All subjects · Physics · WPH13 Practical Skills in Physics I

Planning: variables, range and a fair test

Edexcel IAL Physics · WPH13 Practical Skills in Physics I

Every plan starts by sorting out the variables. Getting this right makes the rest of the plan (instruments, range, graph) fall into place.

Practise this in the app: instant marking and a fix for every mistake. Free, no email needed to start.

Practise this lesson

Three kinds of variable

Independent variable: the one you deliberately change (for example the length of a wire).
Dependent variable: the one you measure to see the effect (the resistance).
Control variables: everything else that could affect the dependent variable, kept constant (temperature of the wire, the material, its diameter).

For each control variable, say how you keep it constant: 'use the same wire', 'switch off between readings so the wire does not heat up'. Naming it alone is often not enough.

Range and number of readings

Take at least six different values of the independent variable for a graph, spread evenly over the widest sensible range.
A wide range makes the trend clear and makes the gradient more reliable.
Repeating each reading and taking a mean reduces the effect of random errors. Be clear which you mean: repeats of the same value for a mean, or different values for a graph. Examiners penalise plans that blur the two.
The specification asks you to discuss whether repeat readings are appropriate. They are not always possible: a reading taken during a continuous change (a cooling liquid, a discharging capacitor) cannot be repeated under the same conditions, so you repeat the whole run instead.
State numbers: 'lengths from 0.20 m to 1.00 m in steps of 0.20 m, three repeats each', not 'many readings'.

Linking to the physics

A good plan always says how the result is obtained: which graph to plot and what its gradient or intercept means. For example, for R = ρl / A, plot R against l; gradient = ρ / A, so ρ = gradient x A.

Wider context

Planning questions can ask about the benefits and risks of the physics, or its wider use, for example why measuring the viscosity of oil matters for engines, or why calibrating sensors matters in medicine. Keep these answers concrete and tied to the context.

Worked example

Plan the variables for investigating how the period of a simple pendulum depends on its length.

Independent: length, from the point of suspension to the centre of the bob, six values from 0.30 m to 1.30 m.
Dependent: period T, from the time for 20 oscillations divided by 20.
Controls: same bob (mass), small amplitude (under about 10°), same point of suspension, no draughts.

Answer: Plot T² against length: a straight line through the origin, gradient 4π²/g

Key idea: Change one variable, measure one, control the rest (and say how). At least six values over a wide range; distinguish repeats from extra values; say which graph and what the gradient gives.

Check yourself

A student measures the extension of a spring for different hanging masses. What is the independent variable?
- The hanging mass
- The extension
- The spring constant
- The length of the clamp
Show answer
The hanging mass. The mass is what the student chooses to change.
In core practical 7 (resistivity), which is a control variable that must be kept constant?
- The length of wire between the contacts
- The resistance of the wire
- The temperature of the wire
- The current at each length
Show answer
The temperature of the wire. Resistance depends on temperature, so the wire must not be allowed to heat up.
Which statement about a control variable would gain credit in a plan?
- Keep the temperature constant by switching off the current between readings
- Keep everything the same
- Make it a fair test
- Control all the variables carefully
Show answer
Keep the temperature constant by switching off the current between readings. A creditworthy answer names the variable and says how it is controlled.
How many different values of the independent variable should a plan include for a reliable straight-line graph?
- Two
- Three
- Exactly ten, never more or fewer
- At least six
Show answer
At least six. Six or more values give a reliable line and show any curvature.
A student wants six equally spaced lengths from 0.20 m to 1.20 m inclusive. What is the step between lengths, in m?
Show answer
0.2 m. Six values have five gaps: (1.20 - 0.20) / 5 = 0.20 m.
Explain why readings should be taken over as wide a range as possible.
Show answer
A wide range shows the trend clearly and gives a more reliable gradient, because the uncertainty in each point is small compared with the change across the range.. Spreading points widely makes the line's slope much less sensitive to the scatter of any one point.
What is the main purpose of repeating each reading and calculating a mean?
- To remove systematic errors
- To reduce the effect of random errors
- To increase the range
- To make the graph pass through the origin
Show answer
To reduce the effect of random errors. Averaging repeats reduces random scatter; it cannot remove a systematic error.
A student records the temperature of a cooling liquid every 30 s. Why can each temperature reading not simply be repeated?
- The temperature is changing continuously, so the same conditions never return; the whole run must be repeated instead
- Thermometers cannot be read twice
- Repeating readings only reduces systematic error
- Temperatures do not have an uncertainty
Show answer
The temperature is changing continuously, so the same conditions never return; the whole run must be repeated instead. The specification asks whether repeats are appropriate: during a continuous change, a moment cannot be repeated, so repeat the whole experiment.

Practise with instant feedback

Sources

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