Millikan's Oil Drop Experiment

This is a simplified simulation of Robert Millikan's oil drop experiment from the early 20th century which determined the charge on an electron to a high degree of accuracy.

Although this will work on almost any screen size, a larger screen and a mouse are recommended to aid the collection of accurate data.

  • Click the "New Drop" button to insert a new drop between the parallel plates.
  • Drops have different masses.  The mass of each drop is given below the button.
  • Drops will usually be negatively charged.  Some drops might be uncharged.
  • Adjust the voltage across the plates with the slider to control the magnitude of the electric field between the plates, and hence the electrical force acting on the drop.
  • The red line to the left of the drop represents the magnitude and direction of the net force on the drop.  It is provided here as an aid to balancing the forces.

The only forces acting on the drop in this simulation are the downwards force of gravity and and the upwards push from the electric field.

This simulation is intended to be used in the context of learning about uniform electric fields between parallel plates.  As such, the most challenging part of the process, measuring the mass of each drop, is avoided and masses are given.  Similarly, the effects of the buoyant force from the air and air resistance on moving drops are ignored.  This means that it is only necessary to balance the downwards gravitational force and upwards electrical force to determine the charge on a drop.  In the real thing, drops must be brought to a complete standstill in order to eliminate the effects of buoyancy and air resistance.