Skip to main content

TR Memescape

  • TalkRational: Don't be all aspie over this, because things like this go to shit when we let that happen.

Show Posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Messages - ppnl

1
Science / Re: Tesla's heat engine (split from DDWFTTW)


Sorry Spork but I have not been paying attention. I have not read all the thread since I was here last but you still seem confused.

When you compress a gas you do two things. You heat the gas and you confine the gas to a smaller volume. All the energy you used to compress the gas shows up as heat. You can use that heat to do work but the efficiency will be very low.

But there is another source of energy to do work. By confining the gas to a smaller volume you have lowered it's entropy. Lower entropy means some of the kinetic energy that was already in the gas is now available to do work. You get that work by allowing the gas pressure to push against a cylinder.

Now it seems like you have got more energy out of the gas than you put in - although much of it in the form of waste heat. And you have. But that's ok because the gas is now at below ambient temperature. The energy equation balances.

Think about the air in a room. Statistically it is possible for all the air molecules to find themselves all on one side of the room at the same time. This violates no law of classic physics but is so statistically unlikely that it is unlikely to happen in billions of times the life span of the universe.

If it happened it would be a violation of the second law of thermodynamics. But that's ok because the second law is only statistical in nature.

But one consequence of such a reduction of entropy of the gas in the room is that there is energy available that was not available before. The energy was there it just wasn't available to do work. Now it is. And as the gas re-expands it will do work on the walls blasting them to splinters. 

Entropy is about how matter is arranged and the consequences for the availability of energy to do work. It is a statistical property of a large number of objects.

2
Science / Re: Tesla's heat engine (split from DDWFTTW)
Ok, I,m out. You people have as much fun as you do in the cart thread.

Don't leave me.  I'm not trying to overturn the 2nd law of thermodynamics or create a perpetual motion machine.  I just want to get a better understanding of the laws that I fully accept.


I have no objection to discussing anything with you. You are reasonable and intelligent. But I have no interest in poking an idiot with a stick. As more and more of the content of this thread is dedicated to poking idiots I will read it less and less.
3
Science / Re: Tesla's heat engine (split from DDWFTTW)
From it's inception, the second law of thermodynamics was on shaky ground. Based on a completely fallacious theory of heat, mathematically inaccurate, eventually largely repudiated by Carnot himself. The "Ideal Gas Law" has no room in it's equations for the inter-molecular forces that make Joule-Thompson cooling of an expanding gas and therefore air-conditioning, refrigeration, heat pumps etc. possible. Modern nano-technology is assaulting the second law on several fronts simultaneously, such as the transmission of heat to outer space we've already discussed and nantennas.

 If you insist that a 200 year old assertion, that it is impossible for a heat engine to operate, drawing heat from a single reservoir, is  absolutely inviolate, there isn't much to talk about here and I suppose it is a waste of your time.

Ok, I,m out. You people have as much fun as you do in the cart thread.
4
Science / Re: Tesla's heat engine (split from DDWFTTW)

Tom Booth,

I cannot see anything special about the thermoacoustic stirling engine. It does not for example appear to be very efficient. So the second law of thermodynamics seems to be safe.

Here is a video that explains the principle step by step.

https://www.youtube.com/watch?v=dkU6QQ1ckUQ

Nobody in any of these videos is claming any mystery or magical effeciency. Why are you?


5
Science / Re: Tesla's heat engine (split from DDWFTTW)
So the good news is that I'm in Maui now, and managed to go kitesurfing before checking into my hotel.  The bad news is that I didn't have a real eureka moment reading this thread on the plane.

Going from memory...
- I understand the math works, and I don't have trouble with the equations.  That's not my issue.
- Some folks are still saying things like "some of the energy is lost as heat".  But from what I understand that's not the case.  The whole issue here is that *all* of the energy used to compress the gas is lost as heat if you give it time to come to ambient temperature.
- I'm more or less familiar with quality of energy, Maxwell's demon, etc.  It has definitely occurred to me that this is a quality of energy issue - or an entropy issue.  Not sure if those two are quite the same, but they seem kind of the same.

If I have to, I guess I can accept that entropy is the magic missing piece that explains why I can compress a gas isothermally, lose all of the energy I put into the gas as heat, and then that gas is left in a manner that it can do exactly the same amount of work as the energy that I put into compressing it - even though that energy was lost.  I guess I can accept that, but it's not intuitive to me yet.


Spork,

you can actually understand what is going on without invoking entropy or any other statistical property of a gas. All you need is classical mechanics.

Say you have a baseball. If you throw it at a wall it will bounce back at at the same velocity as you threw it. The ball has kinetic energy. Temperature is a statistical property of a large number of balls with kinetic energy. But we will stick with classical mechanics.

Now say you throw the ball against a wall that is moving toward you at 10mph. The ball will rebound toward you with an extra 10mph. The moving wall is doing work on the ball thus increasing its kinetic energy. This is how compressing a gas increases its temperature. But don't think about temperature as that is a statistical property. Just think in terms of individual particles with higher velocity and kinetic energy. Classical mechanics.

Now imagine a box with a large number of particles bouncing around in it. The total energy in these particles is simply the sum of the kinetic energy of all the particles. When a particle impacts the wall it produces a force on the wall. With a large number of collisions it can statistically be reduced to the idea of a constant pressure. But keep the classical mechanics view of individual collisions and force in mind.

Now if the number of particles outside the box per unit volume is the same as the number of particles on the inside of the box per unit volume then the number of collisions on the outside pushing in is the same as the number of collisions on the inside pushing out. There is no net force. In statistical terms the pressures are equal.

Now lets move one wall inward thus compressing the gas. Each and every particle that strikes the moving wall bounces off with a little extra kinetic energy. In statistical terms you are heating the gas. The box now contains more energy than it did before. The particle collisions on the wall are more numerous because there are more particles per unit volume. Also the individual particles have a higher velocity. That means there is a force on the wall (pressure) that allows you to extract energy.

But instead lets just let the gas in the box cool off. All the energy of the work you did on the gas is gone. The total kinetic energy of the gas in the box is the same as it was before you compressed it. But there is an important difference. There are more particles per unit volume in the box than outside the box. That means there are more collisions with the wall inside pushing out than outside pushing in. Thus there is still a force on the wall that allows us to do work. Doing so reduces the total kinetic energy to below what we started with. In statistical terms the gas cools.

Thus you are extracting the energy you put in by compressing the gas plus some of the energy already in the gas. But fundamental limitations on efficiency mean you will always get out less work than you put in.

I hope it is clear from a classical mechanics view that you can extract energy from a compressed gas even if it is at ambient temperature. I'm sorry if some of the explanation was overly elementary.

 

Great explanation.

So you "lose" the energy used for compressing a gas as heat. As the gas re-expands it draws on its own internal energy and can do work.

So where does the heat energy from compression go?

"lets just let the gas in the box cool off. All the energy of the work you did on the gas is gone."

Does all that heat have to go to waste?

Obviously that energy is not literally gone. It still exists somewhere. Wantonly or wastefully, we simply let it escape.

Suppose the cylinder was made of some non-heat conducting material so that the heat could not escape, but inside the cylinder we had a material with high thermal density which could readily absorb the heat.

A stainless steel plate perhaps.

Compress the gas, it heats up to 750 F (400 C) The metal plate likewise heats up to the same temperature. Not at all unreasonable IMO as such temperatures can be reached when compressing air in a acrylic tube by hand.

https://www.youtube.com/watch?v=4qe1Ueifekg

probably hotter. What is the auto-ignition temperature of steel wool?

https://www.youtube.com/watch?v=Q3RX64DbciM

So if we got a metal plate in the bottom of the cylinder to absorb the heat, we could remove the plate. perhaps slide it out the side of the cylinder, we could then let the air expand and do work... and the air would get very cold in the process.

We have removed the hot piece of metal, (hot enough to melt lead or tin), we could, presumably do something with that now that we have captured it. We also have an expanding gas we could apparently also get some work out of to do something AND we are left with a cylinder containing some very cold air we might also be able to do something with if we really set our minds to it.

Let's take the cold air left in the tube after the gas re-expands and does some work.

It is possible to operate a Stirling heat engine with cold.

We could, in theory, throw out the hot metal plate we saved the heat in, we can throw out the work potential of the re-expanding air, we still have all around this cylinder of cold air, MORE ambient heat, which is now available to do work by utilizing the "cold hole" we just created in the cylinder.

Sure, we have a "net loss" of energy. Another term for that energy deficit is "cold hole" or heat sink.

How long could a heat engine run on ambient heat with such an energy deficit"?

Remember not all the heat entering the engine goes to fill the "cold hole", at least SOME of the ambient heat energy is converted into work in transit.

I'm not entirely convinced that a Stirling engine running on ambient heat due to such a "cold hole", absolutely could not re-compress another volume of air and repeat the cycle.

Especially with some assistance from the 750 F hot metal plate, and the work output from the re-expanding gas to assist it.

 What exactly are these "fundamental limitations on efficiency"? that so stubbornly prevent us from utilizing the abundance of ambient heat ,free for the taking, in the air around us?

edit: "The fire piston shown easily reaches a compression ratio of 16:1, which suggests a temperature of around 900 K" (1160 F)
source: https://eic.rsc.org/exhibition-chemistry/fire-pistons/2020053.article


You can try to use some of the heat from compression to do work but the efficiency is going to be quit low. A modern coal power plant uses burning coal to get their heat. It uses every technological trick they can think of to efficiently convert that heat into useful work. They top out at about 33%. The 2nd law of thermodynamics tells us that there will always be losses. The Carnot cycle details just how horribly bad those losses will be. Entropy is a bitch.
6
Science / Re: Tesla's heat engine (split from DDWFTTW)
So the good news is that I'm in Maui now, and managed to go kitesurfing before checking into my hotel.  The bad news is that I didn't have a real eureka moment reading this thread on the plane.

Going from memory...
- I understand the math works, and I don't have trouble with the equations.  That's not my issue.
- Some folks are still saying things like "some of the energy is lost as heat".  But from what I understand that's not the case.  The whole issue here is that *all* of the energy used to compress the gas is lost as heat if you give it time to come to ambient temperature.
- I'm more or less familiar with quality of energy, Maxwell's demon, etc.  It has definitely occurred to me that this is a quality of energy issue - or an entropy issue.  Not sure if those two are quite the same, but they seem kind of the same.

If I have to, I guess I can accept that entropy is the magic missing piece that explains why I can compress a gas isothermally, lose all of the energy I put into the gas as heat, and then that gas is left in a manner that it can do exactly the same amount of work as the energy that I put into compressing it - even though that energy was lost.  I guess I can accept that, but it's not intuitive to me yet.


Spork,

you can actually understand what is going on without invoking entropy or any other statistical property of a gas. All you need is classical mechanics.

Say you have a baseball. If you throw it at a wall it will bounce back at at the same velocity as you threw it. The ball has kinetic energy. Temperature is a statistical property of a large number of balls with kinetic energy. But we will stick with classical mechanics.

Now say you throw the ball against a wall that is moving toward you at 10mph. The ball will rebound toward you with an extra 10mph. The moving wall is doing work on the ball thus increasing its kinetic energy. This is how compressing a gas increases its temperature. But don't think about temperature as that is a statistical property. Just think in terms of individual particles with higher velocity and kinetic energy. Classical mechanics.

Now imagine a box with a large number of particles bouncing around in it. The total energy in these particles is simply the sum of the kinetic energy of all the particles. When a particle impacts the wall it produces a force on the wall. With a large number of collisions it can statistically be reduced to the idea of a constant pressure. But keep the classical mechanics view of individual collisions and force in mind.

Now if the number of particles outside the box per unit volume is the same as the number of particles on the inside of the box per unit volume then the number of collisions on the outside pushing in is the same as the number of collisions on the inside pushing out. There is no net force. In statistical terms the pressures are equal.

Now lets move one wall inward thus compressing the gas. Each and every particle that strikes the moving wall bounces off with a little extra kinetic energy. In statistical terms you are heating the gas. The box now contains more energy than it did before. The particle collisions on the wall are more numerous because there are more particles per unit volume. Also the individual particles have a higher velocity. That means there is a force on the wall (pressure) that allows you to extract energy.

But instead lets just let the gas in the box cool off. All the energy of the work you did on the gas is gone. The total kinetic energy of the gas in the box is the same as it was before you compressed it. But there is an important difference. There are more particles per unit volume in the box than outside the box. That means there are more collisions with the wall inside pushing out than outside pushing in. Thus there is still a force on the wall that allows us to do work. Doing so reduces the total kinetic energy to below what we started with. In statistical terms the gas cools.

Thus you are extracting the energy you put in by compressing the gas plus some of the energy already in the gas. But fundamental limitations on efficiency mean you will always get out less work than you put in.

I hope it is clear from a classical mechanics view that you can extract energy from a compressed gas even if it is at ambient temperature. I'm sorry if some of the explanation was overly elementary.

 
7
Science / Re: Tesla's heat engine (split from DDWFTTW)


So what is the difference between the air in a small greenhouse and the air in a large greenhouse?

Is not Earth's atmosphere simply a very large greenhouse heated by the sun, the same as a smaller greenhouse?

What is the critical difference that allows the small one to operate a heat engine but not the larger one?


No important difference.

But a green house is not a closed system. It is getting energy from the sun and thus it surprises no one that you can use it as a source of energy.

Now you seem to be thinking about the entire earths atmosphere as a greenhouse. Ok you can do that. And yes you can derive energy by many different means from the air. For example a wind mill. The energy of the sun unevenly heats the atmosphere creating wind that can be used to generate electricity.

But look at some small part of the atmosphere like the air in my house. The air is very close to being in equilibrium. No heat pump in my house can make use of the heat in my house to generate energy. You need a hot end and a cold end to generate energy. There is no hot end and cold end in my house.

You could change that. You could surround my house with hundreds of mirrors to focus sunlight to make a very hot end. But that's cheating. That's just a solar power plant. Nobody doubts that those work.
8
Science / Re: Tesla's heat engine (split from DDWFTTW)

You are overthinking this.  There needs to be a temperature difference somewhere in the system.

What  exactly are you saying here? Are you really saying that if the inside of my house is the same temperature as the outside of my house then I cannot use my heat pump to heat or cool my house? That is exactly the time that my heat pump is most efficient.
I am saying that somewhere in the system you need a temperature differential or there is no entropy. Try running a combustion engine in a blast furnace. It won't go.

Well yes you need an energy source. I get the energy to drive my heat pump from the powerplant. It is very true that if I unplug my heat pump it will not work. Thank you for pointing that out...
And a place for the energy to go. Just sayin.
I... cannot make any sense of this.
9
Science / Re: Tesla's heat engine (split from DDWFTTW)
Heat pumps do require a temperature difference.
Nope.  They are like refrigerators.  Electrically powered but the sources don't have to be at a different temp. 
In the real world a heat pump doesn't work well when there is little heat (in the air, which is the kind I am speaking of).  It will "work", but not in the sense of keeping your house warm.

https://www.nordicghp.com/2015/12/air-source-heats-pump-cold-climates/

I know this from experience. When it gets shit cold the heat pump will tear itself apart if you don't have supplemental heating, and when it is is really fucking cold they shut off to prevent this.

Doesn't change the theoretical nature of the idea of "will work", but in the real world a heat pump is shit when it gets to -10F


Well yes but but loss of efficiency is not caused by lack of heat in the air. The cause of loss of efficiency is caused by the fact that in cold weather you have to pump the heat across a greater temperature difference. Its like pumping water up a hill. The higher the hill the less water will reach the top at any given power. Only with pumping heat the loss of efficiency is much worse. 
10
Science / Re: Tesla's heat engine (split from DDWFTTW)

You are overthinking this.  There needs to be a temperature difference somewhere in the system.

What  exactly are you saying here? Are you really saying that if the inside of my house is the same temperature as the outside of my house then I cannot use my heat pump to heat or cool my house? That is exactly the time that my heat pump is most efficient.
I am saying that somewhere in the system you need a temperature differential or there is no entropy. Try running a combustion engine in a blast furnace. It won't go.

Well yes you need an energy source. I get the energy to drive my heat pump from the powerplant. It is very true that if I unplug my heat pump it will not work. Thank you for pointing that out...
11
Science / Re: Tesla's heat engine (split from DDWFTTW)
The idea that a heat pump is the reverse of a heat engine.

The two machines are fundamentally different in function and operation. I do not see how one can be "The opposite" of the other.

They are opposites in that a heat pump does work to increase a temperature difference, while a heat engine uses a temperature difference to do work.

Quote
Heat pumps don't CONVERT anything. They don't CONVERT cold into heat or any other form of energy. A heat engine DOES CONVERT heat into other forms of energy. It does not merely MOVE heat from a hot source back into a "cold source."

Again, a heat pump does work to produce or maintain a temperature difference, while a heat engine uses the temperature difference to do work.

Quote
Heat moved by heat pump -> Heat converted by heat engine  -> converted energy out ->

You can do that, but at a net loss. 

Why do you assume it would be at a net loss necessarily? Or how do you know?

Let me ask this; is there any reason one could not use heated air from a greenhouse. Use that hot air to run a heat engine, which would leave colder air. Return the cold air to the greenhouse to be reheated and continue the process? Without a net loss that is.

The first law of thermodynamics tells us that you can't get out more than you put in. The second law tells us that you can't break even. So yes there is a net loss.

As for the greenhouse you are again attempting to build a solar thermal generator. Yes you can get energy out from it but only because the greenhouse is coupled to the sun. Put the greenhouse in the shade and it will not work. You keep trying to sneak other energy sources in.
12
Science / Re: Tesla's heat engine (split from DDWFTTW)

You are overthinking this.  There needs to be a temperature difference somewhere in the system.

What  exactly are you saying here? Are you really saying that if the inside of my house is the same temperature as the outside of my house then I cannot use my heat pump to heat or cool my house? That is exactly the time that my heat pump is most efficient.
13
Science / Re: Tesla's heat engine (split from DDWFTTW)
Thanks for all the responses.  I haven't had a chance to read them yet.  But I have skimmed a few.  I think you're probably right that entropy is what has me tripped up.  This had occurred to me, but I didn't feel like entropy was a good analog for energy, so it didn't comfort me much.  That may not even make sense because, as I said, I haven't really read your replies yet.  But the plan is to print them out tonight and study them on the way to Maui tomorrow.  Hopefully at some point on the plane I'll have a head-slapper and wonder how I could have been so dumb.

I don't think you are being dumb at all.  It was a major head scratcher if one is rusty in thermo.

The thing I think you will want to read up on is "exergy" which is explained fully in this Wikipedia article:

https://en.wikipedia.org/wiki/Exergy

Trust me, by reading this you will learn some cool stuff like "quality of energy" which we haven't touched on before except in the most rudimentary ways.

Another great article which ferrets out the issues are about the "Carnot cycle" and why anything more efficient is a perpetual motion machine and disallowed:

https://en.wikipedia.org/wiki/Carnot_heat_engine

The questions you were asking were a central conundrum and when Maxwell proposed an "intelligent demon" that could stand at an orifice in a jar with hot gas, he could open a door when the high speed molecules came zipping by and let them escape while holding on the the low speed ones (or vice versa).  This allowed a hot or cold reservoir to be created which could do useful work. 

Article here:

https://en.wikipedia.org/wiki/Maxwell%27s_demon

The problem was a real head scratcher for decades as it didn't seem possible but nobody could find an issue with the concept.  Finally, it was determined from statistical mechanics that the processing required to make the decisions for the gate in terms of information used more energy than could be obtained from the sorting.  This concept eventually linked information processing and entropy and carried over into cosmology via black holes! :eek:

There are some real heavy hitters of physics fame who have been involved in this.  Maxwell (Maxwell's equations fame), Szilard (Atomic Bomb fame), Shannon (Claude Shannon of information theory fame), Feynman (Quantum Chromodynamics Fame), and others!  These were not lightweights!

There is still some debate about this in circles where "reversible computing" concepts use no energy to accomplish information so you are now on the hairy edge of thermodynamics, information, computational theory and cosmology! :yes:

Windgrins :grin:


And don't forget quantum computing. Quantum computer calculations must be thermodynamically reversible so you can have a unitary evolution of the wave function. If the quantum logic circuits emitted any heat to the environment then the wave function would decohere and your quantum calculation would collapse. Traditional logic functions are irreversible and so must emit kT ln(2) a heat to the environment. You must use a reversible logic gate that preserves information. One consequence of it being reversible is that all logic gates must have as many outputs as inputs.

In a sense entropy is a simple concept but the implications become very complex very fast and seem to reach the end of time and space.
14
Science / Re: Tesla's heat engine (split from DDWFTTW)


]

When dealing with physical energy bearing substances. I would think it could take less work to gather in the energy than what might be derived from the substance gathered.

For example, we could have rocks that absorb heat from the sun powering a heat engine that drives a conveyor that moves the rocks to where the heat they absorbe can be used by the engine then back out into the sun.

I see little if any difference between hot rocks heated by the sun and hot air.

True you have a situation where there is an excess of hot rocks squirling up the temperature difference but again, being associated with matter, the hot rocks bearing an excess of heat can be moved out or can be excluded from a given area.

I see little if any difference between hot rocks heated by the sun and hot air.

True you have a situation where there is an excess of hot rocks squirling up the temperature difference but again, being associated with matter, the hot rocks bearing an excess of heat can be moved out or can be excluded from a given area.

You seem to be describing a solar thermal power plant. Yes they can work and they even exist. Replace your rocks with molten salt and they work exactly as you describe. But they work off the temperature difference between the sun and the Earth.

What you cannot do is devise a system that taps in to the vast amount of energy in just the atmosphere around you. In order to generate energy you need in some sense a hot end and a cold end. And you cannot use that energy to maintain your cold end. The ambient atmosphere has no hot end and cold end so that vast amount of energy is out of reach.

To understand why you cannot maintain your cold end you need to understand Maxwell's demon.  
15
Science / Re: Tesla's heat engine (split from DDWFTTW)
Thanks for all the responses.  I haven't had a chance to read them yet.  But I have skimmed a few.  I think you're probably right that entropy is what has me tripped up.  This had occurred to me, but I didn't feel like entropy was a good analog for energy, so it didn't comfort me much.  That may not even make sense because, as I said, I haven't really read your replies yet.  But the plan is to print them out tonight and study them on the way to Maui tomorrow.  Hopefully at some point on the plane I'll have a head-slapper and wonder how I could have been so dumb.



Entropy isn't an analog of energy. It is a measure of how much of the energy that is present can actually be used.

There is a massive amount of energy in the hot air around you. But that energy is in a high entropy disordered state. You can't use it for anything. In order to use it you have to lower the entropy. That usually means compressing it into a subset of the available micro-states.

In a deeper sense entropy can be seen to be about the information you have about the system. When you compress a gas it increases the information you have about the position of the individual gas molecules. That information is what makes energy available to do work.

It takes more energy to create low entropy states than you can get from those low entropy states. That pretty much kills any chance for endless energy from the hot air around you.

16
Science / Re: Tesla's heat engine (split from DDWFTTW)
After thinking about it, I'm pretty sure it is all contained in PV=nRT.

Which says, that if you have a fixed amount in moles of gas material, that it will be contained in a fixed size container at a specific pressure and temperature.  If you compress it by making the container smaller, the pressure will rise as well as the temp.  But if you let it cool back down to ambient so it is at the original temperature again, then for the equation to be true with a smaller volume, Pressure will have to be higher.  That higher pressure can do work.  But the total energy contained (which is less than what you started with because some of it was lost to the environment as heat) is still defined by the temperature for that many moles of material at the given pressure and volume.  But if you expand the volume back to the original volume, it will cool and will have to recapture the energy back from the ambient environment to get back the original pressure.

An air compressor on the other hand changes the number of moles of material in the tank but once the compressor stops, and you know how many moles you have in the tank at some pressure level, the energy is defined by the temperature.  Or you can pick two other variables and say the energy is defined by the one remaining.  In an air compressor tank (garage variety), the degrees of freedom are the pressure, temp, number of moles,  but not the volume since it is a fixed tank.


That's all correct, but I don't think it addresses the point spork found confusing, which is this (if I understand him):  after you compress the gas and then let it equilibrate with a surrounding atmosphere, its energy is back to where it started.  How then can it do work on anything? 

The answer is that energy is not the limiting factor here.  After all, the atmosphere has tons of energy in it.  What's potentially lacking is a subsystem that's out of equilibrium, with entropy that's below the maximum value.   Compressing the gas creates such a subsystem even after you allow its temperature to equilibrate.
After thinking about it, I'm pretty sure it is all contained in PV=nRT.

Which says, that if you have a fixed amount in moles of gas material, that it will be contained in a fixed size container at a specific pressure and temperature.  If you compress it by making the container smaller, the pressure will rise as well as the temp.  But if you let it cool back down to ambient so it is at the original temperature again, then for the equation to be true with a smaller volume, Pressure will have to be higher.  That higher pressure can do work.  But the total energy contained (which is less than what you started with because some of it was lost to the environment as heat) is still defined by the temperature for that many moles of material at the given pressure and volume.  But if you expand the volume back to the original volume, it will cool and will have to recapture the energy back from the ambient environment to get back the original pressure.

An air compressor on the other hand changes the number of moles of material in the tank but once the compressor stops, and you know how many moles you have in the tank at some pressure level, the energy is defined by the temperature.  Or you can pick two other variables and say the energy is defined by the one remaining.  In an air compressor tank (garage variety), the degrees of freedom are the pressure, temp, number of moles,  but not the volume since it is a fixed tank.


That's all correct, but I don't think it addresses the point spork found confusing, which is this (if I understand him):  after you compress the gas and then let it equilibrate with a surrounding atmosphere, its energy is back to where it started.  How then can it do work on anything? 

The answer is that energy is not the limiting factor here.  After all, the atmosphere has tons of energy in it.  What's potentially lacking is a subsystem that's out of equilibrium, with entropy that's below the maximum value.   Compressing the gas creates such a subsystem even after you allow its temperature to equilibrate.

Yeah, I think it is entropy that has him tripped up. Take a gas at ambient temp and pressure. The total energy is just the sum of the kinetic energy of all the molecules. But none of that energy is available to do work. Now compress it. In compressing it you are adding kinetic energy to the molecules thus it is hotter. Now use thermocouples to extract the heat as electric current. It can do work. Now the gas is back down to its original temp and thus back down to its original total energy.  But there is a difference. By compressing the gas in a smaller volume you have lowered its entropy and thus some of its energy is now available to do work.

I think it is best to first work out these problems using classical mechanics and then relate it back to the statistical mechanical explanation. Then you see how statistical mechanics is derived from classical mechanics.
17
Science / Re: Better than superglue - surgical glue.
Awesome. Can it be used to glue Trump's mouth shut?

It could. But the resulting orange explosion would cover three states in a layer of cheeto dust three feet thick.
18
Science / Re: Tesla's heat engine (split from DDWFTTW)
I have scanned the thread but remain confused about what you people are confused about.

Temperature is just the average kinetic energy of gas molecules.  When you extract energy from a gas to do work that kinetic energy is what you are drawing from. Drawing from it will cool the gas if no other heat is allowed in to keep it warm.

If you compress a gas then there is more molecules per unit volume. Therefore there is more potentially extraditable energy per unit volume. This is true even if you keep the temperature constant as you compress it. Just do the math. Number of particles per unit mass times kinetic energy per particle.

If you have a compressed gas in a cylinder held in by a piston  then the gas can do work against that piston even if it is at the same temp as the outside and even if it is not allowed to absorb heat from outside. It will cool off because kinetic energy has been removed thus less kinetic energy per molecule which is the definition of cooling off. If you let it absorb heat from outside it will give a little more energy on expansion.  For reasonably small pressure differences it would not give much extra energy.

Well is the energy stored as pressure or temperature? The problem is the question is a semantic mess. All the energy is in the form of kinetic energy of the molecules. Total kinetic energy is just the sum of the kinetic energy of all the molecules. If you know the temperature of the gas and the total number of molecules per volume then you can get the total energy per volume stored. So is energy stored as temperature? You could say that I guess but I wouldn't. Measuring the temperature does not tell you how much energy is in the cylinder. You also need to know the number of molecules per unit volume.

I'm just not sure what the confusion is.

For a thought experiment put one of those dunking birds in a sealed insulated box. What would happen?
19
Science / Re: Sperm counts plummeting

Simple effect of age?

 Or maybe the availability of porn has made men... depleted. Lock a bunch of men up with no access to porn putting them in a preinternet state. You know. Horny. Then test them. Since you are unlikely to get volunteers you will have to kidnap homeless people to experiment on. For science!
20
Right.  The oceans are obviously not rising in average temperature in step with the air, there should be a large lag due to the huge heat capacity of that much water.

Has anyone checked the math for the suggested volume increase of the total ocean water content caused by that temperature increase, then related that to the ocean surface area x the rise in ocean level?

Quote
Sea level rise is very slow
Citation, please!

Quote
New York is expected to see the sea level rise by about three feet by the end of the century. That is if global temps rise by five degree..
Citation, please!

See here!


Simply demanding citations leaves me no clue to what your beef is. But whatever...

http://e360.yale.edu/features/rising_waters_how_fast_and_how_far_will_sea_levels_rise

" Things today are more certain. In its latest report, released on September 27, the IPCC finally could and did put a number on ice flow from the poles. The result was an estimate of sea level rise of 28 to 98 centimeters (a maximum of more than three feet) by 2100 -- more than 50 percent higher than the 2007 projections. "We have our arms around the problem well enough to say there's a limit to how crazy things are going to get," says Ted Scambos, head scientist at the U.S. National Snow and Ice Data Center. "

Again I have no idea what your beef is but this number is all over the internet for anyone to find. There was even a sciam article recently.

I call this a very slow rise and it is in the context of the current discussion. But by historical standards it is very fast.

21
Right.  The oceans are obviously not rising in average temperature in step with the air, there should be a large lag due to the huge heat capacity of that much water.

Has anyone checked the math for the suggested volume increase of the total ocean water content caused by that temperature increase, then related that to the ocean surface area x the rise in ocean level?

Sea level rise is very slow. New York is expected to see the sea level rise by about three feet by the end of the century. That is if global temps rise by five degree.
22
Since water is so available over the earth to be in equilibrium through evaporation / condensation I expect that it is not a threat to be triggered into action by increased CO2 as far as contributing much to "global warming".  It is already a dominant player that (I think) should not be much subject to synergistic interaction.

Trapped methane may be another matter.

But also I think the term "runaway" is not so apt.  A combination of environmental changes may move the global temperature set point a few degrees then settle out.  But the effects on local climate might be significant for the stability of the food supply chain.

The whole point is that water vapor is in equilibrium. Any increase in temperature will shift that equilibrium. In general each one degree of warming will produce about 7% more water vapor in the atmosphere. That will cause more warming. All other things being equal.  
23
Science / Re: Metallic hydrogen
Exactly how would you make a wire out of this stuff? It seems unlikely that it would be very malleable. Also how easily would this stuff burn? Could be an extreme fire and explosion hazard. And good luck with mass producing it.
24
There are three symmetries that are important here.

The first is C symmetry or charge conjugation symmetry. If you replace matter with antimatter then physical laws will appear to be ever so slightly different. This is the violation of C symmetry.



The second is P symmetry or parity symmetry. This is a mirror image symmetry. Again the laws of physics are ever so slightly different in a mirror universe. This is the violation of P symmetry.

The last is T symmetry or time symmetry. If you reverse time then again the laws of physics appear ever so slightly different. This is the violation of T symmetry.

But you can combine them. If you look at an antimatter, through a mirror with time reversed then the laws of physics will appear identical to our matter universe, with no mirror and forward time. Thus CPT symmetry is conserved. As far as they know.

The gravity experiment in the op has little to do with any of this. Observing antimatter falling up would reduce physics to a garbled mess. I'm not sure if they would win the Nobel or just be burned as witches.
25
Philosophy / Re: Weaving the rainbow
TC reminds me of a professional wrestling fan I once knew. He was always talking about the latest soap opera around the latest matches. But mention that it was all fake and he would just walk away. He could not enjoy the narrative unless he could pretend it was all real. I saw that as a sad limitation on his imagination.

Humans are creatures of narrative.  We explore moral possibility space with narrative. That narrative does not have to be real or true in order for it to be powerful and useful.

But having said that it should be noted that grounding your narrative in reality can be very useful. Consider a religious narrative for example. If it is not grounded in reality and you must believe it in order to enjoy it then you might be a creationist. Or Donald Trump supporter. Good luck with that.

As a kid I loved Star Trek. As I grew older I began to understand that faster than light travel as depicted was impossible. And so many technological species in so small a space was impossible. Did that reduce my enjoyment of Star Trek? A little in that it made the narrative look more contrived. Is that a bad thing? I think it's called growing up. People who don't grow up continue to enjoy seeing old fat men in their underwear pretend to throw each other around a ring.

I didn't much care for "unweaving the rainbow" but only because it spent more time criticizing Gould than explaining the beauty of science. It has been many years so my memory may not be trustworthy.