Collecting data in Physics (DP IB Physics): Revision Note
Collecting data in Physics
This is the "doing" phase of your investigation, where you perform the experiment you have carefully designed
Your goal is to gather high-quality raw data that is both accurate and sufficient to answer your research question
This involves making precise measurements and recording all relevant information, including both numbers and observations
Principles of data collection
Collect and record sufficient relevant quantitative data
Quantitative data is numerical data that you measure in your experiment
The foundation of your report is a raw data table
This should be the first table you present and must contain only the direct measurements you take, with no calculations
Designing your raw data table before you start is crucial
A well-designed table must include:
a specific title that describes the experiment
clearly labelled columns for your independent and dependent variables
units and uncertainties in the column headers, not in the body of the table
Data must be recorded to the correct precision of the instrument
This is a common place where students lose marks
For a metre ruler marked in millimetres, readings must be recorded to three decimal places in metres (e.g., 0.550 m)
For a digital multimeter that reads to two decimal places, all voltage readings must be recorded to two decimal places (e.g., 5.00 V, not 5 V)
Sufficient data means collecting enough data points to see a trend
This includes:
data for at least five increments of your independent variable
at least three repeat trials for each increment to ensure reliability
Identify and record relevant qualitative observations
Qualitative data is non-numerical data that you observe during the experiment
These observations provide context and are crucial for your final analysis and evaluation
Do not underestimate the importance of qualitative data
It can help explain unexpected results or errors
Examples of important qualitative data in physics include:
noticing that a pendulum's swing is not planar (i.e., it is swinging in a slight ellipse)
observing a flicker on a digital multimeter, indicating an unstable reading
noting that a wire or component in a circuit experiment felt warm to the touch, indicating heat dissipation by a resistance
seeing that a laser beam spreads out (diffracts) as it passes through a narrow slit
Identify and address issues that arise during data collection
Experiments do not always go perfectly to plan
A key scientific skill is to notice and respond to issues as they happen
If you encounter a problem, do not ignore it
Record the issue in your lab notes
Examples of issues and how to address them:
Unstable readings
If a digital voltmeter's reading is fluctuating, you may need to wait for it to stabilise or record the central value and estimate the fluctuation as an uncertainty
Record how you handled it
An anomalous result (outlier)
If one of your repeat trials gives a result that is very different from the others, record it, and then conduct an additional trial to get a reliable set of results
Do not erase the outlier; you will justify its exclusion later
Difficulty in measuring
If the length of a spring is difficult to measure because it is oscillating, make a note of this
This is a limitation of your method that you will discuss in your evaluation
Worked Example
Research question:
"What is the relationship between the length of a simple pendulum and its period of oscillation?"
Quantitative data:
The raw data would be recorded in a table like the one below, showing the measured length and the raw times for multiple oscillations.
Length / m (±0.001) | Time for 20 oscillations (Trial 1) / s (±0.2) | Time for 20 oscillations (Trial 2) / s (±0.2) | Time for 20 oscillations (Trial 3) / s (±0.2) |
|---|---|---|---|
0.200 | 17.9 | 18.0 | 17.9 |
0.400 | 25.4 | 25.3 | 25.5 |
0.600 | 31.1 | 31.2 | 31.0 |
0.800 | 35.8 | 37.1 (anomalous) | 35.9 |
1.000 | 40.1 | 40.2 | 40.2 |
Qualitative data:
For the longest length (1.000 m), the pendulum bob was observed to swing slightly from side to side, not just in a single plane
The retort stand base was seen to wobble slightly when the pendulum was swinging at the 0.800 m length
Issue addressed during collection:
For the 0.800 m length, trial 2 gave a time of 37.1 s, which was significantly different from trial 1 (35.8 s)
This was identified as a potential outlier, possibly due to the stand wobbling
A fourth trial was conducted, which gave a time of 35.9 s, confirming the second trial was anomalous and could be excluded
Worked Example
Research question:
"What is the relationship between the length of a constantan wire and its electrical resistance?"
Quantitative data:
The raw data would include the potential difference and current for each measured length
Length / m (±0.001) | Potential Difference / V (±0.01) | Current / A (±0.01) |
|---|---|---|
0.100 | 0.45 | 1.52 |
0.200 | 0.88 | 1.51 |
0.300 | 1.35 | 1.53 |
0.400 | 1.81 | 1.52 |
0.500 | 2.24 | 1.51 |
Qualitative data:
The readings on the digital ammeter fluctuated by approximately ±0.02 A before settling
After taking the reading at 0.500 m, the constantan wire felt slightly warm to the touch
Issue addressed during collection:
Initially, the crocodile clips were making poor contact with the resistance wire, causing the multimeter readings to be unstable
The clips were removed, the contact points on the wire were cleaned lightly with sandpaper, and the clips were re-attached firmly
This resulted in stable readings for both current and potential difference
Examiner Tips and Tricks
Record raw data directly.
Never perform calculations in your head or on scrap paper.
Your raw data table must show the actual measurements you took (e.g., potential difference and current, not just the calculated resistance).
Units and uncertainties belong in the headers.
This is the correct scientific convention and makes your tables clear and easy to read.
Avoid writing units after every number in the table body.
Your observations are evidence.
Don't treat qualitative data as an afterthought.
Good observations can be used as evidence in your conclusion and evaluation to explain why your results might differ from theoretical values (e.g., "the wire was observed to be warm, indicating heating may have increased its resistance").
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