Franklin's Lightning Rod Design
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Recording date: 10/24/13 Presenter: Deb Katz In this webinar, we will discuss the use of historical case studies in introductory physics. We'll also cover the characteristics of highly motivated students and what we can do to motivate our introductory physics students. In the broadest sense, a case study is an educational story that is traditionally used in medicine, business and law education. Case studies may be woven into traditional teaching pedagogy or developed for contemporary project-based methods. A student working through a case study is in the role of practitioner. Watch Deb Katz, Physics Professor at the United States Naval Academy webinar, to learn how she creates immediate relevance in her course to motivate and engage students to succeed in her course and throughout their education.
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Franklin's Lightning Rod Design
- 1. Debora M. Katz United States Naval Academy Annapolis, MD
- 2. Physics is the fundamental science. People use the scientific method to examine and discover the truths of the universe.
- 3. Put poetically, physics illuminates the darkness
- 4. 0. It’s a requirement. 1. We live in a high-tech world so we need to understand technology. 2. Physics is a tough course that is used to weed us out.
- 6. • Dogmatic • Devoid of the human element • Does NOT include stories • No mistakes, no missteps, no debate
- 7. •In the broadest sense, a case study is an educational story that is traditionally used in medicine, business and law education. •Case studies may be woven into traditional teaching pedagogy or developed for contemporary project-based methods. •A student working through a case study is in the role of practitioner.
- 8. A student is in the role of a historical scientist, exploring the process by which great discoveries were made or new technologies were invented.
- 9. In the middle of the 18th century, people loved to play with electricity in their homes, but most people thought that lightning was another phenomenon altogether— explosions of atmospheric gas, something like the explosions of gunpowder.
- 10. •Benjamin Franklin thought otherwise. He believed that lightning was a colossal electrical spark just like the small sparks people found so amusing. •Franklin collected charge from his flying kite in a Leyden jar. •Franklin showed that a Leyden jar charged by clouds produced all the same effects as Leyden jars charged in the home. •So he concluded that lightning is an electrical phenomenon, like a giant spark.
- 11. • As people started building taller structures, those structures became more likely targets of lightning strikes, causing fires, destroying property and lives. Once Franklin understood that lightning was a giant spark, he invented a way to protect against lightning strikes—the lightning rod.
- 12. Franklin published the following recommendations for lightning rods in Poor Richard’s Almanac (1753): 1.Just outside each building an iron rod should be planted three to four feet in the moist ground. 2.The rod should extend 6 to 8 feet above the tallest part of the structure. 3.On top of the rod should be a foot of brass wire sharpened to a fine point, like a knitting needle.
- 13. The third point—the shape of the top of the rod—was controversial. Franklin recommended that a pointed lightning rod be used, but another scientist, Benjamin Wilson, recommended a blunt-end lightning rod. The Royal Society of London was asked to evaluate the recommendations and decide which would make a better device. Your goal in this case study is to decide between Franklin’s lightning rod and Wilson’s.
- 14. In order for air to break down and become a conductor, the electric field in the air must be 3 × 106 N/C. Let’s assume that in order for a lightning rod to work, the electric field at its surface must equal that breakdown electric field. Calculate the amount of charge on the surface of each conductor. The one with the least amount of charge is the better design, because a smaller amount of charge on the surface of the conductor means a smaller amount of charge travels through the air.
- 15. Professor Katz, I am really glad you enjoyed my project! It's one of the few things I did right in the many technical courses here at the Academy lol I really did love doing it and I hope your students have enjoyed working on their projects also! … I hope all is well! You are a great teacher and your students are lucky to have you as a Professor! Very Respectfully, Michelle
- 16. Using Historical Case Studies in Introductory Physics 1. 2. 3. Stealing God’s Thunder, Philip Dray 2005 Make to Stick, Chip Heath and Dan Heath 2007 Science Teaching: The Role of History and Philosophy of Science, Michael R. Matthews 1994 (2000) Debora Katz Physics Department United States Naval Academy Annapolis, MD 21402 dkatz@usna.edu http://www.usna.edu/Users/physics/dkatz/
Editor's Notes
- This is a question I asked myself and answered myself.
- Then I asked my students the same question. In fact, I have asked students this question for about 18 years. I have also pestered just about anyone—friends, neighbors, people I know from yoga—about this question. The answers fall in to about 3 broad categories. Almost no one sees physics as ultimate science leading our species through the darkness to illuminate the universe.
- So there is a disconnect between my view of my students’ view of our physics course. To them physics is a hoop they must jump through that leads to something better—the next class, their degree or to the invention of new cell phones.
- The typical physics class does little to change our students’ view of physics. We present laws of physics as though they are doctrine handed down from on-high. We divorce these laws from the human struggle to discover them. We do not tell the stories of the scientific process which is full of mistakes, missteps, debates.
- The human mind enjoys stories, and people find they can easily remember information learned from stories. SOMETHING FROM MAKE IT STICK?
- So a historical case study connects physical laws to their discovery by people who may be very smart, but are in principle no different from any of us. We learn that physics is really a process by which we discover how the universe works.
- Here is an example of a historical case study. From the beginning the human element is introduced. People in 18th century had parties and entertained one another. Here the man suspended from the ceiling has been charged. A women is about to draw a spark from him. Some times this was down with a kiss. People playing such party games knew a lot about static electricity, and they thought that lightning was something else entirely—like an explosion.
- Franklin hypothesized that lightning was an electrical spark. Of course, he needed to test his hypothesis. To support his theory, he suggested that a metal rod be placed on top of a tall structure to capture “electrical fluid”—what we call charged particles. While Franklin was waiting for the completion of Christ Church in Philadelphia (because there were no other tall structures in Philadelphia at the time), he came up with another way to do his experiment. He used a kite with a metal wire attached. The wire was connected to a string, which when wet would act as a conductor. At the end of the string was a metal key connected to a Leyden jar—a device used to store charge. (Leyden jars were among the props normally used in home parties involving electricity at that time.) Franklin held on to a piece of dry silk, which insulated him and then proceeded to collect charge from his flying kite. Franklin showed that a Leyden jar charged by clouds produced all the same effects as Leyden jars charged in the home. So he concluded that lightning is an electrical phenomenon, like a giant spark.
- For some students, knowing that lightning is an electrical phenomenon is not satisfying. They need to see why knowing something about the universe is important. In this case we can do that because Franklin went on to invent the lightning rod. Had he believed that lightning was an explosion, he would not have been able to invent the right device.
- Poor Richard's Almanack (sometimes Almanac) was a yearly almanac published by Benjamin Franklin, who adopted the pseudonym of "Poor Richard" or "Richard Saunders" for this purpose. The publication appeared continually from 1732 to 1758. It was a best seller for a pamphlet published in the American colonies; print runs reached 10,000 per year.[1][2]Franklin, the American inventor, statesman, and publisher, achieved success with Poor Richard's Almanack. Almanacks were very popular books in colonial America, offering a mixture of seasonal weather forecasts, practical household hints, puzzles, and other amusements.[3] Poor Richard's Almanack was also popular for its extensive use of wordplay, and some of the witty phrases coined in the work survive in the contemporary American vernacular.[4]
- Franklin reasoned that during a storm, the atmosphere builds up excess charge much as a glass rod builds up excess charge when it is rubbed with silk. He knew from his experience with charged objects that a spark could make its way through the air. From his indoor experiments he knew that if he used a pointed object (like a knitting needle) to draw charge from an object through the air, only a small spark occurred compared to the spark drawn to a blunt object (like his thumb). He reasoned that when a small charge builds up in the atmosphere, a pointed lightning rod would draw the small charge through the air and into the Earth continuously. If only a small charge traveled through the air, it would not be visible and there would be no giant spark of lightning. Without a lightning rod, charge would still build up in the atmosphere. Since buildings are connected to the Earth, they attract charge of the opposite sign. When sufficient charge builds up, the air acts as a conductor and a large charge is transferred in a giant lightning spark. Such a violent spark causes great damage to building and can be very dangerous.Wilson—a contemporary of Franklin’s—believed that lightning rods should be blunt. He argued that a pointed rod would draw down lightning that might have just passed harmlessly overhead. Effectively Wilson argued that pointed lightning rods were more dangerous than having no rods.
- This case study addresses a number of concepts and problems we often cover in our introductory class. (1) the shape of a conductor matters. (2) Charged particles tend to clump up on pointy parts of conductors, (3) how to calculate the electric field produced by different shaped conductors.
- Students who enjoy case studies. The project, she is referring to is a case study that she wrote and solved on her own. As she says, she is not a particularly good technical student. She likes stories and can learn a technical subject when we appeal to her strengths.
- Michael Matthews has another book about the history of the pendulum.