Physics Required Practicals and Uncertainties
The GCSE Physics required practicals cover specific heat capacity, thermal insulation, resistance of a wire and of components in series and parallel, current-potential difference characteristics, density, force and extension, acceleration, waves in a ripple tank and in a solid, light reflection and refraction, and infrared radiation. Alongside the methods, physics expects the language of measurement, including the resolution of an instrument and the difference between random and systematic error.
Before you start
No specific prerequisites - this is a good place to start.
Method
- State the independent, dependent and control variables for the practical in question before describing the method, and give the range and interval of the independent variable, for example masses from 1 N to 5 N in 1 N steps.
- Choose apparatus by resolution and justify it: a metre rule reading to 1 mm rather than to 1 cm, a digital ammeter reading to 0.01 A, a balance reading to 0.01 g. Say why the resolution matters for the size of the quantity being measured.
- Reduce random error by repeating and taking a mean, and reduce systematic error by checking the instrument reads zero before use, for example zeroing a newtonmeter held vertically or checking the ammeter reads zero with the circuit open.
- Calculate uncertainty from repeats as half the range: uncertainty = (largest value - smallest value) / 2, quoted as the mean plus or minus that value.
- Convert to percentage uncertainty when comparing: percentage uncertainty = (uncertainty / mean) x 100. Two results overlap, and so cannot be said to differ, if their uncertainty ranges overlap.
- For a graph, plot the independent variable on the x axis, use a scale that fills more than half the grid, draw a line of best fit rather than joining the points, and take the gradient from two widely separated points on the line, not from raw data points.
Worked example
A student measures the resistance of a wire five times: 4.8, 5.0, 5.1, 4.9 and 5.2 ohms. Calculate the mean, the uncertainty and the percentage uncertainty.
- Add the readings: 4.8 + 5.0 + 5.1 + 4.9 + 5.2 = 25.0 ohms.
- Divide by the number of readings: 25.0 / 5 = 5.0 ohms, so the mean resistance is 5.0 ohms.
- Find the range: largest minus smallest = 5.2 - 4.8 = 0.4 ohms.
- Uncertainty = half the range = 0.4 / 2 = 0.2 ohms, so the result is 5.0 plus or minus 0.2 ohms.
- Percentage uncertainty = (0.2 / 5.0) x 100 = 4 percent.
- Interpret: a second wire measured as 5.1 plus or minus 0.2 ohms overlaps this range, so the two results cannot be said to be different.
Practice questions
Try each question, then tap to reveal the answer.
Q1Define the resolution of a measuring instrument.Show answer
Answer: The smallest change in the quantity being measured that the instrument can detect.
Q2How is the uncertainty in a set of repeated readings usually estimated?Show answer
Answer: As half the range: (largest reading - smallest reading) / 2.
Q3Readings have a mean of 20.0 cm and a range of 1.0 cm. Calculate the percentage uncertainty.Show answer
Answer: Uncertainty = 0.5 cm, so percentage uncertainty = (0.5 / 20.0) x 100 = 2.5 percent.
Q4Distinguish repeatability from reproducibility.Show answer
Answer: Repeatable means the same person gets the same results using the same method and equipment; reproducible means different people, or different equipment or methods, get the same results.
Q5In the specific heat capacity practical, why is the block insulated?Show answer
Answer: To reduce energy transferred to the surroundings, which would otherwise make the energy supplied appear larger than the energy that raised the block's temperature, giving too high a value for the specific heat capacity.
Q6Why should a line of best fit be drawn rather than joining the plotted points?Show answer
Answer: Individual points contain random error; a line of best fit averages them out and shows the underlying relationship.
Q7Give one reason for taking the gradient from the line of best fit rather than from two data points.Show answer
Answer: Data points may lie off the true trend because of random error, so a gradient taken between two of them can be misleading; the line of best fit uses all the data.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Describe how a student could measure the density of a small irregular stone. Include the apparatus and one precaution.
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A student obtains a value for the specific heat capacity of aluminium that is 15 percent higher than the accepted value. Explain the most likely cause and state whether it is a random or a systematic error.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the physics required practicals and uncertainties worksheet pack - 24 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 9 of GCSE Physics Workbook, the whole course as one free printable PDF.
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