Electric Potential Energy - Introduction
Lesson 13 of 15
Objective: Students will expand their knowledge of work to incorporate electric charges and electric potential energy.
Prior to this class, students have been preparing simple, short demonstrations of electrostatic phenomenon. In the previous class, they rehearsed and some students volunteered to show their demos today.
I have asked students to take on this task for a variety of reasons. First, one of my failings as a teacher is that I am terrible at demonstrations! Second, and more importantly, students enjoy the process of finding and prepping a demo and also seem to internalize many of the fundamental concepts of charges while doing so. Furthermore, the qualitative nature of these demonstrations is a welcome opportunity for those students who struggle computationally - a way of balancing the traditionally computation-intensive study of Physics. Finally, as the concept of small charges is quite abstract, these demonstrations help to make the topic of electrostatics more concrete. As a related note, the sense of fun and playfulness that accompanies this exercise goes a long way toward creating student ownership of the classroom.
These demos are short (5-10 minutes) and students are expected to address the question of the role that charges play in their demonstration. Non-presenters are watching and may ask questions. Here, a student is charging a plastic knife by wiping it through her hair. A plastic straw is positioned on a water bottle, ready to be repelled by the charged knife.
Here the straw begins to move away from the charged knife:
Having identified, in a previous lesson, that work can be defined as the change in energy of an object, I have my students return to the mix of energy transformation problems that we began the other day. Some involve springs, some involve kinetic energies, and some are related to other kinds of energies. Many of these energies are new to the students but it is my hope that they can see the big picture (energy is conserved and can be transformed from one type to another), and not get hung up on the variety of formulas that are presented.
Students work individually or in small groups - these problems are simply for in-class practice and not for homework. I use these problems to formatively assess student progress and spend time circulating in class to work with students. The conversations at these times are instructive to me; if there are some widespread conceptual problems, I can stop the proceedings and provide some whole-group re-direction, otherwise I provide direct, relevant, and timely feedback to individual students and student groups. This in-class practice is invaluable to me, as an instructor, and much appreciated by students.
After about 20 minutes or so, I give students a warning about an upcoming transition. I try to wrap up this segment in another five minutes or so. Students check with one another and with me about the correctness of their answers and I provide an answer key, projected on the board, so that students can either re-assured or re-directed.
Pride Points Problem
Earlier in the year, I created randomized teams of students to compete in occasional challenges called "Pride Points" events. These are challenge problems that require transfer of skills or concept and which are novel in some way. The results of these events count only toward a team's total points - student grades are unaffected. Today I ask my students to assemble into their Pride Points teams and work collaboratively to address the two problems on the handout. The first problem is a familiar one - finding the area under a multi-step function - though one that we have not done in a while. The second problem requires students to make several estimates and to break up a non-standard function into recognizable shapes that fill the appropriate space.
These problems are essentially used as reinforcement of earlier concepts and topics. The competitive aspect of the Pride Points structure changes the atmosphere and adds a level of salience to the proceedings that might not happen if I simply told students that i wanted to revisit some earlier ideas. The teams work together for 20-25 minutes and I begin collecting the best work from each team around this time.
Here's a short snippet of time showcasing what this looks like with my students:
In the final few minutes, I want to build upon the practice problems from earlier in the lesson and link the concept of work to charges and electrostatics. I have a short set of slides to share with students in a lecture mode. It is somewhat of a strategic choice for me to have this section be the last segment of class - the bell rings and prevents this lecture form being too long!
I start by making a parallel argument about the way that, in a gravitational field, the change in gravitational energy can be seen as the minimum work done. I share an image of a simple gravitational potential energy problem and we talk through the idea that an outside agent must "work" to separate the ball from the surface of the Earth and, as a result, that agent does work.
Immediately thereafter, I show a sample electrostatics problem. Here, I want students to wrestle with the idea that this is (or perhaps, in their minds, is NOT) analogous to the gravity problem. I use a "turn & talk " strategy to get students to have semi-private conversations about the issue before trying to have a large group discussion.
The result of the paired conversations is interesting and reveals a cognitive struggle. I solicit student thoughts about this situation and find that, on one hand, students can agree that separating unlike charges requires work to overcome their electrical attraction. On the other hand, they know that the force between the charges diminishes as the distance increases. This seems very different (though it is NOT truly) than the case of gravity, where the naive interpretation is that the gravitational force on a given mass is a constant. This difference is enough to cause doubt about the idea of doing work while overcoming electrical attractions.
We might end the lesson without this dilemma resolved. My intention is to revisit this idea in the next class and our exact ending point will dictate where we resume next time.