Pearson Edexcel International A Level Physics: Unit 3: planning and checking an experiment (YPH11)

This guide is aligned to the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics (YPH11) specification, Issue 3, and covers sections 3.3 Planning and 3.4 Implementation and measurements. Unit 3 is an International AS (IAS) unit. The specification gives it as a written paper of 1 hour 20 minutes and 50 marks, with a minimum of 20 marks awarded for mathematics at Level 2 or above, and it tests the experiments you meet while studying Units 1 and 2.

Use the Edexcel A Level Physics hub for the full course and the printable checklist to tick off what you can do. Pair this guide with the revision notes and the practice set. If you are not sure where your practical skills stand, take a free 10-minute test from the diagnostics page.

What this unit covers

Section	Skills tested
3.3 Planning	Plan an experiment that Pearson sets, without carrying it out: apparatus, range and resolution, calibration, measuring variables, controls, repeats, safety, use of data, uncertainty and systematic error, and the implications and context of the physics
3.4 Implementation and measurements	Judge an experiment done by an inexperienced student: number of readings, range, significant figures, inconsistent readings, and improvements

The experiments come from the Core Practicals in Units 1 and 2, for example free fall (Core Practical 1), the falling-ball method for viscosity (2), the Young modulus (3), the speed of sound in air (4), a vibrating string (5), the wavelength of laser light (6), resistivity (7) and e.m.f. and internal resistance (8). Processing results (3.5) has its own pages. The physics behind them is in Mechanics, Materials and Electric circuits.

3.3 Planning an experiment

A good plan reads like instructions that another student could follow without asking you anything. Work through the same checklist each time.

Apparatus, range and resolution

List each instrument and say what it measures. Then check its range (the smallest to largest value it can read) and its resolution (the smallest interval it shows). The specification names two instruments:

Instrument	Resolution	Typical use
Metre rule	1 mm	Lengths of wires, heights of fall
Vernier calipers	0.1 mm	Diameter of a ball, a rod or a tube
Micrometer screw gauge	0.01 mm	Diameter of a thin wire

Choose the instrument whose resolution is small compared with the quantity. A wire of diameter 0.30 mm measured with a ruler graduated in mm is hopeless, because the reading error is bigger than the wire. A micrometer gives a percentage uncertainty of about 2%.

Reading a micrometer: add the sleeve reading (including the half-millimetre mark if it is showing) to the thimble reading in hundredths of a millimetre. A sleeve reading of 1.5 mm and a thimble reading of 27 gives 1.5 + 0.27 = 1.77 mm.

Reading Vernier calipers: take the main-scale reading just before the zero of the Vernier scale, then add 0.1 mm for each Vernier division number that lines up with a main-scale mark. A main scale reading of 24 mm with the third Vernier mark aligned gives 24.3 mm.

Calibration and zero error

Before you take readings, check that each instrument reads zero when it should. Close the micrometer jaws without force, and use the ratchet. If it reads 0.02 mm, every reading is too large by 0.02 mm, so subtract it. This is a systematic error: it shifts all readings the same way and repeating readings does not remove it. A meter that does not read zero with nothing connected needs the same correction.

Measuring variables with correct technique

Say how, not just what. Examples of specific technique:

• Use a set square or plumb line to make sure a ruler is vertical.
• Read a scale with your eye level with the pointer to avoid parallax.
• Measure the diameter of a wire at several places along it and at right angles to each other, because wire is not perfectly uniform or circular.
• Time many swings or oscillations, then divide, when the single interval is short.
• Use a fiducial marker (a fixed pointer or a bit of tape) as a reference for starting and stopping a timer.

Control variables and fair testing

Name the independent variable, the dependent variable and every other variable that could change the result, then say how you keep each constant. Resistance of a wire changes with temperature, so a plan to find resistivity should keep the current small and switch the supply off between readings so that the wire stays at room temperature. In a falling-ball viscosity plan, the liquid temperature, the ball and the depth of liquid should stay the same.

Repeat readings

Repeats are appropriate when random scatter is likely, such as timing with a stopwatch. Take at least three, find the mean, and look for readings that stand out. Repeating is pointless for a systematic error such as a zero error, and it is not worth repeating a reading from a digital meter that does not change. If a reading depends on a variable you set (for example the length of wire), repeat the whole set and average each pair.

Safety

Name the hazard, then the precaution. Hot wires: low current and short switch-on time. Falling masses: a sand tray and goggles, with feet clear. Springs and wires that may snap: goggles. Spilled glycerol: wipe up at once, because it is a slip hazard. “Be careful” earns nothing.

Using the data

Say what you will plot and how you get the result from it. For a wire, resistance R against length l gives a line through the origin whose slope is ρ/A, so ρ = gradient × A. For free fall from rest, s = ½gt², so a graph of height s against t² has gradient g/2. A graph averages out random scatter and shows an anomalous point.

Uncertainty and systematic error

Quote the uncertainty for each measurement as follows (this matches the specification’s Appendix 10):

• Single reading: half the resolution of the instrument.
• Several readings: half the range of the readings.
• Percentage uncertainty = (uncertainty ÷ measurement) × 100%.

Then say how to reduce it: a better instrument, a bigger quantity (a longer length, many oscillations), or repeats. For systematic error, say what you will check and how you will correct for it.

Implications and context

Be ready to say one benefit and one risk of the physics, and to place it in a social, environmental or historical setting. For example, resistivity data helps engineers choose a material for power cables: a lower resistivity wastes less energy as heat, but mining and refining metals for cables has an environmental cost.

Worked example 1: planning the resistivity of a wire

Task: determine the resistivity of a constantan wire (Core Practical 7).

1. Apparatus: wire about 1 m long taped to a metre rule, micrometer, power supply (low voltage), ammeter, voltmeter, switch, leads, crocodile clips.
2. Diameter: close the micrometer and note its zero error. Measure d at three places, rotating the wire between readings. Suppose the readings are 0.31, 0.32 and 0.31 mm and the zero error is +0.02 mm. Corrected readings are 0.29, 0.30 and 0.29 mm, mean 0.29 mm, half range 0.005 mm, so the percentage uncertainty is 0.005 ÷ 0.29 × 100% = 1.7%.
3. Length: a ruler with 1 mm resolution has uncertainty 0.5 mm. For 800 mm this is 0.0625%, so the diameter dominates the uncertainty.
4. Method: connect one clip at 0 and move the other to lengths of 200, 300, 400, 500, 600, 700 and 800 mm. At each length close the switch, read V and I, switch off, and calculate R = V ÷ I.
5. Controls: low current and short switch-on time keep the temperature constant; use the same wire throughout.
6. Data: plot R against l. The gradient is ρ/A, with A = πd²/4 = 6.6 × 10⁻⁸ m² for d = 0.29 mm, so ρ = gradient × A. A gradient of 7.4 Ω m⁻¹ gives ρ = 4.9 × 10⁻⁷ Ω m.

3.4 Implementation and measurements

In this part of the paper you are given the method or results of an inexperienced student. You act as the checker. Examine four things.

Number of readings

Too few readings cannot show a trend or reveal an anomaly. Three or four points can be fitted by many lines, so for a graph take at least six well spaced values.

Range of measurements

The range should be as wide as the apparatus allows, though the limit of proportionality caps it for a spring or wire. A narrow range makes the gradient uncertain and cannot show whether the line goes through the origin.

Significant figures

Record each reading to the resolution of the instrument, so every reading of one variable has the same number of decimal places. A micrometer reading should be 0.30 mm, not 0.3 mm. Never give a calculated answer with more significant figures than the data supports. The specification says the number of significant figures should usually match the instrument with the fewest in its reading, and that processed values are usually taken to 3 significant figures.

Inconsistent readings

A point off the line is a prompt to check, not to discard. Recalculate it, compare it with neighbours, and repeat the measurement if possible. Look for a misread scale, a transposed digit, a slipped clip, or the load beyond the limit of proportionality. Only exclude it with a reason.

Worked example 2: checking a spring experiment

A student hangs masses from a spring and records extensions with a ruler held next to it.

Load / N	1.0	2.0	3.0	4.0	5.0	6.0
Extension / mm	12	24	36	59	60	72

• Number: six readings is acceptable, but only one reading at each load, so scatter cannot be judged.
• Range: 1.0 to 6.0 N is reasonable if the spring is still within its limit of proportionality.
• Anomaly: the first three points give 12 mm per newton. At 4.0 N the line predicts 48 mm, but 59 mm was recorded, 11 mm too high. A transposed or misread tens digit (49 read as 59) is likely. Repeat the reading.
• Without that point the stiffness is k = 1.0 ÷ 0.012 = 83 N m⁻¹.
• Improvements: use a set square to check that the ruler is vertical, put a pointer on the spring and read at eye level, and repeat with loads taken off as well as put on to check for hysteresis.

Worked example 3: a micrometer diagram

A micrometer measuring a wire shows 1.5 mm on the sleeve and the thimble line at 27. The jaws read +0.02 mm when closed. Reading = 1.5 + 0.27 = 1.77 mm; corrected diameter = 1.77 − 0.02 = 1.75 mm. State it to 3 significant figures (2 decimal places), since the micrometer resolves 0.01 mm.

Common errors

• Writing “repeat and average” without saying how many repeats.
• Quoting a micrometer reading to one decimal place.
• Adding a positive zero error instead of subtracting it, or calling it a random error.
• Discarding an anomalous point without checking it.
• Listing safety as “wear goggles” with no hazard named.
• Giving “human error” as an improvement point instead of a specific cause.

Next steps

Do the practice set under timed conditions, then use the revision notes to fix the gaps.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics, specification Issue 3 (July 2021), Pearson Education Limited: Unit 3 sections 3.3 and 3.4, with Appendix 10 on uncertainties and practical work.