📚 AS Physics Unit 2 Jan 2020 Mark Scheme: Mastering Experimental Investigations | AS物理单元2 2020年1月评分标准：精通实验探究
In the AS Physics Unit 2 examination from January 2020, one of the most heavily weighted sections was the experimental investigation question. Understanding the mark scheme for this question is not only about seeing where marks were awarded, but also about learning how to think like an examiner when planning, presenting, and evaluating an experiment. This article decodes that mark scheme, using a typical resistivity-of-a-wire experiment as a model, and provides actionable strategies to maximise your marks.
在2020年1月的AS物理单元2考试中，实验探究题是最具分量的一部分。理解该题的评分标准，不仅是要看清分数如何分配，更是要学会在计划、展示和评价实验时像考官一样思考。本文将以一个典型的“测量导线电阻率”实验为模型，解密这份评分标准，并提供可操作的策略帮助你在考试中取得高分。
1. Overview of the Jan 2020 Experimental Context | 2020年1月实验题背景概述
The experimental question in the January 2020 AS Unit 2 paper asked students to determine the resistivity of a metal wire using standard laboratory equipment such as a power supply, ammeter, voltmeter, metre ruler and micrometer screw gauge. The mark scheme rewarded a clear logical sequence, correct circuit diagrams, proper data handling, and critical evaluation of uncertainties.
2020年1月AS单元2的实验题要求学生使用电源、电流表、电压表、米尺和千分尺等标准实验室设备测定一根金属导线的电阻率。评分标准奖励清晰的逻辑顺序、正确的电路图、恰当的数据处理以及对不确定度的批判性评价。
2. Designing the Experiment: Key Apparatus and Circuit | 实验设计：关键仪器与电路
The mark scheme emphasised that a circuit diagram must allow the current through the wire and the potential difference across it to be measured independently. A simple series circuit with a variable resistor to control current, an ammeter in series, and a voltmeter connected across the test wire was expected. The metre ruler is used to measure the length between the voltmeter crocodile clips, not the full length of the wire.
评分标准强调，电路图必须能独立测量通过导线的电流及其两端电压。预期使用一个简单的串联电路：包含一个可变电阻以控制电流、串联的电流表，以及并联在待测导线两端的电压表。米尺用于测量电压表鳄鱼夹之间的导线长度，而非整根导线的长度。
Examiners also assigned marks for mentioning that a micrometer screw gauge should be used to measure the diameter d of the wire at several positions, taking the average to minimise the effect of irregularities.
考官还会对提到“需使用千分尺在导线不同位置多次测量直径d并取平均值以减少不均匀性的影响”这一操作给予分数。
3. Variables and Control: Independent, Dependent and Fixed | 变量与控制：自变量、因变量与控制变量
ariable is the resistance R, calculated from the voltmeter and ammeter readings (R = V/I). The mark scheme required candidates to state that the current should be kept small and constant throughout each length measurement to avoid heating effects, which is a crucial controlled variable.
为获取可靠数据，你必须明确自变量是导线的长度L，通过移动鳄鱼夹来改变；因变量是电阻R，由电压表和电流表读数计算得出（R = V/I）。评分标准要求考生说明：在每次长度测量过程中，电流应保持较小且恒定，以避免热效应，这是一个关键的控制变量。
Additionally, temperature is a control variable; the wire must be allowed to cool if it becomes warm. The material and cross-sectional area are constant for a given wire.
此外，温度也是一个控制变量；如果导线变热，必须让它冷却。对于给定的导线，材料和横截面积是恒定的。
4. Data Collection: Minimising Uncertainties | 数据采集：减少不确定度
The mark scheme rewarded repeated measurements for the diameter d using the micrometer, and for resistance at each length. For d, at least three readings at different positions along the wire, with the orientation rotated, to get an average and reduce random error. For R, the advice is to take pairs of V and I, then calculate R, and ideally repeat the resistance measurement at each length after disconnecting and reconnecting to check consistency.
评分标准奖励使用千分尺对直径d进行重复测量，并在每个长度下重复测量电阻。对于直径，至少需要在导线不同位置旋转千分尺测量三次，取平均值以减少随机误差。对于电阻，建议记录成对的V和I值，然后计算R，理想情况下应在每个长度下断开并重新连接后重复测量电阻以检验一致性。
It is also vital to set the ammeter and voltmeter to appropriate ranges to ensure readings take up at least half of the scale, reducing percentage reading errors.
同样重要的是，将电流表和电压表设置在合适的量程，确保读数至少占满量程的一半，以减小百分比读数误差。
5. Graphical Analysis: Plotting and Axis Choice | 图形分析：描点和坐标轴选择
sistivity ρ, you are expected to use the relationship R = ρL/A, where A = π(d/2)² = πd²/4.
为求电阻率ρ，应使用关系式R = ρL/A，其中A = π(d/2)² = πd²/4。
R = (4ρ/πd²) L
gin with gradient m = 4ρ/πd². The mark scheme insisted on the correct labelling of axes with quantities and units, and sensible scales using at least half the graph paper.
上式表明，以R为y轴、L为x轴作图，将得到一条通过原点的直线，其斜率m = 4ρ/πd²。评分标准坚持要求正确标注坐标轴（含物理量和单位），并使用至少占方格纸一半的合理刻度。
Data points must be plotted accurately with small crosses or circled dots; the line of best fit should evenly distribute points above and below it. Anomalous points must be identified and ignored in the best-fit line.
数据点需用细十字或带圈的圆点准确描点；最佳拟合线应使点均匀分布在线两侧。须识别异常点并在线性拟合时将其排除。
6. Gradient Calculation and Resistivity Determination | 斜率计算与电阻率的确定
ts, to avoid transfer errors. The gradient m has units Ω m⁻¹. Then, resistivity ρ = m × (πd²/4). The mark scheme requires substitution of the average d (in metres) to find ρ, and then stating the unit Ω m.
计算斜率时，应使用一个覆盖最佳拟合线至少一半长度的大三角形。从线上读取坐标，而非数据点，以避免传递误差。斜率m的单位为Ω m⁻¹。然后，电阻率ρ = m × (πd²/4)。评分标准要求代入平均直径d（以米为单位）求出ρ，并写明单位Ω m。
Often a follow-up question asks to compare the experimental value with a standard value (e.g., for nichrome) and calculate a percentage difference. Remember not to call it an 'error' unless justified.
后续问题常要求将实验值与标准值（例如镍铬合金）进行比较，并计算百分偏差。除非有充分理由，否则不要称之为“误差”。
7. Uncertainty Analysis: Absolute and Percentage Uncertainties | 不确定度分析：绝对与百分比不确定度
eadings (or the instrument precision if no range). The percentage uncertainty in d is (Δd/d)×100%, and it contributes twice (since area ∝ d²), so percentage uncertainty in area A is 2 × (% und from the difference between worst acceptable gradient lines (steepest and shallowest).
2020年1月的评分标准对估计不确定度分配了分数。对于直径d，绝对不确定度为重复读数范围的一半（若无范围，则取仪器精度）。d的百分比不确定度为(Δd/d)×100%，且此不确定度加倍（因为面积∝ d²），故面积A的百分比不确定度为2倍d的百分比不确定度。对于斜率，其不确定度可通过最陡与最浅可接受梯度线之差求得。
ects a well-structured working leading to the final value, e.g., ρ = (a.b ± c.d)×10⁻⁷ Ω m.
最后，百分比不确定度组合来自m和d²的不确定度，然后求得ρ的绝对不确定度。评分标准常常期待清晰的计算步骤得出最终值，如ρ = (a.b ± c.d)×10⁻⁷ Ω m。
8. Evaluation of Errors: Systematic vs Random | 误差评价：系统误差与随机误差
Candidates must discuss sources of error and whether they are systematic or random. A zero error on the micrometer would systematically shift all diameter readings, which can be corrected by subtracting it. Contact resistance at crocodile clips introduces a systematic error in the resistance measurement, especially for short lengths, which explains why the graph might have a non-zero intercept. Random errors arise from fluctuations in voltage/current readings and parallax when measuring length.
考生必须讨论误差来源及其属于系统误差还是随机误差。千分尺的零误差会系统性地使所有直径读数偏移，可通过减去零误差修正。鳄鱼夹处的接触电阻会引入电阻测量中的系统误差，尤其当长度较短时，这解释了为何图形可能有非零截距。
Published by TutorHao | Physics Revision Series | aleveler.com
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