Nobel laureate, Robert Millikan quantified the unit charge of an electron courtesy of his oil drop experiment in the year 1909. As such, in his experimental setup, he was able to charge minute oil droplets under free fall between charged plates inside a chamber, suspend them courtesy of an existing electric field, momentarily vary the charge quantity thanks to a temporarily varying X-ray illumination and, by simple calculation he realized that the charge quantities were a simple multiple of 1.6 x 10^-19 C. Consequently, he concluded that this was the smallest charge embedded in an electron (Franklin 7).
Similarly, in a simulation experiment, it is possible to demonstrate Millikan’s experiment with a mass of pennies and hence, indirectly determine its unit mass and quantity- the experiment’s objective.
In this experiment, the rule of the thump was that counting was prohibited and, all coins were to be returned prior to a subsequent handful pick. Initially, an empty beaker, a beaker plus all available coins and, a beaker plus half the available coins were to be separately weighed and recorded. At least 15 volunteers (students) were to briefly pick, weigh and record pennies in a beaker and, the smallest difference between the pennies and the most recurring one was to represent the weight of a penny (not those with small differences below 2.4 g). Ultimately, with the recorded data and Millikan’s calculation knowledge, the unit weight of a coin was to be obtained.
Data and observations
|Sorted data (x)||Successive difference in x (d)||No of pennies making the difference /Multiples (f=d/2.7)||Average mass per sample (mps=d/f)||Number of pennies per sample (Npps= x/2.7)|
|The average mass of one penny |
Mass of all pennies
Number of pennies
- Mass of a penny =2.70 g.
- Mass of empty beaker= 170.16 g.
- Mass of beaker plus all pennies= 1850.52 g.
- Mass of beaker plus a half number of bennies=1010.34 g.
Results and Discussion
The smallest difference in the data collected was approximately equivalent to 2.7 g., representing a simple multiple of mass akin to Millikan’s pre-established unit charge. The sorted data of handfuls with varied masses is akin to Millikan’s briefly changing charge quantities with; the students representing varying illumination. As such, the differences between successive counts per sample (Npps) e.g. 3 (35 minus 32) coincides with an alternative and corresponding calculation (d/2.7) i.e. 7.34/2.7 ≈ 3. This underscores the fact that there is a common multiple just like Millikan’s discovery. Hence, to obtain a more accurate value, there is a need to find the mean mass per sample (mps) using whole number multiples (f) prior to finding the average penny’s mass across samples: 2.61 g. obtained from the expression ∑mps/12. Millikan established this figure (unit charge per electron) as 1.6 x 10^-19 C.
Noteworthy, the motion of going for the pennies and briefly stopping to collect pennies depicts Millikan’s process of varying electric field strength and, eventually striking an equilibrium with the force of gravity maintaining an oil droplet airborne.
In a conclusion, the objectives of the experiment were met since using Millikan’s simulated experiment, it was established that the average mass of a penny was 2.61 grams and, indeed the unit charge of an electron as established by Millikan is 1.6 x10^-19 C. (Franklin 4).
Franklin, Allan. (1997). “Millikan’s Oil-Drop Experiments.” The Chemical Educator 2. 1(1997): 1–14. Print.