Welcome to Communication in AS Practical Physics!
Welcome! In Physics, discovering something amazing through an experiment is only half the battle. The other half is communicating your findings clearly, accurately, and honestly to others. Whether you are recording data in a lab notebook, plotting a graph, calculating uncertainties, or writing an exam answer, clear communication is an essential scientific skill.
Don't worry if you have ever lost marks on graph scales or significant figures before. This guide breaks down every rule step-by-step so you can communicate like a professional physicist and pick up every mark available in your AS 3 practical exams!
1. Presenting Data in Tables
A data table is often the very first thing an examiner looks at. A neat, unambiguous table makes your data easy to interpret and verify.
Key Rules for Tabulating Data
• Column Headings: Every column must state the quantity and the unit. Separate the quantity and unit using a forward slash, for example: \(T / \text{s}\), \(V / \text{V}\), or \(L / \text{cm}\).
• Raw Data Consistency: All raw measurements in a particular column must be recorded to the same precision (the same number of decimal places), which corresponds to the resolution of the measuring instrument.
• Calculated Data: Values calculated from raw data (such as \(T^2\) or \(1/d\)) should generally be quoted to the same number of significant figures (or one more) as the least precise raw measurement used in the calculation.
• Repeats and Averages: Always record individual repeated trials (e.g., \(t_1\), \(t_2\), \(t_3\)) and their calculated mean \(t_{\text{mean}}\) clearly in dedicated columns.
Example Table Format:
Length \(L / \text{cm}\) | Time for 20 oscillations \(t_1 / \text{s}\) | Time for 20 oscillations \(t_2 / \text{s}\) | Mean time \(t / \text{s}\) | Period \(T / \text{s}\) | \(T^2 / \text{s}^2\)
\(20.0\) | \(17.92\) | \(18.04\) | \(17.98\) | \(0.899\) | \(0.808\)
\(30.0\) | \(21.98\) | \(22.10\) | \(22.04\) | \(1.102\) | \(1.214\)
Did you know? Writing \(T / \text{s}\) literally means "quantity \(T\) divided by seconds". Because pure numbers have no units, dividing a measured quantity by its unit leaves pure numbers in the table cells below!
Key Takeaway
Always label table columns with Quantity / Unit, keep decimal places uniform for raw readings, and ensure significant figures in calculated values reflect the precision of your raw data.
2. Graph Drawing Skills: The S-A-P-L Method
Graphs allow us to visualise patterns, identify anomalies, and determine physical constants. When drawing graphs, remember the simple mnemonic: S - A - P - L.
S — Scale
• Choose a sensible scale (such as \(1\), \(2\), or \(5\) units per large grid square). Avoid awkward multipliers like \(3\), \(7\), or \(6\).
• Your plotted points must occupy more than 50% of the grid along both the horizontal and vertical axes.
• You do not always need to include the origin \((0,0)\) if it squashes your data into a tiny corner, unless the question specifically requires reading a y-intercept directly.
A — Axes
• Place the independent variable (what you changed) on the horizontal x-axis.
• Place the dependent variable (what you measured) on the vertical y-axis.
• Label both axes fully with the Quantity / Unit, matching your table headers (e.g., \(F / \text{N}\), \(x / \text{m}\)).
P — Plotting Points
• Plot each data point neatly using a small, sharp cross \(\times\) or a dot inside a circle \(\odot\).
• Ensure points are plotted accurately to within half a small grid square.
L — Line of Best Fit
• Use a clear ruler for a straight line or draw a single, smooth stroke for a curve.
• Balance your points evenly: there should be roughly an equal number of points above and below the line.
• Never force the line through the origin unless theoretical physics dictates it must pass through \((0,0)\) and the data supports it.
• Ignore obvious outliers (anomalous results) when positioning your line of best fit.
Key Takeaway
Remember S-A-P-L: Scale (\(>50\%\), sensible steps), Axes (labelled with units), Points (accurate sharp crosses), and Line (balanced, single thin stroke).
3. Extracting Information: Gradients and Intercepts
Many physical laws can be rearranged into the straight-line equation:
\(y = mx + c\)
Where \(m\) represents the gradient (slope) and \(c\) represents the y-intercept.
Calculating the Gradient Correctly
1. Choose two points on the line of best fit that are far apart. Do not use original raw data points unless they happen to lie precisely on the line!
2. Draw a large right-angled triangle on your graph. The hypotenuse must span more than 50% of your drawn line.
3. Read the coordinates: \((x_1, y_1)\) and \((x_2, y_2)\).
4. Calculate the gradient using the formula:
\(m = \frac{\Delta y}{\Delta x} = \frac{y_2 - y_1}{x_2 - x_1}\)
5. Determine the unit of the gradient by dividing the unit of the y-axis by the unit of the x-axis (e.g., \(\text{N} / \text{m} = \text{N m}^{-1}\)).
Relating Gradient and Intercept to Physical Equations
Consider the formula for a falling object starting with initial velocity \(u\):
\(v = u + at\)
Comparing this to \(y = mx + c\):
• If \(v\) is plotted on the y-axis and \(t\) on the x-axis:
• Gradient \(m = a\) (acceleration)
• y-intercept \(c = u\) (initial velocity)
Key Takeaway
Always draw a large triangle (\(> 50\%\) of the line), use coordinates from the line of best fit, show all substitution working clearly, and remember to state the correct unit for the gradient.
4. SI Units, Scientific Prefixes, and Significant Figures
SI Base and Derived Units
All scientific quantities are communicated using standard SI units. The fundamental base quantities and their base units include:
• Mass: kilogram (\(\text{kg}\))
• Length: metre (\(\text{m}\))
• Time: second (\(\text{s}\))
• Electric current: ampere (\(\text{A}\))
• Temperature: kelvin (\(\text{K}\))
Standard Prefixes
When values are very large or very small, use standard prefixes:
• Nano (\(\text{n}\)): \(10^{-9}\)
• Micro (\(\mu\)): \(10^{-6}\)
• Milli (\(\text{m}\)): \(10^{-3}\)
• Centi (\(\text{c}\)): \(10^{-2}\)
• Kilo (\(\text{k}\)): \(10^{3}\)
• Mega (\(\text{M}\)): \(10^{6}\)
• Giga (\(\text{G}\)): \(10^{9}\)
Significant Figures (sf) Rules
• Non-zero digits are always significant (e.g., \(45.2\) has \(3\text{ sf}\)).
• Zeros between non-zero digits are significant (e.g., \(205\) has \(3\text{ sf}\)).
• Leading zeros are not significant (e.g., \(0.0034\) has \(2\text{ sf}\)).
• Trailing zeros in a decimal number are significant (e.g., \(5.600\) has \(4\text{ sf}\)).
• In multiplication and division, your final answer should have the same number of significant figures as the measurement with the fewest significant figures.
Key Takeaway
Convert non-standard units to SI base or derived units during calculations, and match the significant figures of your final answer to the least precise raw measurement.
5. Communicating Experimental Uncertainties
Every measurement in physics carries an uncertainty. Communicating this uncertainty allows others to judge the reliability of your experimental conclusion.
Types of Uncertainty
• Absolute Uncertainty (\(\Delta x\)): The margin of uncertainty with units (e.g., \(L = 25.0 \pm 0.1\text{ cm}\)). For a single reading on an analogue scale, it is usually half the smallest division; for a digital scale, it is \(\pm 1\) the smallest displayed digit.
• Percentage Uncertainty: The absolute uncertainty expressed as a percentage of the measured value:
\(\text{Percentage Uncertainty} = \left( \frac{\Delta x}{x} \right) \times 100\%\)
Combining Uncertainties
• Adding or Subtracting quantities (\(y = a + b\) or \(y = a - b\)): Add the absolute uncertainties:
\(\Delta y = \Delta a + \Delta b\)
• Multiplying or Dividing quantities (\(y = ab\) or \(y = \frac{a}{b}\)): Add the percentage uncertainties:
\(\% \Delta y = \% \Delta a + \% \Delta b\)
• Powers (\(y = a^n\)): Multiply the percentage uncertainty by the power \(n\):
\(\% \Delta y = n \times (\% \Delta a)\)
Quick Example:
If radius \(r = 2.0\text{ cm} \pm 5\%\), the percentage uncertainty in the cross-sectional area \(A = \pi r^2\) is:
\(\% \text{ uncertainty in } A = 2 \times 5\% = 10\%\)
Key Takeaway
Add absolute uncertainties for addition/subtraction; add percentage uncertainties for multiplication/division; multiply percentage uncertainty by the index for powers.
6. Quality of Written Communication (QWC) in Practical Questions
In extended response questions where you describe an experimental method, marks are specifically awarded for clear, coherent, and logical structure.
A Step-by-Step Template for Practical Descriptions
1. Apparatus & Diagram: Name the specific instruments needed (e.g., "micrometer screw gauge" rather than just "ruler", "stopwatch", "voltmeter"). Include a clear, labelled diagram if appropriate.
2. Variables: Identify the independent variable (to be changed), the dependent variable (to be measured), and at least two control variables (to be kept constant).
3. Method: Describe step-by-step how the experiment is performed over a wide range of values (at least 6 different values of the independent variable).
4. Minimising Errors: State practical precautions (e.g., "repeat and calculate a mean", "view the scale at eye level to avoid parallax error", "use a set square to ensure the ruler is vertical").
5. Analysis: Explain clearly which graph should be plotted, what the gradient represents, and how the target quantity is determined from the gradient.
Common Mistakes to Avoid
• Saying "measure time" without naming the instrument ("measure time using a digital stopwatch").
• Forgetting to mention repeats and calculating a mean value.
• Stating "plot a graph" without specifying which variable goes on which axis (e.g., "plot a graph of \(T^2\) on the y-axis against \(L\) on the x-axis").
• Writing imprecise descriptions like "make sure it's accurate" instead of stating an exact physical technique (e.g., "use a fiducial marker at the equilibrium position").
Key Takeaway
Structure experimental plans logically: name specific measuring tools, describe controlling variables, explain how to minimise errors, and finish with exact graph analysis.
Quick Review Summary
• Tables: Label headers as Quantity / Unit and keep decimal places consistent for raw readings.
• Graphs (SAPL): Sensible Scale (\(>50\%\)), fully labelled Axes, neatly plotted Points, balanced Line of best fit.
• Gradients: Use a large triangle (\(>50\%\) of line), calculate \(\Delta y / \Delta x\) with points on the line, and quote correct units.
• Uncertainties: Add absolute values for \(+\)/\(-\), add percentage values for \(\times\)/\(\div\), and multiply by \(n\) for powers.
• QWC: Use precise terminology, state named measuring instruments, describe repeats/controls, and specify graphical analysis.