Electric Energy Transfer Investigation - Day 3
Lesson 8 of 13
Objective: Students continue an investigation of electric energy transformations.
As we continue this multi-day investigation, I want my students to pause briefly to consider their station carefully. This lesson comes immediately after a weekend, therefore students have not seen their materials for four days. To offset aimless wandering as they head back into the investigation, I have designed a simple, short activity to re-familiarize themselves with their work. After a warmup problem, I ask students to "visualize" their stations and to try to generate questions that are important.
Though this investigation is time-consuming, it allows my students to practice many of the eight Science & Engineering Practices of the NGSS and to address the Performance Expectation HS-PS-3-3. Each team is investigating how electrical energy is transformed into some other kind of energy - thermal, sound, chemical, and so on. Time is needed for familiarization and optimization of the testing station.
Today's warmup requires that students adjust their thinking from voltage divider circuits to current dividers. The challenge here is "mental inertia:" all previous warmups and all of the investigation stations are structured as voltage divider circuits. The goal of the warmup today is to disrupt that inertia and to get students to recall the voltage and current relationships specific to this circuit architecture. Furthermore, to create that particular focus, I design the question so that it is heavily conceptual with little to no computation involved.
I give students three or four minutes to consider these questions before providing any sort of assistance or hints. They may work individually or in small groups and I move throughout the room to check in on student progress and to understand the types of issues students are wrestling with.
Students frequently make one of several incorrect relationships:
a) that the voltages split as they do in a voltage divider (this is a result of the metal inertia mentioned above)
b) that the current splits into three equal parts (sometimes, but only under special conditions)
c) that I1 will always have more current because R1 is closer to the battery than the others (Visually, yes, but not necessarily in practice. In addition, it is the value of R1, not its location that will determine the current that flows in that branch.)
To combat these issues requires a bit of time so this warmup cuts into the investigation time a bit. As the investigation will continue for two more class periods, I am comfortable with the investment of time here. One way to ensure that all students are seeing the circuit properly is to have students draw their mechanical analogies (what we've called our "Liftyphus" drawings). After 8-10 minutes, I ask a student to put her Liftyphus drawing on the board, as a way of visualizing the circuit.
Her response shows clearly that there are three ways for items to lose their potential energy (bowling balls in the mechanical drawing, charges in this electric circuit). The energy difference (whether it is from point A to B, or C to D, or E to F) is the same. Therefore, all three voltages are the same as the battery voltage and the current splits into three parts, but not necessarily equal parts.
After looking at this student's response together, we have a fuller conversation as a large group about the warmup problem solutions. Specifically, I take each of the incorrect assumptions mentioned above and demonstrate, using the drawing, how the circuit truly operates. In addition, as some students still have a hard time seeing that the conservation of energy is NOT violated, I ask students to imagine that they are on the third floor of our building and wish to get to the first floor. Thought they may take one of four routes to do so, the loss of potential energy is the same independent of the route chosen. This seems to help satisfy students.
As the discussion ends, I ask students to be ready to reflect on the first two days of the investigation.
Before going back to their stations, I want my students to reflect on the work they're doing. I use a set of visualization prompts to accomplish this. I ask each individual student to record their thoughts to these prompts in their notebooks. While I don't collect their individual responses, I will collect the teams' best thoughts at the end of the day. This way, each student gets a chance to think by herself but can compare thoughts with teammates before contributing to a class document.
I give student just a few minutes to address the prompts, then direct them to find their teammates and their materials and to return to their investigations. During the investigation, I want students to compare, with their teammates, the questions they've just generated and to look for common and strong questions. In principle, this comparison can happen at any time during the investigation though, in practice, it is common for the teams to consult on these ideas right at the end of the investigation, just before committing to the document.
On this third day of the investigation, students teams are wrestling with different needs. Some teams are recovering from faulty equipment issues which have delayed them, while others are ready to link their observed phenomenon (say, the brightness of a light bulb) with power or energy. Despite this issue, all teams are truly engaging in many, if not all, of the Science and Engineering Practices incorporated in the NGSS.
The heavy, and obvious, emphasis of today's work is planning and carrying out an investigation (practice #3). In addition, many teams are developing and using models (practice #2) or using mathematics and computational thinking (practice #5). For example, the team investigating rotational energy with motors is beginning to create a model in their minds about how electrical energy turns the motor and how the addition of a "blade" affects the rotational rate. This ultimately leads to considering a property known as "moment of inertia," a property whose value depends upon the size and shape of the blade itself. As they research this property, they employ mathematical thinking. In one way or another, most students replicate this sort of engagement with the Science & Engineering Practices. Their work is a blend of lab work, Internet research, and conversations with their teammates and me as they try to resolve problems and collect valid data.
Students continue this work for most of the rest of this lesson. With about ten minutes left, I prompt them to clean up their stations and add their best questions to a class document at the SmartBoard.
Share Questions and Clean up
In the final few minutes of class, I ask my students to send a team representative to the board and enter one or two of the most important questions that came to them during their work today. Some of these questions are simply curiosities that do not impact their work while others are central to their success. I am not necessarily interested in generating answers to all of these questions; my real goal is to apply some gentle pressure to be continuously thinking about the nature of the phenomenon they are witnessing.