C.5 reading

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Jan 9, 2024

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C.5: Revised Model of Current Electricity © 2023 PEER Physics C.5 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.5h CCCs – Mechanistic models: Scientists build models that include small-scale mechanisms that can describe how the systems work. Investigating small-scale mechanisms helps scientists understand larger systems and phenomena. Scientific models often include ideas about mechanisms (in other words, processes or interactions) that explain how something occurred. For example, your model of static electricity included ideas about mechanisms when you explained how the tapes, balloons, and fur became charged. Those mechanisms included claims about how negative and positive charges move within and between different kinds of materials. Your model of current electricity also includes ideas about mechanisms, but this time those ideas explain how electric current flows in circuits. Small-scale mechanisms are fundamental processes or interactions that occur very small scales (for example, microscopic, molecular, atomic, or subatomic). Understanding these mechanisms is an important practice in all natural sciences. Scientists use ideas about small-scale mechanisms to help them comprehend larger systems and phenomena. For instance, a chemist can explain large scale chemical reactions by making claims about how atoms and molecules interact. In this activity, you collected evidence about small- scale mechanisms when you used a simulation to explore circuit systems. By collecting evidence about electric current in different circuits and interpreting that evidence using ideas about small- scale mechanisms, you made claims about cause and effect relationships (for instance, between voltage and current). These cause and effect relationships allowed you to make predictions about how other changes in the circuit would affect the flow of electrons within it.

C.5: Revised Model of Current Electricity © 2023 PEER Physics Scientists use simulated models to investigate complex systems. Scientists use different methods and techniques to investigate things they cannot see or directly observe. In many scientific fields there are phenomena that are not directly visible with the naked eye, or even with laboratory equipment. When this happens, scientists must rely on other practices to collect evidence and draw conclusions. Some examples of strategies they might use are analyzing patterns, creating mathematical models, and creating computer simulations. In this activity, you used a simulated model to observe patterns at a small scale that you would not have been able to directly see. By interacting with a computer simulated model, you could observe a representation of how electrons move within circuits and make inferences about their effects on bulbs. In real-world circuits we cannot see individual electrons moving, and studying their behavior is challenging because of their small size. Even though the simulation is only designed to show a model-based representation of a phenomenon, it provided you with a way of visualizing the movement of electrons and of observing the effects of their movement on the system as a whole. C.5i SEPs – Using models to explain cause and effect relationships: A scientific model of a system includes ideas about how different parts of the system relate to and influence each other. Models support hypotheses about cause and effect relationships in systems. Supporting predictions is one of the main purposes of a scientific model. That is why scientists constantly make predictions based on ideas in their models, and then design experiments to test those predictions. When studying cause and effect relationships, scientists design experiments in a way that keeps other variables from influencing their results. The simplest kind of experiment allows a scientist to intentionally change one variable, called the independent variable, to then measure or observe some kind of effect, called the dependent variable. Before conducting the experiment, scientists make predictions about what will happen to the dependent variable when the independent variable is changed in some way. These kinds of predictions are called directional hypotheses, since they focus on the cause and effect relationship between the dependent and independent variables. In this activity, you made directional hypotheses about how a the current in a circuit (the dependent variable) would be affected by the voltage and the resistance (two different independent variables). When studying two (or more) independent variables, it is important to design experiments that keep all but one of the independent variables constant, or unchanged. You kept the voltage (number of batteries) constant while changing the resistance (number of bulbs) to

C.5: Revised Model of Current Electricity © 2023 PEER Physics study the effect resistance has on the current. Then, you kept the resistance constant to measure the effect of voltage on the current. Controlling variables is an important component of experimental designs across all scientific fields. Simulated models allow scientists to test cause and effect relationships. After scientists make predictions, they must test their predictions by collecting evidence to either support or refute them. This can be challenging to do for phenomena that are too big or too small to see directly, like electrons moving inside of conductors. Scientists sometimes use indirect observations to collect evidence for things they cannot see. For example, in this activity you used a simulation to measure the current in different places in a circuit. By comparing these measurements, you could make inferences about how the flow of electrons did not depend on where they are within the circuit. You then created new circuit configurations and measured how the electric current was affected by the things you changed. Using your observations of the computer simulation, you revised your model of current electricity and used it to make claims about how the number of batteries and bulbs affect the current within the circuit. Equations are used to summarize mathematical patterns in evidence. Scientists often find mathematical patterns in their experimental evidence about cause and effect relationships, and these patterns are very important and useful. They allow scientists to simplify complicated relationships into equations and formulas, which can then be applied to multiple different situations. In this activity, you collected and analyzed data about the relationships between voltage, resistance, and current, and you found that these relationships follow simple mathematical patterns of direct and inverse proportionality. Combining these patterns into one formula gives us Ohm's Law, which you may explore in greater depth in the Mathematical Model Building activity. Explaining the cause and effect relationships between the variables in Ohm's Law, as well as describing the meaning of the variables themselves, requires using ideas you've developed and the evidence you've collected throughout this entire chapter. Every formula in physics is built from scientific practices like those you've been engaging in - making predictions, performing experiments, analyzing data, and coming to consensus about ideas and outcomes with a community of peers.

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C.5: Revised Model of Current Electricity © 2023 PEER Physics C.5 3D Q UESTIONS Respond to the following questions individually in your lab notebook: 1. What did the computer simulated model allow you to make observations of that would have been impossible for you to make without it? 2. Why is it important to make a hypothesis before conducting an experiment? In your response, describe how ideas from a scientific model influence both the hypothesis and the experiment. 3. Describe three circuit properties or components that you changed when you used the simulation. What did you measure or observe as a result of those changes? 4. Make a claim about how the current in a circuit system is related to the number of bulbs in the system. Use ideas about the cause and effect relationship between current and resistance to explain how your observations from the computer simulated model support your claim. 5. Describe what happens to the electrons when another battery is added to a circuit system. How would you describe the relationship between voltage and current? In other words, in a circuit, how does current change as voltage increases or decreases? 6. In this activity, you used straws as analogies for wires of different lengths and thicknesses. Reflect on this experiment by answering the questions below. a. In what ways did this experiment represent the different properties of circuit systems (voltage, current, resistance)? b. After making observations with the straws, make a claim about how the electric current flowing through a wire is affected by that wire’s i) thickness and ii) length. 7. How do the values for electric current compare at points A, B, and C in the circuit shown on the right? Explain your answer using a model. 8. Read Theo’s statement below. How would you respond to Theo? Use evidence from the simulation and your model of current electricity to support your ideas. I think that the current is used up when it goes through the bulb. This is what allows the bulb to light.

C.5: Revised Model of Current Electricity © 2023 PEER Physics 9. For each circuit, calculate the total resistance and the total voltage, then calculate the current using Ohm’s Law: a. b. 10. Two flashlights are each powered with four batteries. All of the batteries are 1.5 volts. One of the flashlights has a bulb with a resistance of 6 ohms and the second flashlight has a bulb with a resistance of 4 ohms. a. Draw circuit diagrams for each of the flashlights. Label the battery voltage and bulb resistance for each diagram. b. Calculate the total voltage for each flashlight. c. Using the total voltage and the bulb resistance, calculate the current for each flashlight. d. Which flashlight will be brighter, the one with the 6-ohm bulb or the one with the 4-ohm bulb? Describe your reasoning. 11. Is a short circuit considered an open circuit, closed circuit, both, or neither? Describe your reasoning.

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