Milikan's Oil Drop Experiment (SL IB Physics)

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

Method for Milikan's Oil Drop Experiment

• A fine mist of atom-sized 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 ionised by X-Rays
  • This consequently changes their charge from neutral
  • They will become positively charged if they lose electrons 
  • They will become negatively charged if they gain electrons
• The drops pass into a region between two metal plates and are viewed using a microscope

Equipment Set Up for Milikan's Oil Drop Experiment

In Milikan'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

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 electrical force equation 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
  • The bigger the mass, then the shorter the distance they rise 
  • The gravitational force equation, which comes from Newton's second law is:

• • Where:
    • W = weight of drop (N)
    • m = mass of drop (kg)
    • g = gravitational field strength (m s⁻²)
• Through balancing the electric and gravitational forces on the drops the value of fundamental charge was determined to be 1.60 × 10⁻¹⁹ C
• The magnitude of charge on any object is found to be a multiple of 1.60 × 10⁻¹⁹ C
• This is called the quantisation of charge