Science Core Experiments Directory | Science Home Page | Curriculum Home Page | USOE Home Page

Copyright © by the Utah State Office of Education.

Modeling the Mass of the Electron

Description of Activity

Title: Modeling the mass of the electron.
Overview: Students will investigate the historical contributions by Millikan to the atomic model by determining the mass of a single paper clip without directly massing it. Students will use a process similar to that used in the Millikan oil drop experiment to determine the charge of one electron.
Duration: This experience will take about 40 minutes to complete.
Materials:
1 box of large paper clips per student team
1 Styrofoam cup per student team
decigram or centigram balance

Background

Millikan observed that the charge on any particular oil drop was always an integral multiple of a single quantity, which he took to be the charge carried by a single electron. The ratio of mass to charge of the electron had been determined by Thomson, hence Millikan was able to determine the mass of the electron.

Teaching and Learning Strategies

In order to understand the process that Millikan used to measure the mass of the electron, students must never mass a single paper clip nor should they count the number of clips in or out of the cup at any time. The students can only use their 25 massings to find the answer.

Large paper clips are desirable since any variation in mass is a small percentage of the total mass of clips. Standard size paper clips tend to lead to errors. A digital balance is desirable since the number of measurements needed is considerable.

The following sample data tables may be helpful to students as they organize their data:

Prerequisite Instruction

Students should be familiar with the development of the atomic model and the contributions of some of the scientists involved. A description of the oil drop experiment and the conclusions that Millikan drew frequently brings up the question of just how did he find out how much one electron weighs when looking at oil drops through the telescope.

Invitation to Learn

Millikan observed that the charge on any particular oil drop was always an integral multiple of a singe quantity. How did he do that? Using paper clips instead of electrons, accomplish this same task; determine the mass of a single paper clip "electron" without weighing it directly or counting the paper clip "electrons".

Masses may only be taken of the Styrofoam container (oil drop) holding an unknown number of paper clips. You are to remove or add bunches of clips before each massing. Never add or remove less than five clips. After you remove or replace clips, the total mass of the remaining clips is to be recorded. The number of clips removed and the number of remaining clips may not be determined by counting. A minimum of 25 massings should be recorded. Record and analyze your data to determine the mass of one paper clip.

Safe Operating Procedures

No unusual safety considerations must be considered.

Summary of Learning

Students should construct a data table in which to enter their 25 separate observations.

1. Using only the data in the table, what is the mass of a single paper clip "electron"?
2. Does the mass of one paper clip seem to work with all of your data?
3. In the model that you used what common item represents an electron.
4. What differences exist between models and "the real thing"?
5. What is useful about the model that you constructed?
6. Each model has limitations. What limitations are there in the model you constructed?

2 multiple choice questions

1. Millikan was able to measure the mass of the electron by:

a. Placing one on a balance and determining the mass.
b. Looking for patterns within a series of investigations.
c. Placing a small pile of electrons on a balance, counting them, and dividing to find the average.
d. Looking up the mass in a reference book.

2. Models are useful in the study of science because

a. They are exactly the same as the subject being studied.
b. They are like the subject being studied except for size.
c. They use familiar objects and terms to describe unfamiliar or unknown topics.
d. They are not useful but are interesting to build.

All rights reserved except those which may be granted under Sections 107 and 108 of the Copyright Revision Act of 1976. This document may be freely distributed in its entirety for non-profit purposes provided that the copyright notice is not removed. If you have questions concerning proper use of this material, or if you are interested in obtaining permission, contact the Curriculum Section Reception Desk at 801-538-7698.

This document was submitted for posting to the Internet by the State Science Specialist. Any questions concerning content should be directed to that individual.

Updated September 25 1997 by Michelle Dumas