oPhysics: Interactive Physics Simulations


The Millikan Oil-Drop Experiment

Description
This simulation is a simplified version of an experiment done by Robert Milliken in the early 1900s. Hoping to learn more about charge, Milliken sprayed slightly ionized oil droplets into an electric field and made observations of the droplets. When the voltage is zero and the run button is pressed, the drop will fall due to the force of gravity. It will reach a terminal velocity (vt) as it falls. Pause the simulation while you record the terminal velocity. This terminal velocity can be used to determine the mass of the drop. Use the equation: mass = kvt2 to determine the mass of the particle. The value of k in this simulation is 4.086 x 10-17 kg s2/m2. Once the terminal velocity is recorded and the mass calculated, with the simulation still paused increase the voltage between the plates until the two force vectors are approximately equal length. This will produce an upward field and an upward force on the positive droplets. If the upward force of the electric field is equal to the downward force of gravity, and the drag force is zero, the particle will not accelerate. To be sure that the lack of acceleration is not related to drag forces, the velocity must also be zero as well as the acceleration in order to be sure that the two forces are balanced. Increase and decrease the voltage (use the left/right arrow keys) until both the acceleration and velocity are at zero. The velocity may not stay at exactly zero, but find the voltage that has the velocity changing most slowly as it passes v = 0.

Use the methods discussed above to ultimately determine the charge on ten (or more) different oil-drops. Use V = Ed to calculate the field strength (d = 5 cm = 0.05 m). Use Eq = mg when the velocity is zero to determine the charge q on the droplet. Record all your data in a table or spreadsheet. After you get each q, create a new particle and start again. When you have the table filled in, look at the various values for q. Is there any pattern to them, or are they seemingly random? Can you draw any conclusions from the Q measurements?