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Topic: Kinetic Energy  (Read 2413 times)

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Offline LHM

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Kinetic Energy
« on: April 05, 2011, 11:06:57 PM »
I'm really sorry that I'm asking yet another question about kinetic energy, but I'm still kind of confused.

In answering another one of my posts, Borek said:
I have a cup of a hot tea here, standing on the desk. As far as I can tell, its kinetic energy is zero.

I just realized the other day what it meant but in that case, since the gas is technically confined to whatever container it's in, would the gas - and not the actual gas molecules - have a kinetic energy of zero too?

Also, just to check then, would the actual molecules of gas and liquid at the same temperature have the same kinetic energy because of the formula Ek=3/2kBT?

Offline Borek

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Re: Kinetic Energy
« Reply #1 on: April 06, 2011, 05:25:07 AM »
I just realized the other day what it meant but in that case, since the gas is technically confined to whatever container it's in, would the gas - and not the actual gas molecules - have a kinetic energy of zero too?

Yes.

Quote
Also, just to check then, would the actual molecules of gas and liquid at the same temperature have the same kinetic energy because of the formula Ek=3/2kBT?

This is more complicated. I don't think so. In liquid part of the energy is stored in kinetic energy, part in potential energy. In gas we assume no interaction between molecules (that is, they only collide elastically, but they don't attract each other), so the potential energy part is zero.
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Offline LHM

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Re: Kinetic Energy
« Reply #2 on: April 06, 2011, 08:24:28 PM »
What does the potential energy in liquid have to do with the formula Ek=3/2kBT, since that's the formula for kinetic energy and not total energy?

Also, according to DevaDevil in this link:
http://www.chemicalforums.com/index.php?topic=46364.msg175807#msg175807
Molecules in ice and liquid water have the same kinetic energy at the same temperature because of Ek=3/2kBT. So to set this straight, is it then that molecules in ice and liquid water at the same temperature have the same kinetic energy but molecules of liquid and molecules of gas (or molecules of solid and molecules of gas) at the same temperature have different kinetic energies then?

Sorry but I also have a question about when you said the potential energy of a gas is zero. I see how the logic works, but in the same link above rabolisk had said that molecules of liquids have more potential energy than molecules of solids. So is the gas just different again in this case too, and in increasing order of potential energy, it goes gas<solid<liquid?

Thanks!

Offline Borek

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Re: Kinetic Energy
« Reply #3 on: April 07, 2011, 04:48:09 AM »
Sorry, my mistake. 3/2kT is kinetic.
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Offline LHM

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Re: Kinetic Energy
« Reply #4 on: April 07, 2011, 05:21:13 PM »
So then molecules of liquids and molecules of gases do have the same kinetic energy?

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