1.2.1Development of Practical Skills in Physics
Presenting and Interpreting Results
5 min read 8 parts Lesson 8 of 29 1 quiz
1.2.1 Presenting and Interpreting Results — OCR A Level Physics A notes: Record raw data in a well-structured results table, display it in a suitable form and describe the pattern it shows.Raw readings written on scrap paper are almost useless.
- independent variable
- same number of decimal places
- Line graph or scatter graph
- anomalous results
- Worked example 1
- Worked example 2
Section 1 of 8Why presentation matters
Raw readings written on scrap paper are almost useless. A good results table lets another person see what was measured, in what units, how precisely, and how the readings were repeated. Presentation also helps you: patterns, anomalies and mistakes become obvious once the data are laid out in order.
Before the experiment starts, draw the table with empty cells so that every reading can be written straight in. Leave columns for repeats, for the mean, and for any calculated quantity you will need later, such as the square of a time or the reciprocal of a length.
Section 2 of 8Building a results table
Follow these conventions every time:
Never mix units in a column and never round a raw measurement; round only calculated results.
Section 3 of 8Choosing a form of display
The best display depends on the data type.
A graph should be plotted from the table, never the other way round. Anything that has a numerical relationship should be shown as a graph of one variable against the other, with the independent variable on the horizontal axis.
Section 4 of 8Interpreting the pattern
Describing a trend means stating the shape in words and supporting it with numbers. Useful phrases are: as x increases, y decreases; y is directly proportional to x (straight line through the origin); y increases at a decreasing rate (curve flattening); y is inversely proportional to x (a hyperbola, or a straight line when y is plotted against 1/x).
To test proportionality from a table without drawing a graph, calculate the ratio y/x for each row. If the ratio is constant within the scatter of the data, the relationship is probably proportional. If the product x×y is constant, the relationship is inverse. If neither is constant, try y/x2 or plot a graph.
Always look for anomalous results: a reading that does not fit the pattern. Do not delete it. Mark it, think about its likely cause (misread scale, timing slip, loose connection), and, if possible, repeat that reading.
Section 5 of 8Linking to uncertainty and graphs
The way a table is laid out feeds directly into later skills.
Finally, a conclusion drawn from the table must refer to the data. Writing “the time gets longer” is weak; writing “the period rises from 1.42 s to 2.85 s when the length is quadrupled, so T is not proportional to l” is strong.
Section 6 of 8Worked examples
A student times 10 swings of a pendulum for string length 0.400 m and records 12.6 s, 12.8 s and 12.7 s with a stopwatch reading to 0.1 s. Show how this should be entered in a results table and find the period.
Column headings: l / m, t1 / s, t2 / s, t3 / s, mean t / s, T / s.
Row: 0.400, 12.6, 12.8, 12.7.
Mean t = (12.6 + 12.8 + 12.7) / 3 = 12.7 s (one decimal place, as the stopwatch resolution).
Period T = 12.7 / 10 = 1.27 s (three significant figures, as the readings had three).
A table shows applied force F and extension e of a spring: (2.0 N, 4.1 cm), (4.0 N, 8.0 cm), (6.0 N, 12.2 cm), (8.0 N, 16.0 cm). Is F proportional to e?
Calculate F / e for each row: 2.0/4.1 = 0.49; 4.0/8.0 = 0.50; 6.0/12.2 = 0.49; 8.0/16.0 = 0.50 N cm−1.
The ratio is constant to within about 2%, which is within normal reading scatter, so F is directly proportional to e. The spring constant is about 0.50 N cm−1 = 50 N m−1.
Four readings of current through a resistor at p.d. 1.0, 2.0, 3.0, 4.0 V are 0.20, 0.41, 0.80, 0.81 A. Identify any anomaly and say what to do.
I/V values: 0.20, 0.205, 0.267, 0.203 A V−1. The third value (0.80 A at 3.0 V) is out of line; the expected current is about 0.61 A.
Mark it as anomalous in the table, do not discard it silently, repeat the measurement at 3.0 V, and check for a misread scale (0.80 versus 0.60 is a typical slip).
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Independent variable | The quantity deliberately changed by the experimenter. |
| Dependent variable | The quantity measured as a result of the change. |
| Anomalous result | A reading that does not fit the pattern shown by the rest of the data. |
| Resolution | The smallest change in the quantity that an instrument can show. |
| Directly proportional | Two quantities whose ratio is constant; the graph is a straight line through the origin. |
| Mean | The sum of repeat readings divided by the number of readings. |