Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut the magnetic strip into 20-cm pieces, one per lab group.
2. Distribute the BBs evenly into the medium weighing dishes, about 100 BBs per dish. Note: The weight of 100 BBs without the dish is approximately 33–34 g.

Safety Precautions

The materials in this experiment are considered nonhazardous. Immediately pick up any BBs that may have rolled onto the floor to prevent a person from slipping on them. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in one 45- to 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• Balances with 0.1 g-readability may be used; however, data will not be as precise, resulting in less accurate predictions.
• To enhance the analogy, have student cut the magnet pieces into “teardrop” shapes, keeping the lengths the same.
• Data analysis may be done using a computer spreadsheet program.

Teacher Tips

• This is a good activity for units on atomic structure and the development of the atomic theory of matter. A discussion of Millikan’s Oil-Drop Experiment fits in well with the development of atomic theory from an historical perspective including Dalton’s atomic theory and the discoveries of Thompson, Becquerel, Rutherford, and Roentgen.
• The official website of the Nobel Foundation http://nobelprize.org serves as an “electronic museum” of information concerning the prize-winning scientists and their discoveries. The website includes fact-checked biographies, official award citations and the presentation speeches of the scientists themselves. The presentation speeches are especially helpful in putting the discoveries in the context of the times and in showing the interconnectedness of the work by many different individuals.
• The current accepted value of the elementary charge of an electron is 4.803 x 10^–10 esu (electrostatic units) or 1.602 x 10^–19 C (coulombs). Millikan continued to refine his experiment and in 1913 obtained a value that was in error by only 0.6% from today’s accepted value—a remarkable feat!

Answers to Prelab Questions

1. An analogy in science is an activity or experiment that models or simulates a real-world experiment. Read through the Background section and the complete Procedure. What part of this experiment is analogous to the electrons in Millikan’s Oil-Drop Experiment? What is analogous to the oil droplets? The total charge on a droplet?

   The BBs in this experiment correspond to the electrons in Millikan’s experiment and the magnets correspond to the oil droplets. The mass of the BBs is analogous to the total charge on a droplet.

2. How is the procedure in this experiment similar to Millikan’s procedure? How is it different?

   In both experiments, a measured value of a single item (BBs/electrons) is determined without knowing the quantity of the item being measured. This experiment uses magnetic force acting on BBs; Millikan’s experiment used an electric field acting on charged oil droplets. The BBs are removed from the magnet pieces; Millikan did not remove the electrons from the charged oil droplets. Millikan could not see the electrons on the oil droplets and his experiment was much more complex.

Sample Data

Answers to Questions

1. Arrange the measured masses of BBs into the Sorted Mass column in descending order, starting with the largest recorded mass and ending with the smallest.

   See Sample Data table.

2. Some of the masses in the third column may be the same or nearly the same (within a few hundredths of a gram of each other).
   1. Average the masses that are nearly the same. For example, masses of 3.12, 3.11, and 3.10 grams would average to 3.11 g.
   2. Record only the unique masses and the average masses of those that are nearly the same in the Unique Masses column of the data table. Note: This column will have fewer than 15 data points.

      See Sample Data table.

3. Subtract each unique mass from the one just above it in the Unique Masses column and record each difference in the last column of the data table. Again, this Difference in Mass column will have fewer than 15 data points.

   See Sample Data table.

4. Note the smallest value in the Difference in Mass column between any two samples of BBs. If several of the differences in mass are the same or nearly the same as the smallest difference, find the average smallest difference and record this value at the bottom of the last column. What does this difference represent?

   The smallest difference in mass between any two samples was 0.33–0.35 g. This should be the mass of one BB.

5. Using the average smallest difference in mass calculated above and the recorded mass for the BBs from the final 40-mm magnet piece, predict the number of BBs that were attached to the largest magnet. Record your prediction.

   Predicted number of BBs from largest magnet: 10.40 g/0.34 g/BB = 30.59 or 31 BBs

6. Count and record the number of BBs in the weighing dish from step 14 of the Procedure.

   Actual number of BBs from largest magnet: 31

7. How did the predicted number of BBs compare to the actual number from the largest magnet? What possible sources of error are in this procedure?

   In the sample data, the actual number of BBs was the same as the predicted number. Possible sources of error if the numbers did not match include the limited precision of the balance or the smallest difference in mass may have been two BBs rather than one.

8. One could easily determine the mass of a single BB by weighing it or by weighing a known number and dividing the total mass by the number of BBs weighed. Why couldn’t Robert Millikan determine the charge of a single electron in a similar manner?

   Electrons are too small to be seen. Millikan could not move electrons onto the oil drops one at a time.

9. The predicted number of BBs was checked with the actual number of BBs attached to the largest magnet. Millikan could not verify his results in the same way. Give examples of other scientific research that depends on supporting evidence without the ability to verify results in such a concrete way.

   Answers may vary but may include determining the mass of subatomic particles, atomic structure, and Avogadro’s number.

References

Special thanks to Earl Pearson, Middle Tennessee State University, Murfreesboro, TN, for providing the idea and the instructions for this activity to Flinn Scientific.

Pearson, E. F. Millikan: Good to the Last (Oil) Drop; J. Chem. Ed. 2006, 83, 9, 1312.

Pearson, E. F. Revisiting Millikan’s Oil-Drop Experiment; J. Chem. Ed. 2005, 82, 6 851–854.

Introduction

The electron is an elementary particle of matter having a negative charge. Exactly how much electrical charge does one electron have? In 1911, Robert Millikan (1868–1953) published the results of a series of experiments designed to quantify the charge of an electron. What is amazing about this work is that Millikan determined the charge of a single electron without knowing the number of electrons for which he was gathering data. After all, no one has ever seen an electron! Model Millikan’s famous experiment by determining the mass of one BB without weighing any known quantity of BBs.

Concepts

• Electrons
• Electrical charge
• Quantization of energy

Background

To determine the charge of an electron, Millikan used an apparatus that included a chamber with two metal electrode plates. An atomizer was used to spray tiny droplets of oil into the chamber above the top plate. Friction from the atomizer caused some of the oil droplets to pick up a static charge. Millikan also charged more droplets by exposing the chamber to X-rays. As the droplets fell inside the chamber, a few went through a hole in the top plate. Millikan used a small telescope to view the motion of the droplets (see Figure 1). Knowing the density of the oil, the time for one droplet to fall between two reference points, and the force of friction from the air, Millikan determined the mass and acceleration due to gravity for each droplet observed. Before the droplets reached the bottom electrode, the voltage was turned on, creating an electric field between the two electrodes. This caused the negatively charged droplets to be attracted toward the top positive electrode. As he varied the voltage between the plates, Millikan could suspend a single droplet in the air or cause it to rise or fall in the chamber at different rates. By factoring in all forces acting on the droplet, Millikan was able to calculate the total electric charge on the droplet. Yet Millikan did not know how many electrons were contributing to the total charge on the measured droplet. Millikan reasoned that with enough data—that is, by measuring and calculating the total charges for many different oil droplets—he could deduce the charge of a single electron. The smallest difference between measured charges should correspond to the electric charge of one electron. In 1923, Millikan received the Nobel Prize in Physics for this work.

Experiment Overview

The purpose of this experiment is to determine the mass of a single BB without weighing any known number of BBs. Magnets of varying sizes will be used to attract different unknown quantities of BBs. These unknown quantities will be weighed and the reasoning employed by Millikan in his oil-drop experiment will be used to determine the mass of one BB.

Materials

Prelab Questions

1. An analogy in science is an activity or experiment that models or simulates a real-world experiment. Read through the Background section and the complete Procedure. What part of this experiment is analogous to the electrons in Millikan’s Oil-Drop Experiment? What is analogous to the oil droplets? The total charge on a droplet?
2. How is the procedure in this experiment similar to Millikan’s procedure? How is it different?

Safety Precautions

The materials in this experiment are considered nonhazardous. Immediately pick up any BBs that may have rolled onto the floor to prevent a person from slipping on them. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. Obtain scissors, a metric ruler and a 20-cm magnetic strip.
2. Measure and cut one 40-mm piece from the magnetic strip. Set this large piece aside for step 14.
3. Cut a second piece from the magnetic strip that is 20 mm long.
4. Cut the remainder of the magnetic strip into 13 pieces of varying sizes ranging from 5 mm to 20 mm long. Try to cut as many different sizes as possible. The smallest piece should be no less than 5 mm.
5. Obtain a weighing dish with 100 BBs, a smaller weighing dish and plastic forceps.
6. Choose one magnetic piece at random (not the largest piece) and drop it into the dish of BBs.
7. Use the forceps to move the piece around and turn it over a few times until the magnet is covered with BBs. Note: The BBs may only be attracted to one side of the magnet.
8. Place the small weighing dish on the balance and tare the balance to zero.
9. Gently pick up the magnet and BBs with the forceps, being careful so no BBs are dislodged from the magnet. Cup one hand under the magnet to prevent any BBs from falling onto the floor.
10. Transfer the BBs into the small weighing dish by gently brushing them off the magnet (see Figure 2).
    {12833_Procedure_Figure_2}
11. Record the total mass of the BBs attracted to Magnet Piece 1 under Mass of BBs in the data table. Do not count the BBs.
12. Pour the BBs from the small weighing dish back into the larger weighing dish.
13. Repeat steps 6–12 thirteen more times, each times using a different size of the small magnetic strips cut in step 4. Try to select the magnet pieces at random; that is, do not test them in order from smallest to largest.
14. Repeat steps 6–11 using the 40-mm magnet piece from step 2, only this time leave the BBs in the small weighing dish after recording the mass. Remove the small dish with the BBs from the balance and set it aside for future reference.
15. Complete the data table according to the instructions on the worksheet and answer the questions.

Student Worksheet PDF

12833_Student1.pdf