Unit #1

Activity 13
Cool It! ... Especially if You're Hotter Than Me

Activity Overview

Heat energy can be transferred between two containers by putting them in direct physical contact, and this heat energy will always flow from the hotter container to the cooler container.

Part A: Students fill two containers (plastic bags) with water of different temperature, insert a temperature probe into each bag and monitor the temperature on the computer. Students then repeat the experiment, but put the bags in contact with each other.

Part B: Students kinesthetically model the lab from Part A.

Part C: Based on the students’ previous experience with the kinesthetic modeling of heat transfer between two containers in direct contact, students are predict what the computer model would look like for this situation. They then use a computer simulation of the lab in Part A. Students then compare and contrast their previous kinesthetic experience, with their prediction and actual experience with the computer model.

Learning Objectives

Students will:

• Predict in what direct heat energy will flow between two bodies which are in concact with each other.
• Indicate when thermal equilibrium has been reached between two bodies which are in contact with each other.
• Describe what happens at the molecular level when thermal equilbrium has been reached between two bodies which are in contact with each other

Conceptual Prologue

Macro-Micro Connection

The air inside the hot air balloon is warmer than the air outside of the balloon and is contained by a thin nylon fabric. Because heat energy is due to atomic motion, the atoms inside the balloon can transfer their energy through collisions to air outside the balloon causing the temperature to decrease inside the balloon (and increase outside the balloon). Heat always flows from a place of greater temperature to lower temperature, so the hot air will cool, requiring the need for the burner to repeatedly heat up the air inside.

Other macro connections:

• We can explain, in general, why hotter things tend to get cooler, and cooler things tend to get warmer.

Science Concepts

Heat energy can be transferred between two containers by putting them in direct physical contact, and this heat energy will always flow from the hotter container to the cooler container.

The temperature of a substance is dependent on the kinetic energy [energy of motion] of its atoms or molecules. Because atoms or molecules can transfer some of their kinetic energy by colliding with other atoms, heat energy can be transferred through atomic collisions.

If there are two substances each inside their own container, then heat energy can be transferred between the two substances through the following process:

• The atoms of each substance collide with the atoms of their respective containers.
• If the containers are touching each other, the atoms of one container can collide with the atoms of the other container which can then collide with the atoms of the substance within the container.
• Eventually, the two substances and their containers come to thermal equilibrium [a state in which all atoms in the system have the same average kinetic energy].

During the process of reaching thermal equlibrium you might wonder how atoms know which way to transfer their kinetic energy. In fact, heat is flowing from the hot container to the cold AND from the cold to the hot. However, because the hotter container has a greater portion of atoms with higher kinetic energies, the rate of kinetic energy transfer from the hotter container to the cooler container is faster than the the rate of energy transfer back from the cooler container to the hotter container. The result is a decrease in the temperature of the hotter container and an increase in the temperature of the cooler container.

Eventually, when the temperatures become equal, the rate of energy exchange is equal and it appears that nothing is happening. However, energy is still being exchanged from one container to another. It’s just that the rate of exchange is equal. This state is known as thermal equilibrium.

Naive Conceptions

• There is cold energy.

There is no such thing as "cold energy". Heat energy, on an atomic level, is the same as the kinetic energy of the atoms. The faster the atoms move the more heat energy they have. Students will often describe ice as giving off "cold energy" when they should be saying the ice is absorbing heat energy. The atoms in ice are moving more slowly, so they typically have less heat energy than their surroundings, but they don't radiate "cold energy".

• Heat is a substance.

Heat is not matter. Heat is just a manifestation of the motion of atoms.

• Heat and temperature refer to the same thing.

Heat and temperature are very different. Heat energy is the TOTAL kinetic energy of a group of atoms while the temperature is the AVERAGE kinetic energy of those atoms.

Activity Design and Execution

Major Science Concepts:	• thermal equilibrium • 2nd law of thermodynamics (heat alwasy flows from hot to cold)
Assumed Previous Knowledge:	• Atoms have kinetic energy. • Kinetic energy of atoms is the same as heat energy. • Temperature is a measure of the average kinetic energy of atoms. • Thermal equlibrium happens when all atoms in a system have the same average kinetic energy.
Time:	• Part A: approximately 50 minutes • Part B: approximately 50 minutes • Part C: approximately 30 minutes
Materials:	For part A each group needs: • A computer with some type of data collection software.* • Two temperature probes.* • Two plastic bags. • Hot and cold water. For part B: • Popsicle sticks. For part C: • Computers with Workbench software. *You can substitute standard thermometers and graph paper for these.
Advanced Preparation: (if any)	• Make sure to have a large supply of room temperature water. It may be good to fill a bucket the day before.

Investigative Question: In what direction will heat energy be transferred and why?

Part A:

1. Have students complete the Heat Transfer lab.
2. Discuss the lab questions.
   

Part B:

1. Have students form two square containers as depicted in figure A. These students are the walls of the containers.
2. Divide the rest of the class into two groups and place them inside the containers. These students will be atoms.
3. Give everyone inside the first contianer and the people making up the container the same number of popsicle sticks with small variations.
4. Give everyone inside the second container and the peole making up the second container a larger number of popsicle sticks with small varitions in the number.
5. Remind students that all models have limitations and simplifications. Explain to them that all of the models they have used so far used containers that just showed flat walls. As we know everything is made of atoms, so the wall in the models are made of atoms as well. Normally, these atoms are not shown in order to make the model more simple. However, to understand how heat energy gets transferred from one model to another we need to have the atoms of the walls participate in the model.
6. Tell them that the atoms in this model will behave as in the previous kinesthetic models, and ask them to list what those rules are.
7. Then explain that there will be a couple of additional rules for this model. The atoms making up the walls of the containers will not move, and the atoms of the container walls that are touching must always share their energy sticks equally between them.
8. Have the students of one container call out their various kinetic energies (or number of sticks) and calculate the temperature (the average). Do the same for the other container and place these values on the board.
9. Run the model for a while, and repeat the previous step of calcuating temperatures. Eventually the averages should come out the same.
10. Ask the students how this relates to the heat transfer lab.
11. Ask the students how this is similar or different than their previous ideas of thermal equilibrium.
12. Even when they have reached thermal equilibrium ask them if energy stops being transferred between the containers and to give an example of how they know.
13. Ask students to describe what a computer model of this acitivity might look like.

Part C:

1. Using Workbench software have student open the "Heat Transfer" activity.
2. The students will see two containers constructed from atoms each containing a gas. Beneath each container will be a temperature guage and a control allowing them to add or remove heat energy.
3. The software will then set the temperature of one contianer higher than the other and ask the students to indicate which way the heat energy will flow and how will that affect the temperature of each container.
4. The software will then run the simulation and if the student answered incorrectly ask them to explain why the simulation show the energy going the other way.
5. The software will then allow students to set the starting temperature of two separate containers and predict which way the heat energy will flow.
6. The software will also ask the students to indicate when the model has reached thermal equlibrium by clicking on a button. It will then ask them how they know the system is in thermal equilbrium.
7. The software will then ask the students to change the temperature in either container and describe what they observe happen.
8. Discuss what they observed during the simulation and then discuss the similarities and differences between their kinesthetic modeling experience, their expectations for the computer model, and their actual experience with the model.

Assessment

Have students write several things in their notebooks:

1. When a cup of hot chocolate cools down to room temperature, where did all the heat energy go?
2. The second law of thermodynamics states that energy always flows from hot to cold. How can you explain this on an atomic level?
3. A good thermos can keep cold things cold and hot things hot. How does it know? (Hint: Some thermoses have almost all of the atoms between the inner chamber holding the liquid and the outer walls of the thermos removed. In other words, there is a region of empty space containing no atoms which separates the inside from the outside.)

Internal Notes:
• See computer lab F for mock-up of this simulation.