入极端值（例如 0、90° 或极大质量），检验表达式是否符合物理图景。对于光滑斜面上的物体，沿斜面加速度为 g sin θ。若选项给出 g / cos θ，用 θ = 0 检验：斜面水平时加速度应为 0，但 g / cos 0 = g，显然错误。
In circuit problems, assume a component’s resistance becomes zero (short circuit) or infinite (open circuit) to predict the effect. If the question asks what happens to the current when a parallel resistor is removed, imagine infinite resistance in that branch—current through the remaining branch stays unchanged, and total current decreases. This mental model quickly matches the correct option.
在电路问题中，假设某元件电阻为零（短路）或无穷大（断路）来推测影响。如果题目问，移除一个并联电阻后电流如何变化，想象该支路电阻无穷大——其余支路电流不变，总电流减小。这一思维模型能迅速对应正确选项。
For projectile motion, set air resistance to zero to test statements about horizontal velocity. The horizontal component remains constant because no horizontal force acts. An option claiming horizontal velocity decreases must be false under ideal conditions.
对于抛体运动，可设空气阻力为零来检验有关水平速度的陈述。水平分速度应保持不变，因为水平方向不受力。任何声称水平速度减小的选项在理想情况下必然错误。
3. Eliminating Wrong Answers | 排除错误选项
Begin by scanning for obviously absurd options. For example, if the question asks for the time taken for a stone to fall 20 m, an answer of 200 s is far too large (it takes about 2 s). Eliminate such outliers immediately. Also watch for answers that violate basic principles, such as a net force on a stationary object being non-zero.
先快速扫视，剔除明显荒谬的选项。例如问一块石头下落 20 m 所需时间，若某个选项为 200 s 就大得离谱（实际约 2 s）。立刻排除这类异常值。同时注意那些违背基本原理的选项，例如静止物体所受合力不为零。
Be wary of absolute words like ‘always’ or ‘never’. A statement such as ‘a moving object always has a force acting on it’ is incorrect according to Newton’s first law; an object can move with constant velocity with no net force. Unless you are certain the absolute is physically justified, eliminate that option.
对“总是”、“绝不”这类绝对化词汇保持警惕。像“运动物体一定受力”这样的陈述，根据牛顿第一定律就是错误的；物体可以保持匀速直线运动而不受净力。除非你确信绝对化表述符合物理事实，否则将其排除。
Work backwards by testing each remaining option with a simple numerical example. If the question involves an equation like P = IV, invent simple values: V=2 V, I=3 A, then P=6 W. Plug these into the given expressions to see which one yields 6 W. This can uncover hidden algebraic errors in distractors.
用简单的数值代入每个剩余的选项进行反向检验。如果题目涉及 P = IV 这类公式，可设 V=2 V, I=3 A，则 P=6 W。将这些数值代入待选表达式，看哪个得出 6 W。这样能发现干扰项中隐藏的代数错误。
4. Estimation and Order of Magnitude | 估算与数量级
Your mental library of typical physical magnitudes is a powerful filter. Know that the speed of sound in air is about 340 m/s, the speed of light is 3×10&sup8; m/s, the mass of an apple is roughly 0.1 kg (weight 1 N), and the height of a table is about 0.8 m. If an option suggests the sound intensity level of a whisper is 100 dB, that is far too high—eliminate it.
你脑中储存的常见物理量大小是强大的过滤器。记住空气中声速约 340 m/s，光速 3×10&sup8; m/s，一个苹果质量约 0.1 kg（重约 1 N），桌子高度约 0.8 m。若选项说悄悄话的声强级为 100 dB，那太高了——直接排除。
Use order-of-magnitude checks. If the question asks for the resistance of a lamp filament, typical values are in the range of hundreds of ohms, not milliohms or megaohms. An answer of 2 Ω might be too low for a domestic filament bulb; 200 Ω is more plausible. Quickly seeing that a given number is ten times too large or small can save precious seconds.
进行数量级检查。如果问灯丝的电阻，典型值在几百欧姆范围内，而非毫欧或兆欧。家用灯丝约 200 Ω 是合理的，2 Ω 可能太小。一眼看出某个选项数值大了或小了十倍，就能省下宝贵时间。
Also, practise converting between prefixes such as kilo (10³), mega (10&sup6;), and milli (10&sup-3;). A common trap is mixing up MJ and kJ. If a calculation gives 5000 J, know this equals 5 kJ, not 5 MJ. Incorrect prefix conversions often appear in distractors.
同时，要熟练转换词头，如千（10³）、兆（10&sup6;）、毫（10&sup-3;）。常见的陷阱是将 MJ 和 kJ 混淆。若计算结果是 5000 J，应等于 5 kJ 而非 5 MJ。干扰项中常出现此类词头换算错误。
5. Formula Substitution Checks | 公式代入验证
When the answer choices are algebraic expressions, perform a quick substitution test. For example, a question might ask for the final velocity v of a car that accelerates from rest with acceleration a over distance s. The correct formula is v² = 2as. Take simple numbers: a=2 m/s², s=9 m, then v²=36, so v=6 m/s. See which option gives 6 with those values.
当选项为代数表达式时，进行快速代入验证。例如一道题要求汽车从静止以加速度 a 行驶距离 s 后的末速度 v，正确公式为 v² = 2as。取简单数字：a=2 m/s², s=9 m，则 v²=36，v=6 m/s。看哪个选项用这些数值能得出 6。
If the question provides a data table, you can test each candidate formula against the data. Suppose you are asked which equation links p and V for a fixed mass of gas at constant temperature. Use two data pairs (p₁, V₁) and (p₂, V₂) to check which relationship holds—for Boyle’s law, pV = constant, so p₁V₁ should equal p₂V₂. Options like p ∝ V or p ∝ 1/V² will fail this test.
如果题目给出数据表，可以用数据检验每个候选公式。例如问一定质量气体在恒温下 p 和 V 的关系。取两对数据 (p₁, V₁) 和 (p₂, V₂) 检验哪种关系成立——根据波义耳定律，pV=常数，则 p₁V₁ 应等于 p₂V₂。像 p ∝ V 或 p ∝ 1/V² 的选项就通不过检验。
This technique also works for rearranged formulas. If the answer is ‘Resistance = Voltage / Current’, but a distractor reads ‘Resistance = Current / Voltage’, simply plug in V=6 V, I=2 A. The correct expression gives 3 Ω; the distractor gives 0.33 Ω—clearly wrong.
该方法也适用于公式变形。若正确答案是“电阻 = 电压 / 电流”，而干扰项为“电阻 = 电流 / 电压”，代入 V=6 V, I=2 A，正确表达式得 3 Ω，干扰项得 0.33 Ω，显然错误。
6. Graph and Slope Interpretation | 图像与斜率判读
In multiple-choice items with a graph, identify what the gradient and area represent. On a velocity–time graph, the gradient gives acceleration; the area under the graph gives displacement. If an option states ‘the slope of a v-t graph equals displacement’, it is wrong. Applying these fundamentals lets you delete distractors in seconds.
遇到带图像的选择题，先明确斜率和面积分别代表什么物理量。在速度–时间图中，斜率代表加速度，图线下面积代表位移。若某个选项声称“v-t 图的斜率等于位移”，那就是错误的。运用这些基本知识可以秒杀干扰项。
Check the intercepts and shape. For a component that obeys Ohm’s law, the I–V graph is a straight line through the origin. A graph curving upwards indicates a change in resistance—typical for a filament lamp. If the question describes a ‘fixed resistor’, any option showing a curved I–V characteristic is impossible.
注意截距和曲线形状。遵守欧姆定律的元件，其 I–V 图为过原点的直线。向上弯曲的图线表明电阻在变化——这是灯丝的典型特征。如果题目描述的是“定值电阻”，任何显示曲线特性的选项都不可能正确。
Use the concept of half-life from a graph of activity against time. The time for the count rate to halve can be read straight from the curve. A common distractor proposes the time to drop to zero; since radioactive decay is exponential, it never reaches exactly zero, so such options are easily dismissed.
对于放射性活度–时间图，可直接从曲线上读取计数率减半所需的半衰期。常见干扰项会给出降为零的时间；由于放射性衰变是指数型的，永远不会真正为零，因此这类选项可轻松排除。
7. Symmetry and Conservation Laws | 对称性与守恒定律
Conservation of momentum is a rapid discriminator in collision and explosion problems. Calculate total momentum before the event; any answer where the total momentum afterwards differs is physically impossible. This alone often leaves only one viable choice.
动量守恒在碰撞和爆炸问题中是快速判别利器。先算事件前的总动量；任何使得事件后总动量不同的选项在物理上都不可能。仅凭这一点，往往只剩一个合理选项。
Similarly, apply charge conservation in circuit problems. At any junction, the sum of currents entering equals the sum leaving. If an option claims a current of 3 A flows into a junction but 2 A and 2 A flow out, it violates Kirchhoff’s first law. You can instantly rule it out without complex calculation.
同理，在电路问题中应用电荷守恒。在任何节点，流入的电流之和等于流出的电流之和。若选项声称 3 A 流入节点，却各 2 A 流出，违反了基尔霍夫第一定律，可立即排除，无需复杂计算。
In nuclear equations, both mass number (top) and proton number (bottom) must balance. For α-decay, a nucleus
238₉₂U loses 2 protons and 2 neutrons, giving 234₉₀Th. Any option that does not preserve total mass number or total proton number is incorrect. Scan quickly—it is often the easiest point to check.
在核反应方程中，质量数（上标）和质子数（下标）都必须守恒。α 衰变时，238₉₂U 失去 2 个质子和 2 个中子，生成 234₉₀Th。任何不满足总质量数或总质子数守恒的选项都是错的。快速扫一眼——这通常是最容易验证的一点。
8. Common Traps and Pitfalls | 常见陷阱与误区
Beware of unit conversions embedded in the options. A speed of 36 km/h is equal to 10 m/s (divide by 3.6). A distractor might list 36 m/s as the equivalent, exploiting a missed conversion. Always convert to SI units before selecting your answer if the question mixes km and m, or minutes and seconds.
警惕选项中暗藏的单位换算。36 km/h 等于 10 m/s（除以 3.6）。干扰项可能列出 36 m/s 作为等效值，利用转换遗漏来误导。如果题目混合了 km 与 m，或 min 与 s，务必在选择前统一换算为国际单位。
Vector direction frequently separates right from wrong. If a question asks for the resultant force on an object moving in a circle at constant speed, the net force is towards the centre (centripetal). An option stating the force is tangent to the circle is wrong—that would be a net force causing tangential acceleration, which would change speed.
矢量的方向常常是正误的分界线。如果问匀速圆周运动物体的合力方向，合力指向圆心（向心）。声称合力沿切线方向的选项是错误的——那样的合力会导致切向加速度，从而改变速度大小。
Pay close attention to negative signs, especially in electricity and magnetism. For example, the transformer equation and power calculations must account for direction, but often we deal with magnitudes. However, a question on induced e.m.f. might ask for the direction of the induced current; applying Lenz’s law gives the polarity. An option that ignores the minus sign in Faraday’s law will be wrong.
特别注意负号，尤其在电磁学中。例如变压器公式和功率计算中虽常处理标量，但涉及感应电动势方向时，必须应用楞次定律确定感生电流的极性。忽略法拉第定律中负号的选项就是错的。
9. Time-Saving Strategies | 节省时间策略
Do not get stuck on a single question. If a problem requires lengthy calculation, mark it and move on. There is no negative marking, so you can return later. Often, eliminating just one option raises your guessing probability to 33%. On very tough items, trust the elimination techniques and take an educated guess.
不要在一道题上卡死。如果某个问题需要冗长计算，做好标记先往下做。选择题不倒扣分，之后可以回头再攻。通常，排除哪怕一个选项，猜对的概率就提高到
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