Thermodynamics in the 21st century

[VincentG]

Thank you to Matt Brown for introducing me to Ludwig Boltzmann. He was at the precipice of a major breakthrough in attempting to quantify the molecular energy that we call "heat".

This article seems rather timely for the current state of the forum.

https://scitechdaily.com/how-chaos-theo ... f-physics/

[matt brown]

So many of these past legends never lived to 50, imagine where we might be if they had all lived to 70. Boltzmann passes by unnoticed by most, even within science. Today, we would call him a physicist, but during his time, science wasn't clearly organized. History does not follow a linear path and has many twists and turns. One of the weirdest is that by 1900, mainstream physicists did NOT believe in the molecular theory !!! The nickel tour is that physicists thought energy ruled the universe and subordinated any matter to largely irrelevant (makes me wonder how they got thru each day). However, all chemists had bought into the molecular theory a century earlier ~1800. Boltzmann believed the molecular theory, but struggled to unify it with thermodynamics. He was one of the few guys that kept the kinetic theory alive thruout the later 1800s and early 1900s until Einstein's 1926 Nobel Prize secured the kinetic theory.

It's a sad reflection of our society that so many know a lot about Ludwig Beethoven, but nothing about Ludwig Boltzmann. Another guy that passes by largely unnoticed is James Clerk Maxwell who is usually ranked second to only Newton (Einstein getting third place).

[VincentG]

Agreed Matt.

Can someone help provide insight to #5? Was this written before caloric theory was abandoned and heat was viewed as a thing other than molecular speed itself? How could they assume that the gas molecules lose no energy to impact? That would be the definition of perpetual motion and mean that no entropy would occur.

20230713_201654.jpg (590.12 KiB) Viewed 5838 times

[matt brown]

Agreed Matt.

Can someone help provide insight to #5? Was this written before caloric theory was abandoned and heat was viewed as a thing other than molecular speed itself? How could they assume that the gas molecules lose no energy to impact? That would be the definition of perpetual motion and mean that no entropy would occur.

The kinetic theory was debated for decades, so yes, this was written before the caloric theory was abandoned. According to the Equipartition of Energy theory, all internal energy is kinetic energy (for an ideal gas). A large, heavy, oxygen molecule moves slower than a small, light, hydrogen molecule, but similar to mv^2, each has the same kinetic energy. Since both have the same energy, no energy is transferred during collisions. OK, this covers 2 things in motion colliding, but what about gas molecules hitting a fixed wall (cylinder or piston)? If the moving gas molecules hit a fixed surface, no energy is lost since no...work...was done, but if something moves (think piston) then work is done and gas molecules loose energy. So, yep, a rigid box full of gas molecules is perpetual motion.

[VincentG]

So, yep, a rigid box full of gas molecules is perpetual motion.

To be more clear, I understand that if the temperature inside and outside the box are equal, there is no change in internal energy, and therefore "perpetual". But if the temperature(energy) inside is higher, the gas obviously loses energy upon impact with the walls.

Ok so molecular theory is still half baked then...

[matt brown]

We tend to accept the heating part without question (constant volume heating) as if related to our cooking experiences, but the cooling part makes us question this whole scheme (oops, theory). Yeah, exactly how the 'heat' move back and forth. As per another post, I'm clueless and prefer to accept the conclusions without knowing the process/es.

[VincentG]

It appears Ben & Jerry were onto something of the sort with their thermoaccoustic cooling device.

[VincentG]

A very interesting discussion around the 2nd law.

https://youtu.be/LRvxMV1HHbU?si=swnqsLG9Gouj_L6q

[MikeB]

So, yep, a rigid box full of gas molecules is perpetual motion.

To be more clear, I understand that if the temperature inside and outside the box are equal, there is no change in internal energy, and therefore "perpetual". But if the temperature(energy) inside is higher, the gas obviously loses energy upon impact with the walls.

Ok so molecular theory is still half baked then...

I think you are getting hung up on "perpetual" because we are so used to that term being used to describe engines that in fact/fake an "over unity" device.
Perpetual energy, perpetual momentum, and indeed perpetual motion in a straight line are actually some of the most basic laws of physics.

If you want to look at it another way, consider what would happen if gas molecules did NOT undergo perfectly elastic collisions - the 'lost' energy would presumably have to turn into 'waste heat', and what would that do inside a sealed box? Clearly it would heat up the gas...

[Fool]

How could they assume that the gas molecules lose no energy to impact? That would be the definition of perpetual motion and mean that no entropy would occur.

The term "perfect elastic collisions" is enough to describe the physics equations to use for modeling.

In a perfect elastic collision both momentum and energy are conserved. However, energy and momentum are transferred. The fast moving mass slows down, and the slow mass speeds up, in equal mass collisions. It is true in unequal mass collisions too, but the proportions are taken into account.

So when a fast gas molecule hits a slow wall molecule, the wall molecule speeds up, and the gas molecule slows down. Heat is transfered into the wall, out of the gas. The reverse is true for slow gas and fast wall molecules.

Heat will move in or out depending on how many more are in what direction. If the wall is the same temperature as the gas, the overall average direction of collisions will balance out to zero.

The wall material is subject to a different speed of melting (temperature) than the gas. It can very very slowly evaporate the wall. Evaporating faster, (ground away), if hit by a group of high speed condensed gas molecules.

The above is a glimpse of internal energy.

Entropy is not a loss of energy. It is a transfer of energy to a lower temperature. Because of the second law, the higher the entropy the less the energy can be converted to work. It still can be used 100% to warm up something even colder. It will again be at an even colder temperature. One more mystery is that the entropy of something at zero Kelvin is finite, not infinity.

Struggling with entropy, makes struggling with internal energy a puppy walk.

[Goofy]

I think this video explains the heat/energy transfer in a proper way, even if it´s some sort of a "statistical simulation" :
https://youtu.be/0P0R7D2sMQI?si=gRI9KHhilN3rKXDe

- And here is a gift for you, Fool :

https://carnot-legacy.sciencesconf.org/ ... ex&lang=en

Sorry, I couldn't help it !
But you mention it, in all most every post, so this must be like a wet dream for you. ;-)

BR

[Fool]

Thanks Goofy. Happy Birthday 'Refextions'.