SAM Teachers Guide Heat and Temperature

Transcription

1 SAM Teachers Guide Heat and Temperature Overview Students learn that temperature measures average kinetic energy, and heat is the transfer of energy from hot systems to cold systems. They consider what makes a good conductor. Learning Objectives Students will be able to: Distinguish between heat and temperature at the atomic level. Determine temperature is related to both the speed and the mass of atoms. Explain that temperature is a measure of the average kinetic energy of atoms and molecules in a system. Experiment with the amount of energy and the size of a system and its relationship to a rise in temperature. Compare thermal radiation and heat conduction. Possible Student Pre/Misconceptions Heat and temperature are the same thing. Heat is a form of energy. Heat is a physical substance, such as steam. Heating a substance increases the motion of atoms only. Models to Highlight and Possible Discussion Questions After completion of Part 1 of the activity: Models to Highlight: Page 2 Bottom Model o Use this model to highlight the ways in which the mass of atoms and temperature of the system is related to the motion of the atoms. o Link to other SAM activities: Atoms and Energy. Review the concepts of heat increasing the energy in a system. Page 4 Equilibrium Model o Use this model to discuss the difference in the system from when the divider is present (for the first few seconds) and when the divider is removed. Discuss what happens to the temperature of the two containers once the separator is removed. o Link to other SAM activities: Atoms and Energy. Review the concepts of energy transfer.

2 Possible Discussion Questions: What is the difference between heat and temperature? How does the kinetic energy of a substance depend on temperature and mass? Hint: Refer to the equation KE = ½ mv 2 Have students share real world examples of systems reaching equilibrium and / or of substances that start out at different temperatures and end up the same temperature over time. Ask students to describe what is happening at the atomic level. After completion of Part 2 of the activity: Models to Highlight: Page 6 Heat Conduction o Take this opportunity to discuss the different shape of the conductors and their properties. Analyze the effect of the type of conductor used on the time it takes the two materials to reach a temperature equilibrium. Page 8 Thermal Radiation o Use this model to discuss the presence of photons emitted from vibrational energy and the difference between this and atom toatom contact. o Link to other SAM activities: Energetic States and Photons. Use this to preview the idea of quantized energy levels and photons. Possible Discussion Questions: What properties of a conductor will determine how effective it is at heat transfer? Have students share some examples of very efficient and inefficient conductors. Students can share ideas about why different materials could be used for varied purposes. Have students brainstorm examples of heat transfer and thermal radiation focusing on how they are different.

3 Connections to Other SAM Activities This activity helps students learn that temperature is the average kinetic energy of atoms whereas heat is the transfer of energy from hot systems to cold systems. Atoms and Energy supports this activity. Students learn what kinetic energy is and how energy is transferred between kinetic and potential forms. Heat and Temperature addresses the concept that atoms are in constant motion due to thermal energy. This is evident in almost all of the other SAM activities. However, there are some key lessons where this point is particularly highlighted. In Electricity, an electric current heating a filament causes a light bulb to glow. Understanding heat is essential to understanding this model. In Excited States and Photons, kinetic energy is

4 converted into the excited states of atoms and is released in the form of a photon. Phase Change explores what happens to substance as heat is added and/or removed. The Gas Laws activity explores the relationships between pressure, volume, and temperature within gases. Intermolecular Attractions are illustrated by how tightly or loosely atoms are attracted to one another. This is demonstrated by using heat to break bonds between different atoms. For solids, the amount of solute that can dissolve in a given solvent increases as the temperature is raised, and is shown in Solubility. When analyzing chemical reactions in Chemical Reactions and Stoichiometry, it is evident that increases in temperature usually increase the reaction rate due to more frequent atomic collisions. In Chemical Reactions and Energy, an understanding of heat and temperature is necessary to analyze endothermic and exothermic reactions. Finally, in Four Levels of Protein Structure, the binding strength (strength of attraction) between proteins (or a protein ligand) is based on the ability they have to stay together given thermal motion.

5 Activity Answer Guide Page 1: 1. Compare the motion of the air molecules at high and low temperatures. The air molecules move faster at higher temperatures and slower at lower temperatures. Page 2: 1. The temperature of a substance is related to: (c) 2. A substance composed of atoms A and another composed of atoms B are at the same temperature, but atom A is ten times more massive. How does the speed of A compare to the speed of B, on average? The speed of atoms A will be slower than the average speed of atoms B. The more massive the atoms, the slower they will move when the temperature is held constant. 3. Which type of atom has the greater mass? (a) 4. The yellow and pink atoms in the container are the same temperature. Use what you know about kinetic energy to explain what you observe in the model. The total KE is the same for both the pink and yellow atoms. You can see that the less massive atoms (the yellow) are moving faster because they are affected more by the temperature than the more massive atoms (the pink). Page 3: 1. What did you observe about the kinetic energy (KE) of the atoms? (e) 2. If we add another box to the model, the average kinetic energy of atoms within the box will: (a, b and e) 3. Describe how changing the number of atoms in the box (by changing the size) affects the temperature. (c) 4. How is the average KE affected by adding or subtracting heat? Adding heat increases the average KE because the particles are moving faster and colliding more often. Subtracting heat lowers the average KE as the particles slow down and run into each other less. Page 4: 1. What will happen to the temperatures in the chambers if the temperature of the left one is higher than that of the right one? (d) 2. What do you think happens that causes both chambers to reach the same temperature in the above simulation? The particles are moving randomly and colliding. As they do, they transfer their KE to each other including to atoms in the other container. In an open system, these interactions will continue until the temperature is the same in both. Page 5: 1. Write down the temperatures of the large and small solids after you have hit the "Add energy" button 10 times. Describe the relationship between the readings and the size of the solids. For a large solid, the average temperature was about 400K after hitting the "Add Energy" button ten times. For a small solid, the average temperature was approximately 800K. As the size of the solid increases, it takes more energy to increase the temperature of the system. This is because there are more atoms that need to be "hit" or impacted by the transfer of energy. 2. How many times do you need to click the "Add energy" button on the small solid to reach approximately the same temperature of the large solid after you hit the "Add energy" button 10 times? (b) Page 6:

6 1. Which one in the above results in the fastest heat transfer between the hot and cold solid? (b) 2. A radiator heats a house by pumping a hot liquid through tubes folded as shown in the image on the left. What do you learn from the experiment on this page that can explain why the tube is made in such a shape? Heat conduction will be most efficient using this shape. The thin, short tubes that turn many times offer a lot of surface area for heat conduction with the colder surrounding air to take place. Page 7:. 1. How is thermal radiation different from heat conduction in transferring heat? (Check all that apply.) (b) (c) Page 8: 1. Take a snapshot of the model that shows thermal expansion, and then follow the instruction below to drag in the snapshot image. Sample snapshot: This snapshot of the graph shows an increase in energy when the solid is being heated. Page 9: 1. What is true of the kinetic energy of each individual atom when a substance has reached a certain temperature? (b) 2. Which of the following means of heat transfer explains why Jane's hand was burned after she touched the hot pot? (a) 3. When a hot and cold object are placed in contact, the hot one loses energy. Does this violate energy conservation? Why or why not? This does not violate the Law of Energy Conservation of a system. Although the hot object loses energy, the cooler object will gain energy. As a system, energy is conserved. Sample snapshot: Thermal expansion is shown here because the volume of the heated solid has increased. 2. Take a snapshot of the graph that shows the increasing of energy when heated, and then follow the instruction below to drag in the snapshot image. 4. When you place a hot cup of tea down, why does the cup of tea get cooler and the counter get warmer? Be sure to talk about kinetic energy and temperature in your explanation. Heat is being transferred from the cup of tea (the object with the higher temperature) to the countertop (the object with the lower temperature). The kinetic energy of the atoms in the cup of tea is being transferred to the atoms in the countertop. The temperature, therefore, decreases in the cup and increases in the countertop.

7 5. A vacuum flask (sometimes called a Thermos bottle) is a double-wall container with a vacuum between the two walls. How does the flask keep its contents hotter than the outside air? A vacuum is an extremely poor conductor of heat. Placing a vacuum between the two walls keeps the contents hotter than outside air because the presence of the vacuum limits the transfer of heat. Heat will ultimately be transferred slowly through the cap (for example), but will keep the substance much hotter than another type of container. 6. A gas contains equal numbers of atoms of radon (atomic mass = 222) and helium (atomic mass = 4). In equilibrium, which kind of atom is a) going faster, b) has more average KE, and c) is hotter? Explain your answers. a) The helium atoms are moving faster because they are lighter, according to the equation KE = ½ mv 2. b) The average KE is the same for all of the atoms in the gas because KE is being transferred from atom to atom as they collide. c) The average temperature is the same for all of the atoms in the gas. If heat is added to the system, the KE of the atoms will increase.

8 SAM HOMEWORK QUESTIONS Heat and Temperature Directions: After completing the unit, answer the following questions to review. 1. What is the difference between the terms heat and temperature? 2. How is the movement of air molecules different in the summer than in the winter? 3. Explain the following statement: Temperature in a system is a measurement of the average kinetic energy of the molecules in that system. 4. What happens to the kinetic energy of molecules in a system as they collide? 5. Explain how the snapshot to the right is an illustration of thermal expansion. 6. Heat conduction refers to heat flowing from a hot body to a cold body when they come into contact. Give an example of heat conduction that you have observed in everyday life. Then explain what is happening at the atomic level. Are all substances equally good at conducting heat? 7. Career Connection: Find a picture of a computer model that is modeling some form of heat transfer and give a brief description of the illustration. Examples might be home design to maximize energy efficiency, or new materials that help move heat away from electronics like computer chips.

9 SAM HOMEWORK QUESTIONS Heat and Temperature With Suggested Answers for Teachers Directions: After completing the unit, answer the following questions to review. 1. What is the difference between the terms heat and temperature? Temperature is a measure of average kinetic energy while heat is the transfer of energy from hot systems to cold systems. 2. How is the movement of air molecules different in the summer than in the winter? Air molecules are in greater motion in the summer because the temperature is higher than in winter. Their increased kinetic energy causes more collisions between molecules. 3. Explain the following statement: Temperature is a measurement of the average kinetic energy of molecules in an isolated system. The average kinetic energy is the total kinetic energy divided by the number of atoms. Temperature is a measure of the kinetic energy of all the atoms in the isolated system. 4. What happens to the kinetic energy of molecules as they collide? Some atoms have higher kinetic energy and some have lower kinetic energy. When they collide energy from the atom with the higher kinetic energy is transferred to the molecule with lower kinetic energy. 5. Explain how the snapshot to the right is an illustration of thermal expansion. As a substance is heated, 1) the temperature increases and 2) atoms move apart, thus taking up more space. Thermal expansion, which is an increase in volume, is shown by the atoms over the original line. 6. Heat conduction refers to heat flowing from a hot body to a cold body when they come into contact. Give an example of heat conduction that you have observed in everyday life. Then explain what is happening at the atomic level. Are all substances equally good at conducting heat? If you put a hot cup of soup down on the counter, it gets cold over time. The kinetic energy of the atoms in the cup of soup is being transferred to the atoms in the countertop. No, all substances are not equally good at conducting heat. 7. Career Connection: Find an image of heat transfer modeling and give a brief description. There are many examples of modeling heat flow. One way to find them is to google modeling heat flow and take a look at the images link to show images matching that search phrase.