Radioactivity, Part One
Lesson 12 of 17
Objective: Students will identify and distinguish between new forms of radiation that are the result of nuclear reactions.
The overall goal for today is to return to thinking about "radiation" with some new thoughts to share about "radioactivity." The bulk of today's lesson is dedicated to developing the idea of radioactivity, though only as far as naming and identifying the three modes of radioactive decay (HS-PS1-8). I leave the concept of half-lives for another day.
Before diving into this new Physics, I present my students with a solenoid warmup problem. Though we have introduced the relationship between current and magnetic field and students have collected data at one of their Four Big Ideas stations, we have not yet "closed the loop" by manipulating the relationship to make a prediction based upon data.
I provide the students a hard copy of this problem while it is simultaneously shown at the Smartboard. I urge them to find their notes about this phenomenon as it is critical to any solution. The data are intentionally "noisy" as I wish to make a larger point about imperfect data.
I give students a bit more time than usual; the topic is not familiar, the data are erratic, and we have not yet done a problem like this before. After a few minutes, I put the relationship on the board (B = u * n * I) for any student who, for whatever reason, cannot locate it in his notes. While I wonder about, I check in on students who seem to be making progress. Invariably, they have decided to select one of the data points to calculate the solution. When I ask "why that point" the answer is, typically, simply a random choice. I ask what could make their answer better and students will offer that trying other points and averaging could be helpful. The mini-lesson is just this: when the data are obviously erratic, sampling more than one point can lead to increased confidence.
We take some time to show two solutions. The first features student samples, based on one or more points. The second is based on an "eyeballed" best fit line, whose slope can be used to calculate the value in question. I make the point that the slope of the line represents the contributions of all points without having to calculate the unknown for each data point - one calculation provides the average of all points!
Before proceeding, I remind students that they have collected data just like this, only with greater precision. As such, they should be able to use their data to estimate the number of turns per meter of their actual solenoid.
Free Write: Radioactivity
Once the warmup is complete, I get my students prepared for some new thoughts. I have not yet said the word "radioactivity" because I want to reveal it as part of the free-write exercise which launches this segment of class.
Students take 2-3 minutes to write their own private thoughts, then take a minute or so to turn & talk to a neighbor. This two-step approach provides time for students to get comfortable with their ideas and to recognize that their thoughts are worth sharing. Here is a sample of what the "turn & talk" strategy looks like in my class:
I collect student responses by having them shout out their thoughts while I rapidly record them on the board. While they have many responses, one can see that the amount of science content is quite limited.
Having assembled their thoughts and preconceptions about radioactivity, we are ready to fill in the missing science content. In the next segment of class, students take notes on a template that I provide to them.
In this segment of class, I introduce the three types of radioactive decay, which is the focus of NGSS Performance Expectation HS-PS1-8. I provide students with a template that we is used to elaborate upon the decay modes (alpha, beta, and gamma).
For each decay mode, I identify a generic decay equation to summarize the differences between the parent and the daughter nuclei. When completed, the template has much to offer - atomic masses of the parents and daughter elements, additional particles that are expressed, and an image relevant to the decay mode.
In addition, this is a good time for me to connect to our ongoing discussion of "radiation." These radioactive events comprise both particulate radiation (alpha and beta) and electromagnetic radiation (gamma). I share an image that summarizes the shielding effectiveness of some materials. There are many questions, ranging from dental x-rays to nuclear power plants to comic book superheroes, and I try to honor each question with as full an answer as I can muster. I want students to recognize the richness of this topic and, as we will be returning to writing in our Radiation Journals soon, to be ready to elaborate upon the impact of these idea on the concept of radiation.
In the final segment of class, my students receive a set of radioactivity examples and a set of practice problems. I use the examples to show how the generic rules from earlier in class can be applied to some specific cases. Students then take some time to apply the decay mode rules to a variety of element and decay mode combinations (Thorium undergoes beta decay, for example). Students are welcome to work alone or in small groups. I circulate throughout the room to identify students who may be struggling and to provide affirmation for those who are successful. We continue in this mode until the end of class. These problems are not due as homework as this exercise is intended as a time for students to make meaning of today's lesson and for me to formatively assess student understanding.