Milikan's Oil Drop Experiment (SL IB Physics)

• This experiment was conducted by Millikan and Fletcher in 1909
• It determined the value of the fundamental elementary charge

Method for Millikan's Oil Drop Experiment

• A fine mist of oil drops is sprayed into a chamber 
  • Oil is used instead of water because it does not evaporate quickly
  • This means the mass of the drops will remain constant

• As the drops pass out of the spray nozzle they are charged by friction (alternatively, they can also be ionised by X-rays)
  
  • Some drops lose electrons and become positively charged
  • Some drops gain electrons and become negatively charged

• The drops pass into a region between two metal plates and are viewed using a microscope

Equipment Set Up for Millikan's Oil Drop Experiment

In Millikan's Oil Drop Experiment oil is sprayed into a chamber before passing between metal plates where the electric and gravitational forces are compared

Electric vs Gravitational Force

No Electric Field

• The oil drops fall under gravity between the metal plates
• They reach a terminal velocity when the air resistance and gravitational force acting on the drop are equal

With Electric Field

• A potential difference is applied between the metal plates which creates an electric field
• The charged oil drops begin to rise when the electric field is strong enough
• This means the upward electrical force is greater than the gravitational force
• The equation for electric force is: 

• Where:
  • E = electric field strength (N C^-1)
  • F = electrostatic force on the charge (N)
  • q = charge (C)

• The distance the drops rise depends upon their mass
• With the correct potential difference applied, the electric and gravitational forces can become equal and opposite
• The equation for gravitational force, which comes from Newton's second law, is:

• Where:
  • W = weight of drop (N)
  • m = mass of drop (kg)
  • g = gravitational field strength (N kg⁻¹)

• By equating the electric and gravitational forces of the drops, the value of fundamental charge was determined to be 1.60 × 10⁻¹⁹ C
• The magnitude of the charge on any object is found to be a multiple of 1.60 × 10⁻¹⁹ C 
• Therefore, Millikan's experiment provides evidence for the quantisation of charge