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Pretty much what the title says. My base question is this. Assuming I take a piece of steel, and a piece of PVC plastic and I measure both their temperatures and find they are the same. I then take a look at the vibration speeds of the individual molecules would they be the same as well?

Here's a rough example:

I measure both the steel and PVC and find them both at 100F, and then I measure the vibrations of a molecule in the steel and find it to be moving at 10 miles an hour. Would the PVC molecules also be moving at 10 miles an hour?

I'm sure I'm not using the correct units of measure to measure the vibration, but I didn't know what the correct unit of measure is for something like that. Hopefully it gets the point across.

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up vote 4 down vote accepted

No, because the atoms in steel and plastic have different masses.

Your example is a bit more complicated than it need be because steel (well iron) is an element while plastic is a compound. This complicates things because molecules can have internal motions that contribute to the energy. A better comparision might be between lead and lithium. These are both elements to you just need to consider the motions of the Pb and Li atoms. For a given kinetic energy a lead atom will be moving more slowly than a lithium atom because it's heavier.

You could think of it this way: if you touch the piece of lead to the piece of lithium the atoms will be in contact so they'll swap energy by bashing into each other. If a lead atom hits a lithium atom the Li atom will recoil a lot faster than a Pb atom.

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Its probably simpler to think of gases. From the Wikipedia article on the Kinetic Theory of Gases, the average kinetic energy of molecules in an ideal gas is 3/2 k_b T. But since molecules of nitrogen (28) are lighter than molecules of oxygen (32), it follows that at the same temperature, an "average" nitrogen molecule is moving about 7% faster than an "average" oxygen molecule. – Anonymous Jun 2 '12 at 17:05

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