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Physics 11–12 · Year 12

Modelling Millikan's oil drop experiment: quantised charge from blind mass measurements

Module 8: From the Universe to the Atom (Structure of the Atom)

Practical, model not builtLow risk

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The idea

Charges on oil drops come in whole multiples of one unit, found by balancing a drop's weight against an electric force and looking for the common factor in the results.

What you need

  • Opaque containers of equal mass (film canisters or small boxes), each holding an unknown number (1 to 8) of identical small objects such as coated chocolates or dominoes, sized to fit, and an electronic balance reading to 0.01 g (the NSW Department of Education Module 8 guide method)
  • Optional: school Millikan apparatus with a microscope, plates and 0 to 500 V supply (teacher-operated)

How to do it

  1. Weigh each sealed canister and subtract the mass of an empty one; tabulate the net masses without opening any.
  2. Order the results and look for the smallest difference between them; test whether every net mass is a whole multiple of that unit.
  3. Report the unit mass and the count in each canister; then open them to check.
  4. Simulation: balance the electric force on a drop of set radius against its weight and read the charge; repeat for ten drops and find the common factor.

What you should see

Net masses fall only at whole multiples of one value (if each object is 5.2 g, at 5.2, 10.4, 15.6 g and so on), so the unit mass emerges without ever seeing inside. In the simulation, a drop of radius 0.75 micrometre and density 900 kg/m^3 has mass 1.59e-15 kg and weight 1.56e-14 N; with plates 5.0 mm apart it balances at 487 V for one elementary charge, 243 V for two and 162 V for three, all inside the 0 to 500 V range of the school apparatus, and the charges read 1.602e-19 C multiples.

What changes

What you change
number of objects in a canister (unknown)
What you measure
net mass
What you keep the same
  • identical objects
  • identical canisters
  • same balance

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Charge can take any value; it comes in whole multiples of e.
  • Millikan measured the electron's mass directly; he measured its charge, and mass followed from Thomson's e/m.

Safety card

Low riskLearners carry it out

Hazards

  • school Millikan apparatus uses up to 500 V (teacher-only)

Controls

  • low-voltage model version for learners

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-15PH11/12-5PH11/12-6
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/sites/default/files/noindex/2025-03/physics-stage-6-syllabus-2017.docx
  2. education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/physics/12Physics_-module-8-guide.docx
  3. www.bipm.org/en/measurement-units/si-defining-constants
  4. pdg.lbl.gov/2024/reviews/rpp2024-rev-phys-constants.pdf

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