Introduction to Planning an Experiment

In Unit 3 of your International AS Physics course, you aren't actually standing in a lab with a stopwatch and a beaker. Instead, you are the architect. The exam will ask you to design or "devise" an experiment on paper. Think of it like writing a perfect recipe: if someone else followed your instructions exactly, would they get the right result?

Planning is about making smart choices before you even touch the equipment. We need to decide what to measure, how to measure it accurately, and how to stay safe. Don't worry if this seems like a lot to remember—once you learn the "logical flow" of a plan, it becomes second nature!

1. Choosing the Right Tools (Apparatus)

The first step in any plan is picking your equipment. You must choose instruments with the correct range (can it measure the highest and lowest values expected?) and resolution (is it sensitive enough?).

Key Instruments to Know:

  • Vernier Calipers: These have a resolution of \(0.1\text{ mm}\). Use these for things like the diameter of a glass tube.
  • Micrometer Screw Gauge: This has a much higher resolution of \(0.01\text{ mm}\). This is your go-to for very thin objects, like the diameter of a wire in a resistivity experiment.
  • Metre Rule: Standard resolution of \(1\text{ mm}\). Good for measuring the length of a string or the height of a falling object.

Quick Tip: Always mention a zero check. Before you start, ensure your instrument (like a micrometer or a digital scale) reads exactly \(0\) when it should. This is a simple way to reduce systematic errors.

2. Variables: The "Who's Who" of Your Experiment

To make an experiment a "fair test," you need to manage your variables strictly:

  • Independent Variable: The thing you change (e.g., the length of a wire).
  • Dependent Variable: The thing you measure (e.g., the resistance).
  • Control Variables: Everything else you must keep the same to ensure the results are valid. For example, when measuring the viscosity of an oil using the falling-ball method, you must keep the temperature constant, as viscosity changes with heat.

3. Measuring Technique

The examiners want to see that you know how to use the equipment properly. This is often where students lose easy marks!

Common Techniques to Include in Your Plan:

  • Avoiding Parallax Error: Always state that you will view scales (like a thermometer or a ruler) at eye level and perpendicular to the scale.
  • Using Supports: Mention using a set square to ensure a ruler is perfectly vertical or a laboratory stand is stable.
  • Timing: If you are measuring the period of a vibrating string, don't just time one oscillation. Time \(10\) or \(20\) oscillations and then divide by that number to find the time for one. This reduces the impact of your reaction time.

Analogy: Imagine trying to time a single 100m sprint with a manual stopwatch. Your reaction time at the start and end matters a lot. But if you timed a 10,000m race, that same small reaction error becomes much less significant compared to the total time!

4. Reliability and Repeats

Should you repeat your readings? Yes, always!
In your plan, specify that you will take repeat readings for each value of your independent variable and then calculate a mean (average). This helps you identify anomalies (readings that don't fit the pattern) and improves the precision of your results.

Note: For more detail on how to handle these numbers, see the chapter on "Taking readings: range, repeats and anomalies".

5. Health and Safety

Every good physics plan considers the "what if?" scenarios. You must identify a specific hazard and how to deal with it.

  • High Tension: If a wire is under high tension (like in a Young Modulus experiment), mention wearing safety goggles in case the wire snaps.
  • Heavy Masses: Use a sand tray or "catch box" under falling weights to protect feet and floors.
  • Hot Components: In electricity experiments, wires can get hot. Suggest switching off the circuit between readings.
  • Lasers: When determining wavelength with a diffraction grating, never look directly into the laser beam.

6. Using the Data

A plan isn't finished until you explain what you will do with the numbers you've collected. Usually, this involves a graph.

  • Identify what goes on each axis (usually the independent variable on the \(x\)-axis).
  • Explain how the gradient of the graph will help you find a constant. For example, in a density experiment, a graph of mass (\(m\)) against volume (\(V\)) would have a gradient equal to the density (\(\rho\)), because \(m = \rho V\).

Did you know? In the IAS exam, you are often asked to "Criticise" a student's plan. Look for things they missed: Did they forget to repeat readings? Did they use a ruler when they should have used a micrometer? Being a "Physics Critic" is a great way to practice your own planning skills!

Summary Checklist for a Perfect Plan

When you are writing your answer in the exam, try to tick off these five points:

  1. Apparatus: Name the specific tool and its resolution (e.g., "Use a micrometer with \(0.01\text{ mm}\) resolution").
  2. Variables: State what you change, what you measure, and what you keep the same.
  3. Method: Step-by-step instructions, including a zero check and how to avoid errors like parallax.
  4. Reliability: Mention repeats and calculating a mean.
  5. Safety: Identify one risk and a way to reduce it.

For more information on how to process the numbers once you have them, check out the chapter on "Processing data, significant figures and graphs".