Measurement, Practical Skills and Laboratory Safety
This domain tests the craft of experimental physics: units and dimensions, instrument choice and reading, uncertainty treatment, graph work, and safe practice in the school laboratory. It is the domain most often lost on small slips rather than on physics.
Core rules
- SI base units: metre, kilogram, second, ampere, kelvin, mole, candela. Prefixes run in steps of 10³ (k, M, G above; m, μ, n below).
- Dimensional analysis checks equations but cannot prove them, since dimensionless constants are invisible to it.
- Precision means repeated readings agree; accuracy means readings are close to the true value. Random errors scatter and are reduced by averaging repeats; systematic errors shift every reading and are corrected by calibration or by checking and applying zero error.
- Vernier callipers typically read to 0.1 mm (some to 0.02 mm); a micrometer screw gauge reads to 0.01 mm. Zero error is subtracted with its sign, so a negative zero error is added to the reading.
- Uncertainties: add absolute uncertainties for sums and differences; add percentage uncertainties for products and quotients; multiply the percentage uncertainty by n when a quantity is raised to the power n.
- Graphs: plot the dependent variable on the y-axis, draw a single straight line of best fit, and give the gradient in its derived units. Use a false origin only when labelled and justified.
- Safety: use low-voltage d.c. supplies with no water nearby; handle mercury over a tray and report spills; use tongs for radioactive sources, store them in lead, minimise exposure time and never eat in the laboratory; never look along a laser beam; handle hot objects with insulated gloves and broken glass with a dustpan, not hands.
Worked example
A wire of length 2.000 m ± 0.002 m and diameter 0.50 mm ± 0.01 mm is used to find resistivity. Percentage uncertainty in length = 0.002/2.000 × 100 = 0.1%. Percentage uncertainty in diameter = 0.01/0.50 × 100 = 2.0%. Area depends on diameter squared, so its percentage uncertainty is 2 × 2.0% = 4.0%. Since ρ = RA/L, the percentage uncertainty in ρ is about 4.0% + 0.1% ≈ 4.1%. The diameter dominates, so improving the diameter measurement is where effort should go.
Traps that cost marks
- Reading a micrometer to 0.1 mm, or misreading the vernier's main scale.
- Applying a zero error with the wrong sign.
- Adding percentage uncertainties when dividing quantities rather than adding them - and forgetting to double for a squared quantity.
- Drawing a join-the-dots zigzag instead of a line of best fit.
- Stating a gradient without units.