AQA AS Physics Unit 2 Jan 2020 Exam Report: Practical Investigation Insights | AQA AS物理单元2 2020年1月考试报告：实验探究启示

📚 AQA AS Physics Unit 2 Jan 2020 Exam Report: Practical Investigation Insights | AQA AS物理单元2 2020年1月考试报告：实验探究启示

The January 2020 AQA AS Physics Unit 2 examination paper placed strong emphasis on practical skills and experimental design. The examiner’s report reveals that many students struggled to link theoretical knowledge with hands‑on procedures, particularly when asked to describe improvements or analyse uncertainties. This article breaks down the key practical investigations from the paper, highlighting common errors and offering clear strategies to achieve higher marks.

2020年1月AQA AS物理单元2试卷重点考查了实验技能和实验设计。考官报告显示，许多学生难以将理论知识与动手操作联系起来，尤其是在要求描述改进措施或分析不确定度时表现不佳。本文详细剖析该试卷中的核心实验探究，指出常见错误并提供清晰的提分策略。

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1. Overview of Required Practicals | 必做实验概览

Unit 2 tested a range of core practicals including determination of the coefficient of static friction, measurement of the Young modulus of a wire, resistivity of a metal, internal resistance of a cell, double‑slit interference of light, and standing waves on a stretched string. Students needed to recall specific apparatus, identify independent and dependent variables, and evaluate sources of uncertainty.

单元2考查了一系列核心实验，包括测定静摩擦系数、测量金属丝的杨氏模量、金属的电阻率、电池内阻、光的双缝干涉以及弦上的驻波。学生需要回忆具体器材、识别自变量和因变量，并评估不确定度来源。

Examiners noted that many candidates lost marks by providing vague descriptions or failing to justify their choice of measuring instruments. For example, stating ‘use a ruler’ without specifying a metre rule or a vernier calliper often resulted in no credit.

考官指出，许多考生因描述模糊或未能证明所选测量仪器的合理性而丢分。例如，仅说“用尺子”而不明确是米尺还是游标卡尺，通常不得分。

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2. Determining the Coefficient of Static Friction | 测定静摩擦系数

A classic method involves placing a wooden block on an inclined plane and slowly raising the angle until the block just begins to slide. The coefficient of static friction μₛ is then calculated as μₛ = tan θ, where θ is the angle of inclination measured with a protractor.

经典方法是将木块放在斜面上，缓慢抬升角度直到木块刚好开始滑动。静摩擦系数 μₛ 由 μₛ = tan θ 计算，其中 θ 是用量角器测出的斜面倾角。

To obtain reliable data, the block surface and ramp must be clean and dry. Students were often expected to suggest giving the block a very gentle tap to overcome the threshold of static friction, ensuring that the sliding angle corresponds to limiting friction rather than a stuck condition.

为获得可靠数据，木块表面和斜面必须清洁干燥。题目常要求学生提出轻敲木块以克服最大静摩擦的阈值，确保滑动角对应的是极限摩擦而非卡住状态。

μₛ = tan θ

The examiner’s report highlighted that many candidates forgot to repeat the measurement and take an average, and some used the wrong trigonometric function (e.g., sin θ). Others incorrectly assumed that mass affects the coefficient, which it does not.

考官报告强调，许多考生忘记重复测量取平均值，还有些人使用了错误的三角函数（如 sin θ）。另外一些学生错误地认为质量会影响摩擦系数，而实际上并不影响。

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3. Measuring the Young Modulus of a Wire | 测量金属丝的杨氏模量

The standard setup uses a long, thin wire clamped at one end with a vernier scale attached to measure extension when known masses are added. The diameter must be measured in several places using a micrometer screw gauge, and the original length recorded with a metre rule to the nearest millimetre.

标准装置使用一根长金属丝，一端夹紧，并附有游标尺以测量增加已知质量时的伸长量。直径必须用千分尺在不同位置多次测量，原长用米尺记录，精确到毫米。

Stress and strain are calculated from force/area and extension/original length, then plotted to obtain the Young modulus E from the gradient of the linear portion. A typical precaution is to use a safety shield and to remove the load between readings to check for elastic behaviour.

应力和应变分别由力/截面积和伸长量/原长计算，然后作图，从线性部分的梯度得到杨氏模量 E。典型的安全措施是使用防护屏，并在读数之间卸载以检查弹性行为。

E = (F L₀) / (A ΔL)

Common errors included forgetting to convert diameter to radius, using units inconsistently, and neglecting to measure the diameter in two perpendicular directions to account for non‑uniformity. Examiners wanted candidates to explain why a long wire is used (to make extension measurable) and why a control for temperature was often mentioned.

常见错误包括忘记将直径转换为半径、单位使用不一致，以及忽略在两个垂直方向测量直径以考虑不均匀性。考官希望考生解释为什么使用长金属丝（使伸长量可测），以及为什么经常提到温度控制。

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4. Resistivity of a Metal Wire | 金属丝的电阻率

This experiment requires measuring the resistance R of a uniform wire for different lengths L, while keeping the diameter constant. A standard circuit includes a power supply, ammeter, voltmeter, and a jockey or crocodile clips to vary the effective length along a metre rule.

该实验需要测量不同长度 L 下均匀金属丝的电阻 R，同时保持直径不变。标准电路包括电源、电流表、电压表以及可在米尺上滑动改变有效长度的滑线夹或鳄鱼夹。

Resistivity ρ is determined from the gradient of a graph of R against L, since R = ρL / A, where A is the cross‑sectional area found from the diameter measured with a micrometer. This method minimises systematic errors caused by contact resistance.

电阻率 ρ 由 R 对 L 图形的梯度确定，因为 R = ρL / A，A 是用千分尺测量直径得到的横截面积。此方法能最大程度减小接触电阻带来的系统误差。

ρ = R A / L

The exam report noted that few students could correctly explain why the wire should be straightened without stretching or why the current must be kept low to avoid heating. Many also confused resistivity with resistance and did not quote the final unit as Ω·m.

考试报告指出，极少有学生能正确解释为什么金属丝应当拉直但不能拉伸，或者为什么电流必须保持较低以避免加热。许多人混淆了电阻率和电阻，且未将最终单位写为 Ω·m。

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5. Internal Resistance and EMF | 内阻与电动势

The practical typically uses a cell, a variable resistor, an ammeter and a voltmeter. By altering the external resistance, the terminal potential difference V and current I are recorded. The equation ε = V + Ir leads to the linear relationship V = ε − Ir, where the intercept is the emf ε and the gradient is −r.

该实验通常使用电池、可变电阻、电流表和电压表。通过改变外电阻，记录端电压 V 和电流 I。方程 ε = V + Ir 导出线性关系 V = ε − Ir，其中截距为电动势 ε，梯度为 −r。

To improve precision, a wide range of resistance values should be used, and the switch opened immediately after readings to prevent internal heating. A common error is plotting V against I with reversed axes or ignoring the fact that the voltmeter draws a small current, which introduces a systematic uncertainty.

为提高精度，应使用宽范围的电阻值，并在读数后立即断开开关以防止内部发热。常见错误是将 V 对 I 绘图时坐标轴颠倒，或者忽略电压表会吸取微小电流，从而引入系统不确定度。

V = ε − I r

The examiner’s feedback indicated that students frequently misidentified the emf as the terminal voltage at zero current, and some misread the negative gradient, quoting a positive internal resistance but failing to note the sign.

考官反馈显示，学生经常将电动势错误地等同于零电流时的端电压，还有一些学生误读了负梯度，引用正的内阻值但未注意到负号。

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6. Double‑Slit Interference of Light | 光的双缝干涉

A laser is directed through a double slit onto a screen. The fringe separation w, slit‑to‑screen distance D, and slit separation s are measured to find the wavelength using λ = w s / D. In the January 2020 paper, candidates were asked to describe how to obtain an accurate value for w by measuring across several bright fringes and dividing by the number of spaces.

激光通过双缝射到屏幕上。测量条纹间距 w、缝屏距离 D 和双缝间距 s，利用 λ = w s / D 求出波长。在2020年1月试卷中，考生被要求描述如何通过测量多个亮纹间距并除以间隔数来精确获得 w。

The report highlighted confusion between slit separation and slit width, and a lack of clarity when stating that the screen should be far from the slits (typically >1 m) to make w large enough to measure with an ordinary ruler. Using a travelling microscope or vernier calliper was also rewarded.

报告强调学生混淆了缝间距和缝宽，以及在说明屏幕应远离双缝（通常 >1 m）以使 w 大到能用普通尺子测量时表述不清。使用读数显微镜或游标卡尺测量亦可得分。

λ = w s / D

A further mistake was not converting all distances to metres before calculation. Examiners also expected candidates to recognise that laser light provides a coherent source, which eliminates the need for a single slit.

另一个错误是计算前未将所有距离转换为米。考官还期望考生认识到激光提供了相干光源，因而无需单缝。

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7. Standing Waves on a Stretched String | 弦上的驻波

A vibration generator connected to a signal generator drives a string under tension. When the driving frequency matches a natural harmonic, clear stationary nodes and antinodes are observed. Students had to measure the distance between adjacent nodes to find half the wavelength, then use v = f λ to determine the wave speed.

振动发生器连接信号发生器驱动一根绷紧的弦。当驱动频率与固有谐波频率匹配时，可观察到清晰的驻波波节和波腹。学生需测量相邻波节间的距离以得到半波长，然后用 v = f λ 计算波速。

The exam report criticised candidates who measured between an antinode and a node, yielding one‑quarter wavelength. Properly identifying the central segment with a known mass hanger or knowing how tension affects speed was also tested.

考试报告批评了那些测量波腹到波节之间距离（得到四分之一波长）的考生。试题还考查了如何正确识别中央振段、使用已知质量钩码，或说明张力如何影响波速。

v = √(T / μ)

Many students omitted to mention that the string mass per unit length μ must be known or that the experiment works best with a thin, flexible string to minimise damping. A common improvement suggestion was to use a strobe light to freeze the wave and read positions more accurately.

许多学生未提及必须知道弦的线密度 μ，或实验最好选用细而柔韧的弦以减少阻尼。一项常见的改进建议是使用频闪灯使波形“冻结”，从而更精确地读取位置。

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8. Data Analysis and Uncertainty | 数据分析与不确定度

The Unit 2 paper often includes a section requiring calculation of a gradient, an intercept, and the associated uncertainties. Examiners expect candidates to draw a best‑fit straight line, a worst‑fit line (either steepest or shallowest), and use the difference to quote the absolute uncertainty in the gradient.

单元2试卷通常包含要求计算梯度、截距及相关不确定度的部分。考官期望考生绘制最佳拟合直线和最差拟合直线（最陡或最浅），并用两者之差给出梯度的绝对不确定度。

When using a micrometer, the reading uncertainty is usually taken as ±0.01 mm, but a vernier calliper might be ±0.1 mm. Percentage uncertainties must be combined carefully when quantities are multiplied or divided, while absolute uncertainties add for sum/difference operations.

使用千分尺时，读数不确定度通常取 ±0.01 mm，而游标卡尺可能为 ±0.1 mm。量相乘或相除时需谨慎合并百分比不确定度，而加减运算则直接相加绝对不确定度。

The examiner’s report highlighted that many pupils confused resolution with uncertainty and forgot that taking repeat readings reduces the random uncertainty but not the systematic one. Candidates who simply wrote ‘human error’ without specific justification were penalised.

考官报告强调，许多学生混淆了分辨率与不确定度，并忘记了重复读数只能减小随机不确定度，而不能减小系统不确定度。那些仅写“人为误差”而未给出具体理由的考生被扣分。

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9. Common Examination Mistakes | 常见考试错误

One recurrent mistake was the failure to identify the most significant source of error in a particular experiment. For example, in the resistivity experiment, the largest uncertainty often comes from measuring the diameter because the micrometer reading is squared when calculating area, doubling its percentage uncertainty.

一个反复出现的错误是未能识别特定实验中最重要的误差来源。例如，在电阻率实验中，最大的不确定度通常来自直径测量，因为千分尺读数在计算面积时需平方，其百分比不确定度加倍。

Many candidates also misapplied the concept of parallax error, claiming it affected digital readings or using it as a blanket explanation. Another issue was drawing graphs with uneven scales, missing axis labels, or plotting points that did not cover the full range of data.

许多考生还误用了视差误差的概念，声称其影响数字读数，或将其作为万能解释。另一个问题是绘制图表时比例不均匀、缺少坐标轴标签，或所绘点未覆盖数据的完整范围。

Finally, a significant number of answers showed confusion between independent, dependent, and control variables. In the static friction experiment, for instance, the angle is independent, the point of slip is dependent, and the surface material is a control.

最后，大量答案混淆了自变量、因变量和控制变量。例如，在静摩擦实验中，角度是自变量，滑动瞬间是因变量，而表面材料是控制变量。

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10. Improving Experimental Technique | 改进实验技术

The examiners consistently rewarded suggestions that were specific, practical, and linked to reducing uncertainty. In the Young modulus experiment, using a longer wire increases the extension measured, reducing the percentage uncertainty in ΔL. For standing waves, placing a metre rule behind the string and sighting nodes from eye level removes parallax.

考官一贯奖励那些具体、实用且关联到减小不确定度的建议。在杨氏模量实验中，使用更长的金属丝可增大被测伸长量，降低 ΔL 的百分比不确定度。对于驻波实验，在弦后方放置米尺并从视线水平观察波节可消除视差。

Other useful improvements include using digital sensors (e.g., light gates, force sensors) for more precise data logging, conducting the experiment in a dark room for interference fringes to improve contrast, or clamping equipment firmly to reduce vibration.

其他有用的改进措施包括使用数字传感器（如光门、力传感器）进行更精确的数据记录，在暗室中进行干涉实验以提高条纹对比度，或牢固夹紧设备以减少振动。

Students should also be prepared to discuss safety considerations. In the wire Young modulus experiment, wearing safety goggles and placing a sand tray under the load are expected precautions against wire snap.

学生还应准备好讨论安全注意事项。在金属丝杨氏模量实验中，佩戴护目镜并在悬挂物下方放置沙盘是防范金属丝断裂的预期防护措施。

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11. Summary and Exam Tips | 总结与考试技巧

• Always read the question to decide whether a description of a method, an improvement, or an error analysis is required. Tailor your answer precisely.

  务必审题，明确要求的是方法描述、改进措施还是误差分析，使答案精准切题。

• When asked for an instrument, state its name and its precision. For example, ‘micrometer screw gauge, resolution 0.01 mm’ earns more marks than just ‘micrometer’.

  当被问及仪器时，既要说出名称也要说明精度。例如，“千分尺，分辨率0.01mm”比仅写“千分尺”得分更高。

• Use diagrams to support your description where possible. A clear, labelled sketch can clarify how the apparatus is set up and where measurements are taken.

  尽可能用图表辅助说明。清晰、带标注的草图能阐明装置如何设置以及测量位置。

• Repeat readings and calculate a mean. Always state this explicitly, as it shows good experimental practice and directly addresses random error.

  重复读数并计算平均值。务必明确写出，这体现了良好的实验习惯，可直接应对随机误差。

• Finally, link each precaution to the physical principle it protects. For example, ‘switch off between readings to prevent temperature rise which would change resistance’ demonstrates deeper understanding.

  最后，将每项预防措施与它所维护的物理原理联系起来。例如，“读数间断开电路以防止温度升高导致电阻变化”展示了更深层次的理解。

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