Chemistry · Virtual lab
Millikan's Oil Drop
Catch a charged oil drop, weigh it by timing its fall, and find the charge of the electron as the smallest step every droplet is a multiple of.
Background
Robert Millikan's oil drop experiment (1909) measured the electric charge of a single electron. Tiny oil droplets were sprayed into a chamber and ionised by X-rays. By adjusting the electric field between two plates, a single charged droplet could be suspended in mid-air, balancing the gravitational and electric forces.
By measuring the droplet's terminal velocity with the field off (to find its mass) and the field strength needed to suspend it (to find its charge), Millikan determined that every droplet carried a charge that was an integer multiple of a fundamental unit, the elementary charge e ≈ 1.6 × 10⁻¹⁹ C.
In this virtual lab you catch a charged oil drop, time its fall to measure terminal velocity, apply the electric field to suspend it, and calculate the charge. Repeating with several drops reveals the quantised nature of electric charge.
What you will learn
- Understand the principle of balancing gravitational and electric forces
- Measure the terminal velocity of a falling oil drop
- Suspend a charged drop and determine its charge
- Demonstrate that charge is quantised in units of e
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Related experiments
- Thomson's Cathode Ray Tube Experiment: Deflect a cathode ray with electric and magnetic fields, balance the two forces, and measure the charge-to-mass ratio of the electron.
- Rutherford's α-Particle Scattering: Fire α-particles at a thin gold foil, count where they scatter, and discover the tiny dense nucleus at the centre of the atom.