📚 AS Physics Unit 2: Experimental Investigation from Jan 2022 Mark Scheme | AS物理单元2：从2022年1月评分方案看实验探究技巧
In AS Physics Unit 2, students frequently face an experimental investigation question that asks them to outline a procedure, identify variables, collect data, analyse graphs, and evaluate errors. By studying the January 2022 mark scheme for this unit, we can see exactly what examiners reward and where candidates commonly drop marks. This article breaks down the key skills using a typical resistivity-of-a-wire experiment as a central example, showing you how to write mark‑winning answers.
在 AS 物理单元 2 中，学生经常会遇到实验探究题，要求概述步骤、确定变量、收集数据、分析图像并评估误差。通过研究 2022 年 1 月的单元评分方案，我们可以清晰地看到考官奖励哪些细节以及考生通常在何处失分。本文以典型的导线电阻率实验为核心例子，分解关键技能，展示如何写出得分答案。
1. Decoding the Experimental Question | 解读实验题
Before describing an experiment, you must identify the independent, dependent, and control variables. The mark scheme rewards candidates who state explicitly how each variable will be measured or kept constant. For a resistivity experiment using a metal wire, the independent variable is the length L of the wire, the dependent variable is its resistance R, and the control variables are the cross‑sectional area A (determined by the wire’s diameter) and the temperature.
在描述实验之前，必须确定自变量、因变量和控制变量。评分方案奖励那些明确说明如何测量各个变量或如何保持恒定的考生。对于使用金属丝的电阻率实验，自变量是导线的长度 L，因变量是其电阻 R，控制变量是横截面积 A（由导线直径决定）和温度。
2. Choosing Apparatus and Precision | 选择仪器与测量精度
Selecting the right instrument for each measurement is critical. The mark scheme expects a micrometer screw gauge to measure the wire diameter at three or more different points along the wire, and then to calculate a mean value. A metre rule marked in millimetres is acceptable for length; to reduce parallax error, place the rule on the bench and view the scale perpendicularly. Resistance can be found by measuring voltage and current with a voltmeter and ammeter and applying R = V / I, or using an ohmmeter. Always take repeat readings and find the average to improve reliability.
为每一项测量选择正确的仪器至关重要。评分方案要求用千分尺在导线上的三个或更多不同位置测量直径，然后计算平均值。使用毫米刻度的米尺测量长度是可以接受的；为了减小视差误差，应将尺子平放在桌面上并垂直读数。可以通过伏特计和安培计测量电压和电流并应用 R = V / I 来求出电阻，也可以使用欧姆表。务必重复取读数并求平均值，以提高可靠性。
3. Keeping Control Variables Constant | 恒定控制变量
Simply naming a control variable is not enough; the mark scheme demands a practical technique. To keep the cross‑sectional area constant, use the same piece of wire throughout. To prevent resistance changes from heating, state that the current should be kept small (e.g. by using a variable resistor) and that the circuit should be switched off between readings. Some marks are reserved for checking and correcting any zero error on the micrometer.
仅仅说出控制变量的名称是不够的，评分方案要求给出实际操作技巧。要保持横截面积恒定，应全程使用同一段导线。为防止因发热导致电阻变化，需要说明电流应保持较小（例如使用变阻器），并且两次读数之间应断开电路。一些分值还专门留给检查并修正千分尺的零点误差。
4. Designing a Results Table | 设计结果表格
Mark schemes carefully scrutinise the recording of data. A table must have headings that include the quantity and its unit, separated by a slash, for example Length L / m and Resistance R / Ω. All raw readings must be given to the same number of decimal places consistent with the instrument’s resolution. Repeated values and a calculated mean column should be clearly presented. If you vary L from 0.200 m to 1.000 m in steps of 0.200 m, list every pair of repeat R readings and their average.
评分方案会仔细审查数据的记录方式。表格必须包含带斜线分隔的物理量和单位表头，例如 Length L / m 和 Resistance R / Ω。所有原始读数必须保留与仪器分辨率一致的小数位数。重复值和计算出的平均值列应清晰呈现。如果你从 0.200 m 到 1.000 m 以 0.200 m 为步长改变 L，要列出每一对重复的 R 读数及其平均值。
| L / m | R₁ / Ω | R₂ / Ω | Mean R / Ω |
|---|---|---|---|
| 0.200 | 1.12 | 1.10 | 1.11 |
| 0.400 | 2.23 | 2.25 | 2.24 |
Example of a well‑designed results table | 设计良好的结果表示例
5. Plotting the Graph and Finding Gradient | 绘图与求斜率
The mark scheme requires a graph of R against L, which should yield a straight line through the origin if resistivity is constant. Both axes must be labelled with quantity and unit, scales should be chosen so that the plotted points occupy more than half the grid in each direction, and points must be plotted accurately to the nearest half‑square. Draw a single thin line of best fit. To determine the gradient, construct a large triangle (at least half the line’s length) and read the coordinates correctly, e.g. gradient = ΔR / ΔL with units Ω m⁻¹.
评分方案要求绘制 R 对 L 的图，如果电阻率恒定，应得到一条过原点的直线。两轴必须标注物理量和单位，坐标标度应使描点占据各方向一半以上的网格，点必须精确描到最近的半格。画一条细的最佳拟合线。为确定斜率，需构造一个大三角形（至少占线长的一半），正确读取坐标，例如 斜率 = ΔR / ΔL，单位为 Ω m⁻¹。
6. Calculating Resistivity and Its Uncertainty | 计算电阻率及其不确定度
From the gradient k = R / L, use the resistivity formula ρ = R A / L = k × A. First compute the cross‑sectional area from the mean diameter d: A = π d² / 4. Pay careful attention to unit conversions – diameter in metres. The mark scheme often expects you to estimate the percentage uncertainty in ρ by combining % errors: %ρ = %R + %L + 2 × %d. Alternatively, draw worst‑fit lines to find the range of gradients and hence the absolute uncertainty. State the final resistivity with its uncertainty and the correct unit (Ω m).
根据斜率 k = R / L，使用电阻率公式 ρ = R A / L = k × A。首先由平均直径 d 计算横截面积：A = π d² / 4。要仔细注意单位换算 —— 直径用米。评分方案常要求你通过合成百分误差来估算 ρ 的百分不确定度：%ρ = %R + %L + 2 × %d。或者画出最差拟合线以求得斜率范围，从而给出绝对不确定度。最后需用正确单位（Ω m）给出电阻率及其不确定度。
7. Verifying the Proportional Relationship | 验证正比关系
The straight line passing through the origin confirms that R ∝ L, which is predicted by R = ρ L / A when A and ρ are constant. Any intercept (positive or negative) would suggest a systematic error, such as the resistance of connecting leads or a non‑zero offset on the ohm meter. The mark scheme rewards a concluding statement that explicitly links the graph’s shape to the theoretical relationship.
过原点的直线证实了 R ∝ L，这正是当 A 和 ρ 恒定时公式 R = ρ L / A 所预言的结果。任何截距（正或负）都提示存在系统误差，例如连接导线的电阻或欧姆表的非零偏移。评分方案奖励明确将图形形状与理论关系联系起来的结论性陈述。
8. Evaluating Sources of Error | 误差来源评估
The diameter measurement usually contributes the largest uncertainty because a small absolute error in d appears squared and then doubled in percentage terms. Other candidates for discussion include heating of the wire (causing R to drift), parallax when reading the metre rule, and contact resistance at crocodile clips. A high‑scoring evaluation names each error, explains its effect on the result, and suggests a practical improvement, such as using a longer wire to reduce the fractional error in length or using a digital calliper alongside the micrometer to cross‑check diameter.
直径的测量通常贡献最大的不确定度，因为 d 的微小绝对误差经平方后，百分误差会加倍。其他可讨论的候选因素包括导线发热（导致 R 漂移）、读数米尺时的视差，以及鳄鱼夹处的接触电阻。高分的评估会逐一指出每个误差、说明其对结果的影响并提出切实的改进措施，例如使用更长的导线以减小长度的相对误差，或使用数显游标卡尺与千分尺交叉核对直径。
9. Common Pitfalls Highlighted by the Mark Scheme | 评分方案揭示的常见失分点
Many answers lose marks because they are too vague. Saying ‘keep temperature constant’ without explaining how (low current, switching off) will not earn full marks. Similarly, stating ‘repeat the experiment’ without specifying which readings to repeat or that a mean should be calculated is insufficient. The mark scheme also frequently penalises the omission of a labelled diagram showing the circuit and the placement of instruments. Remember that a diagram can replace many words and must indicate clearly how length is varied and measured.
许多答案因叙述过于笼统而失分。仅说“保持温度恒定”却不解释具体做法（低电流、断电）无法获得满分。同样，陈述“重复实验”而未说明需要重复哪些读数以及应计算平均值也是不够的。评分方案还常对遗漏带标注的装置图进行扣分，图中需显示电路和仪器位置。请记住，一幅图可以替代许多文字，且必须清晰标明如何改变和测量长度。
10. Using the Mark Scheme as a Study Tool | 将评分方案用作学习工具
The January 2022 mark scheme is a blueprint for writing high‑scoring answers. Key phrases that frequently appear include ‘measure diameter in at least three places and calculate mean’, ‘draw a line of best fit through the origin’, and ‘triangle used to determine gradient covers at least half the drawn line’. When you practise, incorporate these exact phrases into your descriptions. By internalising the mark scheme’s vocabulary, you will learn to write precisely what examiners are looking for and avoid dropping procedural marks.
2022 年 1 月的评分方案是写出高分答案的蓝图。经常出现的关键短语包括“至少在三个位置测量直径并计算平均值”、“经过原点画最佳拟合线”以及“用于确定斜率的大三角形至少覆盖所画直线的一半”。在练习时，将这些准确短语融入到你的描述中。通过内化评分方案的用语，你将学会准确写出考官想要的内容，避免丢失步骤分。
Published by TutorHao | Physics Revision Series | aleveler.com
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