Welcome to Practical Physics!

In your IA2 Physics journey, Unit 6 is all about showing that you don't just know the theory—you know how to be a real scientist. This chapter, Implementation, Tables and Units, focuses on the "hands-on" logic of an experiment. Even though you'll be sitting a written exam, you need to demonstrate that you can record data accurately, use units correctly, and spot mistakes in a table of results. These are the "easy marks" that many students miss, so let’s make sure you’re ready!

1. Master the Language: Units and Prefixes

In Physics, a number without a unit is just a lonely digit. For Unit 6, you must be fluent in SI units and the standard prefixes used in Unit 4 and 5 topics (like fields and nuclear physics).

Base Units and Derived Units

Most quantities you measure come from these five base units:

  • Length: metre \((\text{m})\)
  • Mass: kilogram \((\text{kg})\)
  • Time: second \((\text{s})\)
  • Current: ampere \((\text{A})\)
  • Temperature: kelvin \((\text{K})\)

For IA2, you will encounter complex units like the Tesla \((\text{T})\) for magnetic flux density or Becquerel \((\text{Bq})\) for activity. Always ensure your final answers are converted back to SI base units unless the question asks otherwise.

Scientific Prefixes

Physics deals with the very big and the very small. You must know these prefixes by heart:

  • Pico (p): \(10^{-12}\)
  • Nano (n): \(10^{-9}\)
  • Micro (\(\mu\)): \(10^{-6}\)
  • Milli (m): \(10^{-3}\)
  • Kilo (k): \(10^{3}\)
  • Mega (M): \(10^{6}\)
  • Giga (G): \(10^{9}\)
  • Tera (T): \(10^{12}\)

Quick Tip: If you see mass in \((\text{MeV/c}^2)\), remember this is a specific unit used in Nuclear Physics. You might be asked to convert this into \((\text{kg})\) using the conversion factor \(1\text{ u} = 1.66 \times 10^{-27} \text{ kg}\).

Key Takeaway: Always check your units twice. A common exam trick is giving you one value in millimetres \((\text{mm})\) and another in metres \((\text{m})\). Convert them to be consistent before calculating!

2. Implementing the Experiment

When you are given a description of an experiment, you need to judge if the measurements taken were "good enough."

The "Golden Rules" of Readings

  • Number of Readings: For a good graph and to identify trends, you should generally have at least 6 sets of readings.
  • Range: Your readings should cover as wide a range as possible within the limits of the apparatus. For example, if measuring the resonance of a mass on a spring, don't just use \(100\text{g}\) to \(150\text{g}\); try \(100\text{g}\) to \(600\text{g}\).
  • Intervals: Try to keep the gaps between your independent variable readings equal (e.g., \(10\text{cm}\), \(20\text{cm}\), \(30\text{cm}\)...).

Checking for Inconsistencies

If you see a set of repeat readings like \(1.23\text{s}\), \(1.25\text{s}\), and \(1.89\text{s}\), the third reading is an anomaly (an inconsistent reading). In an exam, you might be asked how to improve this. Your answer should be: "Identify the inconsistent reading, discard it, and take a repeat measurement."

3. Perfecting Data Tables

In Unit 6, you are often asked to "clean up" or complete a student’s table. Tables must follow very strict rules.

Table Headers

Headers must contain both the quantity name (or symbol) and the unit, separated by a forward slash.

Correct: \(t / \text{s}\) or \(L / \text{m}\) or \(\text{Temperature } (\text{K})\)

Incorrect: \(t (\text{sec})\) or \(L\) in metres

Significant Figures (SF) and Decimal Places (DP)

This is where most students lose marks. Follow these two rules:

  1. Raw Data (Columns): All numbers in a single column must be recorded to the same number of decimal places. This is determined by the resolution of the instrument. If using a ruler with \(1\text{mm}\) resolution, all readings should be recorded to \(0.001\text{m}\) (e.g., \(0.150\text{m}\), not \(0.15\text{m}\)).
  2. Calculated Data: When you calculate a new value (like \(V^2\) or \(1/x\)), the result should be given to the same number of significant figures as the raw data used, or perhaps one more if it keeps the data consistent.

Common Mistake to Avoid: Don't let your calculator decide the significant figures! If your raw data is \(2.0\text{A}\) (2 SF) and you square it, write \(4.0\text{A}^2\), not just \(4\).

4. Accuracy, Precision, and Sensitivity

To "criticise" or "evaluate" an implementation, you need to use the right vocabulary defined by the exam board:

  • Resolution: The smallest change in the quantity being measured that gives a perceptible change in the reading (e.g., \(0.01\text{mm}\) for a micrometer).
  • Precision: How close repeat readings are to each other. If your repeats are \(5.1\), \(5.2\), and \(5.1\), they are precise. This is affected by random errors.
  • Accuracy: How close a measurement is to the true value. You can't know the true value perfectly, but you can judge accuracy by comparing your result to an accepted value (like \(g = 9.81 \text{ m s}^{-2}\)).
  • Repeatability: If you do the experiment again and get the same results.
  • Reproducibility: If someone else uses different equipment/methods and gets the same results.

Note: For more on how to calculate specific uncertainty values, see the chapter on Compound Uncertainties, Precision and Accuracy (IA2).

Quick Review: Table Checklist

Before moving on, ask yourself these questions when looking at a table in an exam:

  • Are the headers formatted as Symbol / Unit?
  • Does every raw reading in a column have the same number of decimal places?
  • Are there at least 6 variations of the independent variable?
  • Are there repeats for the dependent variable?
  • Are any anomalies clearly marked or excluded from the mean?

Did you know? A "best-fit line" that is expected to go through the origin (0,0) but doesn't is a classic sign of a systematic error, such as a "zero error" on your measuring instrument!

Key Takeaways

  • Units: Use SI base units and standard prefixes. Ensure consistency across all measurements.
  • Record-Keeping: Record raw data to the resolution of the instrument (constant DP).
  • Quality Control: Use a wide range of at least 6 readings and always check for anomalies in repeats.
  • Calculation: Keep significant figures consistent with your raw data measurements.