The purpose of this experiment is to measure the smallest unit into which electric charge can be divided, that is, the charge of an electron e. The method is the one proposed by R.A. Millikan in 1910. A small sphere of mass m having a charge q can be suspended in air by applying an electric field of field strength E to balance the gravitational force on it.
We then have
We neglect here the (very) small buoyant force. The charge q will in general not be the electron charge but rather an integral multiple of it:
When the measurement is repeated several times, e can be found as the largest common denominator of the measured charges q.
In the absence of an electric field, the electrons will reach a constant terminal velocity vT after a short time. The viscous force balances the gravitational force, so that the net force acting on the droplet is zero and we have:
where according to Stoke's law:
with η the viscosity of air (1.83×10−5Nsm−2 at 18∘C), r the radius of the spheres (≈0.50μm). From measuring the terminal velocity vT of free fall, the mass of the spheres can be determined.
Figure 1 shows a schematic sketch of the experimental set-up. A closed chamber is placed between two capacitor plates 0.4 cm apart, in which a uniform electric field E can be built up (remember, E=V/d). The chamber is illuminated by a small lamp. Charged spheres (a suspension of latex in water and alcohol) can be blown into the chamber through a tube and a nozzle, and be viewed there through a telescope with a calibrated scale (spacing of graduations 0.5 mm). Note that the telescope gives an inverted image.
The equipment we have doesn't always work very well. Therefore rather than follow the above procedure we propose you observe the phenomena using the actual setup and then take your data using a simulation: OPhysics Millikan Oil Drop http://www.teachscience.net/2011/02/07/millikan-oil-drop-simulation/
The simulation is very similar to the real experiment. Rather than produced many drops simultaneously you focus one drop at a time. To produce a drop click the button labelled “New Droplet”. You can change the rate at which time passes using the “Time Speed” slider. The terminal velocity can be measured using built in stop watch. The large divisions on the scale on the screen are cm (ie. the plates are about 2.5 cm apart). The stopping potential can be found by using the virtual “Power Supply”. Once you have determined the potential for one droplet, simply summon a new one and repeat until you have enough values.