Advanced Experiment Planning

CIE A-Level Physics· 25 min read

1. Identifying Variables★★☆☆☆⏱ 5 min

📘 Definition

Experiment Variables

All valid physics experiments require three categories of variable: independent (changed), dependent (measured) and control (kept constant) to ensure a fair test of the relationship being investigated.

Example:

For a test of how extension depends on force, independent = force, dependent = extension, control = cross-sectional area of wire

📐 Worked Example

An investigation tests how the rate of cooling of water depends on the volume of water in a container. Identify all relevant variables for this experiment.

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1. Identify the independent variable, which is the variable changed by the experimenter:
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Independent variable = Volume of water
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1. Identify the dependent variable, which is the variable measured to test the relationship:
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Dependent variable = Rate of cooling of water (calculated from temperature change over time)
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1. List all relevant control variables that could affect the dependent variable if not kept constant:
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Control variables = Material of container, initial temperature of water, room temperature, surface area of container

Exam tip:

CIE always allocates 2-3 marks for correct variable identification. Explicitly label each variable to guarantee you get these easy marks.

2. Drawing Experimental Diagrams★★★☆☆⏱ 6 min

CIE Paper 5 always requires a labelled diagram of your planned experimental setup. Diagrams must follow standard scientific conventions, and every instrument you mention in your method must be labelled on the diagram.

📐 Worked Example

Draw and label an experimental setup to test how the resistance of an LDR depends on light intensity. Justify your arrangement.

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1. Arrange core electrical components: Draw a d.c. power supply, with ammeter in series with the LDR, and a voltmeter connected in parallel across the LDR.
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This arrangement allows calculation of LDR resistance using the relationship:
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$R = \frac{V}{I}$
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where $V$ is voltmeter reading and $I$ is ammeter reading.
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1. Add the light source and controls: Add a variable intensity light source at a fixed distance from the LDR, a black tube surrounding the LDR to block ambient light, and a thermometer to monitor LDR temperature.
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1. Label every component clearly. All instruments used in the method are included on the diagram.

Exam tip:

You get 1 mark per correct labelled key component. Do not draw artistic sketches, use standard symbols (e.g. standard circuit symbols for electrical experiments).

3. Error Analysis and Improvements★★★★☆⏱ 8 min

📘 Definition

Systematic vs Random Error

Systematic error causes a consistent shift in all measurements, while random error causes unpredictable scatter of measurements around the true value. Improvements must match the error type to get marks.

Example:

Uncalibrated zero error on a force meter = systematic error, human reaction time timing a pendulum = random error

📐 Worked Example

An experiment to measure the period of a pendulum has random error from human reaction time of 0.2 s when starting and stopping the stopwatch. Suggest an improvement to reduce this error, and explain why it works.

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1. The improvement is to time 10 full complete oscillations of the pendulum, rather than just a single oscillation.
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1. Calculate the average period of one oscillation by dividing the total measured time by 10.
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1. Explanation: Total reaction error is still 0.2 s, so the proportional error in the total time is reduced by a factor of 10. This reduces the overall uncertainty in the calculated value of the period.

Exam tip:

Always link your improvement directly to the error it solves. Examiners do not award marks for generic improvements that are not linked to the specific error.

4. Safety Precautions★★★☆☆⏱ 4 min

CIE almost always allocates 1-2 marks for a valid safety precaution with justification. Generic precautions such as 'wear goggles' or 'work carefully' do not get marks unless they are linked to a specific hazard in the experiment.

📐 Worked Example

An experiment investigates the extension of a metal wire when masses are added to the end. Suggest one relevant safety precaution with justification.

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1. Identify the specific hazard: If the stressed wire snaps under tension, it can recoil and cause serious eye injury.
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1. State the precaution: Wear safety goggles throughout the experiment, and place a safety screen between the wire and observers.
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1. Justify the precaution: This prevents injury from flying fragments or recoil if the wire snaps under load, directly mitigating the identified hazard.

Exam tip:

Use the structure: Hazard → Precaution → Justification to make sure you hit all marking points.

5. Common Pitfalls

Wrong move:

Not explicitly labelling variables as independent/dependent/control

Why:

CIE allocates 2-3 full marks specifically for correct labelling, which you will lose if you only list variables

Correct move:

Always explicitly label each variable: 'Independent variable = x, Dependent variable = y...'

Wrong move:

Suggesting repeating measurements to fix systematic error

Why:

Repeating and averaging only reduces random error, it does nothing to fix consistent bias from systematic error

Correct move:

For systematic error, suggest calibration of instruments, correcting zero error, or using a more accurate experimental method

Wrong move:

Drawing sketches instead of standard scientific diagrams

Why:

Unclear sketches mean examiners cannot identify labelled components, leading to lost marks

Correct move:

Use standard scientific symbols for all components and clearly label every instrument you mention in your method

Wrong move:

Giving generic safety precautions that are not relevant to the experiment

Why:

Examiners only award marks for precautions linked to specific hazards of the experiment

Correct move:

Always name the specific hazard first, then give the precaution and link it to mitigating that hazard

Wrong move:

Forgetting to explain how you will analyse data to reach a conclusion

Why:

CIE allocates 2-3 marks for describing data analysis, including what graph to plot

Correct move:

Always state what graph you will plot, and how you will use the graph to test the relationship being investigated

6. Quick Reference Cheatsheet

Topic	Key Requirement	Mark Tip
Variables	Explicitly label independent/dependent/control, list all controls	2-3 easy marks, just structure it right
Diagrams	Standard symbols, label all instruments from your method	1 mark per label, don't add unnecessary components
Errors & Improvements	Match improvement to error type, explain the link	Random = repeat/average, Systematic = calibrate
Safety	Structure: Hazard → Precaution → Justification	No generic precautions, always link to hazard
Analysis	State graph to plot, interpret gradient/intercept	2-3 marks available, always include this step

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 5
Plan resistivity wire experiment
2023 · 5
Plan pendulum SHM experiment
2024 · 5
Plan LDR intensity experiment

What's Next

Advanced experiment planning is the core skill for CIE 9702 Paper 5, which counts for 20% of your total A-Level Physics mark. Mastering the marking expectations for planning questions is the highest impact revision you can do for this paper. The skills you learned here, including variable identification and error analysis, directly build into the next key Paper 5 topics of data analysis, graph plotting and uncertainty calculation. These practical skills also support your understanding of theory topics by showing how physics relationships are tested experimentally.