Millikan's Experiment
Physics: AQA A Level, topic 12.1.4
Millikan's Experiment
Millikan's experiment found the charge and mass of the electron.
Millikan's oil-drop experiment
- Robert Millikan performed an experiment that discovered the charge on an electron.
- Millikan combined his results with Thomson’s specific charge of the electron:
- This allowed him to discover the mass of the electron.
Stokes' law
- Stokes’ law gives the viscous force on a sphere in a fluid.
- F = 6πηrv
- F is the viscous force.
- η is the viscosity of the fluid.
- r is the radius of the sphere.
- v is the velocity of the sphere.
Method
- Millikan used an atomiser to create a mist of charged oil droplets which fell into a viewing chamber.
- He then applied a potential difference across the viewing chamber to create an electric field.
- He could then study the motion of these charged particles in a variable electric field.
Analysis
- By performing the experiment with no electric field, he could see the motion under only the weight of the drop and viscous force.
- He used the velocity of the particles and Stokes’ law to calculate the radius of the particles.
- Once the electric field was turned back on, he could adjust the field to hold the droplets still.
- This allowed him to equate weight to electric field, and hence calculate the charge on each droplet.
- Each droplet’s charge had a common factor of 1.6×10-19C, which is the charge on one electron.
Test yourself on Millikan's Experiment
What does η represent in Stokes' law?
Millikan's Oil Drop Experiment