According to mainstream physics: Is heat "destroyed" in a heat engine?

[swansont]

3 minutes ago, Tom Booth said:

You contradict yourself saying "it's contrary to mainstream science" then proceed to give an example: "There are examples of work without heat flow - adiabatic compression and adiabatic expansion against pressure or some other impediment, like a spring."

I would simply add to that example the fact that there is a temperature drop within the working fluid.

As you’ve noted, we are discussing a heat engine. There can be work without heat flow, but they are not heat engines. Because: no heat flow.

 

[Ghideon]

11 minutes ago, Tom Booth said:

If there is a simultaneous addition of heat. i.e. Isothermal expansion for example; the working fluid which would otherwise suffer a drop in temperature maintains the temperature due to additional simultaneous heat input alongside the work output. It may even increase in temperature, none of that however negates or contradicts the principle involved as far as heat being converted into work.

For every unit of work output a unit of the heat/energy input to the engine "disappears" or is subtracted from the account so to speak.

(bold by me)

By "account" you mean the total energy input?

(Don't forget about the heat that is rejected from a heat engine.)  

[Tom Booth]

27 minutes ago, studiot said:

Very clear thank you.

Clear that you have classed all combustion engines as not being heat engines.

I realize many people classify any and all engines, gas, diesel, propane etc. Internal combustion engines along with "heat engines" and it is true. These are heat engines, because the product of combustion HEAT, is what generally speaking, causes the expansion of a working fluid producing work output, but the particulars of the reaction that produces the heat is generally pretty irrelevant to the question.

For simplicities sake it might be helpful if the discussion were confined to external combustion heat engines, i.e. Stirling engines, where nothing crosses into or out of the engine other than heat and work.

Sorry for cutting you short, but my statement was in regard to a temperature drop within the working fluid due to work output. I should have been more specific in the heading, but I don't particularly want to get bogged down in debate about specific chemical reactions involved in combustion.

Edited February 14, 2023 by Tom Booth

[Lorentz Jr]

11 minutes ago, Tom Booth said:

For simplicities sake it might be helpful if the discussion were confined to external combustion heat engines, i.e. Stirling engines, where nothing crosses into or out of the engine other than heat and work.

Or steam engines. The working fluid may cool off temporarily, in the specific configuration you're describing now, because there's no input of heat at the moment it's doing work, but it has to be reheated in order to do more work on the next cycle, and that's not the general definition of a heat engine.

Edited February 14, 2023 by Lorentz Jr

[swansont]

11 minutes ago, Tom Booth said:

I realize many people classify any and all engines, gas, diesel, propane etc. Internal combustion engines along with "heat engines" and it is true. These are heat engines, because the product of combustion HEAT, is what generally speaking, causes the expansion of a working fluid producing work output, but the particulars of the reaction that produces the heat is generally pretty irrelevant to the question.

For simplicities sake it might be helpful if the discussion were confined to external combustion heat engines, i.e. Stirling engines, where nothing crosses into or out of the engine other than heat and work.

Sorry for cutting you short, but my statement was in regard to a temperature drop within the working fluid due to work output. I should have been more specific in the heading, but I don't particularly want to get bogged down in debate about specific chemical reactions involved in combustion.

Then let’s look at the Stirling cycle

https://en.m.wikipedia.org/wiki/Stirling_cycle

Diagram from link

1→2 Isothermal heat addition (expansion).

2→3 Isochoric heat removal (constant volume).

3→4 Isothermal heat removal (compression).

4→1 Isochoric heat addition (constant volume).

 

 

https://en.m.wikipedia.org/wiki/Stirling_cycle#/media/File%3AStirling_cycle_pV.svg

 

Heat in, heat out

[studiot]

15 minutes ago, Tom Booth said:

I realize many people classify any and all engines, gas, diesel, propane etc. Internal combustion engines along with "heat engines" and it is true. These are heat engines,

Exactly.

and what do you think the working fluid is in a diesel engine ?

[Tom Booth]

16 minutes ago, Lorentz Jr said:

Your phrasing ("Period.") suggested that there are no exceptions to your rule. @swansont was pointing out that there are plenty of exceptions, because your rule only describes the class of situations where there's no other heat source.

My statement is confined to a specific phenomenon, heat converted to work, period.

When a gas expands and does work, driving a piston in an engine there is a temperature drop in the working fluid as a consequence of the energy (work) output.

Is that not a fact?

You can add some corollaries to that, exceptions or whatever but we are talking about a basic principle.

The work output of the engine results in a temperature drop within the working fluid. I don't think it would be accurate to say adding heat would "prevent" that principle from operating. It could be compensated for but not "prevented" or "avoided" in the general accounting.

15 minutes ago, Lorentz Jr said:

Or steam engines.

? What about steam engines. That would be a different class of engine with its own complications. Latent heat of vaporization within the steam for example.

Strictly speaking, in a steam engine heat is added to the boiler not the working fluid in the cylinder, nevertheless cooling takes place in the cylinder as a consequence of work output.

13 minutes ago, studiot said:

Exactly.

and what do you think the working fluid is in a diesel engine ?

Air mostly.

[Lorentz Jr]

19 minutes ago, Tom Booth said:

My statement is confined to a specific phenomenon, heat converted to work, period.

Your statement is correct if there's no heat source and no other source doing work on the fluid: The fluid will cool off if it's not heated and it does work on a net basis on its external environment.

19 minutes ago, Tom Booth said:

When a gas expands and does work, driving a piston in an engine there is a temperature drop in the working fluid as a consequence of the energy (work) output.

Is that not a fact?

Yes, if there's no other heat source compensating for the work done.

19 minutes ago, Tom Booth said:

You can add some corollaries to that, exceptions or whatever but we are talking about a basic principle.

Not really. The principle is conservation of energy. The heat lost by the fluid is equal to the work it does, but the fluid's temperature is only guaranteed to drop in those specific situations where there's not enough external or internal heat input to compensate for the work done.

19 minutes ago, Tom Booth said:

The work output of the engine results in a temperature drop within the working fluid. I don't think it would be accurate to say adding heat would "prevent" that principle from operating.

It's not a general principle, it's just an outcome that occurs in certain circumstances.

Edited February 14, 2023 by Lorentz Jr

[studiot]

15 minutes ago, Tom Booth said:

Air mostly

 

Nah, it's the air-fuel mixture.

Nothing would happen without that.

And a lot of work is done on that mixture to raise its temperature, in part of the cycle.

Edited February 14, 2023 by studiot

[Tom Booth]

56 minutes ago, Lorentz Jr said:

Your statement is correct if there's no heat source: The fluid will cool off.

Yes, if there's no other heat source compensating for the work done.

Not really. The principle is conservation of energy. The heat lost by the fluid is equal to the work it does, but the fluid's temperature is only guaranteed to drop in those specific situations where there's no external or internal heat source.

It's not a general principle, it's just an outcome that occurs in certain circumstances.

Generally speaking I have no problem with your statements and can heartily agree with the whole, except perhaps the last in part, in that I believe it IS a general principle, It is simply conservation of energy. When energy as work is taken out, an equivalent amount of heat "disappears" (so to speak). But to suggest this is just an outcome that might be avoided seems a bit disingenuous or hair splitting.

Like if I said If you put a match to the wood in a fireplace the wood will burn up.

Well no, that is only a circumstance in a particular instance where there is no additional wood available.

Wood burns, as a general principle.

As you stated: "The principle is conservation of energy. The heat lost by the fluid is equal to the work it does" that is not something that can be sidestepped, or avoided.

In general that means there will be a temperature drop barring some additional intervening circumstance such as the introduction of additional heat.

 

Edited February 14, 2023 by Tom Booth

[Lorentz Jr]

17 minutes ago, Tom Booth said:

When energy as work is taken out, an equivalent amount of heat "disappears" (so to speak). But to suggest this is just an outcome that might be avoided seems a bit disingenuous or hair splitting.

You said the temperature of the fluid drops:

4 hours ago, Tom Booth said:

Work goes out, the temperature goes down. Period. A basic principle, widely accepted.

That's the "rule" that you and swansont were debating, and it's the outcome I was referring to. Maybe you're the one who needs to be less disingenuous.

Edited February 15, 2023 by Lorentz Jr

[swansont]

1 hour ago, Tom Booth said:

When a gas expands and does work, driving a piston in an engine there is a temperature drop in the working fluid as a consequence of the energy (work) output.

Is that not a fact?

Not if it’s an isothermal expansion.

[iNow]

Almost 4 pages in 24 hours. This one has a good pace! /metacommentary 

[Tom Booth]

1 hour ago, swansont said:

Not if it’s an isothermal expansion.

Fantastic.

Since for the most part an isothermal expansion is an unrealizable ideal not possible in a real engine we are pretty much in full agreement.

1 hour ago, Lorentz Jr said:

You said the temperature of the fluid drops:

As a general rule, yes. That is the principle. Isn't it? When heat is converted to work the temperature of the working fluid drops, as a matter of conservation of energy. Of course more heat can sometimes be added either simultaneously or in another cycle.

I already conceded, of course the temperature could be maintained or even increased by adding more heat as the heat is being "consumed" but that is not the question being addressed. The question is:

According to mainstream physics: Is heat "destroyed" in a heat engine?

If I might defer to the father of thermodynamics himself:

Quote

"Heat is simply motive power, or rather motion which has changed form. It is a movement among the particles of bodies. Wherever there is destruction of motive power there is, at the same time, production of heat in quantity exactly proportional to the quantity of motive power destroyed. Reciprocally, wherever there is destruction of heat, there is production of motive power."

Reflections on the Motive Power of Heat/Appendix A

 

 

Someone here seems to think his use of the term "destruction" is a mistranslation.

Quote

When work is transformed into heat or heat into work,
the 'quantity of work is dynamically equivalent to the
quantity of heat.

It has also been expressed in this way :

"When equal quantities of mechanical effect are produced by any means whatever from purely thermal sources,... equal quantities of heat are put out of existence... " (Kelvin).

This law has been confirmed in a variety of ways :

1. The experiments of Joule, Rowland, and others in generating heat by the expenditure of work.

2. The experiments of Him and others, showing that when work is done by a heat-engine heat disappears.

Henry Smith Physics for university students (Volume 2)

OK, so copyright 1904 does the copyright date invalidate the principle expressed?

Here's a good one:

Quote

when heat does work in an engine, a portion of the heat disappears.

James Clerk Maxwell. Theory of heat

 

[Lorentz Jr]

3 hours ago, Tom Booth said:

Someone here seems to think his use of the term "destruction" is a mistranslation.

It's an overstatement, Tom. Heat "disappears", but energy isn't "destroyed".

3 hours ago, Tom Booth said:

As a general rule, yes. That is the principle. Isn't it?

It's a "general" rule for a certain class of situations that don't always occur, but not for other situations. So it's not "the" principle, it's only one specific principle that can be derived from the conservation of energy, which is "the" principle that physicists use in thermodynamics.

Any more questions, Tom? I think all of your current ones have been answered.

Edited February 15, 2023 by Lorentz Jr

[Boltzmannbrain]

56 minutes ago, Lorentz Jr said:

It's an overstatement, Tom. Heat "disappears", but energy isn't "destroyed".

I have been reading this whole time, and as much as "destroyed" is bizarre and possibly outdated description when heat energy is converted into work energy, the term "destroyed" isn't necessarily wrong.  

If there is any possible argument left here against the OP it is that mainstream physics today does not describe such a process this way.  Maybe for some reason "destroy" had different connotations 100 years ago than it does now, especially since the term "energy cannot be created or destroyed" is such a widely known and used phrase.

  

Edited February 15, 2023 by Boltzmannbrain

[Lorentz Jr]

23 minutes ago, Boltzmannbrain said:

the term "destroyed" isn't necessarily wrong.  

That's right, it's not. When a car is "destroyed" in a junk yard, it ceases to "exist" in any usable or recognizable form, even though the parts and materials that it was made of are still there, buried somewhere in the junk heap.

23 minutes ago, Boltzmannbrain said:

the term "energy cannot be created nor destroyed" is such a widely known and used phrase.

And that's why this thread has gone on for four pages in 24 hours: Regular posters took the word in scare quotes at face value and kept trying to explain the conservation of energy to the OP, because that's what physicists care about and it's one possible interpretation of the question.

But the OP wasn't really asking about thermodynamics. Either intentionally or unintentionally, he was asking a question about semantics, or at least his question about physics was semantically ambiguous. So everyone kept talking past the OP, incorrectly assuming that the meaning of his question was well established. That's the problem with civility rules in public discussions: They fail to expose the uncivil motives of trolls and the incoherent thinking of beginners. The result is a waste of everyone's time.

Edited February 15, 2023 by Lorentz Jr

[Ghideon]

5 hours ago, Tom Booth said:

The question is:

According to mainstream physics: Is heat "destroyed" in a heat engine?

That question have ben answered several times already. Note that some of the input heat energy being converted into useful work output. This is the basic principle behind the operation of a heat engine.

[exchemist]

2 hours ago, Boltzmannbrain said:

I have been reading this whole time, and as much as "destroyed" is bizarre and possibly outdated description when heat energy is converted into work energy, the term "destroyed" isn't necessarily wrong.  

If there is any possible argument left here against the OP it is that mainstream physics today does not describe such a process this way.  Maybe for some reason "destroy" had different connotations 100 years ago than it does now, especially since the term "energy cannot be created or destroyed" is such a widely known and used phrase.

  

Agreed. However the trouble with cranks, as some of us know to our cost from previous encounters, is that getting us to accept strange and potentially misleading terminology is quite often a rhetorical ruse to promote their crank ideas. In this case it is likely to be, in some way yet to be disclosed, his notion that the Second Law of Thermodynamics is false or can be broken. (This individual has spent over a decade, off and on, on various forums, obsessing about this.) This is why we are wary of agreeing with his peculiar statement without qualifying it. We would not be nearly so cautious if the poster had a track record of posting in good faith.

 

Edited February 15, 2023 by exchemist

[Genady]

10 hours ago, Tom Booth said:

OK, so copyright 1904 does the copyright date invalidate the principle expressed?

Copyright date does not necessarily invalidate the principle, but it often invalidates the verbiage. Language evolves, including the one used in science.

[Tom Booth]

7 hours ago, Lorentz Jr said:

Regular posters took the word in scare quotes at face value and kept trying to explain the conservation of energy to the O

As has been pointed out, the word in "scare quotes" traces back to Carnot and the question is basically, how should we interpret the meaning of his statement. Is it really true that when work (or "motive power" - Carnot) is produced heat is destroyed?

What exactly did Carnot and others mean by that? A fair question I think, particularly when my discussions are being shut down for the crime of simply restating conservation of energy. Experimental observations just thrown out the window, not just ignored, but  banned, ridiculed, slandered.

When one form of energy is produced, the other form of energy ceases to exist utterly. That does not violated conservation of energy, that IS conservation of energy!

Let's not butter it up and say the heat is mery rolled up, or crushed like a car in a wrecking yard, or it's parts are scattered. No, it is utterly annihilated, not just changed in shape.

It (the heat, as heat) does not linger around. No, it is gone, vanished, disappeared, and the absence of heat is cold. The heat being "destroyed" does not leave some wreckage of scattered or dispersed HEAT behind, it leaves COLD, a drop in temperature.

Well is it just SOME, heat or ALL the heat, that is another question, but certainly ALL of that portion of the heat that was converted to some other form of energy is now gone.

This very strong wording: destroyed, vanishes, ceases to exist, disappears; has been used over and over, including by the various "founding fathers" of thermodynamics, Carnot, Kelvin, Maxwell... and many of the dusty old physics books from my father's book shelf.

Is this concept obsolete? Certainly. At least I think so, in the sense that Heat is now recognized as a transfer of energy, not a thing in itself that can be created or destroyed, but IMO that does not alter the principle, does it?

Heat goes out as "work", a different form of motion. The motion belonged to the gas particles, the "working fluid" and now it belongs to the flywheel.

This IS a rather mind boggling idea or concept. The warmth in the air transformed into the rotation of cold steal, but that is simply conservation of energy, not as some in here would like to portray, the ravings of some perpetual motion "crank". If someone does not understand this basic principle of conservation of energy then they are the perpetual motion crank thinking that heat can be used to run an engine and then just continue on it's merry way through the engine to the other side.

Edited February 15, 2023 by Tom Booth

[Bufofrog]

8 minutes ago, Tom Booth said:

This very strong wording: destroyed, vanishes, ceases to exist, disappears; has been used over and over, including by the various "founding fathers" of thermodynamics, Carnot, Kelvin, Maxwell... and many of the dusty old physics books from my father's book shelf.

Is this concept obsolete? Certainly. At least I think so, in the sense that Heat is now recognized as a transfer of energy, not a thing in itself that can be created or destroyed, but IMO that does not alter the principle, does it?

So you have answered your own question (According to mainstream physics: Is heat "destroyed" in a heat engine?) and the answer is no.  So are we done here?

[Genady]

13 minutes ago, Tom Booth said:

What exactly did Carnot and others mean by that?

This is not a question in physics. Perhaps, in history of science. But not in physics.

[swansont]

8 hours ago, Boltzmannbrain said:

I have been reading this whole time, and as much as "destroyed" is bizarre and possibly outdated description when heat energy is converted into work energy, the term "destroyed" isn't necessarily wrong.  

If there is any possible argument left here against the OP it is that mainstream physics today does not describe such a process this way.  Maybe for some reason "destroy" had different connotations 100 years ago than it does now, especially since the term "energy cannot be created or destroyed" is such a widely known and used phrase.

  

Yes. It has already been pointed out that one does not generally refer to textbooks of 100 years ago but uses current ones as much as possible, because mainstream science is the science of today, not the science of history. It’s not static. We learn things. Language evolves.

A work dated 1904, for example, cannot reflect fully modern notions of energy, since it predates relativity.

[studiot]

12 hours ago, Tom Booth said:

If I might defer to the father of thermodynamics himself:

I'm suprised Tom didn't go back to the realy father of thermodynamics, that ancient Greek who thought the Earth was flat and that there were four elements earth, water, air and heat (fire).

So can I quote them to promote my flat earth leanings ?

Anyway going back to retrieve quotes from Carnot an his contemporaries it is not suprising the they talekd of 'destroying' heat.
After all they thought it to be a substance.

And their word for it was caloric, which  unfortunately survives to this day in French as Chauler, Portugese and Spanish as calor, etc.

Atom was another ancient Greek concept that we now understand quite differently from the original.

So what, our knowlesdge and understanding of physics has improved over the millenia so keeping older ideas is only useful if for engineers if it provides an easy route to calculation or to historians.

Edited February 15, 2023 by studiot