Experimental Techniques Guide | 实验操作指南

📚 Experimental Techniques Guide | 实验操作指南

Mastering experimental techniques is a core component of the WJEC IGCSE Physics qualification. This guide takes you through the essential practical skills, from setting up apparatus to analysing data, helping you approach Paper 3 (Practical Assessment) and the alternative-to-practical paper with confidence. Whether you are measuring the acceleration of free fall or investigating Ohm’s Law, each section below combines the theory of measurement with hands‑on advice. All guidance is aligned with the WJEC specification and includes common pitfalls, error analysis and tips for drawing valid conclusions.

掌握实验操作技巧是 WJEC IGCSE 物理课程的核心内容。本指南带你梳理从搭建仪器到数据分析的关键实验技能，帮助你从容应对 Paper 3（实验考核）或作为替代的实验笔试。无论你是在测量自由落体加速度还是探究欧姆定律，下面每一节都把测量原理和动手建议结合在一起。所有指导内容都与 WJEC 考纲对齐，包含常见易错点、误差分析以及得出有效结论的技巧。

1. Safety and Preparation | 实验安全与准备

Before any practical work, you must conduct a quick risk assessment. Identify hazards such as hot plates, heavy masses, sharp edges or high‑voltage supplies. Tie back long hair, tuck in loose clothing and always wear safety goggles when dealing with projectiles, heated liquids or electrical circuits. Clear the workbench of clutter so that you have enough space to set up apparatus without knocking over a clamp stand or a glass beaker. Check that all equipment is working – if a meter shows a flat battery warning, replace it before taking readings.

每次动手实验前，你要快速做一次风险评估。找出热板、重物、锋利边角和高压电源等危险源。把长头发扎起来，束紧宽松的衣服，处理抛体、加热液体或电路时务必佩戴护目镜。清理工作台，腾出足够空间安置仪器，避免碰倒铁架台或烧杯。检查所有设备是否正常——如果电表显示电池电量低的提示，务必在读数前更换电池。

A well‑prepared table of results, drawn before you start, saves time and reduces mistakes. Label columns with the quantity and unit (e.g. ‘Current I / A’). Leave space for repeat readings and calculated averages. Always record data in pencil so corrections are neat. If you are using a stopwatch, plan how to start and stop it reliably – for oscillations, it is often better to time 10 swings and divide by 10 to minimise reaction‑time errors.

实验开始前预先画好清晰的记录表格，能节省时间并减少错误。为每一列标上物理量和单位（如“电流 I / A”）。留出重复读数和计算平均值的空格。始终用铅笔记录数据，这样修改干净整洁。如果使用秒表，先计划好如何可靠地启动和停止——对于振动，通常计时 10 个周期再除以 10 能减小反应时间误差。

2. Measuring Length Accurately | 长度的精确测量

For distances longer than about one metre, use a metre rule or a tape measure. Ensure the rule is parallel to the object and avoid parallax error by aligning your line of sight perpendicular to the scale. The typical precision of a metre rule is ±1 mm, but you should record readings to the nearest millimetre (e.g. 23.7 cm, not 24 cm) to show the instrument’s resolution.

测量大于一米的长度时，使用米尺或卷尺。确保尺子与被测物平行，并将视线与刻度垂直对齐以避免视差。米尺的典型精度是±1 mm，但你应该记录到最接近的毫米（如 23.7 cm，而不是 24 cm），以体现仪器的分辨力。

For small diameters, such as the thickness of a wire or the diameter of a ball bearing, a vernier calliper or a micrometre screw gauge is required. A vernier calliper typically reads to ±0.1 mm, while a micrometre can resolve to ±0.01 mm. Always check for zero error before use: close the jaws fully; if the zero mark does not align, subtract or add the error from all subsequent readings. Take three to five measurements at different orientations and average them to account for slight non‑uniformity.

对细小的直径，比如导线径或滚珠直径，需要用到游标卡尺或螺旋测微计。游标卡尺的典型读数精度是±0.1 mm，螺旋测微计可分辨到±0.01 mm。使用前一定要检查零误差：将测量面完全闭合，若零刻度未对准，之后所有读数都要减去或加上这一偏差。在不同方位测量 3 到 5 次并取平均，以克服微小的不均匀性。

3. Measuring Time and Mass | 时间与质量的测量

Time measurements in IGCSE experiments often involve a stopwatch or an electronic timer. Human reaction time introduces an uncertainty of about ±0.2 s for a single start‑stop action. To reduce this, measure the time for multiple cycles (e.g. 20 oscillations of a pendulum) and divide by the number of cycles. Always start the stopwatch at a clearly identifiable point, such as the centre of the swing, and use a fiducial marker to aid your consistency.

IGCSE 实验中时间的测量常使用秒表或电子计时器。人体反应时间会给单次启停动作带来约±0.2 s 的不确定度。为减小这一影响，可以测量多个周期的时间（例如单摆振动 20 次）再除以周期数。务必在一个明确易辨的位置（如摆动中心）开始计时，并借助一个参考标记来保持一致性。

Mass is measured with a digital balance or a beam balance. Check the zero before placing any object on the pan. Avoid touching the pan with your hands, as moisture and oils can affect the reading slightly. For very small masses, use the balance inside a draught‑free enclosure. Record the mass to the full resolution of the instrument – for a digital balance reading 0.01 g, write 12.40 g, not 12.4 g. When plotting graphs involving mass, always use kilograms (kg) unless the question states otherwise, because the SI unit of mass is the kilogram.

质量用数字天平或杠杆天平测量。放置物体前先调零。不要用手触碰秤盘，因为水分和油脂会轻微影响读数。对非常小的质量，应在防风罩内使用天平。读取示数时，写出仪器的全部分辨力——若数字天平显示到 0.01 g，就记录 12.40 g 而非 12.4 g。在绘制含质量的图像时，除非题目另有说明，一律使用千克（kg），因为质量的国际单位制是千克。

4. Electrical Circuits: Ammeters, Voltmeters and Rheostats | 电路实验基础：电流表、电压表和变阻器

When building electrical circuits, always connect the ammeter in series and the voltmeter in parallel with the component being tested. Start with the circuit diagram on paper and use it as a guide. Choose the correct ranges: if you are unsure, start at the highest range and step down to avoid damaging the meter. A variable resistor (rheostat) should be wired as a potential divider or in series to sweep through a range of currents smoothly.

搭建电路时，电流表必须串联，电压表必须与被测元件并联。先在纸上画出电路图，再按图接线。选择合适的量程：如果不确定，从最高量程起逐步降低，避免损坏电表。变阻器（滑动变阻器）应接成分压电路或串联接入，以便平稳地改变电流范围。

To investigate Ohm’s Law, you adjust the rheostat to vary the current and record the corresponding voltage across a fixed resistor. Take at least six pairs of readings over a wide range. Reverse the connections to obtain negative values as well, which shows the symmetry of the I–V characteristic. Always switch off the circuit between recordings to minimise heating effects; excessive temperature rise can change the resistance and give a curved graph.

探究欧姆定律时，调节变阻器改变电流，并记录固定电阻器两端的相应电压。至少在较宽范围内采集六组数据。调换接线方向还可获得负值，以此展示 I–V 特性的对称性。每次记录后都应关断电路，以减小热效应；温度过度上升会改变电阻，使图像弯曲。

5. Recording Data and Significant Figures | 数据处理与有效数字

All recorded data must reflect the precision of the instrument. If a metre rule reads to ±1 mm, write 15.0 cm (three significant figures) rather than 15 cm (two). For a digital ammeter showing three decimal places, record 0.234 A, not 0.23 A. The number of significant figures should be consistent throughout a column. Never discard data points just because they do not fit the expected pattern – instead, repeat the measurement or investigate the cause.

所有记录的数据都要如实反映仪器的精度。若米尺读到±1 mm，应记录 15.0 cm（三位有效数字）而非 15 cm（两位）。如果数字电流表显示三位小数，应记作 0.234 A，而不是 0.23 A。同一列数据中有效数字的位数应保持一致。绝不能因为某个数据点不符合预期规律就随意扔掉——应当重测或探究原因。

If you calculate a quantity from raw data, apply the appropriate rules for significant figures. For multiplication or division, the result should have no more significant figures than the least precise measurement used. For addition or subtraction, the result should have the same number of decimal places as the least precise measurement. Always show your working clearly so an examiner can follow your steps.

如果用原始数据计算某个物理量，要遵循恰当的有效数字规则。乘除运算时，结果的有效数字位数不应多于参与运算中最不精确的那个测量值。加减运算时，结果的小数位数应与最不精确测量的位数相同。始终清晰展示运算步骤，让考官能跟踪你的推导。

6. Types of Error and How to Reduce Them | 误差的类型与降低方法

Random errors cause readings to be scattered around the true value. They arise from factors like fluctuating readings on a digital meter, human judgement in reading a scale, or small variations in experimental conditions. The best way to reduce random errors is to take multiple readings and calculate the mean. Plotting a graph with a best‑fit line also smooths out random fluctuations.

随机误差会使读数围绕真值离散分布。其来源包括数字表的波动、读刻度时的人为判断或实验条件的微小变化。减小随机误差的最佳方法是多次测量取平均值。绘制带有最佳拟合线的图像也能平滑随机波动。

Systematic errors shift all measurements in one direction, away from the true value. Examples include a zero error on a vernier calliper, a clock that runs consistently fast, or a ruler that has expanded due to heat. Repeating measurements will not reduce systematic errors; instead, you must identify and correct the source. For instruments, check the zero before each use. For procedure‑related offsets, redesign the experiment – for instance, use a set‑square to ensure a ruler is truly vertical when measuring height.

系统误差会使所有测量值朝同一方向偏离真值。例如游标卡尺的零误差、总是走得快的时钟，或因受热膨胀的尺子。重复测量无法减小系统误差；你必须找出并纠正源头。对仪器，每次使用前检查零位。对操作上的偏差，重新设计实验——比如在测量高度时，借助直角尺确保尺子真正竖直。

7. Drawing Graphs and Lines of Best Fit | 绘制图表与最佳拟合线

A well‑drawn graph is one of the most powerful tools in physics. Use graph paper with sensible scales: the plotted points should cover more than half the page in both directions. Label each axis with the quantity and unit, e.g. ‘Voltage V / V’ and ‘Current I / A’. Plot points as small, sharp crosses (×) or dots with a circle drawn around them. Do not join dot‑to‑dot. Instead, draw a single straight line or a smooth curve that passes as close as possible to all points, with roughly equal numbers of points on either side of the line.

一幅绘制得当的图像是物理学中最有力的工具之一。使用坐标纸并选取合理的标度：绘出的数据点在横纵两个方向上均应占据超过半页的范围。为每个坐标轴标注物理量和单位，如“电压 V / V”与“电流 I / A”。用细小而清晰的叉号（×）或带圆圈的点来标绘数据。不要逐点连线。应画一条尽可能靠近所有点的单一直线或光滑曲线，并使线两侧的点数大致相等。

If a point lies far from the best‑fit line, it is an outlier. Circle it and label it ‘anomalous’. Do not include an anomalous point when positioning the line of best fit, but mention it in your evaluation. The line of best fit may not pass through the origin even if theory predicts it should – this could indicate a small systematic error.

若某个点远离最佳拟合线，它就是异常点。把它圈起来并标上“异常”。定位最佳拟合线时不要纳入异常点，但在评估中应提及它。最佳拟合线可能不通过原点，尽管理论预期如此——这可能表明存在一个小的系统误差。

8. Finding Gradients and Intercepts from Graphs | 从图表计算梯度与截距

To determine the gradient of a straight‑line graph, pick two points that lie on the line of best fit, not necessarily original data points. The two points should be as far apart as possible to improve precision. Use a large triangle on the graph; label the coordinates (x₁, y₁) and (x₂, y₂). The gradient m is given by:

要确定直线图的梯度，选取位于最佳拟合线上的两个点，不一定是原始数据点。这两点应尽可能远，以提高精度。在图中画一个大的三角形，标注坐标 (x₁, y₁) 和 (x₂, y₂)。梯度 m 由下式给出：

m = (y₂ − y₁) / (x₂ − x₁)

Always quote the gradient with appropriate units – if the y‑axis is Voltage / V and the x‑axis is Current / A, the unit of the gradient is Ω (ohm), and you should write the numerical value followed by the unit.

梯度要给出恰当的单位——如果 y 轴是 Voltage / V，x 轴是 Current / A，那么梯度的单位是 Ω（欧姆），写出数值后要跟上单位。

The y‑intercept is read directly where the best‑fit line crosses the y‑axis (x = 0). If the graph does not extend to x = 0, you can calculate the intercept using the equation y = mx + c once m is known. The intercept often has a physical meaning, such as the internal resistance of a cell when plotting terminal p.d. against current.

y 轴截距直接从最佳拟合线与 y 轴（x = 0）交点处读取。如果图中没有延伸到 x = 0，可在已知 m 后通过 y = mx + c 计算截距。截距通常有物理意义，比如绘制路端电压与电流的关系时，截距表示电池的内阻。

9. Determining the Acceleration of Free Fall g | 测定自由落体加速度 g

A classic WJEC experiment uses a free‑fall timer or a simple pendulum to determine g. For the free‑fall method, a small steel ball is released from an electromagnet and its fall time t over a measured height h is recorded by a trapdoor switch or light gate. The equation is:

一个经典的 WJEC 实验利用自由落体计时器或单摆来测定 g。自由落体法中，一颗小钢球从电磁铁释放，下落高度 h 已知，下落时间 t 由活门开关或光门记录。方程为：

h = ½ g t²

Rearrange to give g = 2h / t². To minimise the effect of reaction time, keep the electromagnet current constant so the ball always starts from the same position and marks a precise beginning. Measure h with a metre rule and repeat for at least five different heights, calculating g each time and then finding the mean.

整理得 g = 2h / t²。为削弱反应时间的影响，保持电磁铁电流恒定，使球总是从同一位置释放，标记精准的起始时刻。用米尺测量 h，至少在五种不同高度下重复实验，每次计算 g，最后求平均值。

For the pendulum method, measure the time T for one complete oscillation from the relation T = 2π√(L/g). You measure the period for large numbers of swings (e.g. 20T) and vary the length L. Plot T² against L. The gradient of the straight line will be 4π²/g, so g = 4π² / gradient. This method avoids the need to measure height precisely and typically gives a value around 9.8 m s⁻².

单摆法中，通过 T = 2π√(L/g) 测量一个完整摆动的周期 T。测量大量摆动次数（如 20T）的时间，并改变摆长 L。绘制 T² 对 L 的图像。直线的梯度是 4π²/g，因此 g = 4π² / 梯度。这种方法避免了精密测量高度，通常得出数值在 9.8 m s⁻² 左右。

10. Investigating Ohm’s Law and Resistance | 探究欧姆定律与电阻

Set up a circuit with a power supply, a fixed resistor, an ammeter in series and a voltmeter in parallel with the resistor. Use a variable resistor to vary the current. Record the potential difference V for each current I. At constant temperature, the graph of V against I is a straight line through the origin, confirming V ∝ I. The resistance R is the gradient (R = V / I).

搭建一个包含电源、固定电阻器、串联的电流表和与电阻并联的电压表的电路。用变阻器改变电流。记录每个电流 I 下的电压 V。在温度恒定的条件下，V 对 I 的图像是一条过原点的直线，证实 V ∝ I。电阻 R 即是梯度（R = V / I）。

If the resistor heats up significantly, the line bends, illustrating that resistance depends on temperature. This is a great opportunity to discuss limitations: to keep temperature constant, you should switch off the circuit between readings and use a low‑voltage supply. Reversing the connections and plotting both positive and negative quadrants demonstrates that metallic conductors obey Ohm’s Law in both directions.

如果电阻器显著发热，图像将弯曲，揭示电阻随温度变化。这是讨论实验局限的好机会：为保持温度恒定，每次读数后关断电路并使用低压电源。调换接线方向，绘制正负两个象限的图像，可以证明金属导体在两个方向上都遵循欧姆定律。

11. Measuring Density of Regular and Irregular Objects | 测量规则与不规则物体的密度

Density ρ is mass per unit volume, ρ = m / V. For a regular solid, such as a rectangular block, measure the mass with a balance and calculate the volume using V = length × width × height, each measured with a vernier calliper or metre rule. Repeat each dimension at different positions to get an average volume – this averages out small irregularities.

密度 ρ 是单位体积的质量，ρ = m / V。对于一个规则固体，如长方体块，用天平测量质量，用游标卡尺或米尺测出长、宽、高，再由 V = 长 × 宽 × 高算出体积。在不同位置测量每一个尺寸并取平均体积——这可以均摊微小的不规则性。

For an irregular object (e.g. a stone), use the displacement method. Partially fill a measuring cylinder with water and record the initial volume V₁. Gently lower the object into the water, ensuring it is fully submerged, and read the new volume V₂. The volume of the object is V = V₂ − V₁. Tilt the cylinder and slide the object in to avoid splashing. For objects that float, use a sinker (a heavy object) to pull it under the water, measuring the total displaced volume and subtracting the sinker’s volume later.

对不规则物体（如石块），使用排水法。量筒中先装部分水，记录初始体积 V₁。将物体轻轻放入水中，保证完全浸没，读取新体积 V₂。物体的体积 V = V₂ − V₁。倾斜量筒让物体滑入，防止水花溅出。对漂浮的物体，可用沉子（一个重物）将其拉入水下，测量总排出体积，再减去沉子的体积。

12. Writing a Practical Conclusion and Evaluation | 实验报告的撰写与评估

A strong practical write‑up ends with a conclusion that directly answers the aim of the investigation. Refer to your graph or calculations: e.g. ‘The straight line through the origin confirms that current is directly proportional to voltage, giving a resistance of 47 ± 2 Ω.’ Always quote both the value and an estimate of the absolute uncertainty or percentage uncertainty.

一份扎实的实验报告最后要有直接回应探究目标的结论。引述图像或计算，例如：“通过原点的直线证实电流与电压成正比，得到电阻为 47 ± 2 Ω。”务必同时给出数值和绝对不确定度或百分比不确定度的估计。

In the evaluation, comment on the reliability of your results and suggest at least two specific improvements. For example, ‘A set‑square could be used to ensure the ruler is vertical when measuring the height of fall, reducing systematic error’ or ‘Using a light gate instead of a stopwatch would eliminate reaction‑time errors.’ Avoid generic statements like ‘do more repeats’; instead, identify an actual weakness and propose a practical fix linked to the apparatus or procedure. This critical thinking is exactly what the WJEC mark schemes reward.

在评估部分，对结果的可靠性进行评论，并提出至少两条具体的改进建议。例如：“在测量下落高度时，可使用直角尺确保尺子竖直，以减小系统误差”或“用光门替代秒表可以消除反应时间误差。”避免笼统地说“多做几次重复实验”；相反，要识别一个实际缺陷，并提出与仪器或流程挂钩的切实改进方案。这种批判性思维正是 WJEC 评分标准所鼓励的。

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Experimental Techniques Guide | 实验操作指南