Download Determining the fundamental electric charge (Based on Millikan`s

Determining the fundamental electric charge (Based on Millikan’s procedure)
The simulation is either on the laptop’s desktop or at
http://www.physics.rutgers.edu/~jgoett/millikan/millikan.html
Observation: Open up the simulation and run the simulation multiple times, adjusting
the magnitude of the electric field and observing the resulting motion of the oil droplets.
Model: Devise a model to explain the motion of the droplets. Include sketches,
diagrams, and equations
Application: Devise a procedure to measure the charge on a droplet. Record relevant
data and calculate its charge.
If we assume each droplet only contains a few extra electrons, we can calculate the
charge of an electron. Analyze the data and determine this charge.
In real life, one does not know the mass of the droplets, cannot begin with the droplets at
rest, and cannot conduct the experiment in a vacuum (why?).
Model: Using fluid dynamics, scientists have found the force of air on a sphere of radius
a to have a magnitude
F  6ua v
It acts in the direction opposite the sphere’s motion. F is the force of the air on the
droplet, u is the viscosity of air, 1.73*10-5 N-s/m2, and v is the velocity of the droplet.
Devise a model of a droplet’s motion as it falls.

Application: Devise a method to measure the size of the droplet. Derive any equation
needed to calculate it and specify what data needs to be recorded. (The density of the oil
is effectively 710 kg/m3).
Model: Now, let’s determine the charge on a particle. Use Stokes’ Law to model the
motion of a particle in an electric field. Devise a procedure for measuring the charge on a
droplet. Derive any equation needed to calculate it and specify the data that needs to be
recorded.
Sample data
In an experiment, you measure the downwards v1, the free-fall velocity, and the upwards
velocity of the droplet with an electric field on it, v2, for different droplets. From this data
determine the charge of an electron.
E (V/m)
Time (sec) to fall 2
mm under only
gravity
Time (sec) to fall 2
mm under gravity
and electric force
2000
139.9
213.2
2100
131.6
749.1
1900
93.9
491.4
Millikan Oil Drop Apparatus
q (C)