SAM Teachers Guide Atoms and Energy Overview Learning Objectives: Possible student pre/misconceptions

Transcription

1 SAM Teachers Guide Atoms and Energy Overview Students will explore how the Law of Conservation of Energy (the First Law of Thermodynamics) applies to atoms as well as the implications of heating or cooling a system. This unit focuses on potential energy and kinetic energy as well as energy conservation. The goal is to apply what is learned to both our human scale world and the world of atoms and molecules. Learning Objectives: Students will be able to: Differentiate between kinetic and potential energy. Explain how kinetic energy can be converted to potential energy and vice versa. Analyze energy graphs and the motion of objects in a system to learn about the Law of Conservation of Energy. Apply the first Law of Thermodynamics when heating or cooling a system. Differentiate between energy transfer in an open system and energy conversion. Possible student pre/misconceptions Energy is associated only with movement. Energy is destroyed in transformations from one type to another. Gravitational potential energy is the only type of potential energy. Models to Highlight and Possible Discussion Questions After completion of Part 1 of the activity: Models to highlight: Page 3 Pendulum Model o Discuss how kinetic and gravitational potential energy are converted and vice versa. Point out that gravitational potential energy is not the only type of potential energy in light of common student misconceptions. o Link to other SAM Activity: Gravitational potential energy is compared to electric potential energy in future lesson studying electric current. Page 4 The Model of the Spring (Elastic PE) and Corresponding Graph o Discuss the relationship between the position of the spring and the curves for potential, kinetic and total energy in the graph. Ask students to share ideas for the snapshots they chose to illustrate the point of lowest kinetic energy and energy conservation.

2 Possible Discussion Questions: Have student s brainstorms examples of objects that have kinetic energy and the factors that impact kinetic energy (mass and velocity). List situations besides the pendulum where students can explain how potential energy is converted into kinetic energy and vice versa. Try to encourage students to think of examples in addition to those involving gravitational potential energy. Demo Idea: Use spring or slinky in class to illustrate the principles of elastic potential energy and conversion to kinetic energy. After completion of Part 2 of the activity: Models to highlight: Page 5 Model of Interatomic PE and Corresponding Graph o Point out the relationship between KE and PE as the atoms move both toward and away from each other. Differentiate the importance of atoms being in close proximity to that of the spring example from Day 1. Discuss how total energy still remains constant. o Link to other SAM Activity: Connect interatomic potential energy to future lessons such as Electrostatics (attraction and repulsion between atoms) and Newton s Laws (forces between atoms at work at close enough distances). Page 6 Model of Atomic Motion and Corresponding Graph o Discuss how the color of the atoms indicates the gain or loss of kinetic energy in the system. Stress how the First Law of Thermodynamics can be seen in both the model and the graph. Page 8 Model of Energy Transfer o Differentiate between energy conversion and energy transfer. Discuss what happens as the divider is withdrawn and what the effect is on the temperature of both containers. Link to Other SAM Activity: Models of Pages 6,7,8 link strongly to the next activity, Heat and Temperature. Use discussion of the models to lead into the upcoming differences between heat and temperature. Possible Discussion Questions: Why is the example of interatomic potential energy both similar to and different from the spring example? (Discuss how the sum of total energy is still conserved but how distance between atoms now plays a role.)

3 Have students explain their understanding of the First Law of Thermodynamics and give examples of how it applies with heating and cooling. Have students brainstorm/share examples of energy transfer in an open system. How is this different from a closed system?

4 Connections to Other SAM Activities: The focus of this activity is for students to understand kinetic energy, the interconversion of potential and kinetic energy and how energy is conserved according to the Law of Conservation of Energy. The Atoms and Energy unit is supported by both Atomic Structure and Newton s Laws at the Atomic Scale. Atomic Structure sets up a connection between an atom s composition and how this plays a role in both attraction and repulsion. In Newton s Laws at the Atomic Scale students focus on force, mass and acceleration, which connects to how the forces at work between atoms. Atoms and Energy supports Heat and Temperature. Temperature is defined by the kinetic energy of the substance, where atoms are colliding. Students can recognize that energy is conserved in atomic collisions. In addition, kinetic energy is converted into an excited state in Atoms, Excited States, and Photons. Once atoms are excited, energy can be converted into light, photons. Phase Change is related to the energy atoms have to use to overcome forces of attraction. Finally, Atoms and Energy supports Diffusion, Osmosis and Active Transport and ATP Biological Energy. The connections are more indirect but still present when discussing active transport and ATP. Chemical energy is converted into another form of potential energy, electrostatic potential energy. The fundamental principles of energy conversion and conservation, as shown in the Atoms and Energy unit are highlighted once again.

5 Activity Answer Guide Page 1: Introduction Sample Snapshot 2: Page 2: 1. The faster the ball on the left moved the kinetic energy of the ball... (b) 2. The kinetic energy is related to both an object's speed and its mass. Predict what would happen to the kinetic energy if the ball on the left were twice its current mass but the speed of the ball remains the same. The kinetic energy of the ball on the left would double if the mass were twice the current mass. This is true if the speed of the ball remains constant. Page 3: The kinetic energy is lowest in this snapshot where the spring is fully compressed. 2. Take a snapshot of the graph and label it with the annotation tools to explain how we know that energy is conserved in this spring system. 1. Where is the pendulum when the kinetic energy is maximum? (a) 2. What happens to the kinetic and potential energy as the pendulum swings from its highest point to its lowest point? The potential energy decreases and the kinetic energy increases. Page 4: 1. Take a snapshot of the model on the left that shows one of the 2 possible locations of the balls on the spring when the kinetic energy is lowest. Sample Snapshot 1: The kinetic energy is the lowest in this snapshot because the spring is fully extended. The total energy of the spring system remains constant. The total energy is shown as the straight black line. The graph shows that when potential energy is increasing the kinetic energy is decreasing and vice versa. 3. Use the step back or step forward button to describe the change in kinetic energy as the spring moves from the most compressed position to the most stretched position. At the most compressed position, the kinetic energy is low. When the spring is released, the stored potential energy is converted into kinetic energy to stretch the spring. By the time the spring is fully extended to its most stretched position, the kinetic energy is low again as it has been converted back into potential energy.

6 Page 5: 1. The total energy is the sum of the potential and kinetic energy. What happens to it over time, if you do not adjust the strength of the interaction? (c) 2. Take a snapshot of the graph and annotate it to show the point where there is no force. Page 8: 1. The energy level of the circular area on the right rose because: When the separator between the two containers is withdrawn, heat energy from the container on the left is transferred to the system on the right until the temperature of the two circular areas is almost equal. 2. Give one example of something that comes close to having the same characteristics of energy transfer as seen in the model. Explain your choice. 3. Why does the kinetic energy increase as the atoms move toward each other? The attraction between atoms causes the kinetic energy to increase as they move towards one another. Page 6: 1. As long as you leave the atoms alone, what happens to the total energy of the atoms over time? (c) 2. When atoms collide with each other, they exchange energy. How does the model show this? How does the graph show this? The model shows the exchange of energy because the atoms with low energy (the lighter colors) become darker in color after they collide. The graph shows that as a system the kinetic energy will increase and decrease but the total energy (black line) will stay constant. Page 7: 1. Describe how the energy of the system changes when it is heated or cooled. When the system is heated, the total energy of the system increases. When the system is cooled, the total energy of the system decreases. Answers will vary. One example is putting a hot pot on a cool countertop. The heat from the pot will be transferred to the countertop. The pot's temperature will decrease as the countertop's temperature increases. Page 9: 1. When two or more atoms collide... (e) 2. Explain what it means to say energy is conserved when two atoms move toward and away from each other. Energy will be converted from potential energy to kinetic and back but the total energy in the system of the two atoms is conserved. 3. What happens to an atom's kinetic energy if it collides with an atom that has even less kinetic energy? When two atoms with different amounts of energy collide, energy will be transferred from one atom to the other. 4. How can you increase a cup of water's kinetic energy? Adding heat energy to a cup of water will increase its kinetic energy. The addition of heat to the system will cause the water molecules to speed up their motion and therefore increase their energy.

7 SAM HOMEWORK QUESTIONS: Atoms and Energy Directions: After completing the unit, answer the following questions to review. 1. What does the Law of Conservation of Energy state? 2. The equation for kinetic energy is: KE = ½ mv 2. This means that the kinetic energy of an object is based on what two factors? 3. Look at the pendulum below. Label where you think the kinetic energy will be highest and lowest. Do the same for potential energy. 4. In a closed system, the First Law of Thermodynamics can be seen. Look at the image below. What will happen to the kinetic energy of the individual atoms as they are in motion? What happens to the total energy of the system over time? 5. How would your answers to the above question differ if heat were added to the system? Or, if the system was cooled?