Open, closed, and isolated systems

[studiot]

studiot, on 25 April 2012 - 12:12 PM, said:

 

I cannot accept your definition of a closed system since it effectively disbars the existence of equilibrium along with the establishement of the thermodynamic temperature scale.

 

 

I do not know why you say this. The definition of closed system that I gave is the standard one (check reference [1]) and it is perfectly compatible with equilibrium and with thermodynamic scale.

 

 

Yes your definitition of a closed system as one which does not allow mass flow across its boundary is standard.

 

Your one with dE strictly not equal to zero is not.

 

The point is that it is one of the conditions of equilibrium that no energy flows between systems that are in equilibrium. That is dE = 0.

 

Unless I am misunderstanding your dE?

 

It would help if you would comment on the meaning of your terms.

I have guessed several times here, without feedback.

Edited April 26, 2012 by studiot

[juanrga]

Yes your definitition of a closed system as one which does not allow mass flow across its boundary is standard.

 

Your one with dE strictly not equal to zero is not.

 

I fail to see from where you got this. It cannot be a misunderstanding about the symbols and formalism used, because the article cited in the OP says in words (I add bold face for emphasis):

 

the total energy of an isolated system remains constant, whereas this energy can vary for both closed and open systems.

 

Moreover, I already explained that the article uses standard stuff (see [1]).

 

The point is that it is one of the conditions of equilibrium that no energy flows between systems that are in equilibrium. That is dE = 0.

 

Unless I am misunderstanding your dE?

 

It would help if you would comment on the meaning of your terms.

I have guessed several times here, without feedback.

 

As any textbook in thermodynamics explains the state of thermodynamic equilibrium is a dynamic one. The 'net' or 'macroscopic' variation of energy is zero, but there are 'microscopic' variations of energy due to molecular fluctuations. As a consequence the energy of a finite system at equilibrium with a heat bath is not constant due to fluctuations. Textbooks in statistical mechanics explain how to compute those variations in the energy using the canonical ensemble.

 

As you correctly notice thermodynamic equilibrium is given by [math]dE=0[/math], this is not the same than [math]d\tilde{E}=0[/math]

 

Precisely, I am preparing another article, this is about stochastic variables and fluctuations and explains the difference between quantities and the usual averaged quantities used in thermodynamics and other disciplines.

Edited April 27, 2012 by juanrga

[studiot]

As any textbook in thermodynamics explains the state of thermodynamic equilibrium is a dynamic one.

 

I'm sorry I do not have access to your cited authors works.

 

I would say that any textbook that states this as a basis for definition is a very poor textbook.

 

It is true that one of the triumphs of 20th century thermodynamics has been the development of stat mech and demonstrating that this approach results in the same equations on average as the exact and rigorous definitions and analysis developed in the 19th century and further useful theory besides.

 

However the earlier 19 century definitions and analysis still stand because thermodynamics must perforce cover all conceivable systems, including ones where stat mech does not apply.

 

I think I've already said that the trick is in the definition/specification of a system and it is a point worth repeating.

 

Again I preach the value of exhibiting examples over swopping opposing statements.

 

It is instructive to revisit James Prescott Joule's original barrel in the light of modern knowledge.

 

1)Let us consider this barrel, first well insulated so that conditions approach adiabatic, and half full of fluid.

Let the fluid be mechanically stirred, perhaps by a modern magnetic non contact stirrer, so that all the work input is converted to a fluid temperature rise.

The first law tells us that all the mechanical energy appears within the system as an increase in internal energy. Without statistical variation. It does not describe fully the distribution of this increase in U however.

 

Is this system open or closed?

What is the heat flow in this situation?

 

2)Now let the barrel be well refrigerated so that conditions approach isothermal and again be stirred mechanically.

Now there is no change in internal energy as all the mechanical energy supplied is carried away as heat.

 

Is this system open or closed?

What is the change in U?

 

 

3)Now let the barrel be well refrigerated so that conditions approach isothermal but this time instead of stirring the fluid let us pour in some fluid at identical temperature to that already in the barrel, very slowly and gently.

 

Is this system open or closed?

What is the change in U?

 

4)Now let the barrel be well refrigerated so that conditions approach isothermal but this time instead of stirring the fluid let us pour in some additional fluid at higher temperature to that already in the barrel, very slowly and gently.

 

Is this system open or closed?

What is the change in U?

 

5)Now let the barrel be well refrigerated so that conditions approach isothermal but this time instead of stirring the fluid let us pour in some fully miscible fluid that does not react chemically and is at the same temperature to that already in the barrel, very slowly and gently.

 

Is this system open or closed?

What is the change in U?

 

6)Now let the barrel be well refrigerated so that conditions approach isothermal but this time instead of stirring the fluid let us pour in some fully miscible fluid that does not react chemically and is at higher temperature to that already in the barrel, very slowly and gently.

 

Is this system open or closed?

What is the change in U?

 

7) Consider two flatt-topped blocks of perfect crystals standing side by side on a level table at absolute zero.

Let there be a third block of perfect crystal standing on the left hand block and consider the solitary right hand block as the system.

Now slide the upper left hand block across onto the right hand block and therefoe into the system.

 

Is this system open or closed?

What is the change in U?

Edited April 27, 2012 by studiot

[juanrga]

I'm sorry I do not have access to your cited authors works.

 

I wrote "As any textbook in thermodynamics..."

 

I would say that any textbook that states this as a basis for definition is a very poor textbook.

 

I could not disagree more!

 

It is true that one of the triumphs of 20th century thermodynamics has been the development of stat mech and demonstrating that this approach results in the same equations on average as the exact and rigorous definitions and analysis developed in the 19th century and further useful theory besides.

 

However the earlier 19 century definitions and analysis still stand because thermodynamics must perforce cover all conceivable systems, including ones where stat mech does not apply.

 

It is not true that thermodynamics covers systems outside the scope of statistical mechanics (statistical thermodynamics).

 

I think I've already said that the trick is in the definition/specification of a system and it is a point worth repeating.

 

No trick here! Before starting the study of some system you would first define what your system is.

 

Again I preach the value of exhibiting examples over swopping opposing statements.

 

It is instructive to revisit James Prescott Joule's original barrel in the light of modern knowledge.

 

1)Let us consider this barrel, first well insulated so that conditions approach adiabatic, and half full of fluid.

Let the fluid be mechanically stirred, perhaps by a modern magnetic non contact stirrer, so that all the work input is converted to a fluid temperature rise.

The first law tells us that all the mechanical energy appears within the system as an increase in internal energy. Without statistical variation. It does not describe fully the distribution of this increase in U however.

 

Is this system open or closed?

What is the heat flow in this situation?

 

2)Now let the barrel be well refrigerated so that conditions approach isothermal and again be stirred mechanically.

Now there is no change in internal energy as all the mechanical energy supplied is carried away as heat.

 

Is this system open or closed?

What is the change in U?

 

 

3)Now let the barrel be well refrigerated so that conditions approach isothermal but this time instead of stirring the fluid let us pour in some fluid at identical temperature to that already in the barrel, very slowly and gently.

 

Is this system open or closed?

What is the change in U?

 

4)Now let the barrel be well refrigerated so that conditions approach isothermal but this time instead of stirring the fluid let us pour in some additional fluid at higher temperature to that already in the barrel, very slowly and gently.

 

Is this system open or closed?

What is the change in U?

 

5)Now let the barrel be well refrigerated so that conditions approach isothermal but this time instead of stirring the fluid let us pour in some fully miscible fluid that does not react chemically and is at the same temperature to that already in the barrel, very slowly and gently.

 

Is this system open or closed?

What is the change in U?

 

6)Now let the barrel be well refrigerated so that conditions approach isothermal but this time instead of stirring the fluid let us pour in some fully miscible fluid that does not react chemically and is at higher temperature to that already in the barrel, very slowly and gently.

 

Is this system open or closed?

What is the change in U?

 

You ask questions about a system without first saying what is the system is. The barrel? The barrel + fluid? The Barrel + fluid + air? The Barrel + fluid + air + the added extra fluid? The Barrel + fluid + air + the refrigerator? The Barrel + fluid + the added miscible fluid? Any other?

 

7) Consider two flatt-topped blocks of perfect crystals standing side by side on a level table at absolute zero.

Let there be a third block of perfect crystal standing on the left hand block and consider the solitary right hand block as the system.

Now slide the upper left hand block across onto the right hand block and therefoe into the system.

 

Is this system open or closed?

What is the change in U?

 

Closed.

 

I assume that by "absolute zero" you really mean [math]T \rightarrow 0K[/math]. Perfect crystal and 0⁺K would imply no friction, no compressibility, no external gravity effects... I assume that all the blocks are at thermal equilibrium and that there is not electric or magnetic effects. The only reliable source of energy become from electrostatic effects, but would average close to zero due to total neutrality of the crystals. Therefore the change in U would be zero.

Edited April 27, 2012 by juanrga

[studiot]

If a system is defined as initially comprising one block and I add a second, by definition I have an open system.

Why do you say it is closed?

 

Why do you bother to reproduce my post without reply to the other questions?

 

I think you are simply dodging the issue, which is that I have exhibited open and closed systems with zero change in U and in one case zero q and w as well.

[juanrga]

If a system is defined as initially comprising one block and I add a second, by definition I have an open system.

Why do you say it is closed?

 

Why do you bother to reproduce my post without reply to the other questions?

 

I think you are simply dodging the issue, which is that I have exhibited open and closed systems with zero change in U and in one case zero q and w as well.

 

You wrote "consider the solitary right hand block as the system". It is evident that the system (the right hand block) is closed. Now, if you change the definition of system in arbitrary ways and at any time that you want, then it is not a surprise that you get into troubles .

 

If you read my previous message you would find my response to your other questions. You pretended others to say properties of systems which you did not even defined . This is weird.

 

I think that I already said to you two or three times (another poster emphasized the same here) that the law dU = dQ + dW is valid for closed systems but not for open systems. I will not repeat more.

Edited April 28, 2012 by juanrga

[studiot]

You wrote "consider the solitary right hand block as the system". It is evident that the system (the right hand block) is closed. Now, if you change the definition of system in arbitrary ways and at any time that you want, then it is not a surprise that you get into troubles .

 

Don't be ridiculous.

 

When you add mass to any system that it did not start out with you could say this, which could only mean that there is no such thing as an open system.

 

Of course you can take a system and add mass. Then you have a system with more mass.

I did this by adding a block to one system and pouring liquid into a barrel in another.

 

Both are perfectly valid open systems.

 

If you read my previous message you would find my response to your other questions. You pretended others to say properties of systems which you did not even defined . This is weird.

 

 

I'm sorry but this statement seems more meaningless than weird. I certainly don't understand it.

 

I think that I already said to you two or three times (another poster emphasized the same here) that the law dU = dQ + dW is valid for closed systems but not for open systems. I will not repeat more

 

Stating something without proof many times to does not increase its validity.

 

In the barrel example that you so rudely ignored, the internal energy increases by exactly the heat content of the added liquid, thus completely satisfying your presentation of the first law.

 

I provided variations on this example with the intention of introducing Gibbs paradox to the discussion, but my attempts to offer constructive comments seem to be falling on deaf ears.

 

I am sorry about that.

Edited April 28, 2012 by studiot

[juanrga]

Don't be ridiculous.

 

When you add mass to any system that it did not start out with you could say this, which could only mean that there is no such thing as an open system.

 

Of course you can take a system and add mass. Then you have a system with more mass.

I did this by adding a block to one system and pouring liquid into a barrel in another.

 

Both are perfectly valid open systems.

 

I'm sorry but this statement seems more meaningless than weird. I certainly don't understand it.

 

Stating something without proof many times to does not increase its validity.

 

In the barrel example that you so rudely ignored, the internal energy increases by exactly the heat content of the added liquid, thus completely satisfying your presentation of the first law.

 

I provided variations on this example with the intention of introducing Gibbs paradox to the discussion, but my attempts to offer constructive comments seem to be falling on deaf ears.

 

I am sorry about that.

 

Evidently if you can add mass to a system the system is open. But this was not your example:

 

7) Consider two flatt-topped blocks of perfect crystals standing side by side on a level table at absolute zero.

Let there be a third block of perfect crystal standing on the left hand block and consider the solitary right hand block as the system.

Now slide the upper left hand block across onto the right hand block and therefoe into the system.

 

The system is, your own words, the "solitary right hand block" (aka the X) and if you put another block B above it

B  -->  B
BX --> BX

you have not added mass to the system (aka the original block X).

 

About the barrels, you completely ignored that I wrote.

 

The proof that dU=dQ+dW is not valid for open system is trivial (just consider an open system with zero work, zero heat, add or remove mass and measure its internal energy). Many textbooks emphasize that dU=dQ+dW is valid for closed but not for open systems (I have cited a textbook that does).

 

You ignore what is being said to you and insist on applying a closed-system law to an open system

 

Your "the internal energy increases by exactly the heat content of the added liquid" is plain wrong.

 

The Gibbs 'paradox' is a well-known misapplication of the thermodynamic formalism. Any decent reference explains why there is not paradox. Moreover, this has nothing to see with this thread. Maybe you could open a new thread for discussing that.

Edited April 28, 2012 by juanrga

[michel123456]

So, if I understand correctly juanrga:

by definition, you cannot add energy to a closed system, so when he (juanrga) reads "consider the solitary right hand block as the system" he understands "the solitary right hand block is a closed system". Which was not said.

Quite the contrary it was said "and therefore into the system".

 

IOW it is a question of proper definition of what the system is right from the beginning.

 

----------------

edit:

when it is stated that "the solitary right hand block is the system", it is not said, but should be said, that the system has an "aura" around it that allows change such as adding mass.

Edited April 28, 2012 by michel123456

[juanrga]

So, if I understand correctly juanrga:

by definition, you cannot add energy to a closed system,

 

You can add energy to a closed system. You cannot add energy to an isolated system. See the references cited. E.g. Ref. [1].

 

so when he (juanrga) reads "consider the solitary right hand block as the system" he understands "the solitary right hand block is a closed system". Which was not said.

 

This attempt to read inside my mind is fascinating, but that was not what happened.

 

Quite the contrary it was said "and therefore into the system".

 

My post, which you are replying, contains the exact quote and it was:

Now slide the upper left hand block across onto the right hand block and therefoe into the system.

 

You assume that "therefoe" was a typo but "into" was not. I never was able to move a crystal block into another crystal block . If you know how to do it, please explain me how.

 

Moreover I interpret "onto the right" as "onto the right". I will draw again the diagram where the system is denoted by the X and the moving block ("onto the right") by the upper B

B  -->  B
BX --> BX

 

You can draw an alternative diagram, if you disagree with this.

 

IOW it is a question of proper definition of what the system is right from the beginning.

 

----------------

edit:

when it is stated that "the solitary right hand block is the system", it is not said, but should be said, that the system has an "aura" around it that allows change such as adding mass.

 

I could not rebate such scientific arguments as "should be said, that the system has an "aura" around it". I have not still studied the thermodynamic of systems with "auras" and my textbooks have not such chapter

Edited April 28, 2012 by juanrga

[studiot]

My apologies for my earlier imprecision.

 

Whether the system in example 7 is and ever shall be or not one single crystal is not worth disputing.

 

I'm sure you know that you cannot specify any system without also specifying the system process. In this case adding another block by pushing it across was the system process.

Of course you also have to specify the boundary conditions.

 

In the other examples the half full barrel was the system, the system process as described and the boundary conditions the refirgeration or insulation as appropriate.

 

A final requirement for the full description of a system is to allow what is excluded.

Very often if a quantity plays no part in the system process iot is excluded, although present. A good test of this is if the same quantity appears unchanged on both sides of an equation. So for instance the internal energy of molecular bonding is not included in any of my examples.

 

It is not a good idea to keep repeating someone elses' statement (eg from a book) like a religous catechisim without critical examination.

 

I have not seen your sources but, since they take a stat mech approach, would expect them to list the underlying assumptions of that discipline.

 

Some are

 

The system comprises an assembly of distinct independent particles that are weakly coupled.

Each particle has access to a manifold of energy states.

Conservation of number and total energy applies.

 

If you feel that the internal energy rises by some other value in my examples 4 and 6 perhaps you would exhibit a calculation showing this?

[juanrga]

My apologies for my earlier imprecision.

 

Whether the system in example 7 is and ever shall be or not one single crystal is not worth disputing.

 

At contrary, an unambiguous definition of what is the system is needed to answer if the system is open or closed.

 

I'm sure you know that you cannot specify any system without also specifying the system process. In this case adding another block by pushing it across was the system process.

 

By a question of pure logic you cannot specify a "system process" without first saying what the "system" is.

 

You claim now that the system process was "adding another block by pushing it across", but adding to what? Across what? To a ill-defined system which you did not even can say if was a single crystal or two? Does this system include the "aura" that Michael123456 attributes to it?

 

In the other examples the half full barrel was the system, the system process as described and the boundary conditions the refirgeration or insulation as appropriate.

 

Just after reading your ill-defined examples I wrote:

 

You ask questions about a system without first saying what is the system is. The barrel? The barrel + fluid? The Barrel + fluid + air? The Barrel + fluid + air + the added extra fluid? The Barrel + fluid + air + the refrigerator? The Barrel + fluid + the added miscible fluid? Any other?

 

You continue being imprecise. Is "half full barrel" the same than "The barrel"? "The barrel + fluid"? "The Barrel + fluid + air"?...

 

It is not a good idea to keep repeating someone elses' statement (eg from a book) like a religous catechisim without critical examination.

 

See (*)

 

I have not seen your sources but, since they take a stat mech approach

 

(*) Is doing statement about sources that you "have not seen" your idea of "critical examination"?

 

would expect them to list the underlying assumptions of that discipline.

 

Some are

 

The system comprises an assembly of distinct independent particles that are weakly coupled.

Each particle has access to a manifold of energy states.

Conservation of number and total energy applies.

 

Conservation of total energy is not an assumption but a law.

 

It is untrue that statistical mechanics is based in your assumptions. In any case this has nothing to see with this thread. Open a new thread if you want discuss that kind of stuff.

 

If you feel that the internal energy rises by some other value in my examples 4 and 6 perhaps you would exhibit a calculation showing this?

 

This again? The internal energy of what? Of a system which you have not still defined?

Edited April 28, 2012 by juanrga

[swansont]

Conservation of total energy is not an assumption but a law.

 

It's both, really. They aren't mutually exclusive statements. We assume that the laws of physics do not change over time (and test this assumption), which gives rise to the law of conservation of energy.

[michel123456]

(...)

B  -->  B
BX --> BX

 

You can draw an alternative diagram, if you disagree with this. (...)

The circle is the system. At first, the system contains only X. When you add B, the system changes. I am not aware of of a way to make a change without making a change.

 

I could not rebate such scientific arguments as "should be said, that the system has an "aura" around it". I have not still studied the thermodynamic of systems with "auras" and my textbooks have not such chapter

You should. When you increase the temperature of X and X dilates (occupies more space), is it still your original system or is it not?

[Iggy]

My apologies for my earlier imprecision.

 

Whether the system in example 7 is and ever shall be or not one single crystal is not worth disputing.

I don't believe you have any reason to apologize, and I think someone should point out that disputing imprecise statements is the whole character of Juanrga's argument. From the blog:

 

Walter Greiner, Ludwig Neise, & Horst Stöcker write that "the energy is not longer a conserved quantity" for closed or open systems 5. This is not true; the law of conservation of energy, diE=0, holds for closed and open systems as well.

 

-Juanrga's blog

 

This is the part of Greiner's textbook being dissimulated,

 

Systems, phases and state quantities

[...]

b.

Closed systems

 

Here one allows only for the exchange of energy with the surroundings, but not for the exchange of matter. Thus, the energy is no longer a conserved quantity. Rather, the actual energy of the system will fluctuate due to the energy exchange with the surroundings...

 

-Thermodynamics and Statistical Mechanics

 

clearly Greiner is talking about the energy of the system... the internal energy. If there were any genuine confusion it would have been quickly cleared up by DH when he pointed it out. Like you say, Studiot, it isn't worth disputing simple, and quite possibly intentional, misunderstandings. Nobody gains anything from this.

[juanrga]

The circle is the system. At first, the system contains only X. When you add B, the system changes. I am not aware of of a way to make a change without making a change.

 

Very beautiful diagram, but the original problem was:

 

7) Consider two flatt-topped blocks of perfect crystals standing side by side on a level table at absolute zero.

Let there be a third block of perfect crystal standing on the left hand block and consider the solitary right hand block as the system.

Now slide the upper left hand block across onto the right hand block and therefoe into the system.

 

You are lacking a third block over which the block is being moved. It was said that the system is "the solitary right hand block", but you are re-defining the system as the original block plus an arbitrary space around. That is another system.

 

You should. When you increase the temperature of X and X dilates (occupies more space), is it still your original system or is it not?

 

What has this to see with the "aura" that you claim is around a block of crystal?

[questionposter]

It's both, really. They aren't mutually exclusive statements. We assume that the laws of physics do not change over time (and test this assumption), which gives rise to the law of conservation of energy.

 

If it is really true though, how did the universe get here?

[studiot]

By a question of pure logic you cannot specify a "system process" without first saying what the "system" is

 

There are several more examples of plain nit picking argument instead of cooperative constructive discussion in the post this was selected from.

 

But you have tripped your own self up here.

 

I can readily define the system process as "adding 100 joules of heat" to the system before defining any system and know that I can apply this process to a great many systems. Chemical engineers do this daily for a living.

 

@michel

 

The answer to your dilemma is simple.

 

The whole point of a system process and the laws of thermodynamics is to change the system in some way.

The process may change the physical boundaries of the system as well as the energy content temperatuure or pretty well any parameter we choose. Obviously we home in on significant ones.

Edited April 28, 2012 by studiot

[michel123456]

Very beautiful diagram, but the original problem was:

 

 

 

You are lacking a third block over which the block is being moved. It was said that the system is "the solitary right hand block", but you are re-defining the system as the original block plus an arbitrary space around. That is another system.

 

 

What has this to see with the "aura" that you claim is around a block of crystal?

 

bolded mine

That is another system.

 

Yes, for you it clearly is but for others it may not be so evident.

 

If the system was not a crystal but a glass of water, we could put sugar in the system. I guess for your understanding (trying to go into your mind again) that would be a different system, for someone else the same system would have changed.

[juanrga]

I don't believe you have any reason to apologize, and I think someone should point out that disputing imprecise statements is the whole character of Juanrga's argument. From the blog:

 

This is the part of Greiner's textbook being dissimulated,

 

clearly Greiner is talking about the energy of the system... the internal energy. If there were any genuine confusion it would have been quickly cleared up by DH when he pointed it out. Like you say, Studiot, it isn't worth disputing simple, and quite possibly intentional, misunderstandings. Nobody gains anything from this.

 

As is clearly stated in the 'blog', E is the total energy of the system. Therefore your emphasis on "clearly Greiner is talking about the energy of the system" is welcomed but does not add much to this thread, really.

 

The difference between the internal energy U and the total energy E was already noticed in this same thread. It was also mentioned that textbooks dealing with classical thermodynamics only consider systems at rest in absence of external fields. For those systems dE=dU. For more general thermodynamic systems this is not the case.

 

The confusion about the conservation laws was also discussed before in this same thread and in the 'blog'.

 

There are several more examples of plain nit picking argument instead of cooperative constructive discussion in the post this was selected from.

 

But you have tripped your own self up here.

 

I can readily define the system process as "adding 100 joules of heat" to the system before defining any system and know that I can apply this process to a great many systems. Chemical engineers do this daily for a living.

 

Does "adding 100 joules of heat" to an isolated system makes sense for you?

 

If your system is isolated the above 'system process' makes no sense. Period. This is why we need to define first what is the system under study.

 

bolded mine

That is another system.

 

Yes, for you it clearly is but for others it may not be so evident.

 

If the system was not a crystal but a glass of water, we could put sugar in the system. I guess for your understanding (trying to go into your mind again) that would be a different system, for someone else the same system would have changed.

 

Yes, I am convinced that your glass of water, is a different system that studiot solitary perfect crystal.

Edited April 28, 2012 by juanrga

[Iggy]

Juanrga, I mentioned that this was discussed previously in the thread. The point is that DH and Greiner are both most certainly correct. The energy of a closed system does change. When you say this,

 

Contrary to your claims, it is self-evident that total energy E is conserved in both closed and open system. Again this is all well-known even at the undergrad level and I do not need to go into details.

and this,

 

Walter Greiner, Ludwig Neise, & Horst Stöcker write that "the energy is not longer a conserved quantity" for closed or open systems 5. This is not true; the law of conservation of energy, diE=0, holds for closed and open systems as well.

you are objecting to a dissembled interpretation. DH and Greiner are saying that the internal energy of a closed system changes.

 

With DH, Greiner, and Studiot alike, you've found some detail of what they've said -- misunderstood it -- and objected to the misunderstanding with a rather complicated argument. I don't undestand the relevance or usefulness of doing that. It is probably also irrelevant for me to keep pointing it out though, so I'll step out of the way.

[juanrga]

Juanrga, I mentioned that this was discussed previously in the thread. The point is that DH and Greiner are both most certainly correct. The energy of a closed system does change.

 

I welcome again your contribution, but both in the 'blog' and in this same thread I have said many times that the energy of a closed system can vary. For instance in #16 I said that for a closed system [math]dE \neq 0[/math]. And in the 'blog' I give as example of a closed system a thermometer exchanging heat with surrounds.

Edited April 28, 2012 by juanrga

[studiot]

If your system is isolated the above 'system process' makes no sense. Period. This is why we need to define first what is the system under study.

 

 

What a shame. Another attempted nit.

 

I clearly stated to "a great many systems" not to all systems.

 

Far better to have used my crystal example 7 to discuss the interesting fact that with this example we can make the system open or closed depending upon where we place our boundaries.

 

By initially restricting the system to one block we have an open system.

 

By initially including the movable block we have closed system.

Edited April 28, 2012 by studiot

[Iggy]

I welcome again your contribution, but both in the 'blog' and in this same thread I have said many times that the energy of a closed system can vary. For instance in #16 I said that for a closed system [math]dE \neq 0[/math]. And in the 'blog' I give as example of a closed system a thermometer exchanging heat with surrounds.

Yes, Juanrga. I know you maintain that the energy of a closed system varies. Greiner is talking about the energy of a closed system when he says,

 

[in a closed system] the energy is no longer a conserved quantity. Rather, the actual energy of the system will fluctuate due to the energy exchange with the surroundings.

He is talking about the internal energy being variable. You agree but you make this objection,

 

Walter Greiner, Ludwig Neise, & Horst Stöcker write that "the energy is not longer a conserved quantity" for closed or open systems 5. This is not true; the law of conservation of energy, diE=0, holds for closed and open systems as well.

 

Greiner isn't talking about d_iE. By analogy, if Greiner said "red and yellow make orange" it would be a nonsensical argument to reply "no, that is wrong, red and blue make purple! "

[juanrga]

What a shame. Another attempted nit.

 

I clearly stated to "a great many systems" not to all systems.

 

Ambiguous and imprecise statements as this can be eliminated from science when, as I suggested before, we define what is the system under study and what are its properties.

 

Far better to have used my crystal example 7 to discuss the interesting fact that with this example we can make the system open or closed depending upon where we place our boundaries.

 

By initially restricting the system to one block we have an open system.

 

By initially including the movable block we have closed system.

 

An example of how we can initially restrict the system to one block and obtain a closed system was given above (ASCII diagram included). The lesson here is that if you are ambiguous and imprecise in the definition of the system, you will obtain ambiguities and imprecisions.

 

Yes, Juanrga. I know you maintain that the energy of a closed system varies. Greiner is talking about the energy of a closed system when he says,

[in a closed system] the energy is no longer a conserved quantity. Rather, the actual energy of the system will fluctuate due to the energy exchange with the surroundings.

He is talking about the internal energy being variable. You agree but you make this objection,

 

 

Greiner isn't talking about d_iE. By analogy, if Greiner said "red and yellow make orange" it would be a nonsensical argument to reply "no, that is wrong, red and blue make purple! "

 

I agree with your colourful example, but it is not what is happening here. The criticism to Greiner et al is of a different kind. It is more like if they had said "red and yellow are not colours" and I had replied "no, that is wrong, red and yellow are colours! "

 

The law of conservation of energy is stated as d_iE=0, which holds for both open and closed systems. This law is often written in the equivalent form dE=d_eE and named the law of conservation of energy in textbooks.

 

Greiner et al confound d_iE=0 with dE=0 and then make the incorrect claim that energy is not conserved in open and closed systems.

 

That energy E is conserved in both closed and open systems is well-known (or would be well-known). In #11, I already cited the section "15.4 Energy conservation in open system" of a celebrated textbook, but it seems that you did not notice.

 

Not only Greiner et al are confused about the conservation of energy, but they are also confused about matter. They make the claim that the number of particles is conserved in closed systems. The 'blog' explains why this is not true. I notice that neither you nor DH have commented about this point.

Edited April 29, 2012 by juanrga