📚 A-Level Physics Unit 3 Experiment Investigation: Insights from January 2020 Examination Report | A-Level 物理 Unit 3 实验探究：2020年1月考试报告解读
Unit 3 of the International A-Level Physics examination tests your practical skills through written questions. The January 2020 examiner’s report highlighted common strengths and weaknesses in how students planned experiments, recorded data, processed results, and evaluated uncertainties. Understanding these examiner comments is one of the most effective ways to improve your performance. This article distils the key messages from that report and provides actionable guidance for every stage of a typical Unit 3 experiment investigation.
国际 A-Level 物理 Unit 3 考试通过书面形式考查你的实验技能。2020 年 1 月的考官报告揭示了学生在实验设计、数据记录、结果处理和不确定度评价中常见的优点与不足。理解这些考官评语是提高成绩最有效的途径之一。本文提炼该报告的核心信息，并为你提供针对 Unit 3 典型实验探究各环节的实用指导。
1. Understanding the Role of Unit 3 in IAL Physics | 理解 Unit 3 在 IAL 物理中的地位
Unit 3 (WPH13) assesses practical skills without a hands-on lab session. It tests your ability to design an investigation, tabulate readings, plot graphs, calculate uncertainties, and critique experimental procedures. The January 2020 paper followed this format, with questions ranging from micrometer screw gauge readings to the determination of the acceleration of free fall using a pendulum.
Unit 3 (WPH13) 不设动手实验环节，却全面考查实验能力。试卷要求学生设计探究方案、列表记录读数、绘制图表、计算不确定度并评价实验步骤。2020 年 1 月的试卷延续这一形式，题目涵盖从千分尺读数到利用单摆测定重力加速度等多种问题。
The examiner noted that candidates who scored highly had a systematic approach: they identified independent, dependent and control variables right from the start. Lower-scoring scripts often confused these, especially in unfamiliar contexts like a light-bulb filament resistance experiment where temperature was a key control variable.
考官指出，成绩优异的考生从一开始就采取系统方法，明确自变量、因变量和控制变量。得分较低的答卷往往混淆这些变量，特别是在不熟悉的场景中，例如灯泡灯丝电阻实验中温度是关键控制变量。
2. Planning a Valid Experiment | 设计有效的实验
A well-structured plan was a distinguishing factor. The January 2020 paper required candidates to describe how they would measure the resistivity of a metal wire. Successful plans included a circuit diagram, a labelled diagram of the wire mounted on a metre rule, and specific steps such as measuring the diameter of the wire at several points with a micrometer to find an average.
计划是否结构清晰是得分的分水岭。2020 年 1 月试卷要求考生描述如何测量金属丝的电阻率。成功的方案包含电路图、安装在米尺上导线标记图，以及具体步骤，例如用千分尺在导线不同位置测量直径取平均值。
The examiner emphasised that vague statements such as ‘take readings’ were not credited unless accompanied by exact details of what to measure, what instrument to use, and how to ensure repeatability. A plan should always state how to keep control variables constant — for resistivity, temperature was mentioned by noting that the current should be kept low to avoid heating.
考官强调，诸如“记录读数”之类的模糊描述不会被给分，除非同时给出测量什么、使用什么仪器以及如何确保可重复性的具体细节。方案应始终说明如何保持控制变量不变——对于电阻率实验，温度的控制体现在写明电流应保持较小以避免加热。
3. Selecting and Using Apparatus Correctly | 正确选择和使用仪器
Selecting the correct measuring instrument with an appropriate resolution was a key skill. In a pendulum timing experiment, many candidates proposed using a stopwatch but failed to mention measuring the time for 10 or 20 oscillations to reduce reaction-time error. The January 2020 report stressed that a stopwatch has a typical reaction-time uncertainty of about 0.2 s, so timing multiple oscillations is essential.
选择具有合适分辨率测量仪器是一项关键技能。在单摆计时实验中，不少考生提议使用秒表，却未提及测量 10 或 20 个摆动周期的时间以减小反应时间误差。2020 年 1 月报告强调，秒表的典型反应时间不确定度约为 0.2 s，因此计时多个摆动周期至关重要。
For length measurements, a metre rule could be used if the length was long enough, but for the diameter of a wire only a micrometer would give the required precision. The report drew attention to common zero-error mistakes: candidates often read the micrometer or vernier calliper without checking or recording the zero error.
在长度测量方面，若长度足够大可使用米尺，但导线直径仅能用千分尺才能给出所需精度。报告重点指出了常见的零误差错误：考生使用千分尺或游标卡尺时，往往不检查或不记录零误差。
4. Recording Raw Data Accurately | 准确记录原始数据
Raw data tables must be headed with the quantity and the unit, separated by a solidus or written as ‘quantity / unit’. The January 2020 examiner observed that many candidates lost marks because they wrote column headings like ‘current (A)’ which is acceptable in some exam boards, but in Edexcel IAL the preferred format is ‘I / A’. Both styles were generally accepted, but inconsistent units or missing headings were penalised.
原始数据表必须标明物理量及其单位，用斜线分隔或写成“量 / 单位”。2020 年 1 月考官观察到，许多考生因栏目抬头写成“current (A)”而丢分，这虽在某些考试局可接受，但在 Edexcel IAL 中推荐格式为“I / A”。两种格式通常都接受，但单位不一致或缺失抬头会被扣分。
All raw readings should be recorded to the same number of decimal places that reflect the precision of the instrument. For a digital ammeter reading of 0.25 A, it must be recorded as 0.25 A, not 0.250 A, unless the instrument actually displayed 0.250 A. The examiner found many instances of over-specification or truncation of readings.
所有原始读数应记录到相同的小数位数，以反映仪器的精度。对于数字电流表的 0.25 A 读数，必须记录为 0.25 A，而非 0.250 A，除非仪器实际显示为 0.250 A。考官发现很多过度精确或随意截断读数的情况。
5. Handling Data Tables and Significant Figures | 处理数据表格与有效数字
Calculated values, such as the mean of repeated readings, must be given to an appropriate number of significant figures. The report noted that many candidates did not know how to round means correctly, especially when adding or averaging. They often kept all the digits from a calculator display, which does not reflect the uncertainty of the original measurements.
计算值，如重复读数的平均值，必须以适当的有效数字给出。报告指出，许多考生不会正确舍入平均值，特别是进行加法或平均时。他们常常保留计算器显示的所有数字，这不能反映原始测量的不确定度。
A key message from January 2020 was the rule that calculated quantities should generally be quoted to the same number of significant figures as the least precise measurement used in the calculation. For example, if a diameter is measured as 0.36 mm (2 s.f.) and a length as 100.0 cm (4 s.f.), the cross-sectional area should be expressed to 2 s.f.
2020 年 1 月的一个重要信息是，计算的物理量通常应与计算中使用的精度最低测量值的有效数字位数一致。例如，若直径测量为 0.36 mm（2 位有效数字），长度测量为 100.0 cm（4 位有效数字），则横截面积应表示为 2 位有效数字。
6. Plotting Graphs for Maximum Marks | 画图以获取最高分
Graph work was a major area where marks were dropped. The examiner insisted that axes must be labelled clearly with both the quantity and unit, using the same solidus format: e.g., ‘T² / s²’. The scale must be linear, cover more than half the graph grid, and be easy to read (avoid multiples of 3, 7, etc.).
图表绘制是丢分较多的主要领域。考官坚持要求坐标轴必须清晰标明物理量和单位，同样使用斜线格式，如“T² / s²”。坐标刻度必须线性，占据多于半张图网格，且易于读取（避免使用 3、7 等倍数）。
Plotting points accurately with small crosses or encircled dots was expected. The January 2020 report mentioned that many students plotted points that were too large, causing ambiguity. The line of best fit should balance the points, not necessarily pass through the origin unless the physics demands it. A common error was forcing the line through (0,0) when the intercept had a physical meaning that needed interpretation.
要求使用小十字叉或带圈圆点精确描点。2020 年 1 月报告指出，许多学生描的点过大，造成位置不明确。最佳拟合线应均衡各点，不一定非得通过原点，除非物理规律要求。常见错误是强行让直线通过 (0,0)，而此时截距具有物理意义需要解读。
7. Calculating Quantities from Gradients and Intercepts | 从斜率和截距计算物理量
Once a graph is drawn, the gradient should be calculated using a large triangle that covers at least half the drawn line. The coordinates of the two chosen points must be read from the line of best fit, not from data points. The examiner found that some candidates used data points or tiny triangles, yielding large percentage errors in the gradient.
图表绘制完成后，应使用至少覆盖拟合线一半范围的大三角形计算斜率。所选两点的坐标必须从最佳拟合线上读取，而不是来自原始数据点。考官发现，部分考生使用数据点或微小三角形，导致斜率产生很大的百分误差。
In the pendulum experiment to find g, the relation T = 2π√(L/g) is often rearranged as T² = (4π²/g)L. The gradient of a T² vs L graph is 4π²/g, so g = 4π²/gradient. The report highlighted that students frequently made algebra errors when rearranging the formula, or they forgot to include units in their final answer for g.
在利用单摆测定 g 的实验中，关系式 T = 2π√(L/g) 常被改写为 T² = (4π²/g)L。T²–L 图的斜率为 4π²/g，因此 g = 4π² / 斜率。报告强调，学生经常在改写公式时出现代数错误，或者忘记在 g 的最终答案中包含单位。
g = 4π²L / T²
8. Uncertainty Analysis and Error Propagation | 不确定度分析与误差传递
A basic uncertainty calculation was usually required. For a single measurement, the uncertainty is ± the smallest scale division or the stated accuracy of the instrument; for a digital instrument it is ± the last significant digit. The percentage uncertainty is (absolute uncertainty / measured value) × 100%. The January 2020 paper rewarded candidates who correctly combined percentage uncertainties when quantities were multiplied or raised to a power.
通常需要进行基本的不确定度计算。对于单次测量，不确定度为 ± 最小刻度分度或仪器的标称准确度；对于数字仪器，则为 ± 最后一位有效数字。百分不确定度 = (绝对不确定度 / 测量值) × 100%。2020 年 1 月试卷对正确组合乘法或乘方运算中百分不确定度的考生给予了加分。
Examiner comments indicated that many candidates incorrectly added absolute uncertainties from different sources instead of using percentage uncertainties when the measured quantities were multiplied. For example, if v = s/t, the percentage uncertainty in v is the sum of the percentage uncertainties in s and t, not the sum of their absolute uncertainties.
考官评论显示，许多考生错误地将不同来源的绝对不确定度相加，而若测量量是相乘的关系，就应使用百分不确定度。例如，若 v = s/t，则 v 的百分不确定度是 s 和 t 百分不确定度之和，而非绝对不确定度相加。
9. Evaluating the Experiment and Suggesting Improvements | 评价实验并提出改进建议
The evaluation section asks you to identify the main sources of uncertainty and to suggest realistic improvements. The January 2020 report noted that generic comments like ‘human error’ or ‘use more accurate instruments’ were almost never credited. Students needed to identify a specific procedural flaw and propose a precisely targeted refinement.
评价部分要求你识别不确定度的主要来源并提出切实可行的改进建议。2020 年 1 月报告指出，类似“人为误差”或“使用更精确的仪器”这种泛泛而谈的评论几乎从不给分。学生需要指出来自特定步骤的缺陷，并提出精确、有针对性的改进措施。
For instance, in the resistivity experiment, a major uncertainty was the heating effect of the current changing the resistance. A valid improvement was to use a low-current source and take readings quickly, or to immerse the wire in a water bath to maintain constant temperature. Such specific, physics-based proposals earned full marks.
例如，在电阻率实验中，一个主要不确定度是电流热效应导致电阻变化。有效的改进措施是使用低电流源并快速读取数据，或将导线浸入恒温水浴以保持温度恒定。这种具体、基于物理原理的提议获得了满分。
10. Common Pitfalls from the January 2020 Report | 2020年1月报告的常见陷阱
The examiner summarised several recurring errors that prevented candidates from reaching the top band. These included: forgetting to zero a micrometer before use; not repeated measurements for time; plotting T against L instead of T² against L; mislabelling axes with units in brackets; using a single data point to calculate gradient; and failing to compare experimental value of g with the accepted value using percentage difference.
考官总结了几类反复出现、阻碍考生进入高分段的问题，包括：使用前忘记给千分尺调零；没有对时间进行重复测量；绘制 T–L 图而非 T²–L 图；坐标轴用括号标注单位；仅用单个数据点计算斜率；未用百分偏差对比实验得到的 g 值与公认值。
Another subtle mistake was in drawing error bars. When asked to show uncertainty on a graph, many candidates drew bars that were too short or omitted them entirely. The report reminded that the length of the error bar represents the absolute uncertainty in that quantity, and it must be clearly visible.
另一个不易察觉的错误是绘制误差棒。当要求在图上表示不确定度时，许多考生画的误差棒过短或完全遗漏。报告提醒，误差棒的长度代表该物理量的绝对不确定度，必须清晰可见。
The final common error was poor presentation of the final determined value. The examiner expected the value to be expressed as (best estimate ± absolute uncertainty) with correct units, for example g = 9.7 ± 0.4 m s⁻². Omitting the unit or the ± sign would lose a mark even if the numerical work was perfect.
最后一种常见错误是最终测定值的表达不当。考官希望看到该值表示为（最佳估计值 ± 绝对不确定度）并附有正确单位，例如 g = 9.7 ± 0.4 m s⁻²。即使数值计算完美，遗漏单位或±号也会丢分。
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