``` ← Physics ```
```

EDEXCEL INTERNATIONAL A LEVEL PHYSICS • UNIT 3

Physics 03

Master practical physics through experimental planning, measurements, data analysis, uncertainties and exam-focused practice.

Practical Skills Uncertainty Data Analysis Core Practicals Exam Practice
```

UNIT OVERVIEW

Practical Skills in Physics I

Unit 3 focuses on the practical skills developed while studying Units 1 and 2. You are not simply tested on remembering experiments. You must be able to plan investigations, evaluate measurements, process data and draw scientifically justified conclusions.

Exam focus:

Unit 3 is a written practical-skills examination. Questions can involve planning, measurements, calculations, graphs, uncertainties, evaluation and scientific reasoning.

Assessment at a glance

TOPIC 01

Planning an Experiment

Planning questions ask you to design an experiment that can investigate a specified relationship between variables.

Independent Variable

The variable deliberately changed by the experimenter.

Dependent Variable

The quantity measured in response to the independent variable.

Control Variables

Other relevant quantities that should be kept constant so that the investigation is a fair test.

Strong planning answer:

State what will be changed, what will be measured, what will be controlled, what apparatus will be used, how measurements will be taken and how the results will be analysed.

Typical Planning Structure

  1. Identify the independent variable.
  2. Identify the dependent variable.
  3. Identify important control variables.
  4. Select suitable apparatus.
  5. State the range of values to investigate.
  6. Explain how measurements will be taken.
  7. Repeat measurements where appropriate.
  8. Explain how the data will be processed.
  9. Identify relevant safety precautions.
  10. Identify possible sources of uncertainty.

TOPIC 02

Apparatus and Measuring Instruments

Choosing appropriate measuring equipment is an important practical skill. The instrument should have suitable range and resolution.

Resolution

Resolution is the smallest change in a quantity that an instrument can normally display or measure.

Vernier Calipers

Vernier calipers can be used for measurements such as diameter, internal diameter and depth.

Typical resolution considered in Unit 3: 0.1 mm

Micrometer Screw Gauge

A micrometer is suitable for measuring very small dimensions, such as the diameter of a thin wire.

Typical resolution considered in Unit 3: 0.01 mm

Calibration and Zero Error

Before taking measurements, check whether the instrument reads zero when it should. A non-zero initial reading can introduce systematic error.

Exam tip:

Do not simply say "use a better instrument". Explain why the alternative instrument improves the measurement.

TOPIC 03

Implementation and Measurements

In Unit 3 you may be given an experiment performed by another student and asked to evaluate the quality of the measurements.

Number of Readings

A suitable number of readings should be taken to establish the relationship clearly and reduce the effect of random variation.

Range of Readings

The independent variable should normally cover a sufficiently wide range to make the relationship easier to identify.

Repeats

Repeating measurements can help identify anomalous readings and reduce the influence of random errors.

Significant Figures

Measurements and calculated quantities should be recorded to a sensible number of significant figures consistent with the measuring instrument and data provided.

Anomalous Results

An anomalous result is one that does not fit the general pattern of the other measurements.

Important:

Do not automatically delete an anomalous result. Identify it, investigate possible causes and explain whether it should be excluded from further analysis.

TOPIC 04

Uncertainty and Error

Measurements are never perfectly exact. Unit 3 requires you to understand uncertainty and distinguish it from systematic error.

Random Error

Random errors cause readings to vary unpredictably around an underlying value.

Systematic Error

Systematic errors shift measurements consistently in one direction. Repeating the measurement does not normally remove this type of error.

Reducing Random Error

Reducing Systematic Error

TOPIC 05

Percentage Uncertainty

Percentage uncertainty allows the size of an uncertainty to be compared with the measured quantity.

Percentage uncertainty = (absolute uncertainty / measured value) × 100%

Single Measurement

For a single measurement, the specification uses half the instrument resolution as the uncertainty where appropriate.

Absolute uncertainty ≈ half the instrument resolution

Repeated Measurements

When multiple readings are available, the half-range method can be used to estimate uncertainty.

Absolute uncertainty = (maximum − minimum) / 2
Unit 3 point:

For the Unit 3 uncertainty calculations covered here, you are not expected to compound percentage uncertainties.

TOPIC 06

Processing Experimental Data

Experimental results often need to be transformed before a physical relationship becomes clear.

Tables

Calculations

Use the correct equation, substitute values carefully and give the final answer with appropriate units and significant figures.

Graphs

Gradient

The gradient of a graph can be used to determine a physical constant or establish the relationship between variables.

Gradient = change in y / change in x

When finding a gradient from experimental data, use a large triangle on the line of best fit rather than using two nearby experimental points.

TOPIC 07

Identifying Relationships from Graphs

Unit 3 may require you to determine a relationship between variables using experimental data.

Direct Proportion

y ∝ x

A graph of y against x should be a straight line through the origin for a direct proportionality.

Linear Relationship

y = mx + c

The gradient and intercept can provide physical information about the experiment.

Testing a Relationship

If theory predicts a relationship involving a power or transformed quantity, plot the appropriate variables and examine whether the resulting graph is linear.

Example

Suppose theory predicts that y is proportional to x². A plot of y against x² should produce a straight line through the origin if the model is supported by the data.

TOPIC 08

Evaluation and Improving Experiments

Evaluation questions ask you to identify weaknesses in an investigation and suggest realistic improvements.

Weak Evaluation

"The experiment has errors."

Strong Evaluation

Identify the specific source of error, explain how it affects the result and give a realistic method of reducing it.

Good evaluation structure:

Problem → effect on measurement → improvement.

Common Improvements

CORE PRACTICAL 01

Acceleration of a Freely Falling Object

Aim

Determine the acceleration of an object undergoing free fall.

Key idea

Measure appropriate quantities such as distance and time and use a suitable model of motion to determine gravitational acceleration.

Important considerations

  • Measure distance carefully.
  • Use an appropriate timing method.
  • Repeat measurements where possible.
  • Reduce timing and distance uncertainties.
  • Consider the effect of air resistance.

CORE PRACTICAL 02

Determining the Viscosity of a Liquid

Aim

Use a falling-ball method to investigate the viscosity of a liquid.

Principle

A small sphere falling through a viscous liquid experiences resistive forces. Measurements can be used with the appropriate model to determine the viscosity.

F = 6πηrv

Important considerations

  • Use a suitable spherical object.
  • Measure the radius accurately.
  • Measure the terminal-speed region.
  • Control temperature because viscosity depends on temperature.
  • Reduce parallax and timing errors.

CORE PRACTICAL 03

Determining Young Modulus

Aim

Determine the Young modulus of a material by measuring force, extension and the original dimensions of a sample.

Key relationships

Stress = Force / Cross-sectional area
Strain = Extension / Original length
Young modulus = Stress / Strain

Important measurements

  • Original length.
  • Diameter or cross-sectional dimensions.
  • Applied force.
  • Extension.

Important errors

  • Small extension can produce a large percentage uncertainty.
  • Diameter measurements are especially important because area depends on diameter squared.
  • Parallax can affect scale readings.
  • Zero errors should be checked.

CORE PRACTICAL 04

Speed of Sound Using an Oscilloscope

Aim

Determine the speed of sound in air using a two-beam oscilloscope, signal generator, speaker and microphone.

Principle

The wavelength can be determined from the distance between positions corresponding to a known phase relationship, while the frequency is provided by the signal generator.

v = fλ

Key considerations

  • Use a suitable frequency.
  • Measure distances carefully.
  • Identify repeated phase positions where appropriate.
  • Use multiple wavelength intervals to reduce percentage uncertainty.

CORE PRACTICAL 05

Vibrating String or Wire

Aim

Investigate how the frequency of a vibrating string or wire depends on its length, tension and mass per unit length.

v = √(T / μ)

The investigation can be designed to vary one quantity while controlling the others.

Variables to investigate

  • Length of the vibrating section.
  • Tension in the string.
  • Mass per unit length.

Practical considerations

  • Keep the other relevant variables constant.
  • Identify the fundamental frequency or a known harmonic.
  • Take measurements over a useful range.
  • Repeat measurements where appropriate.

CORE PRACTICAL 06

Wavelength of Light Using a Diffraction Grating

Aim

Determine the wavelength of light from a laser or another suitable light source using a diffraction grating.

nλ = d sinθ

Measurements

  • Grating spacing.
  • Distance from grating to screen.
  • Position of diffraction maxima.
  • Order number of the maximum.

Reducing uncertainty

Measuring the separation between symmetric diffraction maxima can reduce the relative uncertainty in the position measurement.

Safety:

Never look directly into a laser beam. Follow appropriate laboratory safety procedures.

CORE PRACTICAL 07

Electrical Resistivity of a Material

Aim

Determine the electrical resistivity of a material by measuring its resistance and dimensions.

R = ρL / A

Rearranged form

ρ = RA / L

Measurements

  • Resistance.
  • Length.
  • Diameter or cross-sectional area.

Important consideration

If the material heats up significantly, its resistance may change. Keep the current and measurement conditions controlled where necessary.

CORE PRACTICAL 08

e.m.f. and Internal Resistance of a Cell

Aim

Determine the electromotive force and internal resistance of an electrical cell.

e.m.f.

The e.m.f. represents the energy supplied by the source per unit charge.

Internal Resistance

A real cell has internal resistance, so the terminal potential difference decreases when current is drawn.

V = E − Ir

A graph of terminal potential difference against current can be used to determine the e.m.f. and internal resistance.

Graph interpretation:

The y-intercept gives the e.m.f., while the magnitude of the gradient gives the internal resistance.

TOPIC 09

Graphs, Gradients and Intercepts

Many Unit 3 questions provide experimental data and ask you to determine a physical quantity from a graph.

Gradient

Gradient = Δy / Δx

Select two well-separated points on the best-fit line rather than automatically using two experimental data points.

Intercept

The intercept may represent a physical constant or another quantity predicted by the experimental model.

Line of Best Fit

A good best-fit line should represent the overall trend of the experimental data, with points distributed reasonably around it.

Using a Graph to Find a Constant

First determine the equation represented by the graph. Then compare the gradient or intercept with the theoretical equation to identify the required physical constant.

TOPIC 10

Units and Significant Figures

Correct presentation of experimental results is essential.

Units

Always identify the appropriate unit for a physical quantity.

Significant Figures

Avoid giving calculated results with unjustifiably excessive precision.

Scientific Notation

Scientific notation is useful for very large or very small physical quantities.

Example: 0.0000032 = 3.2 × 10⁻⁶

TOPIC 11

Health and Safety

A practical plan should identify hazards and explain how the associated risks can be reduced.

Good Safety Answers

Example

If a heated component is used, the hazard is the hot surface. A suitable control measure would be to allow it to cool before handling and avoid unnecessary contact.

TOPIC 12

Evaluating Experimental Conclusions

A conclusion should be based on the evidence collected rather than simply repeating the expected theory.

Good Conclusion

Remember:

A result can support a model without proving that the model is universally true.

PRACTICE

Unit 3 Mixed Practice

1. Explain why repeated measurements are useful in an experiment.
Show answer

Repeated measurements help identify random variation and anomalous results and can allow a more reliable mean value to be obtained.

2. A ruler has a resolution of 1 mm. Estimate the absolute uncertainty associated with a single measurement.
Show answer

Using half the resolution, the uncertainty is approximately ±0.5 mm.

3. A length is measured as 50.0 mm with an uncertainty of 0.5 mm. Calculate the percentage uncertainty.
Show answer

Percentage uncertainty = (0.5 / 50.0) × 100 = 1.0%.

4. State two ways of reducing random uncertainty.
Show answer

Repeat measurements and calculate a mean. Taking more measurements over a suitable range can also improve the reliability of the result.

5. Explain why systematic error is not normally reduced by repeating a measurement.
Show answer

A systematic error shifts measurements consistently in the same direction, so repeating the measurement reproduces the same bias.

6. State two features that should be included on a scientific graph.
Show answer

Clearly labelled axes and appropriate units should be included. A suitable scale and accurately plotted data are also required.

7. Why should a large triangle be used when calculating a gradient from a line of best fit?
Show answer

A large triangle makes the changes in x and y easier to measure accurately, reducing the percentage uncertainty in the calculated gradient.

8. Give the equation used to determine the speed of a wave.
Show answer

v = fλ

9. A wire has resistance R, length L and cross-sectional area A. State the equation for resistivity.
Show answer

ρ = RA / L

10. In an experiment investigating Young modulus, explain why measuring the diameter of a wire accurately is important.
Show answer

The cross-sectional area depends on the square of the diameter, so an uncertainty in diameter can significantly affect the calculated area and therefore the Young modulus.

11. A graph of terminal potential difference V against current I has equation V = 1.50 − 0.80I. What are the e.m.f. and internal resistance?
Show answer

Comparing with V = E − Ir: e.m.f. = 1.50 V and internal resistance = 0.80 Ω.

12. Give three features of a strong experimental plan.
Show answer

A strong plan identifies suitable apparatus, controls relevant variables, explains how measurements will be taken, uses an appropriate range, considers repeats and uncertainty, and includes suitable safety precautions.

QUICK REVIEW

Unit 3 Key Ideas

Planning

Measurements

Data Analysis

Uncertainty

Core Practicals

← Back to Physics ⌂ Home