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6.2.2 Millikan

By the end of this lesson you can:

  • Describe Millikan's oil drop experiment
  • Explain how the charge of the electron was found
  • Explain what it means for charge to be quantised
Find your course: NSW Year 11NSW Year 12: Module 8, IQ2QLDVICAPIB: D.2 Electric and magnetic fields
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Learn the idea: a short explanation, then a check.

In 1897, J.J. Thomson successfully determined the charge-to-mass ratio (q⁄m) of the electron. However, because neither the electron's charge nor its mass could be measured independently, both fundamental values remained unknown at the time.

To solve this, American physicist Robert Millikan designed his famous oil drop experiment in 1909 to determine whether electric charge existed in discrete, fundamental quantities. By measuring the motion of microscopic, charged oil droplets suspended between two parallel metal plates under the influence of gravity and an electric field, Millikan proved that electrical charge is quantized—occurring only in integer multiples of a fundamental unit (e ≈ 1.602 × 10-19 C).

Combining this newly determined elementary charge with Thomson’s charge-to-mass ratio allowed physicists to calculate the mass of the electron (me ≈ 9.109 × 10-31 kg) for the very first time.

 
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Test your predictions in a simulation.

Interactive

​Here is a simple yet instructive interactive for Millikan's experiment, from Physics Aviary

Best process is 

1. Record both voltage and radius of oil droplet in a spreadsheet 

2. For each droplet, determine the mass by calculating the volume and using the supplied density

3. Work out the charge for each by equating the two forces (electric field and gravitational field) (You will have to determine the electric field from the voltage)

4. Carefully examine the charge values - they are multiples of a set value

This activity allows students to simulate a simplified version of Robert Millikan's Oil-Drop experiment. Instructions are given under the simulation. ​By Tom Walsh