Planning a Physics investigation

1. Definition

An experimental plan is a reproducible method for answering a stated question with measurements. It identifies what is changed, what is measured, what is controlled, how the evidence is processed and how significant risks are reduced.

2. Key Ideas

• The independent variable is deliberately changed.
• The dependent variable is measured or calculated in response.
• Control variables are influential factors kept constant for a fair comparison.
• Apparatus must have suitable range, resolution and access for the measurement.
• The plan must state how results will be used, not stop after data collection.
• A safety statement pairs a credible hazard with a specific control.

3. Detailed Explanations

A. Turn the question into variables

For “How does wire length affect resistance?”, change wire length, determine resistance from voltage and current, and control material, diameter and temperature. Do not list quantities that cannot affect the result merely to increase the number of controls.

B. Write a method another candidate could follow

Use an operational sequence:

1. assemble the apparatus and state where each measuring instrument is connected or positioned;
2. state the initial value of the independent variable and how it is measured;
3. measure the dependent quantity using named apparatus;
4. repeat readings where random variation is significant and calculate a mean;
5. choose several well-spaced independent-variable values over a safe, feasible range;
6. keep each named control variable constant;
7. state the graph, calculation or comparison used to answer the question.

Avoid memorising “five values and three repeats” as an unconditional rule. The number and range must be adequate for the task, available time and apparatus.

C. Select apparatus with a reason

“Use a metre rule” is incomplete when instrument choice is being assessed. State why its range and millimetre divisions suit the expected length. A finer-resolution instrument is not helpful if the object does not fit or the technique introduces a larger error.

D. Connect risk to control

Write “hot water may scald; keep the beaker on a heatproof mat and use eye protection when transferring it,” not “take care.” Do not invent a severe hazard for harmless apparatus.

4. Common Mistakes

• Changing two independent variables together.
• Saying “keep everything else constant” without naming relevant controls.
• Naming apparatus without explaining the measurement technique.
• Collecting readings without stating how they lead to a conclusion.
• Giving “repeat and average” as the only improvement to a systematic error.
• Writing a precaution that does not reduce the named risk.

5. Exam Tips

Use a compact chain: change → measure → control → repeat → process → conclude → protect. Check that every noun in the question appears in a measurable step and that your proposed graph places the independent variable on the horizontal axis.

6. Worked Examples

7. Mind Stretchers

8. Practice and next step

Draft a plan from a familiar topic without looking at notes, then check it against the seven-part chain above. Continue to Manipulation, Measurement and Observation.