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An excited atom can give up its stored energy in a collision
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You have learned that atoms can have two kinds of energy: kinetic energy due to their motion and a kind of internal energy if they are in an excited state. These two kinds of energy are interchangeable: they can be swapped back and forth. So if two atoms crash into each other one or both of them might be excited and then the two atoms will move more slowly after the collision. In that case the energy has been converted from kinetic energy to the internal energy of the excited atoms.
The opposite process can happen too: an excited atom can give up its stored energy in a collision with another atom and the energy released will make them both move faster. The internal energy has been converted to kinetic energy.
In the model below we've added two more purple atoms. We've also made the blue atom steerable, meaning that when you press the arrow keys on the keyboard you will push it left, right, up, or down. First try to get all the purple atoms into the excited state. When you succeed, stop the animation and take a snapshot. Then try to de-excite them all back to the ground state and take another snapshot when you succeed. This task is much harder than exciting them; if you want help, press the Hint button.
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<b><font size="5" face="Trebuchet MS">An excited atom can give up its stored energy in a collision</font></b>
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When you push on the steerable atom you either speed it up or slow it down.
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This adds or subtracts energy from the system of atoms.
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The more energy you add the more likely you are to excite the atoms.
Hint
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