In this second Investigation in the LabLearner CELL Investigating Heat, students observed and measured heat transfer between various objects. At the end of this PostLab, students will learn to calculate the Average Rate of Heat Transfer.

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A. Begin the analysis of this Investigation by encouraging students to summarize the experiments they conducted in the lab and the purpose of those experiments. Pose the following questions to prompt student discussion:

1. Ask students: What were the main questions we wanted to investigate in this lab? The main questions investigated in the lab were: How is heat transferred between different substances? How do materials differ in their ability to transfer heat? Which materials are conductors and insulators of heat? Can you determine how quickly heat is transferred?

2. Ask students: How would you summarize the types of experiments that you performed to investigate these questions? Students tested the ability of a metal cube and an acrylic cylinder to conduct heat in order to explore the difference between an insulator and a conductor of heat and to determine whether some materials were better conductors of heat than others. Students then tested the ability of a brass fastener, a gram bear, a glass stirring rod, a woodblock, gravel, and aluminum foil to conduct heat to examine how different materials conduct heat. Students constructed thermoses from beakers, aluminum foil, paper, and plastic bags to investigate how using different materials to insulate a container affects the rate of heat transfer. These experiments also provided students the opportunity to observe the three types of heat transfer: radiation, convection, and conduction.

B. Begin the analysis of the lab by directing students’ attention to the results of Trial 1. The following discussion is designed to encourage students to think about how conductors and insulators differ in their ability to conduct heat.

1. Ask students: Which item was a conductor? Which item was an insulator? How could you tell the difference? The metal cube was a conductor. The acrylic cylinder was an insulator. Both the metal cube and the acrylic cylinder felt similar in temperature in the beginning of the experiment. The difference between the two was that the metal cube became colder to the touch while the acrylic cylinder did not feel as if it had changed temperature after both were placed in cold water.

2. Direct students to look at the definition of heat in their Scientist’s Glossary. Ask students: Does the definition of heat tell you in which direction heat will be transferred? Can you describe this in your own words? The definition of heat indicates that energy is transferred from molecules with greater kinetic energy to molecules with lesser kinetic energy. Students should be able to indicate that this means heat is transferred from substances or objects of higher temperature (hotter) to lower temperature (colder).

3. Ask students: Why would you classify the metal cube as a conductor? How did it conduct heat? In which direction was heat transferred? Student answers may vary. Students should indicate an understanding that heat is a transfer of kinetic energy from a substance of higher temperature to a substance of lower temperature, and not simply related to the substance being warm or hot. This heat was transferred from the metal cube to the ice water. This resulted in the cube becoming colder to the touch after immersion. If the transfer of heat had occurred in the opposite direction (from ice water to the metal cube) the cube would have felt warmer after immersion than before immersion. As the cube was warmer than the ice water before it was immersed, and as heat moves from areas of high temperature to areas of low temperature, the heat transfer must have occurred from the metal cube to the ice water. As a result, the metal cube became colder after immersion in the water.

4. Ask students: Why did you describe the acrylic cylinder as an insulator? Can you explain your answer in terms of heat transfer? An insulator is a substance that does not transfer heat easily. The acrylic cylinder was at room temperature, a temperature warmer than the cold water when it was placed in the cold water. If heat were easily transferred from the cylinder to the water, as with the metal cube, the cylinder would have felt cold when it was removed from the water. However, the acrylic cylinder felt warmer than the metal cube, suggesting that it did not conduct heat as easily as the metal cube. Therefore, it was an insulator of heat.

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5. Ask students: Can you recall, from discussions before the lab, the different methods of heat transfer? How would you describe each?

a. Conduction is the process of transferring heat from one material to another through the contact of molecules. Conduction occurs between solids and can occur between solids and liquids and between solids and gases as the molecules of each come in contact with each other.

b. Convection is the process of heat transfer in fluids such as liquids or gases. During convection portions of fluids have greater kinetic energy than others. When this happens, warmer and cooler areas of fluid begin to move and heat is transferred from the warmer to the cooler areas.

c. Radiation is the process of transferring heat from an object through electromagnetic waves. Examples of electromagnetic waves include ultraviolet (UV), light, and infrared waves. Most of the heat transferred by radiation occurs through electromagnetic waves. Radiation only occurs across a gas or vacuum, not through a solid or liquid.

6. Was the transfer of heat between the water and the metal cube a result of conduction, convection, or radiation? Why? The transfer of heat between the water and the metal cube was the result of conduction, caused by contact between molecules on the surface of the metal cube and the molecules of the water surrounding it. Kinetic energy also moved from the molecules at the center of the cube outward as the outer temperature of the cube dropped. The transfer of heat between the metal cube and the water was not due to convection because solids are not involved in convection. However, the heat moved through the water by convection. Radiation was not involved in the transfer of heat between the metal cube and the water because radiation only occurs through a gas or vacuum.

7. Direct students to record their answers in Problem 4 of their Student Data Record.

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C. Continue the discussion on insulators and conductors by encouraging students to consider what they learned from Trial 2.

1. Ask students: Which objects were good conductors? Which were good insulators? Were some objects better conductors than others? The brass fastener, the aluminum foil ball, the gravel, and the glass rod were good conductors of heat. The gram bear and the woodblock were good insulators.

2. Ask students: Were there any patterns in the types of materials that made good conductors? What about materials that made good insulators? Students should indicate that objects that were good conductors were made of glass, metal, or stone. Objects that were not good conductors of heat were made of plastic or wood. Direct students to record their answers in Problem 5 of their Student Data Record.

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3. Encourage students to think about how these materials are used in a kitchen, where heat is a common occurrence.

a. Ask students: What are the spoons in your family’s kitchen made of? What happens if you leave them in a pan of hot food? Student answers will vary. Some students will indicate that spoons are plastic. Others will indicate that spoons are metal. Others will indicate that spoons are wood. Some students may indicate that the bowls of their spoons are metal but have wood or plastic handles. Students should indicate that plastic and wooden spoons are made of materials that are insulators, and stay cool to the touch when they are left sitting in hot foods. Students should indicate that metal is a conductor of heat, and spoons made entirely of metal will be hot to the touch after sitting in hot food. The handles of metal spoons with wood or plastic handles will be cool to the touch, even when the bowls of these spoons are hot.

b. Ask students: What happens to car door handles and seat belt buckles in the summer? What does that tell you about their ability to conduct heat? Car door handles and seat belts get very hot in the summer. This indicates that these items are made of materials that conduct heat.

c. Ask students: In the winter, would you rather sit on a wood bench or an iron bench? Why? A wood bench would be preferable as a seat in winter. Iron is a metal and is a very good conductor of heat. Metal benches conduct heat away from the body, so would cause a person to feel cold, especially where they are in contact with the metal bench. Wood is an insulator, so the wood bench would not conduct heat away from the body, making it a more comfortable seat.

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D. Continue the discussion by directing students to rejoin their pairs/groups from the lab.

1. Encourage students to describe their thermos designs to the class and share their results from Trial 3.

2. Ask the following questions to promote discussion:

a. Why did you choose your particular materials? Student answers will vary but should reflect their considerations and conclusions from Trials 1 and 2. For example, students should indicate a preference for paper or plastic because of their insulating properties. Some students may demonstrate a preference for aluminum foil. Aluminum foil is capable of reflecting heat, and therefore would be a good choice as an outermost or innermost layer. However, because of its ability to conduct heat as well as reflect it, it is a poor choice for insulation when used alone.

b. What else could you add to your thermos design to help improve its ability to keep the water cold? Student answers will vary. Suggestions should include the addition of an insulated lid as an improvement. Students should also suggest that the bottom of their thermoses should be insulated if they failed to protect the bottom of the beaker with their initial design.

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E. This section of the analysis introduces students to the calculation of the average rate of heat transfer as a method of quantifying the heat gained by the ice water in the thermos construction. Quantifying the heat gained in each beaker allows students to accurately compare the heat transfer into the wrapped beaker with the heat transfer into the unwrapped beaker.

1. Ask students: How could you determine if your thermos was effective in keeping your ice water cold? Student answers will vary. Students may indicate that the effectiveness of their thermoses can be evaluated by comparing the final temperatures of the wrapped and unwrapped beakers.

2. Explain that the effectiveness of the thermoses can also be determined by comparing the average rate of heat transfer between the beaker’s contents and its surroundings. The average rate of heat transfer is more useful than the difference in final temperature between an insulated and an uninsulated container or the total change in temperature for the container, as neither value accounts for the time needed for the temperature to change. In contrast, the average rate of heat transfer factors in the amount of time required for the temperature to change. This provides a method for scientists to determine how long a thermos or other insulated device will effectively and safely keep its contents hot or cold.

3. Tell students that scientists can calculate the average rate of heat transfer to determine how quickly heat or kinetic energy was transferred from one substance or area to another. The rate of heat transfer can be used in this Investigation to determine how effective students’ thermoses were insulating the cold water.

4. Direct students’ attention to the definition of the rate of heat transfer in their Scientist’s Glossary. 

Rate of Heat Transfer: The change in temperature over a specific time period.

Ask students: Based on this definition, what values do you think you will need to calculate the average rate of heat transfer? The values needed to calculate the average rate of heat transfer are higher temperature, lower temperature, and time.

5. Refer to the equation on the slide, and explain that the equation is an example of a formula, one of the tools from their Procedural Toolbox. Students may have had previous experiences in using a formula to calculate the amount of work done when lifting an object. The concept this formula represents is how quickly heat is transferred. Formulas are ways of representing concepts, either through symbols or numbers. As you discuss the equation, remind students that rate refers to a change over time.

Therefore,

6. Example on the slide: a beaker of water has an initial temperature of 6 ^oC. After 45 minutes, the final temperature is 18 ^oC. The higher temperature is 18 ^oC and the lower temperature is 6 ^oC.

7. Direct students to calculate the average rates of heat transfer for both beakers in Problem 6 of their Student Data Record. Assist students as necessary during this process.

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Conclude this part of the Investigation by encouraging students to rehearse what they know about heat transfer. Pose the following questions to prompt discussion:

1. Which direction is heat transferred? Why? Can you list examples from your experiments? Heat is transferred from areas of high temperature to areas of low temperature because kinetic energy moves from areas of higher kinetic energy to areas of lower kinetic energy. Examples from the lab include the metal cube, the gravel, the glass rod, the brass fastener, and the aluminum foil becoming cooler after being immersed in cold water. In the thermos experiment, heat was transferred into the beaker from the air, because the temperature of the water increased. This indicates that the temperature, and thus the kinetic energy, of the air molecules, were higher than the temperature (kinetic energy) of the water molecules.

2. What types of heat transfer did you observe in the metal cube, brass fastener, gravel, glass rod, and ball of aluminum foil? The metal cube, brass fastener, gravel, glass rod, and aluminum foil all demonstrated heat transfer by conduction. Kinetic energy was transferred to the water from the molecules of the objects as the water molecules collided with the surfaces of the objects. Kinetic energy inside the objects also moved toward the outer edges by conduction as the kinetic energy of the outer surfaces became lower than the kinetic energy in the inner portions of the objects.

3. What types of heat transfer did you observe in the uninsulated beaker? Which types were affected by the presence of the insulating materials around the beaker? Students observed conduction, convection, and radiation. Conduction occurred between the air and the sides of the beaker and between the tabletop and the bottom of the beaker. Heat was transferred between the beaker and the water by conduction. Convection occurred between the surface of the water and the air and through the water. Radiation occurred from all sides of the container as the heat was radiated from students’ bodies, other objects, and lights in the laboratory. The insulating materials slowed the transfer of heat by radiation into the room from the sides of the beaker and conduction between the air and the sides of the beaker. If the students wrapped the bottom of the beaker, conduction between the base of the beaker and the tabletop was also inhibited.