Required practicals: method, variables and evaluation — AQA GCSE Physics revision

Planning a practical

• The independent variable is the one you deliberately change; the dependent variable is what you measure; control variables are kept the same to make it a fair test.
• A good method should be repeatable (you can repeat it yourself and get similar results) and reproducible (others can repeat it and get similar results).
• Repeating readings (usually 3+ times) and calculating a mean improves reliability and helps identify anomalous results to discard.
• Choosing an appropriate range and interval for the independent variable ensures a reliable pattern can be seen in the results.

Common physics required practicals

• Density: measure mass with a balance and volume by displacement (irregular objects) or by measuring dimensions (regular objects), then calculate density = mass ÷ volume.
• Specific heat capacity: use a joulemeter or measure current/voltage/time to find energy supplied by a heater, and measure temperature change of a known mass, to calculate specific heat capacity.
• Resistance: vary potential difference across a component using a variable resistor, measure current with an ammeter and potential difference with a voltmeter (connected in parallel) to plot I-V graphs.
• Waves: use a ripple tank or vibration generator with a stroboscope to measure wavelength, and calculate frequency, then use wave speed = frequency × wavelength.
• Force and extension: add masses to a spring, measure extension with a ruler, and check whether the spring obeys Hooke's law (extension proportional to force) up to the limit of proportionality.

Key terms

Independent variable

The variable you deliberately change in an experiment.

Dependent variable

The variable you measure as an outcome of the experiment.

Control variable

A variable kept constant to ensure a fair test.

Repeatable

Getting similar results when the same person repeats the method under the same conditions.

Reproducible

Getting similar results when a different person or method is used.

Anomaly

A result that doesn't fit the general pattern of the data.

Common exam traps

• Mixing up independent and dependent variables in a method description.
• Confusing precision (consistency) with accuracy (closeness to the true value) — a result can be precise but not accurate.
• Suggesting 'be more careful' as an improvement instead of a specific, actionable change.

Practice questions with answers

1. 1. What is the independent variable?

   • • What you measure
   • • What you deliberately change
   • • What you keep the same
   • • The final conclusion

   Answer: What you deliberately change

   The independent variable is the one the experimenter changes on purpose.

2. 2. What is a control variable?

   • • The variable you change
   • • The variable you measure
   • • A variable kept constant for a fair test
   • • The final result

   Answer: A variable kept constant for a fair test

   Control variables are kept the same so that only the independent variable affects the dependent variable.

3. 3. Why should you repeat readings and calculate a mean?

   • • To save time
   • • To improve reliability and spot anomalies
   • • It is required by law
   • • To make the graph longer

   Answer: To improve reliability and spot anomalies

   Repeating readings reduces the effect of random error and helps identify anomalous results.

4. 4. How would you measure the volume of an irregularly shaped object for a density practical?

   Answer: By displacement in a measuring cylinder

   Submerge the object in water in a measuring cylinder and record the volume of water displaced.

5. 5. In a specific heat capacity experiment, what do you need to measure to calculate energy supplied?

   • • Only temperature
   • • Current, potential difference and time (or use a joulemeter)
   • • Only mass
   • • Only volume

   Answer: Current, potential difference and time (or use a joulemeter)

   Energy supplied = current × potential difference × time, or read directly from a joulemeter.

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