Introduction to Practical Investigations

Welcome to the world of experimental Physics! In this chapter, we focus on the skills you need to think and act like a real scientist. While many people think Physics is just about formulas and numbers, those numbers come from practical investigations. In your Edexcel IGCSE exam, about 20% of the marks come from AO3 (Experimental skills). This means you don't just need to know the facts; you need to know how to design, perform, and check an experiment.

Think of an experiment like a recipe: you need the right ingredients (equipment), a clear method (the steps), and a way to make sure the result is perfect (accuracy and reliability).

1. Planning an Investigation: The Variables

Before you start any experiment, you must identify your variables. These are the things that can change or be kept the same. Understanding these is the secret to a "fair test."

  • Independent Variable: This is the variable you change to see what happens. (Example: The mass you add to a spring.)
  • Dependent Variable: This is the variable you measure as a result. (Example: The extension of the spring.)
  • Control Variables: These are the things you must keep the same to make sure the test is fair. If you change more than one thing at a time, you won't know which one caused the result! (Example: Using the same spring and the same ruler for every measurement.)

Quick Tip: If an exam question asks you to "Design an experiment," always list at least two things you will keep the same (control variables) to get full marks!

2. Writing a Method and Staying Safe

A good scientific method is like a set of instructions that anyone could follow to get the same result. When describing a method, remember to:

  1. List the equipment you will use (e.g., a "stopwatch," a "metre rule," or a "Newton-meter").
  2. Explain how you will use it.
  3. State what you are measuring.
  4. Mention safety precautions.
Safety First!

Examiners love to ask how to make an experiment safe. Don't worry if it seems simple—common sense is key!
- Dealing with weights? Wear sturdy shoes so you don't crush your toes if they fall.
- Dealing with heat? Use a heat-proof mat and wear goggles.
- Dealing with electricity? Ensure your hands are dry and wires are insulated.

3. Accuracy, Reliability, and Validity

These three words sound similar, but they mean very different things in Physics. Let's break them down:

Reliability (Repeat and Average)

An experiment is reliable if you can do it again and get the same result. The best way to improve reliability is to repeat the experiment (usually three times) and calculate an average (mean). This also helps you spot anomalies (results that don't fit the pattern).

Accuracy (The Right Tool for the Job)

An experiment is accurate if the result is very close to the true value. You can improve accuracy by using better equipment. For example, using a digital caliper is more accurate than using a wooden ruler because it can measure smaller differences.

Validity (The Fair Test)

An experiment is valid if it actually measures what it is supposed to measure. This is only possible if you have controlled all your variables properly. If your experiment is "unfair," it isn't valid!

4. The Prescribed Practicals (The "Must-Knows")

The syllabus lists specific experiments that you are expected to know. You might be asked to describe these in detail or analyze data from them.

Core Practicals (Examinable on Paper 1 and Paper 2)
  • 1.5 Motion: Investigating the motion of everyday objects (like toy cars or tennis balls) using light gates or stopwatches.
  • 1.22 Extension: How the extension of a spring or rubber band varies with force (Hooke’s Law).
  • 3.17 & 3.19 Light: Investigating refraction and finding the refractive index \(n\) of glass using a rectangular block.
  • 4.9 Thermal Energy: Investigating how heat moves via conduction, convection, and radiation.
  • 5.4 Density: Determining the density of regular and irregular solids and liquids using \(density = mass / volume\).
  • 6.6 Magnetism: Mapping magnetic field patterns using a compass or iron filings.
  • 7.6 Radioactivity: Investigating the penetration power of alpha, beta, and gamma radiation through different materials.
Paper 2 Only Practicals (Look for the 'P' reference)
  • 2.23P Static Electricity: Charging insulating materials by friction.
  • 3.25P Speed of Sound: Measuring how fast sound travels in air.
  • 3.27P Sound Waves: Using an oscilloscope and microphone to find the frequency of a sound wave.
  • 5.11P & 5.14P Thermal Physics: Creating temperature-time graphs during changes of state and measuring specific heat capacity.

5. Summary and Key Takeaways

Key Review:
- Independent Variable: The one you change.
- Dependent Variable: The one you measure.
- Control Variable: The one you keep the same for a fair test.
- Reliability: Repeat the test and calculate a mean!
- Anomalies: Weird results that you should ignore when calculating your average.

Don't worry if this seems like a lot to remember! Most of the time, the exam will give you a scenario and ask you to apply these rules. Just remember: stay organized, be safe, and always repeat your measurements!

Next Step: Now that you know how to plan an investigation, check out the chapter on "Data, graphs and evaluation" to see how to handle the results you collect!