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SAM E1 Model 1
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The Total Energy of Atoms
The total energy is the sum of KE and PE.
Use the graph below to compute the total energy curve.
Do this by reading the KE and PE at several distances and numerically adding the values.
For instance at 0.6 nm the PE (the blue curve) is about –0.6 eV and the KE is about 0.6 eV.
What is the sum of these two?
Repeat this when the distance is 0.8, 0.9, and 1.0 nm.
Put your results on the graph using the annotation feature.
Right-click (Windows) or command-click (Macs) on the graph to access the annotation tool.
The model below makes a graph of the PE, KE, and total energy of the red atom.
Once again, the green ball is nailed down and the red ball has to move in the blue area.
Do this:
Before running the model, predict what the three curves (KE, PE, and total energy) will look like.
Run the model.
Use the "tape" controls below the model to explore the relationship between the model and the graphs.
Take a snapshot of the graph. (Use the camera icon in the menubar.)
Show on the snapshot where on the graph the total energy is greatest.
Red: KE of the red atom
Blue: PE of the red atom
Green: Total energy of the red atom.
By changing the starting positions and using the two buttons, take snapshots of graphs that look approximately like each of the following.
A B C
Super Challenge:
So far, we have looked at only one pair of atoms. What happens when there are more?
The next activity let's you experiment with three or more atoms.
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What is the total energy at 0.7, 0.8, 0.9, and 1.0 nm?
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Zero. Always.
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Explain why the red atom can move, even if it has zero total energy.
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Potential energy become negative as the two atoms attract. This makes it possible to have positive kinetic energy while their sum is zero.
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How much does the total energy change during a run?
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It stays constant. It does not change.
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Describe the motion of the red atom when the total energy is negative.
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It is trapped!
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Describe the motion of the red atom when the total energy is positive.
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It is free to move about the container.
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Can you make the graph below?
I started with the red ball trapped at rest 6 A from the green one. Using the "High Seed Start" button, the red ball eventually got free. How did I do it?
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Press the "High Speed Start" each time the red ball has traveled as far as it can to the left. Repeat seven times.
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It is like pushing a swing. You have to push at the right part of the swing.
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