I found some contradiction on my book

[little boy]

I found some contradiction on my book

 

the book states "PE of gas>PE of liquid>PE of solid"

 

but also states "In a gas, where the intermolecular forces are weak, the internal energy is entirely molecular KE."

 

why is the PE of gas not significant while it is largest among the three state?? HELP!

[Cap'n Refsmmat]

Are you sure that the book doesn't instead state "KE of gas > KE of liquid > KE of solid"?

 

Perhaps you could quote that section; maybe it words things strangely.

[Realitycheck]

This seems kind of weird because I thought Capn would have caught this oversight. I may be missing something, but doesn't the inequality state that the solid has less PE than the gas, when in all actuality, the opposite would be true?

[timo]

I don't know what the potential energy of gas/liquid/solid is supposed to be. But these substances certainly have something called binding energy (which is often computed from pair potentials, so that's probably what the book means with "potential energy"). It is common to count binding energies as negative for a bond holding particles together (the stronger they are bound the more negative the values, i.e. the smaller the value). For a gas the binding energy is about zero, which is negligible. For liquids and solids it is some non-zero negative number and not negligible anymore.

[little boy]

does first law of thermodynamics require the gas being ideal?

[Realitycheck]

What's the name and authors of the book? Where was it written?

[little boy]

What's the name and authors of the book? Where was it written?

 

the book is for (China)Hong Kong A level physics, the author is a chinese

[Realitycheck]

Sorry Capn, I somehow missed that it was spelled out from the start. I would venture to state that its just wrong, on both counts. Can't explain why, but matter has potential energy comprising its makeup. More matter equals more energy. The other statement doesn't make sense either.

Edited August 14, 2011 by Realitycheck

[timo]

does first law of thermodynamics require the gas being ideal?

No, it doesn't even require a gas. But I fail to see how that has anything to do with your original question. It would be nice if you told us whether your original issue has been resolved or not.

[little boy]

After severa days of thinking, I found two possible explanations.

 

The first statement "PE of gas>PE of liquid>PE of solid" is right,

 

but the second statement"In a gas, where the intermolecular forces are weak, the internal energy is entirely molecular KE." is ambiguous

 

 

first explanation:

 

the statement should be rewrote as "In a ideal gas, where the intermolecular forces are weak(or doesn't exist), the internal energy is entirely molecular KE."

 

second explanation:

 

the statement should be rewrote as "In a gas, where the intermolecular forces are weak, the change of internal energy is entirely change of molecular KE."

because the change of volume of gas doesn't involve change of state and the intermolecular forces are weak means insignificant work done on PE

 

I guess the second explanation is the right one.

Edited August 16, 2011 by stupid boy

[Realitycheck]

How do you arrive at the conclusion that the first statement is right? The more matter there is, the more energy is involved, whether you're talking about binding energy or the energy created from splitting the atoms. Maybe you just have your arrows mixed up. Do you have your arrows mixed up? Here's a way you can remember which is which. The small number always eats the big number. Sorry if that sounds too much like kindergarten for you. Secondly, there is no kinetic energy in a gas. In order for something to have kinetic energy, work must be being done. There is no way that matter, by itself, can have kinetic energy, in any form, unless the gas was blowing out of a nozzle real fast or something, but that wasn't stated. Something must be having some effect on something else in order for it to be kinetic energy.

Edited August 16, 2011 by Realitycheck

[little boy]

How do you arrive at the conclusion that the first statement is right? The more matter there is, the more energy is involved, whether you're talking about binding energy or the energy created from splitting the atoms. Maybe you just have your arrows mixed up. Do you have your arrows mixed up? Here's a way you can remember which is which. The small number always eats the big number. Sorry if that sounds too much like kindergarten for you. Secondly, there is no kinetic energy in a gas. In order for something to have kinetic energy, work must be being done. There is no way that matter, by itself, can have kinetic energy, in any form, unless the gas was blowing out of a nozzle real fast or something, but that wasn't stated. Something must be having some effect on something else in order for it to be kinetic energy.

 

Isn't there internal energy in a gas, the molecular kinetic energy? In the gas I meant doesn't involve binding and splitting of particles.

[Realitycheck]

If the gas happened to be flammable, then it would have potential energy. If it was burning, then it would have kinetic energy being transferred to the space around it. I don't see anything kinetic about gas, in general.

 

http://en.wikipedia.org/wiki/potential_energy

http://en.wikipedia.org/wiki/kinetic_energy

Edited August 16, 2011 by Realitycheck

[timo]

I don't see anything kinetic about gas, in general.

A gas is some large quantity of usually small objects moving around, each having a kinetic energy that is usually non-zero.

[mississippichem]

If the gas happened to be flammable, then it would have potential energy. If it was burning, then it would have kinetic energy being transferred to the space around it. I don't see anything kinetic about gas, in general.

 

http://en.wikipedia.org/wiki/potential_energy

http://en.wikipedia.org/wiki/kinetic_energy

 

See Maxwell-Boltzmann distribution for further elaboration on what Timo stated.

 

A gas is some large quantity of usually small objects moving around, each having a kinetic energy that is usually non-zero.

 

This is the way we find the probability of finding a gas particle with a given kinetic energy. Thought I would add this for clarity.

[Realitycheck]

Yeah, but isn't the wind responsible for moving gas, air currents, gravity, and all?

 

I guess if you want to get technical, the electrons flying around the nuclei have kinetic energy. You could also check out Kinetic theory of gases, but it looks like you've got a handle on it. Sorry if you took offense, but I actually have a lot of respect for Oriental philosophy and Hong Kong, as well.

Edited August 17, 2011 by Realitycheck