📚 Year 12 CIE Physics: Practical Exam Key Points | Year 12 CIE 物理：实验/实践考核要点
The Cambridge International AS Level Physics practical assessment (Paper 3) is designed to test your ability to carry out experimental investigations, record and analyse data, and evaluate procedures. This paper accounts for 23% of the AS qualification and requires careful preparation of hands-on skills, measurement techniques, and logical reasoning.
剑桥国际 AS 物理实验考核（试卷 3）旨在考查你进行实验探究、记录分析数据以及评估实验方案的能力。该试卷占 AS 总成绩的 23%，需要你认真准备动手操作技能、测量技术和逻辑推理能力。
1. Understanding the Practical Paper | 理解实验试卷结构
Paper 3 lasts 2 hours and contains two compulsory questions, each worth 20 marks. One question typically involves mechanics, waves or oscillations, while the other focuses on electricity, thermal physics or optics. Each question is divided into several parts that guide you through setting up apparatus, collecting data, drawing a table, plotting a graph, and analysing the results.
试卷 3 时长 2 小时，包含两道必答题，每道 20 分。一道题通常涉及力学、波或振动，另一道则侧重于电学、热学或光学。每道题都分成多个部分，一步步引导你搭建装置、收集数据、绘制表格、描点作图并分析结果。
You are expected to provide clear, logical answers and show all working. Although no prior knowledge of specific experiments is examined, you must be familiar with standard laboratory techniques and the use of common instruments. Marks are awarded for correct measured values (within a tolerance), sensible use of significant figures, and proper presentation of data.
你需要给出清晰、有逻辑的答案，并展示所有求解步骤。虽然不考查对具体实验的预先记忆，但你必须熟悉标准的实验室技巧和常用仪器的操作方法。分数会根据正确测量值（在容许范围内）、有效数字的合理使用以及数据呈现的规范性给出。
2. Common Instruments and Their Uncertainties | 常用仪器及其不确定度
The table below lists typical instruments used in the CIE practical and their standard absolute uncertainties (AU). For analogue instruments, the absolute uncertainty in a single reading is usually half the smallest scale division; for instruments with a digital display, it is the resolution (e.g., ±0.01 V for a voltmeter reading to 0.01 V).
| Instrument | Typical Range / Scale | Absolute Uncertainty |
|---|---|---|
| Metre rule | 0–1 m, 1 mm divisions | ±1 mm |
| Vernier caliper | 0–150 mm, 0.02 or 0.1 mm | ±0.1 mm (or ±0.02 mm) |
| Micrometer screw gauge | 0–25 mm, 0.01 mm division | ±0.01 mm |
| Protractor | 0–180°, 1° division | ±1° |
| Stopwatch (analogue) | 0–60 s, 0.2 s division | ±0.2 s (or human reaction time) |
| Digital ammeter | 0–10 A, read to 0.01 A | ±0.01 A |
| Digital voltmeter | 0–20 V, read to 0.01 V | ±0.01 V |
下表列出了 CIE 实验考试中常用的仪器及其标准绝对不确定度。对于指针式仪表，单次读数的绝对不确定度通常为最小分度值的一半；对于数字显示仪表，则取显示分辨率（如精确到 0.01 V 的电压表，不确定度为 ±0.01 V）。
Always state the unit with every measurement and ensure that you read the instrument perpendicularly to avoid parallax error. When taking a repeat reading, judge whether the instrument’s inherent uncertainty or random variation dominates, and report the spread of values appropriately.
每次测量都要标明单位，并确保视线与仪器刻度垂直，以避免视差。当进行重复测量时，需要判断是仪器本身的不确定度还是随机变化占主导，并合理报告数值的散布范围。
3. Calculating and Combining Uncertainties | 不确定度的计算与合成
Percentage uncertainty is a key concept for comparing the precision of different measurements. It is given by:
Percentage uncertainty = (Δx / x) × 100%
其中 Δx 是绝对不确定度，x 是测量值。百分比不确定度是衡量测量精度的关键。
When two quantities are added or subtracted, the absolute uncertainties add directly:
If R = A + B or A − B, then ΔR = ΔA + ΔB
当两个量相加或相减时，绝对不确定度直接相加：若 R = A + B 或 A − B，则 ΔR = ΔA + ΔB。
When quantities are multiplied or divided, the percentage uncertainties add:
If Q = A × B or A / B, then %ΔQ = %ΔA + %ΔB
当量相乘或相除时，百分比不确定度相加：若 Q = A × B 或 A / B，则 %ΔQ = %ΔA + %ΔB。
If a quantity is raised to a power, multiply the percentage uncertainty by that power. For example, the percentage uncertainty in A² is 2 × (%ΔA). Always quote the final calculated uncertainty to one or two significant figures and make sure it is consistent with the number of decimal places of the derived quantity.
如果对物理量进行乘方运算，则百分比不确定度要乘以该指数。例如，A² 的百分比不确定度为 2 × (%ΔA)。最终报告的不确定度保留一到两位有效数字，并确保与导出量的小数位数保持一致。
4. Recording Data Effectively | 高效记录数据
All numerical data must be presented in ruled tables with clear headings. Each column heading should show the quantity and its unit, separated by a solidus (e.g., ‘l / cm’, ‘T / s’). Record values to the same number of decimal places throughout a column, based on the instrument’s precision.
所有数值数据都必须填写在带有清晰表头的表格中。每个列表头应写明物理量和单位，用斜线分隔（如 “l / cm” “T / s”）。同一列中的所有数据应保持相同的小数位数，其位数取决于仪器的精确度。
Repeat measurements are essential for assessing reliability. For each independent reading, take at least three repeats and calculate the mean. If a reading appears anomalous, repeat it again and consider whether it should be discarded – but do not ignore a result without reason. Always comment on whether your data shows good consistency.
重复测量对评估数据的可靠性至关重要。每个独立的读数至少应重复三次并计算平均值。如果某个读数明显异常，应再次测量并考虑是否舍弃——但决不能无故忽略任何结果。始终要对自己数据的一致性进行评论。
5. Graph Plotting Techniques | 图表绘制技巧
Always use a sharp pencil for graphs. Label each axis with the quantity and its unit, using the format ‘Quantity / unit’ (e.g., ‘t / s’ for time in seconds). The scale must be simple – choose multiples of 1, 2, 5 or 10 that spread your plotted points over more than half the graph grid in both directions.
绘图时务必使用尖铅笔。为每个坐标轴标注物理量及其单位，格式为 “物理量 / 单位”（如时间用 “t / s” 表示）。比例尺要简单，选择 1、2、5 或 10 的倍数，使描点在两个方向上都占据坐标网格的一半以上。
Plot each data point as a small cross (×) or a dot surrounded by a circle. Do not use just a dot. If the question requests error bars, draw them to represent the absolute uncertainty in each variable. Draw either a single best-fit straight line or a smooth curve through the points; do not force the line through the origin unless there is a valid theoretical reason or the question instructs you to do so.
每个数据点用细小十字 (×) 或带圆圈的圆点标出，不要只用一个小点。如果题目要求画误差棒，则依据各变量的绝对不确定度画出。过描点画一条最佳拟合直线或光滑曲线；除非有可靠的理论依据或题目明确要求，否则不必强行让线通过原点。
Avoid unnatural ‘kinks’ in a straight line and ensure the line is balanced with roughly equal numbers of points on either side. A transparent ruler is very helpful for judging the best position of a straight line.
避免在直线上出现不自然的折点，并保持直线两侧的描点数量大致均衡。透明的直尺对判断最佳直线位置很有帮助。
6. Determining Gradient and y-intercept | 求解斜率和截距
To find the gradient of a straight-line graph, select two well-separated points that lie on the best-fit line (not the original data points). Mark them with a triangle or square, and show the rise and run clearly. Use the formula:
Gradient = Δy / Δx = (y₂ − y₁) / (x₂ − x₁)
计算直线斜率时，要在最佳拟合线上选取两个间距较大的点（不是原始数据点），并用三角形或方框标出，清楚标注纵轴变化量和横轴变化量。使用公式：斜率 = Δy / Δx = (y₂ − y₁) / (x₂ − x₁)。
Always quote the gradient to a suitable number of significant figures and include its unit if appropriate. The y-intercept can be read directly from the graph at x = 0 if the scale includes zero, or calculated using y = mx + c after finding the gradient and substituting one point on the line.
斜率的有效数字位数要恰当，必要时要注明单位。如果坐标轴比例尺包含零点，可在 x = 0 处直接读取纵截距；否则可利用 y = mx + c，将斜率及线上一点代入求出截距。
If the graph is a curve, you may be asked to draw a tangent at a specific point and determine its gradient. Use a mirror or a pair of set squares to get the perpendicular to the curve, then draw the tangent accurately.
如果图形是曲线，可能要求你在某点画切线并求其斜率。可利用镜子或一对三角板先确定曲线的法线，再精确画出切线。
7. Identifying and Reducing Errors | 识别并减少误差
Random errors cause readings to be scattered around the true value. They can be reduced by taking multiple measurements and averaging, or by using an instrument with a higher resolution. Systematic errors shift all readings in the same direction – common examples include zero error on a micrometer or ammeter, parallax error, and an incorrectly calibrated device.
随机误差导致读数在真值周围散布。可通过多次测量取平均值或使用更高分辨率的仪器来减少。系统误差会使所有读数朝同一方向偏移——常见例子包括千分尺或电流表的零误差、视差以及仪器校准不当。
When you are asked to criticise an experiment, identify whether the main sources of uncertainty are random or systematic. For instance, measuring the period of a pendulum with a stopwatch is dominated by reaction-time error (random), whereas measuring the diameter of a wire with a worn micrometer can introduce systematic error.
当你被要求评述一个实验时，要辨别主要的不确定度来源是随机的还是系统的。例如，用秒表测量单摆周期主要受反应时间（随机误差）影响，而用磨损的千分尺测导线直径可能引入系统误差。
A useful way to minimise parallax is to view the instrument perpendicularly, or use a mirror behind the scale (where available). For electrical experiments, ensure all connections are clean and tight to avoid contact resistance.
减少视差的有效方法是垂直读取仪器刻度，或使用带镜面的仪表。在电学实验中，确保所有接线清洁且牢固，以避免接触电阻。
8. Suggesting Improvements to Experiments | 提出实验改进
Typical limitations mentioned in CIE practicals include: difficulty in determining the exact centre of oscillation, human reaction time when using a stopwatch, small extension or small angle making percentage uncertainty large, and resistance of connecting leads being comparable to the resistor under test. You must go beyond just stating the limitation and provide a specific, workable improvement.
CIE 实验题中常提到的局限性包括：难以确定振荡的准确中心、使用秒表时的人为反应时间、微小伸长或小角度导致百分比不确定度偏大，以及导线电阻与被测电阻可比拟等。你不仅要指出局限性，还必须给出具体、可行的改进方法。
For example, to improve timing precision, use a light gate connected to a data logger. To measure the length of a pendulum more accurately, use a set square to align the point of suspension and the centre of the bob. If friction is an issue, lubricate the pulley or use an air track. In electrical circuits, use a higher-voltage supply to increase the current and reduce the percentage uncertainty in ammeter readings.
例如，为提高计时精度，可使用光门配合数据采集器。为更精确地测量单摆摆长，可用三角板对准悬点与摆球中心。如果存在摩擦问题，可润滑滑轮或使用气垫导轨。在电路中，使用较高的电源电压来增大电流，从而降低电流表读数的百分比不确定度。
Always link the improvement to the underlying physics and explain what new data you would obtain and how it would reduce the specific uncertainty.
务必将改进方法与背后的物理原理相联系，解释你将获得哪些新数据，以及如何降低特定的不确定度。
9. Typical Experiments to Revise | 需复习的典型实验
Although CIE does not require memorisation of a fixed set of experiments, being familiar with the following classic tasks will build your confidence and data-handling skills:
虽然 CIE 并不要求死记固定实验，但熟悉以下经典任务将增强你的信心和数据处理能力：
- Determining g using a simple pendulum: Measure length L and period T for different lengths; plot T² against L; gradient = 4π²/g.
- 用单摆测定重力加速度 g：测量不同摆长 L 下的周期 T；绘制 T²-L 图线；斜率 = 4π²/g。
- Determining g by free fall: Drop an object past two light gates; record fall time t and the distance s; use s = ut + ½at² or plot s/t against t.
- 通过自由落体测 g：使物体下落通过两个光门；记录下落时间 t 和距离 s；用 s = ut + ½at² 或绘制 s/t-t 图线。
- Resistance of a wire (resistivity): Measure the diameter d with a micrometer, length L with a metre rule, and resistance R from V/I; plot R against L; resistivity ρ = (gradient × πd²)/4.
- 导线电阻（电阻率）：用千分尺测直径 d，用米尺测长度 L，用 V/I 求出电阻 R；绘制 R-L 图；电阻率 ρ = (斜率 × πd²)/4。
- Wavelength of light using a diffraction grating: Measure the distance D from grating to screen and the fringe spacing x; λ = xd/D (for small angles), where d is the grating spacing.
- 用衍射光栅测光波波长：测量光栅到屏幕的距离 D 和条纹间距 x；对于小角度，λ =
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