We start the day with a warmup that features the architecture of a voltage divider circuit. Instead of providing the entire network of resistors, I turn this problem around a little bit by providing the measure of current that flows out of the battery. By doing so, I can keep students from simply "remembering the steps" and force them to consider the bigger concepts embodied in circuit analysis.
I give students, either individually or in small groups, a few minutes to try this problem at their desks, then suggest that there are multiple ways in which it can be solved. When ready to discuss this problem, I look for students to articulate their approaches. I call on students who express confidence in their work; one approach is this:
a) Use I to calculate the voltage across the 10-ohm resistor using Ohm's Law (22.7 volts) then
b) subtract that voltage from the battery to find V2 (27.8 volts) and
c) use that voltage and the current to find R2, again using Ohm's Law (12.2 ohms).
Students who recognize the nature of current in this circuit (that the current out of the battery is the current through each of the resistors in the circuit, it's a single loop) try the above approach. Some do not recognize this but recognize that the series connection of the resistors yields a total resistance of (10 + R2) which can be used this way:
a) Use Ohm's Law for the entire resistance network to fnd R2 (12.2 ohms) then
b) Use Ohm's law for R2 to find V2 (27.8 volts).
This problem provides a nice opportunity to highlight multiple, successful, approaches to problem-solving. This kind of flexible thinking is valuable as we begin our next investigation, which I introduce immediately after this exercise.
I want to introduce a complex, multi-day investigation of energy transformations, all of which feature electrical energy as the input. Before beginning a multi-day investigation, I share with my students the goals and expectations. This allows me to evaluate how well my students can demonstrate the NGSS Performance Expectation HS-PS3-3:
There are multiple goals. I want students to explore the content of energy transformation. I want students to iterate through their particular lab station, making improvements to their approaches that lead to better data and a better understanding of their station. I also want them to be aware of the Science and Engineering Practices that we employ over the course of the investigation.
In order to achieve these goals, I take some care with this initial conversation, centered around the Electric Energy Transfer handout. I am transparent about the product (a shared Google presentation) and the assessment (based on our school's Learning Expectations). I reassure students about time - we will take large portions of the next five classes to implement our work. Finally, because this work is so authentic to the work of real scientists, I share with them that this project will comprise 50% of their mid-term exam grades.
I also introduce a "Day 1 Learnings" document which allows me to arrange groups publicly and foreshadow a need to report out on what advancements get made today. To arrange groups, I ask to see who might be interested in the first station. I record the number of interested students and move the next station. When done, I ask groups larger than four to reconsider and we quickly rearrange the numbers. I then remind students that, before the day is over, they'll need to commit to an entry in the final column.
Finally, I emphasize that we are in "stage one" of this multi-day process. This stage allows for time to simply get the station working and to wrestle with the process of data collection. The prompts for each station are very brief, mirroring those in the handout, leaving the students plenty of room for thinking and problem-solving.
Once we have established groups, we move to the back of the room to distribute the necessary materials.
Students move to their stations in the lab section of the room. I provide materials for each team to explore the energy transfer specific to their station. Each station has its unique material needs which are described in the handout for students. Numerous questions abound and I circulate to address teams as quickly as possible. Teams try to collect reliable and abundant data points.
On day one, the focus is to ensure that the station is working well. Essentially, students need to experience the transfer of electrical energy into sound, light, heat, etc. before they can be expected to gather data on the phenomenon. Once they see that transfer, they can shift their focus to measuring the physical values in the units that are natural for that event: degrees for thermal events, lumens for light output, decibels for sound, etc.
Here, a team is getting started with a small buzzer. Another team begins to measure the temperature of a resistor. We stay in this mode until about ten minutes are left in class.
With roughly ten minutes left in class, I ask students to do two things. First, clean up their stations, returning materials to a storage area in the back of the room. Second, I want each team to enter their thoughts into the last column of the Day 1 Learnings document. They can access the document at the SmartBoard and write in their thoughts using my computer. Here's a sample response from one of my sections:
These ideas help us to start in the right place when the investigation continues in our next class.