Millikan Experiment and the Elementary Charge

• As research in the 1800’s continued, two questions remained.
• Is there a smallest possible charge from which all other charges are just whole number multiples?
• If so, what is this elementary charge
• Millikan reasoned that the smallest possible charge should be on the smallest possible object, the electron.
• He also reasoned that charged objects would have charges that are whole number multiples of the charge on an electron.
• Oil drops from an atomizer have a charge on them.
• To measure the charge on these, Millikan balanced the force of gravity on them by an electric field between two plates.

• From this equation, if the mass of the drop can be determined, then charge can be determined. All other values can be measured (i.e. d and B).
• Mass is determined by the volume of the drop and density of the oil. Density is determined by using:

· Measure 25 ml of oil and measure its mass to get density

· Volume of the oil drop today can be measured by using a calibrated eyepiece to determine radius.

• Mass of the drop is calculated by substituting values from the previous two points into

and solving for mass.

• As a consequence, the quantity of charge on the oil drop can be determined.

• Millikan didn’t have a calibrated eyepiece and so he used Stokes’s law to determine the radius of the drop and therefore its volume. To use Stokes’s law, Millikan had to measure the terminal velocity of the oil drop when it fell freely in air.
• Millikan performed the experiment 100’s of times and noted that all charges were simple multiples of:

e = + or – 1.602 X 10^-19 C

• This is referred to as the elementary charge.
• +e is the charge on the proton and –e is the charge on the electron.
• Electric charges are explained simply by an excess or deficit of electrons. Therefore, by knowing the charge, the number of electrons involved can be calculated.

e.g. An object has a charge of –5.77 X 10^-18 C. How many excess electrons does
this represent?

(1 electron/X) = (-1.602 X 10^-19 C/-5.77 X 10^-18 C)
X = 36 electrons.