OCR A-level Physics A (H556) · Module 1: Development of Practical Skills
Mini-Lesson
Development of Practical Skills
Module 1 is assessed on every written paper as well as in the Practical Endorsement. It covers planning, implementing, analysing and evaluating — and the uncertainty arithmetic that underpins all four.
% uncertainty = (absolute uncertainty ÷ measured value) × 100every number you ever quote should have an uncertainty attached to it
The examiner's favourite distinction:precision is about the spread of your repeats; accuracy is about closeness to the true value. You can be beautifully precise and completely wrong — that is exactly what a systematic error does to you.
Work through each screen, answer the questions as you go (several are full A-level calculations) and collect ⭐ stars. Press Start when you're ready.
Module 1 · planning
Planning an experiment
A good plan names the variables and defends the method:
Independent variable — the one you deliberately change. Dependent variable — the one you measure. Control variables — everything else, held constant.
Choose instruments with a suitable resolution. Measuring a 0.40 mm wire with a millimetre ruler is hopeless — use a micrometer (resolution 0.01 mm).
Choose a range and enough values (usually at least six) to establish a trend, with repeats at each.
Identify the hazards and state a proportionate control measure.
Reduce percentage uncertainty by measuring more: time 20 oscillations, not one; measure across ten interference fringes, not one; use a long wire in a Young modulus experiment. Same absolute uncertainty, ten or twenty times the quantity — so the percentage uncertainty falls by that factor.
Module 1 · errors
Random error, systematic error, precision and accuracy
Random error — unpredictable scatter either side of the true value (reaction time, judging a scale, fluctuating readings). Reduce by repeating and taking a mean.
Systematic error — shifts every reading by the same amount or in the same direction (zero error, a mis-calibrated meter, a consistently wrong angle of view). Repeating does not help at all — you must correct or recalibrate.
Zero error is the classic systematic error: check the instrument reads zero before you start, and subtract the offset if it does not.
Parallax error is systematic if you always view from the same wrong angle, random if you view carelessly from different angles each time.
Anomalies: a point well off the trend should be investigated and re-measured — never silently deleted. If it is confirmed and unexplained, say so; if it was a misread, correct it and note that you did.
Sort it
Random, systematic, or a way of reducing uncertainty?
Tap an item, then tap the category it belongs in.
🎲 Random error
📐 Systematic error
✅ Reduces uncertainty
Quick check
Precise or accurate?
?A student measures the same length five times and gets 24.7, 24.7, 24.8, 24.7 and 24.7 cm. The true value is 25.9 cm. How should the results be described?
Module 1 · uncertainty
Absolute, percentage and combined uncertainties
absolute uncertainty of a set of repeats ≈ (max − min) ÷ 2quoted about the MEAN of the repeats
To combine uncertainties through a calculation, OCR uses three rules:
Adding or subtracting → add the absolute uncertainties.
Multiplying or dividing → add the percentage uncertainties.
Raising to a power n → multiply the percentage uncertainty by n.
Worked example — g from a pendulum
g = 4π²l / T². The length l has a 2% uncertainty; the period T has a 1% uncertainty.
T is squared, so it contributes 2 × 1% = 2%.
Total: 2% (from l) + 2% (from T²) = 4%
Which measurement should you improve? The one contributing the largest percentage uncertainty — especially any quantity that is squared or cubed, since its contribution is doubled or tripled.
Calculate
Your turn — percentage uncertainty
1A current is measured as 2.50 ± 0.05 A. Calculate the percentage uncertainty.
%
Hint: (0.05 ÷ 2.50) × 100.
Calculate
Your turn — uncertainty in a mean
2A student times an event four times: 4.5, 4.7, 4.6 and 4.4 s. Estimate the absolute uncertainty in the mean using half the range. Give your answer in s.
3The mean of those four readings (4.5, 4.7, 4.6, 4.4 s) is 4.55 s with an absolute uncertainty of 0.15 s. Calculate the percentage uncertainty. Give your answer in % to 2 significant figures.
%
Hint: (0.15 ÷ 4.55) × 100 = 3.30%.
Calculate
Your turn — combining uncertainties
4The acceleration due to gravity is found from g = 4π²l / T². The length l has a percentage uncertainty of 2% and the period T has one of 1%. Calculate the percentage uncertainty in g.
%
Hint: T is squared, so it contributes 2 × 1% = 2%. Add that to the 2% from l.
Module 1 · analysis
Graphs, gradients and logarithmic plots
Rearrange every relationship into y = mx + c, plot it, and let the gradient give you the physics.
Gradient = Δy ÷ Δx, taken from a large triangle using points on the line, not raw data points.
Always include error bars, then draw the line of best fit and the steepest and shallowest lines that still pass through all the bars. Uncertainty in the gradient ≈ (mmax − mmin) ÷ 2.
power law y = kxⁿ → log y = log k + n log xplot log y against log x: gradient = n, intercept = log k
exponential y = y₀e^(−kx) → ln y = ln y₀ − kxplot ln y against x: gradient = −k
Why bother with logs? A straight line is the only shape the eye can judge reliably. Log plots turn an unfamiliar curve into a straight line, and the gradient tells you the power — which is often exactly the physics you were trying to test.
Calculate
Your turn — reading a gradient
5A straight line of best fit passes through the points (2.0, 5.0) and (8.0, 20.0). Calculate its gradient.