Millikan experiment
About this app
Pick an oil droplet in the microscope, adjust the voltage until it hovers and save the measurement. The histogram grows block by block and multiples of e appear. With the balancing method, the rise and fall method, worked solutions and exam tasks.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
What the Millikan experiment shows
From 1909 Robert Millikan measured the charge of single oil droplets in a parallel-plate capacitor. His result: electric charge occurs only as a whole-number multiple of the elementary charge, q = n·e with e = 1.602·10⁻¹⁹ C. While a droplet hovers, the electric force q·V/d balances its weight. Because the mass of such a small droplet is unknown, you also measure the fall speed and get the radius from Stokes’ law.
What the app shows
You look through a microscope at glowing droplets between two capacitor plates. The selected droplet shows arrows for weight, electric force and drag. The droplets follow Stokes’ law and jitter through Brownian motion, so the measurements scatter as they do in the lab. Each measurement drops into a histogram of charges as a block. A comb with equal spacing shows the multiples of e.
Try it yourself
Pick a droplet and adjust the voltage until it hovers. Save the measurement and read the working with your own values. Recharge the droplet and watch the balancing voltage jump. Have more droplets measured until clusters and gaps appear, then drag the comb onto the clusters. Prediction questions and exam tasks with random values test the balancing method and the rise and fall method.
Frequently asked questions
- What did Millikan prove with the oil drop experiment?
- He showed that electric charge is quantised. All measured charges were whole-number multiples of a smallest charge, the elementary charge e = 1.602·10⁻¹⁹ C.
- How do you calculate the charge of a hovering oil droplet?
- The fall speed without a field gives the radius through Stokes’ law: r = √(9ηv₀ / (2(ρ_oil − ρ_air)g)). That gives the weight minus buoyancy. While hovering, q = F_g·d/V.
- What is the difference between the balancing method and the rise and fall method?
- The balancing method uses the balancing voltage and the fall speed without a field. The rise and fall method measures, at the same voltage, the rising speed and the falling speed after reversing the polarity. It needs no exact balance point.
- What can I change in the app?
- You pick droplets, set the voltage, reverse the polarity, recharge droplets and switch the method. You can also measure ten droplets automatically, drag the comb in the histogram and work through exam tasks.
Subject: School physics
More from School physics
- The exposure triangle
- Density and buoyancy
- The Doppler effect
- Additive and subtractive colour mixing
- Franck–Hertz tube (3D)
- Photoelectric effect (3D lab)
- Newton’s cradle and collisions
- Particle in a box (3D)
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