12.1.4

# Millikan's Experiment

Test yourself

## 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.