Paper 5 Planning, Analysis & Evaluation — common mistakes

In Q2, when calculating the uncertainty in a derived quantity like $k = x/y^{2}$, first find the percentage uncertainty: $\%Δk = \%Δx + 2(\%Δy)$. Then, convert this back to an absolute uncertainty using $Δk = (\%Δk / 100) * k$. Examiners look for this correct propagation of uncertainties.

In Paper 5 Question 1 (Planning), when asked to evaluate or suggest improvements to an experiment, consider sources of systematic and random errors. For example, suggesting the use of a more precise instrument (e.g., a micrometer instead of a ruler) addresses random error, while suggesting a method to check for a zero error addresses a systematic error.

Always use a 'large triangle' when calculating a gradient from a graph. This means the points you choose on the line should be separated by at least half the length of the drawn line. This minimises the uncertainty in your calculation.

No, this is a common misconception. For Cambridge A-Level Physics, you are almost always expected to calculate and draw error bars for the y-axis quantity only. The uncertainty in the independent variable (x-axis) is usually considered negligible in comparison. Only draw x-axis error bars if the question explicitly instructs you to or if their uncertainty is unusually large and significant.

Your diagram must be a clear, labelled line drawing showing the essential apparatus in its correct arrangement for the experiment. It does not need to be artistic, but it must be functional and unambiguous. All key components mentioned in your method must be present and labelled (e.g., power supply, voltmeter, ammeter, variable resistor, test wire, metre rule). If a circuit is involved, a separate, standard circuit diagram is often the clearest approach.

The WAL, by definition, must pass through every single error bar. If yours does not, it is not a valid 'worst' line and will not score marks. The most common error is drawing a line that is simply 'a bit steeper' than the best fit line. The correct technique is to 'pivot' your ruler on an extreme of an error bar at one end of the graph (e.g., the very top of the first error bar) and rotate it until it just touches an extreme of an error bar at the other end (e.g., the very bottom of the last error bar). This ensures it cuts through all error bars in between.