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Benjamin Franklin and Electricity

Introduction

Benjamin Franklin was one of the most innovative and versatile men of his time—a successful practitioner in science, journalism, statesmanship, diplomacy, and business. Franklin’s concern for the public good led him to devise practical and effective inventions to address real problems. For example, he conducted complex experiments that helped to advance knowledge of the properties regarding electricity. He also invented several important devices to make electricity safer, including the lightning rod.

In this lesson, students learn about some of the important discoveries Franklin made through his electricity experiments. There are two activities on static electricity, one is an experiment with balloons, and the other an observation of the workings of a signal-bell similar to the one invented by Franklin. [Note: Combined, these activities may take up to three class periods. You may also conduct the two activities at different times, or modify them to meet the needs of your students and your instructional time frame.]

Materials

First Activity Second Activity (All materials provided by the teacher)
  • Two empty aluminum soft drink cans
  • Plastic or wood ruler
  • Several inches of sewing thread
  • Two-square foot sheet of aluminum foil
  • Cellophane tape
  • Two electrical wires, each several feet long
  • Working television or CRT computer monitor

Strategy

Slide Presentation
You may either present the Franklin and Electricity QuickTime Presentation on a computer projector or introduce the information yourself. Before beginning the lesson:

  1. If your computer does not have QuickTime Player software, download and install the free QuickTime Player.
  2. Download the Franklin and Electricity QuickTime Presentation (file size approx. 580K) to your computer. Connect your computer to an LCD projector or a television screen in the classroom.
  3. Show students the Franklin and Electricity QuickTime Slide Presentation. Pause each slide for as long as necessary and share the related information from the Franklin and Electricity QuickTime Presentation—Teacher Notes.

First Activity
Franklin was one of many 18th-century scientists who did not see the natural phenomena as based on superstition and folklore, but rather on objectivity and reason. Using the scientific method, Franklin conducted many experiments that led him to conclude that lightning was a form of electricity. He theorized that electricity was composed of positive and negative charges that flowed through objects.

In this activity, students “play” with static electricity to gain an understanding of its properties. They will then conduct an experiment using the scientific method, test variations of the experiment, and answer worksheet questions. For this activity, each student group will need one balloon and a piece of wool cloth.

  1. Divide students into small groups of two or three.
  2. Give each student a copy of the Student Handout—Conducting Balloon Experiments. Give each group a balloon and a piece of wool cloth.
  3. Have each group inflate and tie off its ballon, then follow the directions for Part One on the handout.
  4. After students have completed the steps in Part One, discuss what happened. Explain to students that most materials have roughly an equal number of positive and negative electrons, which gives them a “neutral” status. Objects that have a larger number of either type of electron can attract the opposite type of electrons in other objects. So, when the balloon is rubbed against the wool cloth, many of the positively charged electrons move to the cloth, leaving the balloon with mostly negatively charged electrons. When the balloon is placed near a person’s hair, which is “neutral,” the positive electrons in the hair are attracted to the balloon's negative electrons, causing the hair to stand on end.
  5. Next, have students take the experiment one step further. Part Two explores the questions “How strong is the static electricity in the balloon?” and “Is it strong enough to hold up the balloon against gravity?” Have students again rub the balloon with the wool cloth, place the balloon against a wall, and observe what happens. [Note: The balloon should stick to the wall.] Explain to students that the reason is as Franklin observed: Electricity is composed of positive and negative electrons that can flow from one object to another. In this case, negatively charged electrons flowed from the wool cloth to the balloon. When the balloon was placed against the wall, it stayed there because its negatively charged electrons were attracted the positively charged electrons in the wall. Finally, ask students why they think the balloon falls after a few minutes. [Note: Once the amounts of positive and negative electrons in the balloon are equal, there is no longer an attraction and it falls.]
  6. Following the instructions in part three of the handout, have students conduct the wall experiment again. This time, they should record the length of time the balloon sticks to the wall when it is rubbed for different lengths of time with the wool cloth. Are the results the same? Why or why not? What conclusions can be made regarding the length of time the balloon was rubbed with the cloth?
  7. Inform students that Benjamin Franklin conducted experiments like this one. Though he did not use balloons, he did experiment with devices that produced static electricity and was able to transfer it from one object to another. His experiments led him to devise a battery that held an electrical charge indefinitely.

Debriefing Activity

  1. Conduct a class discussion in which students recount what they learned in the first activity about the movement of electrons. First, review a few of the principles of static electricity:
    • When objects have roughly the same number of positive and negative electrons, they are said to be in a neutral state.
    • Objects can obtain and hold more of one type of electron when they are “charged.” [Like the balloon was when it was rubbed against the wool cloth.]
    • The imbalance of electrons in an object can cause other objects to be attracted to it. [Like human hair or the wall was attracted to the balloon.]
    • Once the electrons have moved from one object to the other, the balance returns to neutral, and objects cease to be attracted to one another. [Like the hair fell away from the balloon, or the balloon dropped away from the wall.]
  2. On the overhead, show students the Primary Source of the Month depicting Benjamin Franklin. While students examine the image, give them some background information on Franklin’s experiments with lightning (kite experiment, lightning bell apparatus, and invention of the lightning rod). Explain that the device shown to his left is the lightning bell signal device he developed to alert him to upcoming lightning storms.

Second Activity
This month's featured primary source is an image of Benjamin Franklin sitting next to his lightning bell apparatus. Franklin devised this mechanism to enable him to detect electrically charged clouds near his house. The following experiment has students observe a replica of the signal-bell apparatus and draw conclusions. Operating the signal-bell is completely safe, but tell students that they should have adult supervision if they construct their own device.

To construct the signal-bell apparatus, have the materials listed above ready before class begins. Use the following directions:

  1. Remove the pull-tabs from both soda cans. Save one pull-tab and discard the other.
  2. Set the cans approximately two inches apart on top of or near a television or a computer monitor.
  3. Tie one end of the thread to the pull-tab, and the other end to the middle of the ruler.
  4. Place the ruler on top of the two cans so the threaded pull-tab hangs between the cans. Make sure the pull-tab hangs freely and does not touch either can or the surface on which they rest.
  5. Tape the sheet of aluminum foil to the television screen or computer monitor.
  6. Use tape or an alligator clip to connect one wire to the side of one of the cans. Connect the other end of this wire to the aluminum foil sheet on the television screen or computer monitor.
  7. Connect the second wire to the other can in the same manner as the first wire. Connect the other end of this wire to a ground, such the metal frame of the television or computer.
  8. Test the device. IMPORTANT: DO NOT TOUCH THE CANS WHEN THE MONITOR IS ON. Doing so is not harmful, but you may get a static shock. Turn on the television or computer monitor. The hanging pull-tab should clang back and forth between the two cans. If it does not, turn off the television, move the cans a bit closer together, and try again.

Briefly explain to students how you constructed the soda can signal-bell apparatus, then demonstrate the device and explain how it works.

WHAT HAPPENED? The television screen or computer monitor creates a static charge of electrons. The charge is sent down the wire to the first can charging it with electrons. These electrons repel the electrons in the pull-tab and push it away from the can. As soon as the pull-tab touches the second can, electrons shift to the pull-tab and push it away. The process continues as long as the current is present. Turn off the television or computer monitor and the process stops.

Debriefing Activity
Debrief the class on the signal-bell device experiment by asking the following questions:

  • The televison screen or computer monitor emits negative electrons. How do they get to the first can?
  • Once the first can is charged with negative electrons, what effect does it have on the pull-tab? Why does it have this effect?
  • When the pull-tab touches the first can, what happens to the balance of positive and negative electrons?
  • When the pull-tab touches the second can, what happens?
  • Why does the pull-tab keep hitting the cans until the television or computer monitor in turned off?
  • How does this experiment relate to benjamin Franklin’s lightning bells?

Lesson Extension
Explain that the battery was first developed by Benjamin Franklin in his experiments with electricity. By placing panes of window glass between thin lead plates, he was able to store static electricity. For instructions on how students can construct a lemon or potato battery, visit the Hila Research Center Web site.


This lesson was written by Greg Timmons, freelance writer and education consultant, Missoula, Montana.



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