Millikan’s Oil Drop Experiment E-practical

A realistic 3D virtual physics experiment

Determination of the charge on an electron using Millikan's method. Measure the separation of the plates in the electrostatic chamber, introduce polymer spheres to the chamber, adjust the HT and polarity, record the voltage, observe the motion of the spheres - just as you would in the laboratory.

Realistic experimental technique

E-practicals are 3d, highly realistic emulations based on a blend of theory and the actual practicals, giving an authentic experience of doing the actual practical in a laboratory. Students operate the apparatus, make measurements and obtain their own results. It is designed to reproduce the experience of performing the practical rather than simply demonstrating the underlying physics. Like a real experiment, it can be done badly. Careless setup or measurements can produce poor results, so students have to apply experimental technique and take care with their measurements.

Useful when laboratory access is limited

Ideal for preparation before a practical, revision afterwards, missed practicals, home study, or demonstrating an experiment when setting up the real apparatus isn't practical.

Works directly in a browser

No installation is required. Windows, Mac and suitable mobile devices can run the WebGL e-practical.

Watch it in action

This shows how the e-practical can be used. In particular it shows how the student or teacher interacts with the apparatus and how the interaction can be controlled using the mouse or the alternative on-screen controls.

What you get

The application is supplied as a PowerPoint which provides background, a synopsis of what the student will learn, a description of the Physics involved, a description of the apparatus, the independent and dependent variables, the method, a link to the interactive online e-practical, analysis of the results and further discussion.

School/Department licence £9.99

Valid for 1 year

Introduction

This experiment is one of the most important ever performed. Thomson had identified the electron as a charged particle in 1897 and had measured its charge to mass ratio. The next step was to obtain a value for either the mass or the charge.

Robert Millikan was born in Illinois in 1868 and he obtained his Physics doctorate in 1895, just two years before Thomson’s experiment.

In our version of the experiment tiny (1.02 microns across) polymer balls are used instead of oil. We use an equation of motion that balances the forces due to gravity, drag and the electrostatic. As with Millikan’s version, we get a distribution of the number of charged electrons on our polystyrene balls.

What students do

Find the separation of the plates in the electrostatic chamber. You can ‘explode’ the chamber into its constituent parts by clicking on it. When you do this a micrometer screw gauge will automatically move to measure the ring that separates the plates. The chamber will only ‘explode’ when the power is switched off).
From the top, you can see the retaining bolts, the top plate, the separating ring with the micrometer screw gauge and the bottom plate which is just above the body of the chamber.
Rotate the barrel of the gauge to obtain an accurate reading for the height of the ring which corresponds to the separation of the plates.
You can zoom and out in using the plus and minus keys on the number pad or main keyboard. With the mouse over the rotating barrel of the micrometer, use the mouse wheel to tighten the jaws of the micrometer onto the separating ring. When the barrel will not move any further you can then read the micrometer. When you have finished measuring the width with the micrometer a second click on the chamber will remove the micrometer and reassemble the chamber.
switch on the power supply by clicking on its on/off button at the lower-right of the unit.
You can change the voltage and polarity using the switches on the main unit beneath the chamber.
You can read the voltage from the meter next to the power supply.
Clicking on the bulb of the atomiser with the left mouse button will introduce the polymer spheres into the chamber.
Each division in the reticle is 2.5 x 10-4 m.
Create a table with headings for the time, the number of divisions the velocity in divisions per second, the velocity in metres per second, the force due to drag and the charge. Your table should look like this:

Want to experience an e-practical yourself?

Try our free Magnetic Field for a Coil e-practical.

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This section is adapted from material developed by Dr Robert Lucas and is related to the book High School and Undergraduate Physics Practicals, published by CRC Press.