Heat and Temperature
Physical Science
MS
Heat Propogation Activity 111
Temperature vs. Heat Activity 110
Temperature of Mixing Water Activity 112
Depending on computer and probe availability relative to the number of students per class, the activities in this unit could be done by individual students up through small groups (probably no more than 3 per group). It will be important for students to know how to run the models, including each of the model features and how they are activated or modified. In the Temperature of Mixing Water activity, insure that the temperature of the water used does not pose a safety hazard.
No special set-up is needed for this unit. Insure that students understand how to run the models, including all of their features and how they can be modified.
Assuming that class periods are 45-47 minutes in length, the Propagation of Heat and Temperature Versus Heat activities can be completed in one period. The Temperature of Mixing Water activity could take longer than one period depending on time for experimental setup and connection speed to the Internet.
Heat
Even after some years of physics instruction, students do not distinguish well between heat and temperature when they explain thermal phenomena. They are closely related but not identical. Everyday language confuses the concepts. For instance, "hotter" means a higher temperature, not more heat.
Heat is said to "flow", but heat energy moves from one place to another by conduction without any material flow. However material does "flow" in convection, meaning that a hot or cold liquid or gas is moved from one place to another. And radiation can transfer heat energy but by an entirely different mechanism -- electromagnetic radiation.
Students usually do not distinguish these different mechanisms. Few middle- and high-school students understand the molecular basis of heat conduction even after instruction. During instruction, upper elementary-school students use ideas that give heat an active drive or intent to explain observations of convection currents.
Content objectives
The student will be able to: (1) Define temperature as the rate of molecular motion. (2) Describe heat energy as the kinetic energy of molecular motion. (3) Describe the process of heat transfer as the imparting of energy from the collision of a more energetic molecule to a less energetic molecule. (4) Formulate a rule for the mixing of quantities of liquids of differing temperatures.
Inquiry objectives
The student will be able to: (1) Use appropriate tools and techniques to gather, analyze, and interpret data. (2) Develop descriptions, explanations, predictions, and models using evidence. (3) Think critically and logically to make the relationships between evidence and explanations. (4) Use mathematics in all aspects of scientific inquiry.
If you leave your spoon in a cup of hot chocolate why does the end of the spoon not in the hot liquid eventually feel hotter?
Since the metal conducts heat well, the spoon and the liquid come into equilibrium at the same temperature, which is close to the temperature of the hot liquid.
If your mother prepares a cup of hot chocolate for you that is too hot to drink, what would you do so that you could drink it?
(1) wait to drink it in order to let the hot chocolate cool down, (2) add some milk from the refrigerator to the hot chocolate to allow it to cool down, (3) move the air above the hot chocolate using your hand to fan it, (4) add a piece of ice. Use the answers for this question to have a discussion about how each of these cooling processes works. The first includes conduction and convection. The second adds molecules at a lower temperature that must be warmed up by the hot chocolate. The third is cooling by convection. The fourth removes the heat required to melt the ice -- change of state.
What happens to the temperature of a substance when heat is added:
The temperature of the substance increases.
At the molecular level, what happens when heat is added to the substance?
the molecules in the substance begin to move faster.
In the models, why are the higher temperature molecules moving faster than the lower temperature molecules?
In the model, temperature is calculated as the average kinetic energy, which is related to the square of the velocity.
When you mix two equal volumes of water of differing temperatures together, the temperature of the mixture is about the average of the two temperatures. What is happening to the heat content of the molecules when the two volumes mix? How are the motions of molecules reflected in the observed temperature of the mixture?
The molecules of the higher temperature water impart some of their energy to the molecules in the lower temperature water. This causes the molecules in the higher temperature water to be less energetic and the molecules in the lower temperature water to be more energetic. Over time, the range of energies becomes less and less and settles on the average of the two extremes. As the two volumes mix, the observed temperature may vary in the mixture, but over time, the variation in temperature decreases and approaches the average of the two extremes.
All forms of energy can be classified as potential or kinetic. The amount of heat stored in a substance is a measure of the kinetic energy of the molecules that make up that substance. The greater the kinetic energy of the molecules, the more heat that is stored in the substance. Heat plays a significant role in the phase changes undergone by matter. The changes in phase from solid to liquid, liquid to gas, or solid to gas by sublimation are only possible by addition to heat energy.
Temperature is measure of the frequency of collisions by molecules of a substance with a thermometer or other temperature-sensing device. Because of the relationship between the frequency of those collisions and the kinetic energy of the molecules, temperature is also an indirect measure of the kinetic energy of molecules. The higher the temperature, the greater is the average molecular kinetic energy of the substance.
When two equal volumes of water of differing temperature are mixed together the resulting temperature of the mixed water is the average of the temperatures of the two waters.
If the volumes are unequal, the resulting temperature is the weighted average. Here is an example: Suppose that 100 milliliters of water (A) at 70 degrees C is mixed with 200 milliliters of water (B) at 50 degrees C. The resulting temperature is calculated using the following method. The proportion of A to the total volume is 100/300 or 1/3 and the proportion of B, 200/300 or 2/3. The resulting temperature of the water is calculated using this equation:
(1/3) x 70 degrees C + (2/3) x 50 degrees C = 56.7 degrees C
Propagation of Heat
How does the type of material affect heat propagation? What kinds of materials make good conductors? What kinds of materials make good conductors of heat? Q: What causes heat to propagate?
Heat propagates as a result of the collision of higher kinetic energy molecules with lower kinetic energy molecules.)
When hot and cold objects are placed in contact, the hot one loses energy. Does this violate energy conservation? Why or why not?
The Law of Conservation of Energy is not violated in this instance. The hot object loses energy as the cold object gains energy.
How did the temperatures of the large and small solids differ after added energy 10 times. Describe the relationship between the readings and the size of the solids.
The temperature difference was inversely proportional to the size of the solid.
A radiator heats a house by pumping a hot liquid through tubes folded as shown in the image on the below. What did you learn from the activity that can explain why the tube is made in such a shape?
The coiled radiator in the picture more effectively propagates heat than a similar length of tube uncoiled because the hotter body is more concentrated and thus heats a larger volume of air through conduction. Teachers: You may want to clarify that older steel and cast iron radiators produced a combination of radiant heat and convective heat -- the hot water ran top-down through the components and the steel panels radiate the heat into a home. But newer radiators are primarily convector types, where the hot water circulates through a tube surrounded by small fins that act to increase the contact surface with surrounding air. They work only via convection, but still oddly use the name "convector radiator".
Temperature Vs. Heat Analysis Questions
What do you think happened that caused both chambers to reach the same temperature?
The average kinetic energy of the molecules in both chambers approached the same value over time.
How does the motion of the air molecules at high and low temperatures compare?
At high temperatures molecular motion is much greater than at lower temperatures.
Is the temperature of a substance related to only the speed of the atoms, the mass of the atoms, or both the speed and mass of the atoms?
Temperature is related to the kinetic energy of the molecules and thus temperature depends on both the speed and mass of the molecules. Teachers: It may be useful here for students to review the meaning of kinetic energy as the energy of motion. Kinetic energy of an object is measured by an equation that relates the mass of the object to the square of its speed.
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.
Students may not be able to articulate the scientific components that underlie heat transfer. Acceptable responses will include recognition that kinetic energy is transferred from the warmer body to cooler bodies, including the environment. As a result, the temperature of the warmer body decreases while the temperature of the counter increases. Astute students may recognize that objects in contact tend to seek a balance (we know this as thermal equilibrium).
Temperature of Mixing Water Analysis Questions
Is the temperature of your first mixture (two quantities that are about equal) close to what you expected?
Yes, the amount of molecules in each volume is approximately the same and when mixed the temperature of the mixed water is a measure of the arithmetic average of the kinetic energy of both water volumes.
Is the temperature of your second mixture (two quantities that are not equal) close to what you expected? Explain.
Yes, the temperature is more influenced by the larger volume of water than the smaller volume of water because there are molecules of water in the larger volume, which means that it has more influence on the temperature of the mixed water.
Can you come up with an equation that could help someone else accurately predict the final temperature for mixing two equal volumes of water, knowing only the initial temperatures? Compare this with your measured result.
R1 is the ratio of one of the volumes of water to the total mixed volume with a temperature of T1. R2 is the ratio of one of the volumes of water to the total mixed volume with a temperature of T2. The final temperature Tf of the mixed water is: (T1 + T2)/2.
Using the same reasoning, can you write an equation for the final temperature for mixing two parts cold with one part warm water? Compare this with your measured result.
Tf = (1/3) x T1 + (2/3) x T2)
Can you come up with an equation that could help someone else accurately predict the final temperature of a mixture of two different volumes of water, if they know the initial temperatures and volumes?
R1 is the ratio of one of the volumes of water to the total mixed volume with a temperature of T1. R2 is the ratio of one of the volumes of water to the total mixed volume with a temperature of T2. The final temperature Tf of the mixed water is: R1 x T1 + R2 x T2.
The diagram shows the variation in atmospheric temperature with height above the surface of the Earth. At the bottom of the atmosphere in the troposphere, air temperature gradually decreases with height. Also, we normally think of space as having very low temperatures and we would expect that the temperature would also be very low near the top of the atmosphere. However, as the diagram shows, air temperature steadily increases with height in the Thermosphere near the top Earth's atmosphere. Based on what you have learned about heat and temperature in this unit
What explains the decrease in temperature with height in the troposphere?
Earth's surface is acting as a source of heat for the lowermost part of the atmosphere and its influence on atmospheric temperature decreases with height.
What explains the steady increase in temperature within the thermosphere?
The energy of the molecules increases with height through this layer because of incoming ultraviolet radiation.