International A-Level Physics Unit 1: Experimental Investigations – January 2021 Examiner’s Report Insights | 国际 A-Level 物理第一单元实验探究：2021年1月考官报告深度解析

📚 International A-Level Physics Unit 1: Experimental Investigations – January 2021 Examiner’s Report Insights | 国际 A-Level 物理第一单元实验探究：2021年1月考官报告深度解析

The January 2021 International A-Level Physics Unit 1 examiner’s report highlighted crucial areas where candidates can improve their experimental and investigative skills. This article distils the key feedback on measurement techniques, uncertainty handling, graph plotting, and the interpretation of results in core practicals such as determining the acceleration of free fall and the Young modulus.

2021 年 1 月国际 A-Level 物理第一单元的考官报告指出了考生在实验探究技能方面可以提升的关键领域。本文提炼了关于测量技术、不确定度处理、图表绘制以及核心实验（如测定自由落体加速度和杨氏模量）结果解读方面的核心反馈。

1. Using Equations of Motion Correctly for Free Fall | 正确运用运动方程测量自由落体

Candidates frequently misapplied the equation s = ut + ½ at² when analysing free‑fall data. The examiner noted that many assumed the initial velocity u to be zero without justifying that the object was released from rest. If the falling mass was given a slight push or if timing started after the object had begun moving, the calculated value of g would be underestimated or overestimated.

考生在分析自由落体数据时经常错误地使用 s = ut + ½ at²。考官指出，许多人未证明物体从静止释放就直接假设初速度 u 为零。如果下落物体被轻轻推动，或者在物体已经开始运动后才开始计时，计算出的 g 值就会偏低或偏高。

A safer approach is to use two light gates fixed at known heights to measure the time interval between them, thereby eliminating the need for an initial velocity assumption. The acceleration can be found from v² = u² + 2as or by timing over two different distances.

更安全的方法是使用两个固定在不同已知高度的光门，测量它们之间的时间间隔，从而无需假设初速度。加速度可以通过 v² = u² + 2as 或在两个不同距离上计时来求得。

g = 2(s₂/t₂² − s₁/t₁²) / (t₂ − t₁) (if u unknown)

2. Systematic Errors in Electromagnetic Release Mechanisms | 电磁铁释放机构中的系统误差

The report highlighted that many candidates did not account for the residual magnetism in electromagnetic release systems. Even after the current is switched off, a short delay can occur before the steel sphere falls freely, introducing a systematic timing error.

报告强调，许多考生没有考虑到电磁铁释放机构中的剩磁效应。即使在电流切断后，钢球自由下落前仍可能出现短暂延迟，从而引入系统性的计时误差。

To minimize this, candidates should use a mechanical release or, if using an electromagnet, ensure that the sphere is not magnetised and that the release is verified by a secondary sensor. Recording the time interval with a light gate immediately after release bypasses the delay.

为了尽量减少这种误差，考生应使用机械释放装置；若使用电磁铁，则应确保球体未磁化，并通过一个辅助传感器验证释放动作。在释放后立即用光门记录时间间隔则可以绕过延迟。

3. Measuring Diameter: Micrometer vs Vernier Caliper Precision | 测量直径：千分尺与游标卡尺的精度

One common weakness was the inappropriate choice of measuring instrument. For a thin wire in the Young modulus experiment, the diameter must be measured with a micrometer screw gauge, not a vernier caliper, to achieve a precision of ±0.01 mm. Vernier calipers (typically ±0.1 mm) lead to unacceptably large percentage uncertainties in cross‑sectional area.

一个常见的不足之处是测量仪器的选择不当。在杨氏模量实验中，测量细丝的直径必须使用千分尺（螺旋测微器），而不是游标卡尺，以达到 ±0.01 mm 的量测精度。游标卡尺（通常精度为 ±0.1 mm）会导致截面积的百分不确定度过大。

The examiner observed that candidates who used a vernier caliper for a wire of diameter 0.3 mm obtained a percentage uncertainty of about 30% in area, rendering the calculated Young modulus unreliable. The report stresses the need to take multiple diameter readings at different orientations and to record the zero error of the micrometer.

考官发现，对于直径约 0.3 mm 的导线，使用游标卡尺的考生获得的面积百分不确定度约为 30%，使计算出的杨氏模量不可靠。报告强调，必须在不同方向上多次测量直径，并记录千分尺的零误差。

4. Extension Measurement: Travelling Microscope and Fiducial Marks | 伸长量测量：读数显微镜与参考标记

When measuring the extension of a wire under load, many candidates failed to use a fiducial mark (e.g., a piece of tape on the wire) and a travelling microscope accurately. The report noted that extension values were often read from a metre rule with poor resolution, ignoring the need for sub‑millimetre precision.

在测量负载下导线的伸长量时，许多考生未能准确使用参考标记（如导线上的胶带）和读数显微镜。报告指出，伸长量的读数往往来自分辨率较差的米尺，忽略了亚毫米级精度的需求。

A vernier scale or travelling microscope should be aligned with the fiducial mark so that the smallest change in length can be detected. The original length of the wire should be measured with a metre rule, but the extension itself requires a finer scale. Moreover, waiting for the wire to stop creeping before taking readings is essential.

应使用游标刻度或读数显微镜对准参考标记，以便能检测到微小的长度变化。导线的原长可用米尺测量，但伸长量本身需要更精细的标度。此外，在读数前等待导线停止蠕变也很重要。

5. Eliminating Parallax Error in Scale Readings | 消除标尺读数中的视差

Parallax error arises when the eye is not positioned directly in front of the scale. The report cited examples where candidates

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