A-Level Physics Unit 3 January 2020 Question Paper: Key Concepts Explained | A-Level物理Unit 3 2020年1月试卷概念解析

📚 A-Level Physics Unit 3 January 2020 Question Paper: Key Concepts Explained | A-Level物理Unit 3 2020年1月试卷概念解析

The January 2020 Edexcel A-Level Physics Unit 3 paper (WPH03/01) assesses students on practical skills, data analysis and experimental techniques. This article unpacks the core concepts examined, from uncertainties to graph interpretation, and provides revision strategies rooted in the question paper’s typical demands.

2020年1月爱德思A-Level物理Unit 3试卷（WPH03/01）重点考查实验技能、数据处理与实验技术。本文深入解析试卷涉及的核心概念，涵盖不确定度、图表分析等，并结合试卷常见题型提供复习思路。

1. Understanding the Role of Uncertainty in Measurements | 理解测量中的不确定度作用

Every experimental measurement carries an uncertainty, which reflects the range within which the true value is expected to lie. In Unit 3, you must be able to identify sources of uncertainty and express them appropriately, e.g., ±0.1 cm for a ruler or ±0.01 mm for a micrometer.

每个实验测量值都带有不确定度，表明真值可能落入的范围。在Unit 3中，你需要能够识别不确定度的来源并合理表达，例如直尺的±0.1 cm或千分尺的±0.01 mm。

A key skill is combining uncertainties when quantities are added, subtracted, multiplied or divided. For addition and subtraction, absolute uncertainties add. For multiplication and division, percentage uncertainties add. This is frequently tested in the January 2020 paper when students had to propagate errors in derived quantities such as density or resistivity.

组合不确定度是一项关键技能。加减运算时，绝对不确定度相加；乘除运算时，百分数不确定度相加。2020年1月试卷中常要求推导密度或电阻率等复合量时进行误差传递。

Uncertainty propagation rules can be summarised:

不确定度传播规则可总结如下：

Operation	Rule
Z = A ± B	ΔZ = ΔA + ΔB
Z = A × B or A ÷ B	%ΔZ = %ΔA + %ΔB
Z = Aⁿ	%ΔZ = n × %ΔA

Always express uncertainties to 1 significant figure, and match the decimal places of the measured value to the uncertainty. The January 2020 mark scheme penalised candidates who gave excessive decimal places inconsistent with their uncertainty estimates.

不确定度通常保留一位有效数字，测量值的末位应与不确定度的末位对齐。2020年1月阅卷标准对小数位数与不确定度估计不符的答案有扣分。

2. Systematic vs Random Errors: Identification and Minimisation | 系统误差与随机误差：识别与最小化

Random errors cause readings to scatter about the true value and can be reduced by taking repeat measurements and calculating a mean. The January 2020 paper asked students to explain why repeating a timing experiment reduced the effect of random errors.

随机误差导致读数在真值附近波动，可通过重复测量取平均值来减小。2020年1月试卷曾要求学生解释为何计时实验重复进行能降低随机误差的影响。

Systematic errors, in contrast, shift all readings in one direction, often due to faulty equipment or poor experimental design. Examples include a zero error on a micrometer (not reset to zero before measuring) or a meter scale not calibrated correctly. Simply repeating readings does not eliminate systematic errors; you must adjust the apparatus or apply a correction.

系统误差则使所有读数朝同一方向偏移，常由仪器故障或实验设计缺陷引起。例如千分尺的零误差（测量前未调零）或仪表刻度未正确校准。单纯重复测量不能消除系统误差，需调整装置或进行修正。

In a typical Unit 3 question on a pendulum experiment, a student may use a stopwatch with a known lag. The resulting period values are all too large. This is a systematic error. Recognising this distinction is vital for the final evaluate questions.

在典型的单摆实验中，学生若使用有已知延迟的秒表，测量出的周期值都会偏大，这就是系统误差。准确区分两类误差对最后的评估题至关重要。

3. Precision and Accuracy in Experimental Data | 实验数据的精密度与准确度

Precision refers to the closeness of repeated measurements to each other, while accuracy describes how close they are to the accepted true value. The January 2020 exam distinguishes between them through data analysis tasks.

精密度指重复测量值彼此接近的程度，准确度则反映它们与公认真值的接近程度。2020年1月考试通过数据分析任务对二者进行区分。

If a student records diameters of a wire as 0.52 mm, 0.53 mm, 0.52 mm, the measurements are precise (small spread) but could be inaccurate if the micrometer had a zero error of +0.04 mm. Understanding this helps in suggesting improvements: recalibrate the instrument to improve accuracy; use a more sensitive instrument to improve precision.

若学生测得导线直径分别为0.52 mm、0.53 mm、0.52 mm，测量很精密（离散小），但倘若千分尺存在+0.04 mm零误差，则不准确。理解这一点有助于提出改进建议：重新校准仪器改善准确度；使用更灵敏的仪器提高精密度。

You might be given a target percentage uncertainty and asked to choose the most appropriate measuring device. For example, to keep the percentage uncertainty in a length of 20 cm below 1%, a ruler with millimetre markings (absolute uncertainty ±1 mm) is sufficient.

题目可能会给出目标百分数不确定度，并要求选择最合适的测量工具。例如，要使20 cm长度的百分数不确定度低于1%，使用毫米刻度直尺（绝对不确定度±1 mm）即可满足。

4. Using Vernier Callipers and Micrometer Screw Gauge | 游标卡尺与千分尺的使用

The vernier calliper (resolution 0.1 mm or 0.02 mm) and micrometer screw gauge (resolution 0.01 mm) are essential in Unit 3 for measuring lengths like the diameter of a wire or the thickness of a sheet. The January 2020 paper included a practical context where the zero error of a micrometer had to be recorded and accounted for.

游标卡尺（分辨率0.1 mm或0.02 mm）和千分尺（分辨率0.01 mm）是Unit 3中测量导线直径或薄片厚度等长度的基本工具。2020年1月试卷包含需记录并考虑千分尺零误差的实际情境。

To read a micrometer: note the main scale reading on the sleeve (0.5 mm divisions), then add the thimble scale reading. Always check for zero error before taking measurements. If the zero marking is offset, record the error as positive or negative and subtract algebraically from all readings.

读取千分尺时：先记录套管上的主标尺读数（每格0.5 mm），再加上微分筒上的读数。测量前务必检查零误差。若零刻度线偏移，记录正或负零误差，并在所有读数中代数相减。

A common pitfall is misreading the half-millimetre lines partly obscured by the thimble. In January 2020, a common mistake was reporting the reading as 5.23 mm when the correct value was 5.73 mm due to the 0.5 mm line being revealed.

常见错误是漏读被微分筒边缘部分遮挡的半毫米刻度线。2020年1月中有学生因半毫米线已露出而误读为5.23 mm，正确读数应为5.73 mm。

5. Interpreting Graphs and Calculating Gradients | 图表解读与梯度计算

Plotting data and drawing a best-fit straight line is a central skill. Use at least six points to minimise the impact of anomalies; draw error bars if required. The gradient and intercept are usually linked to physical constants, such as g or resistivity.

绘制数据点并作出最佳拟合直线是一项核心技能。至少使用六个点以减小异常值的影响；若题目要求，需画出误差棒。梯度和截距通常与物理常数相关联，如g或电阻率。

For a graph of V against I for a fixed resistor, the gradient equals resistance R. In a free-fall experiment, a graph of v² against 2h yields a gradient equal to g. The January 2020 paper asked students to determine the gradient from a graph and state the corresponding physical quantity.

对定值电阻绘制V-I图，梯度等于电阻R。在自由落体实验中，v²-2h图的梯度为g。2020年1月试卷要求从曲线求出梯度并说明所对应的物理量。

When calculating the gradient, use a triangle that covers more than half the line. Show the coordinates you read from the axes; do not use data points unless they lie exactly on the line. The gradient’s unit must be derived from the axes units, e.g., Ω for V/A.

计算梯度时，使用覆盖直线大半的三角形。记录从坐标轴读取的坐标值；除非数据点正好在直线上，否则不要直接使用数据点。梯度的单位须根据坐标轴单位导出，如V/A即Ω。

Formula for gradient:

gradient = (y₂ – y₁) / (x₂ – x₁)

梯度公式：

梯度 = (y₂ – y₁) / (x₂ – x₁)

6. Determining Uncertainty in Gradient and Intercept | 梯度和截距的不确定度计算

To find the uncertainty in a gradient, draw the steepest and shallowest plausible straight lines through the error bars. Denote their gradients as m_max and m_min. The uncertainty is half the range.

求梯度的不确定度时，需画出通过误差棒的最陡和最缓可行直线。记它们的梯度为m_max和m_min，不确定度为二者差值的一半。

Δm = (m_max – m_min) / 2

For the intercept c, extend the two lines to the y-axis and similarly calculate Δc = (c_max – c_min)/2. The January 2020 marking guidelines accepted either the ‘max-min’ method or statistical spread if error bars were symmetrical.

对于截距c，将这两条直线延伸至y轴，同样计算Δc = (c_max – c_min)/2。2020年1月评卷标准接受最大-最小法，若误差棒对称也接受统计离散度方法。

Express the final result as (m ± Δm) with consistent significant figures. For example, if m = 9.78 and Δm = 0.06, write 9.78 ± 0.06 m s⁻². This was assessed in the January 2020 evaluate section where students compared their value with the accepted value of 9.81 m s⁻² and commented on accuracy.

最终结果表示为(m ± Δm)，并保持有效数字一致。例如m=9.78、Δm=0.06，应写为9.78 ± 0.06 m s⁻²。2020年1月评估题就考查了这一表达，要求学生与公认值9.81 m s⁻²对比并评论准确度。

7. Linearisation Techniques for Non-Linear Relationships | 非线性关系的线性化技术

Many experiments yield non-linear relationships. A common Unit 3 task is to suggest how to modify axis variables to obtain a straight line. The January 2020 paper included a question on the period of a simple pendulum, where T is proportional to √L.

许多实验产生非线性关系。Unit 3的常见任务是指出如何修改坐标轴变量以获得直线。2020年1月试卷含有一道单摆周期问题，已知T正比于√L。

Since T = 2π√(L/g), T² = (4π²/g) L. By plotting T² against L, you obtain a straight line through the origin with gradient = 4π²/g, allowing g to be determined. This linearisation circumvents the difficulty of fitting a curve and improves reliability.

由T = 2π√(L/g)可得T² = (4π²/g) L。绘制T²-L图可得到一条过原点的直线，梯度为4π²/g，从而可以算出g。这种线性化方法避免了拟合曲线的困难，提高了可靠性。

Other examples: for a capacitor discharge, V = V₀ e^(-t/RC), plot ln V against t to get a straight line with gradient -1/RC. For the resistivity experiment, R = ρL/A, a graph of R against L yields gradient = ρ/A, giving ρ if cross-sectional area A is known.

其他例子：电容器放电V = V₀ e^(-t/RC)，绘制ln V-t图可得斜率为-1/RC的直线。在电阻率实验中，R = ρL/A，R-L图的梯度为ρ/A，若截面积A已知可求出ρ。

8. Experiment: Measuring the Resistivity of a Wire | 实验：测量导线电阻率

The resistivity ρ of a metal wire is found by measuring resistance R, length L, and diameter d (to calculate cross-sectional area A = πd²/4). The January 2020 paper had a detailed question on this standard experiment, including circuit assembly and uncertainty analysis.

金属导线的电阻率ρ通过测量电阻R、长度L和直径d（计算截面积A = πd²/4）求得。2020年1月试卷对该标准实验有详细考查，包括电路组装和不确定度分析。

The circuit should be set up with an ammeter in series and voltmeter parallel across the wire. Take readings of V and I for several lengths, keeping current low to avoid heating. Calculate R = V/I. Plot R against L; the gradient m = ρ/A.

电路应将电流表串联、电压表并联在导线两端。针对多个长度测量V和I，保持电流较小以避免发热。计算R = V/I。绘制R-L图，其梯度m = ρ/A。

Hence ρ = m × A. The percentage uncertainty in ρ is %Δρ = %Δm + %ΔA. Here %ΔA = 2 × (Δd/d) since A ∝ d². The micrometer’s resolution and zero error directly affect Δd. In January 2020, many candidates forgot to include the 2× factor for the diameter uncertainty, leading to underestimated error bars.

因此ρ = m × A。ρ的百分数不确定度为%Δρ = %Δm + %ΔA。其中%ΔA = 2 × (Δd/d)，因为A∝d²。千分尺的分辨率和零误差直接影响Δd。2020年1月不少考生忘记直径不确定度的2倍因子，导致误差棒偏小。

Careful technique includes measuring d at several points along the wire and averaging to account for non-uniformity. Use the micrometre’s ratchet to avoid compressing the wire.

操作技巧包括：沿导线多点测量直径取平均以考虑非均匀性，并使用千分尺的棘轮防止压扁导线。

9. Experiment: Determining the Internal Resistance and EMF of a Cell | 实验：测定电池内阻与电动势

This classic experiment uses a variable resistor, a cell, and a voltmeter across the cell terminals. Measure terminal p.d. V for different circuit currents I. The relationship is V = E – Ir, where E is the emf, r the internal resistance.

这个经典实验使用可变电阻、电池和并联在电池两端的电压表。改变电路电流I，测量端电压V。关系式为V = E – Ir，其中E为电动势，r为内阻。

A graph of V against I gives a straight line with gradient = -r and y-intercept = E. The January 2020 paper frequently required students to draw this graph, determine E and r, and discuss why the line does not pass through the origin when a significant internal resistance exists.

绘制V-I图可得一条直线，斜率为-r，y轴截距为E。2020年1月试卷多次要求考生绘制此图、求出E和r，并讨论当存在显著内阻时为什么直线不过原点。

When evaluating, note that the voltmeter must have a very high resistance to avoid drawing current and affecting the readings. Using a digital multimeter minimises this systematic error. Also, switch the circuit off between readings to avoid heating the cell, which would alter r.

评估时需注意：电压表内阻应极大，以免分流而影响读数；使用数字万用表可减少这种系统误差。此外，每次读数后应断开电路，防止电池发热导致内阻变化。

The internal resistance can also be found from a graph of 1/I against R, but the V-I method is more intuitive. Uncertainties can be derived by drawing max/min lines, as described earlier.

内阻也可通过1/I-R图求得，但V-I法更直观。不确定度可通过如前所述的最大最小线法求得。

10. Experiment: Young’s Double-Slit Interference | 实验：杨氏双缝干涉

The fringe width w in a double-slit experiment is given by w = λD/s, where λ is the wavelength, D the slit-to-screen distance, and s the slit separation. Unit 3 often tests your ability to suggest improvements and estimate uncertainties.

双缝实验中的条纹间距w满足w = λD/s，其中λ为波长，D为缝到屏距离，s为双缝间距。Unit 3常考查提出改进建议和估算不确定度的能力。

In the January 2020 paper, a variant experiment used a laser and a diffraction grating. The grating equation nλ = d sin θ was central, with the angle θ measured from the grating to the nth bright spot. Since measuring angles directly is tricky, the distance x from the central maximum to the nth order and the distance D to the screen are used: tan θ = x/D. For small angles, sin θ ≈ tan θ, so λ = (d x)/(n D).

2020年1月试卷中有一变体实验使用激光和衍射光栅。核心公式为nλ = d sin θ，θ是从光栅到第n级亮点的角度。由于直接测角困难，改用中央明纹到第n级亮纹的距离x和屏幕距离D：tan θ = x/D。小角度下sin θ≈tan θ，故λ = (d x)/(n D)。

Key sources of uncertainty: measuring x with a ruler (±1 mm), measuring D with a metre rule, and the grating’s slit spacing d (often given as lines per mm, so uncertainty from manufacturing). To improve the accuracy, measure across several fringe widths and divide by the number of fringes to get an average w.

主要不确定度来源：用直尺测量x (±1 mm)、用米尺测量D，以及光栅间距d（常以每毫米线数给出，制造误差）。要提高准确度，可跨越多个条纹间距测量总宽度再除以条纹数得到平均w。

The January 2020 paper highlighted that using a larger D reduces the percentage uncertainty in D, and measuring the distance to a higher order (n) reduces the percentage uncertainty in x. These improvements were frequently examined in evaluation questions.

2020年1月试卷强调，增大D可减小D的百分数不确定度，测量更高级次的距离可减小x的百分数不确定度。这些改进方法在评估题中常被考查。

11. Critically Evaluating Experimental Procedures | 批判性评估实验步骤

Every Unit 3 paper includes a question asking you to evaluate the method and suggest refinements. For January 2020, common themes included: reducing parallax error, minimising heating effects, using dataloggers for fast-changing variables, and allowing apparatus to settle.

每份Unit 3试卷都有要求评估实验方法并提出改进的题目。2020年1月常见主题包括：减小视差、降低热效应、对快变量使用数据采集器、让仪器稳定等。

A strong evaluation comment is one that explains how an improvement reduces a specific uncertainty. For instance, ‘Measure the diameter of the wire in three perpendicular directions along its length and take the mean, which reduces the effect of non-uniformity and provides a more reliable average cross-sectional area.’

高质量的评估意见会说明某项改进如何减小特定不确定度。例如，“沿导线长度在三个正交方向测量直径取平均，这能减少非均匀性的影响，提供更可靠的平均截面积。”

The January 2020 mark scheme rewarded students who linked each criticism to the physics, not just listing generic faults. Avoid vague statements like ‘do the experiment more carefully’. Instead, specify: ‘The ruler should be clamped vertically and a set-square used to align the eye with the meniscus to avoid parallax errors when measuring the length of the wire.’

2020年1月阅卷中，凡能针对物理原理提出批评而非空泛罗列缺点的答案都获加分。避免“实验更细心”之类的空话，而应具体：“应将直尺垂直固定，并用三角尺使视线与液面弯月面平齐，以避免测量线长时的视差。”

12. Common Pitfalls in Unit 3 Questions | Unit 3 题目中的常见陷阱

Drawing conclusions without quoting uncertainties: In the January 2020 paper, if a calculated value overlaps the accepted value within experimental uncertainty, the result is consistent, not necessarily accurate.

忽略不确定度就下结论：2020年1月试卷中，如果计算值在实验不确定度范围内与公认值重叠，则结果是一致的，而非必然准确。

Misidentifying anomalies: An anomaly is a point that lies significantly off the trend line considering the error bars. Do not discard it without justification. Repeat the measurement first.

异常值误判：异常值是明显偏离趋势线且超出误差棒范围的测量点。不可无理由剔除，应首先复测。

Confusing resolution with uncertainty: The resolution of a digital balance is 0.01 g, but the uncertainty may be larger due to zero drift or air currents. Always state ± the instrument’s resolution as a minimum, but consider other factors.

混淆分辨率与不确定度：数字天平分辨率为0.01 g，但不确定度可能因零漂或气流而更大。至少应注明±最小分辨率，但需考虑其他因素。

Units: Forgetting to convert mm to m when using formulas can lead to a factor of 1000 error. The January 2020 paper caught out candidates who calculated g as 0.0098 m s⁻² instead of 9.8 m s⁻² due to units mix-up.

单位问题：代入公式时忘记将mm换算为m，可能导致1000倍的错误。2020年1月就有考生因单位混淆而算得g=0.0098 m s⁻²，正确值应为9.8 m s⁻²。

By mastering these concepts, you will be well-prepared for the style and rigour of the Unit 3 practical paper. Consistent practice with past papers, especially from January 2020, reinforces the logical structure expected by examiners.

掌握这些概念后，你将能从容应对Unit 3实验试卷的风格与严谨性。通过持续练习历年真题，尤其是2020年1月的试卷，可以强化考官所期待的逻辑结构。

Published by TutorHao | Physics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

A-Level Physics Unit 3 January 2020 Question Paper: Key Concepts Explained | A-Level物理Unit 3 2020年1月试卷概念解析