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

Quick recall for sections 3.3 Planning and 3.4 Implementation and measurements of the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics (YPH11) specification, Issue 3. Unit 3 is an International AS (IAS) unit and tests experiments from Units 1 and 2. The study guide has the full explanations and worked examples.

Related pages: Edexcel A Level Physics hub, checklist, practice set and the free diagnostics.

Planning (3.3): the ten-point checklist

Point	What to write
Apparatus	Every item, with the quantity it measures
Range and resolution	Check each instrument can cope with the size of the quantity
Calibration	Zero check before readings; correct for any zero error
Measuring	Technique, not just the instrument
Controls	Name each other variable and how it is held constant
Repeats	Appropriate where there is random scatter
Safety	Hazard and precaution as a pair
Use of data	What is plotted and how the result is found
Uncertainty and systematic error	Source, size, and how to reduce or remove it
Implications and context	Benefit and risk, social, environmental or historical

Instruments

Instrument	Resolution	Uncertainty in a single reading
Metre rule	1 mm	0.5 mm
Vernier calipers	0.1 mm	0.05 mm
Micrometer screw gauge	0.01 mm	0.005 mm

The uncertainty in a single reading is half the resolution, as the specification’s Appendix 10 explains.

Reading a micrometer

Sleeve reading (to the nearest 0.5 mm) + thimble reading (hundredths of a millimetre) − zero error.

Example: sleeve 2.0 mm, thimble 45, zero error +0.03 mm. 2.0 + 0.45 = 2.45 mm; 2.45 − 0.03 = 2.42 mm.

Reading Vernier calipers

Main-scale reading before the Vernier zero + 0.1 mm × the number of the Vernier mark that lines up.

Example: 32 mm and the seventh mark aligned gives 32.7 mm.

Calibration and zero error

• A zero error is systematic: every reading is shifted by the same amount.
• Positive zero error (reads above zero when it should read zero): subtract it.
• Negative zero error: add its size.
• Repeating and averaging does not remove it.
• Check before the experiment, not after.

Example: a voltmeter reads −0.04 V when disconnected. A reading of 1.12 V corrects to 1.12 + 0.04 = 1.16 V.

Fair test

Independent variable	Dependent variable	Examples of controls
Length of wire	Resistance	Same wire, low current, short switch-on time, same temperature
Height of fall	Time of fall	Same object, same release method, same timer
Load on spring	Extension	Same spring, ruler vertical, same zero point

Repeats

Appropriate: timing by hand, varying readings, scatter in diameter along a wire. Not useful: a zero error, or a digital reading that does not change.

Method: take at least three, discard any anomaly only with a reason, average the rest.

Uncertainty quick rules

Situation	Uncertainty
One reading	Half the resolution
Repeated readings	Half the range
Percentage uncertainty	(uncertainty ÷ measurement) × 100%

Worked reminder: times of 12.4 s, 12.8 s and 12.6 s give a mean of 12.6 s. Half range = (12.8 − 12.4) ÷ 2 = 0.2 s. Percentage uncertainty = 0.2 ÷ 12.6 × 100% = 1.6%.

Reminder: a percentage uncertainty is a good way to compare which measurement limits the result. A 0.01 mm micrometer on a 0.45 mm wire gives about 1.1% from a half-resolution of 0.005 mm. A ruler on the same wire gives 111%, so it is useless.

Ways to reduce uncertainty:

• Use an instrument with a finer resolution.
• Measure a larger quantity (a longer length, many swings timed together).
• Repeat and average.
• Use a graph gradient instead of a single calculated value.

Safety, as hazard and precaution

Hazard	Precaution
Wire gets hot	Low current, switch off between readings
Mass falls	Sand tray, goggles, feet clear
Spring or wire snaps	Goggles, keep loads within limit
Liquid spill (glycerol, oil)	Wipe at once; slip hazard
Mains or high-voltage supply	Use a low-voltage supply; check leads for damage

Implications and context

Give one benefit, one risk and one setting. Example: reliable resistivity data lets engineers choose a cable material that wastes less energy as heat (benefit, social and environmental), but extracting and refining the metal has costs for land and energy (risk, environmental).

Checking another student’s work (3.4)

Use the same four questions every time, then finish with improvements.

1. Number of readings: at least six for a graph; one reading per value means scatter is hidden.
2. Range: wide enough to show the trend and fix the gradient; limited by the limit of proportionality.
3. Significant figures: same decimal places for one variable; match the resolution of the instrument.
4. Inconsistent readings: check, repeat, and explain; do not simply delete.
5. Improvements: name the apparatus and what it fixes.

Typical improvements to quote:

• A set square to make sure a ruler is vertical when measuring the extension of a spring.
• A pointer or fiducial marker, and eye level with the scale, to avoid parallax.
• A micrometer instead of a ruler for a thin wire.
• A light gate or data logger instead of a stopwatch for short times.
• Repeating at each setting and averaging.

Anomaly worked reminder

Data: load 1.0, 2.0, 3.0, 4.0, 5.0 N; extension 11, 22, 33, 54, 55 mm. The first three points give 11 mm per newton, so at 4.0 N the line predicts 44 mm. The recorded 54 mm is 10 mm too high. Check the reading for a slipped digit, re-measure and repeat.

Must-know distinctions

Pair	Difference
Resolution and uncertainty	Resolution is the smallest interval shown; uncertainty in a single reading is half of it
Accuracy and precision	Accuracy is closeness to the true value; precision is the closeness of repeated readings
Random and systematic	Random scatter is reduced by repeating; systematic shift is not
Repeatable and reproducible	Same operator and method, against different operators or apparatus
Error and mistake	An error is the difference from the true value, not a slip by the operator

Quick self-test

1. Which instrument should measure the diameter of a 0.45 mm wire, and what is its resolution?
2. A micrometer shows 2.0 mm on the sleeve and 45 on the thimble. The zero error is +0.03 mm. State the diameter.
3. Vernier calipers show 32 mm on the main scale with the seventh mark aligned. Give the reading and its percentage uncertainty (half the resolution).
4. Times are 12.4 s, 12.8 s and 12.6 s. Give the mean time with its uncertainty.
5. A voltmeter reads −0.04 V when disconnected. Correct a reading of 1.12 V.
6. Why does repeating a reading not remove a zero error?
7. A 250 mm length is measured once with a ruler of 1 mm resolution. What is the percentage uncertainty?
8. Give two actions that reduce the parallax error when reading a scale.
9. A student gives extensions 3.0 cm, 4.5 cm and 6.25 cm using a ruler marked in mm. What is wrong?
10. Why is a graph gradient better than one calculated value?
11. A point sits well off the line. What are the first two actions?

Answers

1. Micrometer screw gauge, 0.01 mm.
2. 2.0 + 0.45 = 2.45 mm; 2.45 − 0.03 = 2.42 mm.
3. 32.7 mm; 0.05 ÷ 32.7 × 100% = 0.15%.
4. 12.6 s ± 0.2 s.
5. 1.12 + 0.04 = 1.16 V.
6. It shifts every reading the same way, so each repeat contains the same shift.
7. 0.5 ÷ 250 × 100% = 0.2%.
8. Read with the eye level with the scale; put the pointer close to the scale, or use a mirror strip.
9. The last value has too many decimal places: 6.25 cm cannot be read with a mm ruler, so quote 6.2 or 6.3 cm.
10. It uses all the readings, averages out random scatter and shows anomalies.
11. Check the reading and calculation, then repeat the measurement.

Where marks are usually lost

• Writing “wear goggles” with no hazard named.
• Giving “use a more accurate instrument” without naming the instrument or its resolution.
• Reading a micrometer correctly but forgetting the zero error.
• Quoting readings to different numbers of decimal places in one column.
• Saying “do it again” without saying how many repeats or what the mean is for.
• Suggesting “human error” instead of a specific cause such as parallax or reaction time.
• Deleting an anomalous point with no check or reason.
• Not stating the quantity plotted and what the gradient represents.

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.