Instructor Information
JCE Classroom Activity: #82
Millikan: Good to the Last (Oil) Drop Earl F. Pearson Dept. of Chemistry, Middle Tennessee State University, Murfreesboro, TN 30132;
[email protected] In this Activity, students simulate Millikan’s oil drop experiment using drop-shaped magnets and steel BBs. Students determine the mass of a single BB analogous to the way Millikan determined the charge of a single electron.
fold here and tear out
Michael Faraday’s experiments with electrolysis had shown that there were tiny units of electric charge that could not be divided into smaller charges. Robert Millikan confirmed this and found a way to measure the quantity of charge on an electron—our name for that smallest unit of charge. Millikan used charged droplets of oil sprayed from an atomizer. The droplets were so small that a microscope was needed to see them. An oil droplet could be negatively charged (excess electrons), positively charged (deficiency of electrons), or uncharged. This Activity uses a procedure analogous to Millikan’s (1). In this analogy, drop-shaped pieces of magnet represent oil drops, BBs represent the electrons, and the mass of a BB represents the charge of an electron. BB samples are composed of individual BBs just as the charge that resides on oil drops consists of individual electrons. The smallest possible difference in mass between any two samples of BBs should be the mass of a single BB just as the smallest difference in charge between any two oil drops was the charge of a single electron. A more thorough discussion of Millikan’s experiment has been published in this Journal (2) and a general discussion of this analogy is also in this issue of JCE (3).
Integrating the Activity into Your Curriculum This is a good Activity to use after a discussion of the concept of atoms, including identification of electrons, protons, and neutrons, and how atoms of one element are different from the atoms of another. This is often done early in the course with a discussion of Dalton’s atomic theory. Often atomic theory is developed along a historical perspective that includes the work of Thompson, Millikan, Becquerel, Rutherford, and Roentgen. Another interesting analogy to Millikan’s oil drop experiment was published in this Journal (4). Eckey uses the price of candies to illustrate how the price of an individual piece of candy can be determined by factoring the price of the entire bag by the number of pieces.
perforated
About the Activity Students cut 10–15 drop-shaped pieces from a roll of magnetic tape. Refrigerator magnet material and the magnetic backings for business cards do not work well because their magnetic field is not strong enough to adequately hold the BBs onto the drop. Magnetic tape is found at office supply and discount stores. BBs are available from outdoor suppliers and discount stores. Steel BBs (4.5 mm) were used in testing. Do not purchase plastic pellets. Instructors may wish to substitute a very small high-field neodymium magnet for the final large drop. These magnets can be purchased from a science supplier (5) or may be available at a radio or television supply store. All student groups could share a single high-field magnet. (If a high-field magnet is used for the final drop, it is recommended that a 0.001 g balance be used. If a 0.01 g balance is used, it is possible that the large number of BBs picked up could cause an incorrect calculation of the number of BBs.) Data analysis could also be done using a spreadsheet program.
Answers to Questions 1. We can see BBs and manipulate them individually. Electrons are so small that it is not possible to see or manipulate individual electrons. Millikan was able to measure charge accurately enough to determine the charge corresponding to a single electron, but could not move charges onto the oil drops one at a time. 2. Answers may vary: the prediction may not match the actual number if the smallest mass difference was the mass of two BBs rather than one; the prediction of the number of BBs on the last large drop would be half the actual number. 3. Often, scientists are not able to physically verify results in such a concrete way as counting the exact number of BBs. Millikan could not see and count the number of electrons on an oil drop. Often, supporting evidence is strong enough that we can believe what we determine without being able to physically verify results.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
photos by J. J. Jacobsen and R. J. Wildman
Background
References, Additional Related Activities, and Demonstrations 1. 2. 3. 4. 5.
Millikan, Robert A. Phys. Rev. 1911, 32, 349–397. Pearson, Earl F. Revisiting Millikan’s Oil-Drop Experiment. J. Chem. Educ. 2005, 82, 851–854. Pearson, Earl F. Magnet and BB Analogy for Millikan’s Oil-Drop Experiment. J. Chem. Educ. 2006, 83, 1313–1316. Eckey, Doris A. A Millikan Oil Drop Analogy. J. Chem. Educ. 1996, 73, 237–238. Educational Innovations. 888/912–7474. http://www.teachersource.com/catalog/index.html (M-150, neodymium magnet, small disk) (accessed Jun 2006). JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 83 No. 9 September 2006 •
Journal of Chemical Education
1312A
JCE Classroom Activity: #82
Student Activity
Millikan: Good to the Last (Oil) Drop What do mass, energy, and charge have in common? All matter is “grainy”—made up of small particles. The simplest possible particle of matter is called an atom. The mass of a sample of matter is determined by the number of atoms in the sample and the kinds of atoms present. Atoms of different elements have different masses. Light energy is also “grainy” and made up of small particles. We refer to the particles of energy as photons. The total energy that is moved from one place to another depends on the number of photons transmitted and their wavelength. Photons with a long wavelength have lower energy than that of photons with a shorter wavelength. Robert Millikan showed that electric charge is also “grainy” and made up of small particles. We call these particles of charge electrons. Millikan measured the charge residing on several small oil drops. He argued that the minimum difference in charge between any two of the oil drops must correspond to the charge of a single charged particle (electron). In this Activity, you will simulate Millikan’s experiment using drop-shaped pieces of magnet and steel BBs. You will determine the mass of a single BB without measuring the mass of any known number of BBs, just as Millikan determined the charge of a single electron without measuring the charge of any known number of electrons. You will need: strip of magnetic tape, scissors, ruler or Student Activity template, small dish or beaker, steel BBs, weighing boat, balance that is precise to at least ± 0.01g, tweezers (optional). Data Collection _ 1. Using scissors and a ruler (or the template below), cut a strip of magnetic tape into 10–15 dropshaped pieces of varying sizes from ~ 5 mm wide ⫻ 8 mm long to about 10 mm wide ⫻ 15 mm long. These pieces represent oil drops. It is important to construct many different drop sizes. Cut one very large drop ~ 12 mm wide ⫻ 30 mm long to use as the final measurement. (Alternatively, your instructor may supply a small high-field magnet for the final drop.) _ 2. Create a data table with four columns: Measured Mass, Sorted Masses, Unique Masses, Difference in Mass. _ 3. Fill a small dish or beaker approximately 1/3 full of steel BBs. _ 4. One at a time, place each of the 10–15 drop-shaped pieces into the dish or beaker of BBs. Gently shake the dish so that the drop-shaped piece is covered with BBs. Place a weighing boat on a ± 0.01 g balance and tare to zero. Carefully pick up the drop with its attached BBs (you may wish to use a tweezers). DO NOT COUNT THE BBs. Wipe off the attached BBs into the weighing boat. Measure and record the mass of BBs in the first column. DO NOT COUNT BBs. Pour the BBs back into the dish or beaker. Repeat with the remaining drop-shaped pieces. _ 5. Repeat step 4 using the very large drop or small high-field magnet. After you measure and record the mass of the BBs, leave the BBs in the weighing dish for use in step 11. Data Analysis _ 6. Generate a second column of data (Sorted Mass) in your data table by arranging all the measured masses in descending order, except for the final mass from the large drop. _ 7. You may have two masses in the second column that are the same or nearly the same. What do these two masses represent? Average the values that are the same or nearly the same. Copy only the unique masses and the averages to a third column (Unique Masses). _ 8. Subtract each unique mass from the one just above it in the third column and record this difference in the fourth column (Differences in Mass). _ 9. Using your collected data, calculate the mass of a single BB. _10. Using the mass you calculated for a single BB in step 9 and the mass you recorded for the BBs attached to the final large drop, predict the number of BBs that were attached to the final large drop. _11. Count and record the number of BBs in the weighing dish from step 5.
photo by J. J. Jacobsen and R. J. Wildman
Try This
Questions 1. One could easily determine a single BB’s mass by simply weighing it or by weighing 100 BBs and dividing the mass by 100. Why couldn’t Robert Millikan use this procedure to measure the charge of Templates, actual an electron? size drops: 5 mm 2. Compare your step 10 prediction to the actual number of BBs attached to the final drop. Explain any ⫻ 8 mm, 10 mm ⫻ differences. 15 mm, 12 mm ⫻ 3. You were able to check whether your calculated mass of a single BB was correct or not. How is this 30 mm different from what many scientists are able to do? Why could Millikan not physically check his answer as you did?
Information from the World Wide Web (accessed Jul 2006) More on Millikan’s oil drop experiment. http://chemistry.umeche.maine.edu/~amar/fall2004/Millikan.html Robert A. Millikan–Biography. http://nobelprize.org/physics/laureates/1923/millikan-bio.html This Classroom Activity may be reproduced for use in the subscriber’s classroom.
1312B
Journal of Chemical Education
• Vol. 83 No. 9 September 2006 •
www.JCE.DivCHED.org
