All subjects · Physics · WPH16 Practical Skills in Physics II

Evaluating experiments and conclusions at A2

Edexcel IAL Physics · WPH16 Practical Skills in Physics II

The last part of a Unit 6 question usually asks you to compare, conclude or improve. These marks need numbers and explicit statements.

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Comparing with an accepted value

1. Find the percentage uncertainty of your result (compounded correctly).

2. Calculate the upper and lower limits of your result.

3. State whether the accepted value lies within them, and conclude.

Alternatively, compare the percentage difference (accepted value on the bottom) with your percentage uncertainty: if %D is less than %U the result is consistent. WPH16 mark schemes accept either method, but you must state both numbers being compared. Examiners consistently penalise limits calculated from the accepted value, and conclusions with no explicit comparison.

Spotting systematic error

The accepted value lies outside your limits, and repeats agree closely: suspect a systematic error.
An intercept where the theory predicts none.
A result consistently too big or too small (heat losses make a specific heat capacity too large; friction makes a measured g too small).

Say which way the error pushes the result and why.

Improvements

Target the measurement with the largest percentage uncertainty (remember powers).
Distinguish a technique (how to use existing apparatus better) from a modification (new or different apparatus), and give what the question asks for.
Link each improvement to the error it reduces: 'measure the diameter with a micrometer (resolution 0.01 mm) instead of calipers, reducing the largest percentage uncertainty'.
Extend the range or number of readings when data is insufficient.

Worked example

A student finds L = 3.48 x 10⁵ J/kg ± 3% for ice. The accepted value is 3.34 x 10⁵ J/kg. Evaluate.

Limits: 3.48 x 0.97 = 3.38 and 3.48 x 1.03 = 3.58 (x 10⁵ J/kg)
3.34 x 10⁵ J/kg is below the lower limit, so the result is not consistent.
The result is too large: likely energy from the room melting extra ice (no control funnel, or the control was not identical).

Answer: Not consistent; a systematic error from heat gained from the surroundings, removed with a control funnel

Key idea: Limits from your own compounded uncertainty, compared explicitly with the accepted value. Systematic errors push results one way. Improve the largest uncertainty, and give techniques or modifications as asked.

Check yourself

A result is 3.48 x 10⁵ J/kg with a percentage uncertainty of 3%. Find the lower limit, in units of 10⁵ J/kg, to 3 s.f.
Show answer
3.38 x 10⁵ J/kg. 3.48 x 0.97 = 3.3756, so 3.38 x 10⁵ J/kg.
Measured k = 25.0 N/m ± 4% (limits 24.0 to 26.0 N/m). The manufacturer states 25.5 N/m. What is the conclusion?
- Consistent: 25.5 N/m lies within the limits
- Not consistent, as the values differ
- Not consistent, as 4% is too large
- No conclusion is possible
Show answer
Consistent: 25.5 N/m lies within the limits. 25.5 lies between 24.0 and 26.0.
A measured specific heat capacity is 4450 J kg^-1 K^-1; the accepted value is 4180 J kg^-1 K^-1. Find the percentage difference, to 2 s.f.
Show answer
6.5 %. |4450 - 4180| / 4180 x 100 = 6.5%.
A measured specific heat capacity is consistently larger than the accepted value. What is the most likely cause?
- Random errors in the temperature readings
- Energy lost to the surroundings, so less of the input energy heats the block
- Too many readings
- A thermometer with a fine resolution
Show answer
Energy lost to the surroundings, so less of the input energy heats the block. Losses mean the temperature rise is smaller than it should be, so c = E / mΔθ comes out too large.
Percentage uncertainties: time 0.5%, mass 0.1%, diameter 1.5% (cubed in the formula). Which improvement helps most?
- Time more oscillations
- Use a better balance
- Repeat the mass measurement
- Measure the diameter with a finer-resolution instrument
Show answer
Measure the diameter with a finer-resolution instrument. The diameter contributes 3 x 1.5% = 4.5%.
A question asks for a modification to improve the timing of a falling ball. Which answer fits?
- Read the stopwatch at eye level
- Use light gates connected to a data logger
- Start timing on a countdown
- Take repeat readings
Show answer
Use light gates connected to a data logger. A modification adds or changes apparatus; the others are techniques.
A graph of p against 1/V has a small positive intercept on the p axis when theory predicts it should pass through the origin. Suggest what this indicates.
Show answer
A systematic error, such as a zero error on the pressure gauge (or the volume of gas in the connecting tube not being included), which adds a constant to every reading.. An unexpected intercept is the signature of a systematic error.

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Last updated 8 November 2026. Spotted a mistake? Tell us.