Heat Transfer 2

Interactive, scaffolded model

Overview and Learning Objectives

This model provides an atomic-scale explanation for heat transfer that can help explain fundamental properties of heat and temperature. It is fairly clear from running the model and experimenting with it that hot atoms can warm up cold atoms by exchanging energy through vibration.

This activity requires an appreciation of the random nature of atom motion and its relation to temperature. Activity 238 (Heat and Temperature)is a single page that can be used to develop these ideas. Activity 74 (Heat versus Temperature) addresses these same concepts with a more scaffolded and tested series of pages.

Students will be able to:

• Provide an atomic-scale description of heat flow and thermal equilibrium.
• Estimate the equilibrium temperature of hot and cold objects placed in contact.
• Predict the shape of temperature-time graphs of hot and cold objects placed in contact.
• Explain the origin of temperature fluctuations in small numbers of atoms.

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Assessment

The following questions are asked in the activity:

Can you explain the overall shape of the graphs and their final values? [The kinetic energy graphs tend toward a final value, moving quickly at first and gradually more slowly. Mathematically, these are negative exponentials. Superimposed on these trends are random fluctuations that are always present for small numbers of atoms.]

What changes do you observe in the atoms in the model? [The green ones start with lots of motion and slow down. The blue ones start at rest and speed up a bit.]

What causes the changes in the blue atoms? [All the atoms hit their neighbors. This tends to spread out the starting rapid motion of the green atoms, slowing the green ones and speeding up the blue ones.]

Does anything flow from the green to the blue atoms? [No atoms flow, but the kinetic energy does flow. This is also called heat flow.]

What do you predict if you start with fewer green atoms? [The shape will be similar, but the final value will be lower, because there is less energy available.]

Which type of atoms has greater fluctuations? Why? [The green ones fluctuate more because there are fewer of them.]

What changes could you make in this model to increase the fluctuations in the green graph? [By running a model with half as many green atoms or fewer, the fluctuations of the green graphs can be increased.]

Additional questions that could be asked:

What do you predict if you started with a billion of each kind of atoms? [The shape will be similar, but the fluctuations would be absent.]

If you double the number of green atoms will the final kinetic energy be doubled? What if you start with ten times as many green atoms? [No. The green and blue atoms share their kinetic energy, so the final temperature must always be between the starting energies of the green and blue atoms.]

Why are most things in this room at the same temperature? [As this model shows, heat flows from hot to cold substances until they reach the same temperature.]

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Classroom Practice

This activity is a single page with open-ended questions. Deceptively simple, it can profitably be studied and used for experimentation that gets to the heart of the idea of heat flow and the difference between heat and temperature.

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Central Concepts

Key Concept:

Heat flows from hot to cold substances.

In equilibrium, two objects have the same average temperature or kinetic energy.

Additional Related Concepts

Physics/Chemistry

• Heat energy

Systems

• Equilibrium

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Benchmarks and Standards

AAAS

• COMMON THEMES: Constancy and Change - A system in equilibrium may return to the same state of equilibrium if the disturbances it experiences are small (Full Text of Standard)
  
  
• THE PHYSICAL SETTING: ENERGY TRANSFORMATIONS - Heat energy in a material consists of the disordered motions of its atoms or molecules (Full Text of Standard)
  
  
• THE PHYSICAL SETTING: ENERGY TRANSFORMATIONS - Transformations of energy usually produce some energy in the form of heat, which spreads around by radiation or conduction into cooler places (Full Text of Standard)
  
  

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Additional Info

Additional Background

Interesting point for modelers:

These models often start with a precipitous drop in kinetic energy of any heated atoms. This confused us for quite a while until we figured out that this was an artifact of the way we heat atoms. When you heat atoms using the sun-like heater, you increase the atoms' starting kinetic energy (KE), but not their potential energy (PE). It would be difficult to change their PE, because that would involve moving them. But in thermal equilibrium, there is as much energy in KE as in PE. So atoms heated by giving them only KE rapidly give half of their energy to PE, causing the big initial drops in KE. We have avoided this in the initial setup of this model by saving as its initial state the state of the model after it had run for a few steps. That is, we set up the model, blasted its green atoms with 50 pulses of heat, ran it, stopped it, backed up the model to the second time step, and saved that as the initial state.

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Macro Micro Link

What is seen in this atomic model accurately models what happens at the macroscopic scale when you put, for instance, a hot cup on a cold counter. The only difference would be the absence of fluctuations in the graphs.

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Activity Credits

Created by CC Project: Molecular Workbench using Molecular Workbench

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