C.3 reading

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C.3: Model of Charges in Conductors © 2023 PEER Physics C.3 N ATURE OF S CIENCE R EADING Instructions: The purpose of this Nature of Science reading is to contextualize and formalize the Crosscutting Concepts and Science Practices from this activity. Physics principles (Disciplinary Core Ideas) were formalized in the Scientist’s Ideas reading. These three pieces– Crosscutting Concepts (CCCs), Science Practices (SEPs), and Disciplinary Core Ideas (DCIs) - are often referred to as “the Three Dimensions” of science learning. As you read, consider the ways you engaged in and with the three dimensions throughout this activity. C.3e CCCs - Models can be used to explain the properties of materials: When revising scientific models to account for two kinds of materials, one way scientists examine differences between them is by applying the model to explain why, in the same situation, one mat erial changes and the other doesn’t. Scientists closely examine the properties of materials to reveal their function in different systems. In this activity, you made observations to investigate electric charges and how they behave in conductors and insulators. You observed the way different materials interact with charged (tape) and neutral (paper) objects, and then revised your model to represent what you saw. Comparing your observations allowed you to examine the properties of each of these different materials (metallic vs. nonmetallic and conductor vs. insulator), and to explain how those properties allow charges to move differently through different materials. For instance, you observed that after being rubbed, a Styrofoam plate became charged and attracted pieces of paper confetti while an aluminum pie tin did not. This new pattern in your observations gave you a reason to add details to your model of static electricity. Before this experiment you had only explored insulators , and your model of static electricity described those kinds of objects in terms of how they interact with positive and negative charges. Since the aluminum pie tin does not become charged in the same way that insulators do, you can use your model to classify it as a different kind of object - a conductor . Your observations of the aluminum and plastic apparatus gave you more evidence to support claims about how negative charges seem to be able to move (or flow) within conductors, but not in insulators. The simulation represented this same idea and expanded the term conductor to also include the human body. Our models give us specific effects to try and observe (like attraction and repulsion) and specific entities to make inferences about (like positive and negative charges). These kinds of specifics are useful, because we can analyze situations more easily if our attention is focused on what our model says is important. That is one of the reasons why scientific models are used - as tools to highlight what scientists should be thinking about and observing when they investigate phenomena.

C.3: Model of Charges in Conductors © 2023 PEER Physics When making new observations, scientists apply their models to try and explain why some things change and some stay the same. In this activity you observed different kinds of materials being affected in different ways when the same things were done to them. In one experiment you rubbed both the Styrofoam plate and the aluminum pie tin with fur, in another you observed the interactions between a rubbed Styrofoam plate and the aluminum and plastic apparatus. Each of these experiments showed you insulators and conductors being affected differently by the same interaction. In the experiment with the Styrofoam plate and the aluminum pie tin, the aluminum pie tin remained stable (in other words, it did not become charged) when you rubbed it. However, when you rubbed the Styrofoam plate, it changed (in other words, became charged). Applying your model of static electricity to explain these differences pushed you to think about how electric charges might do different things within each material. In this process you might have changed some ideas in your model, and you might have kept other ideas the same. You didn’t throw away your scientific model and make a new one from scratch. Instead, you expanded and deepened the one you already had, and now it can now account for more situations than it could before. C.3f SEPs – Revising models using new evidence from multiple sources: Scientists reflect on how new observations can be explained by using or revising the ideas already present in their models. To make a valid explanation, scientists need to consider evidence from a variety of sources. Scientists need to revise their model when new observations cannot be explained with the model in its current form. If the predictions we make using our models end up being incorrect, that is a sign for us to revise our model in some way. This is an example of how new observations, or data, can impact a scientific model - they can show scientists that something could be missing from the model. The Styrofoam plate and aluminum pie tin experiment highlighted that your model’s explanation for how insulators become charged does not apply to all objects. You therefore deepened your model to include the concept of conductors, as well as ideas about how electric charges move within them. You had not investigated conductors prior to this activity, and so this new evidence pushed you to revise your scientific model for static electricity. Scientists engage in argumentation in order to come to consensus about what is the best model to explain a variety of observations. It is not one person’s job to decide which ideas should be changed and which new ideas should be included - those decisions are made by a community of scientists that are all studying the same phenomenon. Your small group, as well as your whole class, are

C.3: Model of Charges in Conductors © 2023 PEER Physics examples of this kind of community. While conducting the experiments in this activity and sharing your scientific arguments with your classmates, you may have noticed that other people had different ways of explaining the same things you saw. For instance, the Styrofoam plate and aluminum pie tin experiment showed you that the aluminum pie tin did not become charged when it was rubbed, but there may be more than one way of explaining why. One person could say that no negative electric charges were transferred to the pie tin, while someone else could say that the pie tin just gave away whatever negative charges it got. Since it is possible for people to explain the same observation in more than one way, it is very important for scientists to share their explanations with each other through the process of scientific argumentation. When scientists are clear about the ideas and evidence that they are using in their explanations, other scientists have an easier time comparing those explanations with their own. This makes it easier for the community to come to consensus on explanations and model revisions.

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C.3: Model of Charges in Conductors © 2023 PEER Physics C.3 3D Q UESTIONS Respond to the following questions individually in your lab notebook: 1. How did the Styrofoam plate and aluminum pie tin experiment give you evidence to revise your model of static electricity? Provide specific examples of observations or findings that influenced the revisions you made. 2. Two students are discussing their inferences about the Styrofoam plate and aluminum pie tin experiment. Read their ideas: Theo Amina a. Propose an experiment to help these students come to consensus about whether the aluminum pie tin is actually becoming charged through rubbing. Explain i) how your experiment would address Theo’s idea; and ii) what you would observe in your experiment if Amina ’s idea is actually correct. b. Make a claim about the effect of rubbing the aluminum pie tin with fur and describe two observations you made in the Styrofoam plate and aluminum pie tin experiment that support your claim. Hint - think about the confetti and the T and B tapes. 3. In the questions below, you will be reflecting on the aluminum and plastic apparatus. a. What are the parts / components of the systems (aluminum apparatus and plastic apparatus) you explored? b. What are the purposes of each part / component of these two systems? c. How does a change in one part of the system affect the other parts connected to the system? Brainstorm two possible changes in one part of the system that can affect the system as a whole, and provide pictures to represent those changes. The aluminum pie tin is still neutral after we rubbed it with fur. I think this is because we were holding it with our hands, so the electrons we transferred to the pie tin were just flowing out through our bodies. How are we supposed to know the aluminum pie tin was ever having electrons transferred to it at all? What if we just can’t charge metal by rubbing it in the first place?

C.3: Model of Charges in Conductors © 2023 PEER Physics 4. Revisit your Handouts from the aluminum and plastic apparatus experiment. a. Describe how you represented the positive and negative charges in your Handouts, making sure to explain differences between your drawings for the aluminum and plastic apparatus. b. Explain how your way of representing the positive and negative charges aligns with your evidence from prior activities. Hint - think about the “Balloons and Static Electricity” simulation from the prior activity. 5. Based on your understanding of insulators and conductors, make a prediction about alternative materials that could be used in the aluminum and plastic apparatus (instead of the Styrofoam cup, plastic bottle, and aluminum can) to get similar observations or results. Explain your reasoning behind the choice of materials, considering their properties as insulators or conductors. 6. The “John Travoltage” simulat ion you used in this activity provided you with a visual representation of a scientific model. a. In what way(s) did the simulation provide you with new evidence to revise your model for charges in conductors? b. What would you point out as some limitations, or weaknesses, of the “John Travoltage” simulation? Use your model for static electricity to point out way(s) in which this simulation is potentially misleading. 7. In this activity you explored the “John Travoltage” simulation and made observations of the behavior of electrons in a human conductor. Apply the ideas in your model for charges in conductors to develop an explanation for why the electrons in the simulation flowed out of John Travoltage’s body when his finger was brought near the metal doorknob. What questions do you have about this phenomenon? 8. In the experiments you conducted in activities C.1 and C.2, you used materials known as insulators, and in activity C.3, you investigated the behavior of electrons in conductors. a. What questions have your explorations raised for you about conductors? b. How do you think your observations of the aluminum apparatus would have been different if the can were made of a material with greater conductivity than aluminum? Make a claim, and then support your reasoning by integrating evidence from that experiment with ideas about attraction / repulsion and the Conductivity Spectrum (Key Idea C.3d).

C.3: Model of Charges in Conductors © 2023 PEER Physics 9. Fill in the comic strip below with model-based pictures demonstrating electric charges and how they move as a person rubs their feet across the carpet and then gets an electric shock. Explain what is happening to the electric charges throughout each panel of the comic strip. Rubbing Pre-shock During shock After the shock Reasoning: Reasoning: Reasoning: Reasoning: 10. Use your model of static electricity to explain the purpose of the Styrofoam cup in the aluminum apparatus. How do you think your observations might have been different if the cup were also made of aluminum? 11. Scientific models commonly include ideas about what scientists think might be happening at scales too small for them to see directly, and sometimes, these ideas involve assumptions that might not turn out to be true. Consider your observations of the Styrofoam plate and aluminum pie tin experiment when answering the following questions. a. How does your model explain the small-scale mechanisms that caused the Styrofoam plate to become charged? b. How does your model explain why the aluminum pie tin remained neutral? Describe the assumptions that guided your explanation. c. Explain what new empirical evidence could support your model’s explanation for the aluminum pie tin observation.

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