Measured the elementary charge of the electron, 1.602 x 10^-19 coulomb, by balancing charged oil drops in an electric field.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
In 1909 Robert Millikan suspended charged oil droplets between two plates by balancing gravity against an upward electric force. By adjusting the voltage until a drop hung motionless, he determined the charge on each drop. Every charge he measured was an integer multiple of a basic value, 1.602 x 10^-19 C, which showed that electric charge is quantized. Combining this charge with Thomson's charge-to-mass ratio gave the electron's mass. The experiment established the elementary charge as the smallest unit of electric charge.
Key ideas: Charged oil droplets were held stationary between plates when the electric force balanced gravity: qE = mg, so q = mg / E. Measured charges were always whole multiples of 1.602 x 10^-19 C, showing charge is quantized. Combined with e/m, this gives the electron mass 9.11 x 10^-31 kg.
Common mistakes and connections: Students believe the experiment measured the mass of the electron directly, but it measured charge, with mass obtained afterward from Thomson's e/m. Another error is using the drop mass from density without correction for air resistance (Stokes law is used). It completes the electron's description begun by cathode rays.
Measured oil drop charges are always whole multiples of 1.6e-19 C, showing electric charge is quantized in units of the electron charge. The interactive lesson shows this as a live animation or chart.
Drop charges of 3.2 x 10^-19 C and 4.8 x 10^-19 C are 2e and 3e, respectively.
Measured the elementary charge of the electron, 1.602 x 10^-19 coulomb, by balancing charged oil drops in an electric field.
Millikan's Oil Drop Experiment is a intermediate topic in Atomic Structure & Periodicity. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Atomic Structure & Periodicity first: Molar Mass, Mass Spectrometry, Cathode Rays.
The next topics in Atomic Structure & Periodicity are Electromagnetic Radiation and Wavelength and Frequency.
Yes. The interactive lesson includes a self-check question on Millikan's Oil Drop Experiment, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Millikan's Oil Drop Experiment is topic 31 of 115 in Atomic Structure & Periodicity.