Mini Physics
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Writing the Experimental Solution (IPhO Experimental)

IPhO experimental lesson on presentation: diagrams, tables, graphs, calculations, conclusions, and evaluation written for marks under time pressure.

  • International Physics Olympiad preparation

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You can do correct physics and still lose marks if the examiner cannot see your logic. IPhO experimental solutions are graded like arguments: you need clear definitions, labelled diagrams, a traceable calculation path, and a conclusion that answers the question with uncertainty.

Prerequisites (quick refresh)

1. Definitions (Must Know)

  • Diagram: a labelled sketch of the setup showing what is measured and key lengths/angles.
  • Data table: recorded values with units and uncertainties.
  • Working: equations, substitutions, and intermediate results that allow checking.
  • Conclusion: final result stated with uncertainty and units, plus a comparison if relevant.
  • Evaluation: identification of dominant uncertainty sources and realistic improvements.

2. Key Ideas (What Earns Marks)

A complete write-up has a visible spine
  1. Model and what will be plotted (linear form).
  2. Diagram and measurement plan (what, how, with what uncertainty).
  3. Data table with units and uncertainties.
  4. Graph with best-fit line, gradient, uncertainty, and units.
  5. Parameter extraction, with propagated uncertainty.
  6. Final statement: parameter ± uncertainty (units), and a short evaluation.

3. Detailed Explanations

A. Diagrams that earn marks

Your diagram should show:

  • measured lengths, angles, or positions (with labels)
  • where the instrument is applied (ruler positions, sensor placement)
  • direction conventions (positive directions, current direction, motion direction)

The goal is not art; it is unambiguous geometry.

B. Tables that are easy to mark

Minimum table columns:

  • quantity name and symbol
  • unit in header
  • uncertainty in header or noted once for the column

If you have repeats, show:

  • repeated measurements
  • mean value
  • an uncertainty estimate method (e.g. half-range)

C. Calculations that are traceable

Use the pattern:

  1. write formula
  2. substitute numbers with units
  3. compute value
  4. compute uncertainty (one clear line)

Avoid a wall of arithmetic without units.

D. Conclusions and comparisons

If there is a theoretical value Q₀, compute the difference and compare it with Δ Q. A high-scoring line is:

  • “Result agrees with theory within uncertainty” or
  • “Result does not agree; likely dominated by systematic X”

4. Common Mistakes

  • No units in tables or graphs.
  • Missing statement of what slope/intercept means physically.
  • Using a gradient but never translating it into the asked parameter.
  • Writing improvements that do not address the dominant uncertainty.
  • Listing “human error” instead of naming the mechanism (parallax, drift, friction, alignment).

5. Exam Tips

  1. Use consistent symbols throughout (don’t switch L to l mid-solution).
  2. If you used a transform, state it near the graph: “Plot T² vs L”.
  3. Keep the evaluation short but specific: one systematic, one random.
  4. If time is short, prioritise: diagram, graph, slope extraction, final result.

6. Worked Examples (use Toggle)

1) A model-to-graph sentence that clarifies everything

Suppose your model predicts V = IR and you want R.

Write: “Vary I and measure V. Plot V (V) against I (A). The gradient equals R (ohm).”

This single sentence tells the examiner what the graph is doing and what the slope means.

2) A clean conclusion template (with comparison)

Example structure:

  • “From the gradient, R = (12.6 ± 0.4) Ω.”
  • “The accepted value is 12.0 Ω; the difference is 0.6 Ω which is within the uncertainty.”
  • “Dominant uncertainty is from the voltmeter resolution; using a higher-resolution voltmeter or a larger range of voltages would reduce this.”

The same structure works for g, f, k, ρ, and other parameters.

7. Mind Stretchers (use Toggle)

1) What do you write when results disagree beyond uncertainty?

Your result is Q = 1.32 ± 0.03 but theory says Q₀ = 1.40.

A good response is not “mistakes were made”. It is:

  • identify a plausible systematic (e.g. calibration offset, friction, heat loss)
  • state direction of effect (would lower/raise Q)
  • propose a fix (calibrate, reversal, improved insulation, longer timing, etc.)

8. Practice

  1. Write a 4-sentence “spine” for measuring k of a spring using a graph. Include what you plot and what the slope gives.
  2. You measured g = (9.52 ± 0.15) m s⁻². Comment on agreement with g₀ = 9.81 m s⁻².
  3. List one systematic and one random uncertainty for a lens focal length experiment, and one realistic improvement for each.

Back to IPhO Experimental Skills Hub

Answer key and feedback

  • A complete systematic-effect statement names the affected measurement, propagates the sign through the equation and concludes explicitly that the reported Q would be raised or lowered. Merely naming an error source earns no causal feedback.
Syllabus and review details

No official syllabus alignment is listed for this lesson.