Measurement and Observation
PhysicsΒ· Unit 15: Practical Skills (AS)Β· 15 min read
1. Measuring Instruments and Precisionβ β ββββ± 5 min
All practical measurements require selecting an instrument with appropriate precision for the quantity being measured. The precision of an instrument is set by its smallest scale division, which also determines the absolute uncertainty in a single raw reading.
Smallest Scale Division
The smallest increment of measurement marked on an instrument's scale, equal to the absolute uncertainty in one raw reading from the instrument
Example:
A standard metre ruler has a smallest division of 1 mm, so uncertainty is mm.
State the appropriate instrument and uncertainty for measuring: (a) the diameter of a thin copper wire, (b) the length of a 25 cm wire sample, (c) the mass of a 50 g mass hanger.
- 1
(a) A thin copper wire typically has a diameter of 0.1β0.5 mm. The appropriate instrument is a micrometer screw gauge, with a smallest division of 0.01 mm. Uncertainty is mm.
- 2
(b) A 25 cm wire length can be measured accurately with a metre ruler, which has a smallest division of 1 mm. Uncertainty is mm.
- 3
(c) A 50 g mass hanger is weighed on a standard top-pan balance with 0.1 g precision, so uncertainty is g.
Exam tip:
Always match instrument precision to the size of the measured quantity; examiners penalise selecting an unnecessarily imprecise instrument.
2. Systematic Errors and Zero Error Correctionβ β β βββ± 5 min
Systematic errors are consistent, repeatable errors that shift all measurements in the same direction from the true value. They cannot be reduced by averaging repeated measurements, but they can be completely eliminated by correction.
Zero Error
A common systematic error that occurs when an instrument gives a non-zero reading when the true measured quantity is zero
Example:
A micrometer that reads +0.02 mm when its jaws are fully closed has a positive zero error of +0.02 mm.
A student measures the diameter of a wire with a micrometer. When the jaws are closed, the reading is mm. The measurement on the wire is 2.47 mm. Calculate the corrected diameter.
- 1
First, identify the zero error: the reading at zero is mm, so the zero error is mm.
- 2
- 3
Substitute values to get the corrected diameter: mm.
3. Accuracy, Precision and Significant Figuresβ β β βββ± 6 min
CIE exams regularly test the distinction between accuracy and precision, and the correct use of significant figures when recording measurements. These are easy marks if you remember the key definitions.
Property | Affected by | How to reduce error |
|---|---|---|
Accuracy | Systematic errors | Calibrate instruments, correct zero error |
Precision | Random errors | Average multiple repeated measurements |
A student takes 5 measurements of a pendulum's period: 1.42 s, 1.43 s, 1.41 s, 1.44 s, 1.42 s. The true accepted value is 1.31 s. Comment on the accuracy and precision of the measurements.
- 1
First, check the spread of repeated measurements: all values are within 0.03 s of each other, so measurements are very closely grouped.
- 2
Next, calculate the average measurement and compare it to the true value: s, which is 0.11 s higher than the true value of 1.31 s.
- 3
Conclusion: The measurements are precise (closely grouped) but not accurate (far from the true value).
4. Techniques for Reliable Observationsβ β ββββ± 4 min
Many CIE practical questions ask you to describe techniques to improve measurement reliability. Common reliable techniques for different experiments are:
Repeat measurements and calculate an average to reduce the effect of random error
Align instruments correctly to avoid parallax error when reading scales
Measure multiple identical small quantities (e.g. 10 pendulum oscillations, 10 sheets of paper) to reduce percentage uncertainty
Wait for a system to stabilise (e.g. a thermometer to reach thermal equilibrium) before taking a reading
Describe a technique to reduce the percentage uncertainty when measuring the period of one pendulum oscillation.
- 1
Instead of timing a single oscillation, time 10 complete full oscillations, starting and stopping the stopwatch as the pendulum passes a fixed reference point (usually the lowest point of the swing, where speed is highest to reduce timing error).
- 2
Divide the total measured time by 10 to get the period of one oscillation.
- 3
This increases the total measured time, so the absolute timing uncertainty from the stopwatch becomes a smaller percentage of the total measurement, reducing overall percentage uncertainty.
5. Common Pitfalls
Wrong move:
Recording raw measurements to more significant figures than the instrument's precision allows
Why:
Examiners expect raw readings to match the instrument's precision; extra significant figures are incorrect
Correct move:
Record raw readings to the same number of decimal places as the instrument's smallest division
Wrong move:
Incorrect zero error correction: adding a negative zero error instead of subtracting it
Why:
The correction formula is often misremembered, leading to wrong values
Correct move:
Use the formula: . For a zero error of mm, this becomes mm
Wrong move:
Claiming averaging reduces systematic error
Why:
Averaging only reduces random error; systematic errors are consistent across all readings
Correct move:
State that systematic errors are reduced by calibration or zero error correction, not averaging
Wrong move:
Measuring a single small quantity instead of multiple to reduce uncertainty
Why:
The absolute uncertainty from the instrument becomes a large percentage of the small reading, leading to high overall uncertainty
Correct move:
Measure 10 or 20 repeats of the small quantity, divide the total by the number of repeats to get a lower percentage uncertainty
Wrong move:
Assuming precise measurements are automatically accurate
Why:
Precision and accuracy are independent properties
Correct move:
Always evaluate accuracy against the true value and precision against the spread of repeats separately
6. Quick Reference Cheatsheet
Instrument / Term | Key Value / Definition |
|---|---|
Metre ruler | Smallest division: 1 mm, Uncertainty: mm |
Vernier caliper | Smallest division: 0.1 mm, Uncertainty: mm |
Micrometer screw gauge | Smallest division: 0.01 mm, Uncertainty: mm |
Stopwatch | Smallest division: 0.01 s, Uncertainty: s |
Accuracy | Closeness of measurement to true value |
Precision | Closeness of repeated measurements to each other |
Zero error correction |
7. Frequently Asked
How many significant figures should I record for raw measurements?
Raw measurements should be recorded to the precision of the measuring instrument, i.e. the same number of decimal places as the instrument's smallest division. For example, a 1 mm ruler gives readings to 1 decimal place in cm, or 3 significant figures for lengths under 1 m.
Can I get full marks if I mix up accuracy and precision?
No. CIE examiners explicitly test this distinction, and you will lose marks if you confuse the two terms. Always remember: accuracy is about the true value, precision is about repeatability.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Select instruments for density measurement
- 2023 Β· 2
Zero error correction for micrometer
- 2024 Β· 2
State significant figures for raw data
Going deeper
What's Next
Mastering measurement and observation is the first critical step to succeeding in all CIE AS Physics practical assessments. The skills you learn here underpin every experiment you will carry out or analyse in practical papers, from simple density measurements to more complex oscillations and electricity experiments. Understanding how errors affect your measurements will also help you when you analyse uncertainties and plot graphs of your results, which make up the majority of the marks in practical papers. Next, you will build on this foundation by learning how to calculate and combine uncertainties in processed data, and how to identify sources of error in different experiment types.