Publication No. 12833
Discovering the Charge of an Electron
Student Laboratory Kit
Materials Included In Kit
BBs, copper-coated steel, 1500
Forceps, polypropylene, 15
Magnetic strip, ½" x 10'
Weighing dishes, medium, 15
Weighing dishes, small, 15
Additional Materials Required
Balance, centigram (may be shared)
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.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesDeveloping and using models
Planning and carrying out investigations
Analyzing and interpreting data
Using mathematics and computational thinking
Disciplinary Core IdeasMS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
Scale, proportion, and quantity
Systems and system models
Answers to Prelab Questions
Answers to Questions
Special thanks to Earl Pearson, Middle Tennessee State University, Murfreesboro, TN, for providing the idea and the instructions for this activity to Flinn Scientific.
Discovering the Charge of an Electron
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.
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.
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.
BBs, 100 in a medium-size weighing dish
Magnetic strip, 20 cm
Weighing dish, small
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.
Student Worksheet PDF