📚 AS Physics Unit 2 Jan 2019 Mark Scheme: Experimental Investigation Essentials | AS物理Unit 2 2019年1月评分标准：实验探究核心要点
The January 2019 AS Physics Unit 2 paper includes a substantial experimental investigation question, often worth 6–9 marks, testing your ability to plan, implement, record and analyse data. By examining the mark scheme closely, you can uncover exactly what examiners look for in a top-band response. This article breaks down those marking points through a classic example: determining the resistivity of a wire.
2019年1月的AS物理Unit 2试卷中包含一道分值6–9分的实验探究题，考查你计划、实施、记录和分析数据的能力。通过仔细研究评分标准，你可以发现阅卷官在高分答案中究竟寻找什么。本文通过一个经典实验——测定导线的电阻率——逐一拆解这些评分要点。
1. Understanding the Mark Scheme Structure | 理解评分标准的架构
The experimental investigation in this paper is usually marked against three or four assessment areas: method and apparatus (planning), recording data, analysis of results, and evaluation. For the resistivity question, the mark scheme allocates marks for a clear step-by-step procedure, correct instrument choices, a well‑designed results table, a suitable graph, correct use of gradient, and sensible improvements with error considerations.
该试卷中的实验探究通常按三到四个评估领域评分：方法与器材（计划）、数据记录、结果分析以及评估。对于电阻率问题，评分标准将分值分配到：清晰的步骤说明、正确的仪器选择、设计合理的记录表格、合适的图像、恰当使用斜率，以及合理的改进建议与误差考量。
2. Identifying Variables and Forming Hypotheses | 识别变量与建立假设
The independent variable is the length L of the wire (varied by moving crocodile clips). The dependent variable is resistance R, calculated from R = V / I. Control variables include the diameter (or cross‑sectional area) of the wire, the temperature (avoid excessive current), and the material. The hypothesis: resistance is directly proportional to length, i.e. R ∝ L.
自变量是导线的长度L（通过移动鳄鱼夹改变）。因变量是电阻R，由R = V / I计算得出。控制变量包括导线的直径（或截面积）、温度（避免过大电流）以及材料。假设为：电阻与长度成正比，即R ∝ L。
3. Selecting Appropriate Apparatus | 选择合适的实验器材
Choice of apparatus is a key marking point. You need a metre rule (with mm markings) to measure length, a micrometer screw gauge to measure the wire diameter at several points, a voltmeter and ammeter (both with suitable ranges, e.g. 0–5 V, 0–1 A), a variable resistor or rheostat, a switch, connecting leads and a low‑voltage power supply. The mark scheme rewards stating why a micrometer gives a more precise diameter than a ruler.
仪器的选择是关键评分点。你需要一把米尺（带毫米刻度）来测量长度，一把千分尺在多个位置测量导线直径，合适的电压表和电流表（量程如0–5 V、0–1 A），一个滑动变阻器，一个开关，连接导线和低压电源。评分标准奖励说明为何千分尺比直尺测量直径更精确。
4. Describing a Valid Method Step-by-Step | 逐步描述有效方法
To secure full marks, describe the circuit clearly: a series circuit containing the power supply, ammeter, the test wire (with crocodile clips at a fixed length), and a variable resistor, with the voltmeter connected in parallel across the wire. Then explain: set the length to, say, 10 cm, close the switch, adjust the rheostat to keep current low, record V and I, calculate R, repeat twice without changing the length, increase the length in steps of 10 cm up to 100 cm, and tabulate all readings.
为了获得全部分数，需清楚地描述电路：一个串联电路包含电源、电流表、被测导线（在固定长度处用鳄鱼夹连接）和滑动变阻器，电压表并联在导线两端。然后说明：将长度设为10 cm，闭合开关，调节变阻器使电流较小，记录V和I，计算R，不改变长度重复两次，以10 cm为步长增加长度至100 cm，并将所有读数填入表格。
The mark scheme also expects you to mention that the wire should be kept taut and that a small current must be used to avoid heating, which would change resistance. Include a diagram if space is provided.
评分标准还期望你提及导线需拉直，必须使用小电流以免发热改变电阻。如果答题空间允许，应附上电路图。
5. Designing a Results Table | 设计结果记录表格
A high‑quality table, as rewarded by the mark scheme, includes columns for all raw and processed data: length L (cm), length L (m), voltage V (V), current I (A), resistance R (Ω) for each trial, and a final column for mean R. Each heading must show the quantity and its unit, often separated by a slash, e.g. L / m. Repeated measurements for at least six lengths demonstrate good practice.
评分标准所奖励的高质量表格，需包含所有原始和处理数据列：长度L（cm）、长度L（m）、电压V（V）、电流I（A）、每次的电阻R（Ω），以及最后的平均R列。每个表头需同时标明物理量和单位，通常用斜线分隔，如 L / m。对至少六个长度值进行重复测量，体现出良好的实验习惯。
6. Recording Measurements with Precision | 精确记录测量数据
All voltmeter and ammeter readings should be given to the same number of decimal places consistent with the instrument’s precision. The diameter must be measured at three or more positions along the wire using a micrometer (resolution 0.01 mm) and averaged. The length should be measured from the inside edges of the crocodile clips, and the ruler reading given to the nearest mm. This precision is explicitly checked in the mark scheme.
所有电压和电流读数需要保留到与仪器精度一致的小数位数。必须使用千分尺（分辨力0.01 mm）在导线三个或更多位置测量直径并取平均值。长度应从鳄鱼夹的内侧边缘量起，尺子读数精确到毫米。评分标准会明确检查这些精度细节。
7. Plotting a Graph for Analysis | 绘制分析用图
Plot a graph of mean resistance R on the y‑axis against length L on the x‑axis. The mark scheme insists on labelled axes with units, a sensible scale that covers more than half the graph paper, and points plotted using small crosses or encircled dots. Draw a best‑fit straight line that should pass through the origin (since R = 0 when L = 0 for a uniform wire).
绘制平均电阻R（y轴）对长度L（x轴）的图像。评分标准要求：坐标轴标注量和单位，选用合理的刻度（占据图纸一半以上），数据点用小叉号或带圈的点描绘，并画一条最佳拟合直线，对于匀质导线，该直线应过原点（因为当L = 0时，R = 0）。
8. Determining the Quantity from the Graph | 从图中确定待求量
The relationship R = (ρ / A) L shows that the gradient m equals ρ / A. Use a large triangle to calculate:
gradient m = ΔR / ΔL
Carefully read two points far apart on the best‑fit line, not data points. Then calculate resistivity:
ρ = m × A = m × (πd² / 4)
The mark scheme awards marks for showing the gradient calculation, using the cross‑sectional area correctly, and giving the final answer with an appropriate unit (Ω m).
由关系式R = (ρ / A) L可知，斜率m等于ρ / A。用一个顶点距离较大的三角形计算：
斜率 m = ΔR / ΔL
仔细读取最佳拟合线上相距较远的两点坐标，不要用原始数据点。然后计算电阻率：
ρ = m × A = m × (πd² / 4)
评分标准给分点包括：展示斜率计算、正确使用横截面积、给出带合适单位（Ω·m）的最终结果。
9. Handling Uncertainties and Errors | 处理不确定性及误差
tainty than a ruler. Repeat readings reduce random error. To find the uncertainty in the gradient, draw a “worst acceptable line” t that fits the error bars), calculate its gradient, then determine absolute uncertainty = |gradient_of_best_fit – gradient_of_worst| sistivity can be obtained by combining the percentage uncertainty in gradient and in area.
2019年1月的评分标准通常要求说明如何减小不确定度。用千分尺测量直径，其绝对不确定度小于直尺。重复读数可减少随机误差。为求斜率的不确定度，画出“最差可接受线”（穿过误差棒的最陡或最浅线），计算其斜率，再求绝对不确定度 = |最佳拟合斜率 – 最差线斜率| / 2。电阻率的百分不确定度可通过合成斜率和截面积的百分不确定度得到。
10. Suggesting Improvements and Risk Assessment | 提出改进建议与安全评估
Common suggestions valued by the mark scheme include: using a longer wire to increase the magnitude of the measured length, which reduces the relative error; keeping the wire in a temperature‑controlled bath; using a digital ohmmeter for direct resistance readings; and taking more diameter readings along the wire. For risks, mention that sharp ends of the wire can cause cuts and that the low‑voltage supply should be disconnected when changing connections.
评分标准看重的常见改进建议有：使用更长的导线以增大长度测量值，从而降低相对误差；将导线置于恒温水浴中；使用数字欧姆表直接读取电阻；沿导线测量更多位置处的直径。安全方面，应提及导线端头锋利可能造成割伤，以及更换连接时应断开低压电源。
11. Key Takeaways from the January 2019 Mark Scheme | 2019年1月评分标准关键收获
Examiners consistently award marks for a circuit diagram with the voltmeter in parallel and ammeter in series, a micrometer used for the diameter rather than a ruler, repeating readings without changing the independent variable, a table with physical quantity/unit headings, a force‑through‑origin best‑fit line, gradient calculation with a large triangle, and a final resistivity value quoted in Ω m with the correct number of significant figures (usually 2 or 3).
阅卷官持续给分的要点包括：电路图中电压表并联、电流表串联；使用千分尺而非直尺测量直径；在不改变自变量的前提下重复读数；表格标题采用物理量/单位格式；通过原点的最佳拟合直线；用大三角形计算斜率；最终电阻率值以Ω·m为单位，并使用正确的有效数字位数（通常为2或3位）。
Published by TutorHao | Physics Revision Series | aleveler.com
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