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Tools and Inquiry — IB Physics skills

I.2.3 Interpreting results in Physics — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 20 of 22 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Lesson summary

I.2.3 Interpreting results in Physics — IB PHYSICS SL notes: Interpret data and graphs, explain trends and anomalies, and assess accuracy, precision, reliability and validity.This is the "sense-making" phase — analysing processed data for patterns, trends and relationships.The go…

  • y-intercept
  • Area under the curve
  • Error bars
  • Link the graph to theory
  • Use the gradient to find the physical quantity
  • Highlight it on the graph

Part 1 of 7Aim of interpretation

1This is the "sense-making" phase
  • This is the "sense-making" phase analysing processed data for patterns, trends and relationships.
2
The goal is to decide what the results say, to answer the research question.
3
It almost always involves a graph of the independent against the dependent variable.

Part 2 of 7Qualitative and quantitative data together

Qualitative and quantitative data together
Qualitative observations are evidence that explains quantitative results.
A resistivity higher than the accepted value may be explained by a wire that felt warm.
  • Current heated the wire, raising its resistance and the calculated resistivity.

Part 3 of 7Features of a graph

Features of a graph
Gradient
  • Gradient can represent a key physical quantity.
  • For I against V of an ohmic resistor the gradient is 1/R.
y-intercept
  • y-intercept the value of the dependent variable when the independent variable is zero.
  • It can reveal systematic errors, such as a zero error.
Area under the curve
  • Area under the curve a total quantity.
  • The area under a velocity–time graph is displacement; under force–distance it is work done.
Error bars
  • Error bars show the precision of the data.
  • Large bars suggest significant random error.
  • A line passing through all error bars indicates a good fit.

Part 4 of 7Describing and explaining trends

Describing and explaining trends
Two steps.
  1. Describe the trend using terms such as directly proportional, linear positive correlation, inversely proportional or exponential increase.
  2. Explain the trend with physical principles and equations — this is the most important part.
Link the graph to theory
  • T2 against L is a straight line through the origin because T2 = (4π2/g)L.
Use the gradient to find the physical quantity
  • Use the gradient to find the physical quantity e.g. g = 4π2/gradient — and compare it with the accepted value.

Part 5 of 7Anomalous results

1
An anomalous result clearly does not fit the overall trend.
2Highlight it on the graph
  • Highlight it on the graph never delete data without comment.
3
Justify it using physics, not "it looks odd".
  • Link it to a likely experimental error, e.g. a delay in starting the stopwatch.
4
Exclude it from the average and the graph after justifying.

Part 6 of 7Accuracy, precision, reliability and validity

TermMeaningAffected by
AccuracyHow close the final result is to the accepted valueSystematic errors
PrecisionHow close repeat measurements are to each otherRandom errors
ReliabilityConsistency of results when repeatedRepeats and consistent method
ValidityFair test with relevant, controlled variablesControl of other variables
Comment on accuracy only when a literature value is available.
A small spread in the data shows high precision.
Repeating and averaging improves accuracy and shows anomalies.
Reliability question
  • Reliability question would similar conclusions be reached if someone repeated the experiment?
Validity question
  • Validity question how relevant is the experiment to the original research question?
  • Charles' law needs pressure to be kept constant.

Part 7 of 7More from the specification

More from the specification
Simple pendulum period and length
  • Simple pendulum period and length the period squared of a simple pendulum is directly proportional to its string length T2 ∝ L.
Percentage uncertainty calculation
  • Percentage uncertainty calculation calculated by dividing absolute measurement uncertainty by the recorded value and multiplying by one hundred percent.
Repeated measurement trials
  • Repeated measurement trials conducting multiple experimental trials reduces random errors, yielding a more reliable mean result to record in data tables.
Timing Oscillations in Pendulum Experiments
  • Measuring the time for multiple complete swings in each trial reduces the percentage uncertainty in the period T.
    • (e.g. 10 or 20 oscillations)
Propagating Uncertainties in Physics Equations
  • When squaring or taking square roots of experimental variables, the fractional uncertainty is scaled by the power.
    • (such as length L in pendulum formulas)
Error Bars and Worst-Fit Gradients
  • Experimental data plotted on Cartesian axes with error bars allow construction of maximum and minimum lines of best fit to determine gradient uncertainty.
Raw Data Recording Standards
  • Raw measurements must be recorded to the resolution of the measuring instrument, with consistent decimal places and explicit uncertainty statements.
Simple pendulum
  • A small bob on a light string of length L
Repeat trial
  • Repetition of a measurement to improve reliability.
Uncertainty
  • An estimate of the range in which the true value is expected to lie.
Length
  • Resistance is directly proportional to the length of a wire.
Pendulum
  • A weight swinging on a string, exchanging GPE and KE.
Period
  • Time for one revolution.
Decimal place
  • A digit to the right of the decimal point.
Potential difference
  • Work done by a unit charge passing through a component.
Significant figures
  • Digits carrying meaningful information in a number.
Percentage uncertainty
  • The absolute uncertainty divided by the value, times 100%.

Key terms

TermMeaning
GradientChange in y divided by change in x; often a physical quantity.
y-interceptValue of y at x = 0; can reveal systematic error.
AccuracyCloseness of a result to the accepted value.
PrecisionCloseness of repeated measurements to each other.
ReliabilityConsistency of results when repeated.
ValidityWhether the experiment is a fair test of the research question.

Avoid these mistakes

1
Describing a trend without explaining it with physics.
2
Confusing accuracy (closeness to the true value) with precision (closeness of repeats).
3
Commenting on accuracy when there is no accepted value to compare.
4
Deleting anomalous points without comment.
5
Ignoring a non-zero y-intercept that points to a systematic error.

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