How accurate is the definition of heat?

[mech-eng]

I have always had trouble with formulas. Now the trouble is about a definiton. I would like to ask you how accurate the definition of heat is. It does not seem to me completely accurate. I think it is partly accurate.

From the Fundamental of Engineering Thermodynamics by Sonntag\Borgnakke

Heat is defined as the form of energy that is transferred across the boundary of a system at a given temperature to another system (or the surroundings) at a lower temperature by virtue of the temperature difference between the two systems

The definitions in other books are the same with different words.

Of course the above definition is true but how accurate is it?

Heat pumps can transfer heat from a lower temperature system to a higher temperature system. So I can't see how the definition applies in this case. Would you please explain how definition covers this situation?

Do not heat pumps prove wrong, or falsify, the "temperature difference" part of the definition?

 

[DrClaude]

Heat pumps can transfer heat from a lower temperature system to a higher temperature system. So I can't see how the definition applies in this case. Would you please explain how definition covers this situation?

If you take the heat pump as a black box, then your statement in correct. However, if you look at the inner workings, the temperature of the working substance is below the temperature of the low-T reservoir when heat is taken from it and it is higher than the high-T reservoir when dumping heat into it. So it is compatible with the definition of heat.

(Note that I am skipping over the phase transitions that happen in most real-world heat pumps, which involves latent heat also.)

 

[sophiecentaur]

Do not heat pumps prove wrong, or falsify, the "temperature difference" part of the definition?

If you think you have found a 'flaw' in the definition then I have the following suggestion which may reconcile your problem. A heat pump will have two heat exchangers; one will operate at a temperature below the temperature of the 'cold' side and the other will operate at a temperature above that of the 'hot' side. At each side of the heat pump there will be a process which follows the description that is giving you a problem. There will be a source of Energy that maintains this temperature inequality and that will involve some mechanical or electrical energy transfer.

 

[Chestermiller]

If you take the heat pump as a black box, then your statement in correct. However, if you look at the inner workings, the temperature of the working substance is below the temperature of the low-T reservoir when heat is taken from it and it is higher than the high-T reservoir when dumping heat into it. So it is compatible with the definition of heat.

(Note that I am skipping over the phase transitions that happen in most real-world heat pumps, which involves latent heat also.)

I think you meant to say that heat is transferred from the outside to the working fluid (with the outside at a higher temperature than the working fluid) and then a greater amount of heat is transferred from the working fluid to the room (with the room at a lower temperature than the working fluid).

 

[DrClaude]

I think you meant to say that heat is transferred from the outside to the working fluid (with the outside at a higher temperature than the working fluid) and then a greater amount of heat is transferred from the working fluid to the room (with the room at a lower temperature than the working fluid).

The OP for discussing the case where heat is transferred from low-T to high-T, so I explained it as when the heat pump works to cool down the room, not heat it up.

 

[Lnewqban]

Heat pumps can transfer heat from a lower temperature system to a higher temperature system. So I can't see how the definition applies in this case. Would you please explain how definition covers this situation?

Do not heat pumps prove wrong, or falsify, the "temperature difference" part of the definition?

The indoor air, or your skin, does not know the difference between a cold coil and a block of ice, it will yield some of its thermal energy to that cold source.

The outdoor air does not know the difference between a hot coil and a rock heated by the Sun, it will gain some of their thermal energy from that hot source.

In order to keep the cold coil cold and the hot coil hot (avoiding indoors and outdoors thermal equilibriums being naturally reached), additional external energy must be constantly used.

Please, see:
https://en.m.wikipedia.org/wiki/Second_law_of_thermodynamics

http://labman.phys.utk.edu/phys136core/modules/m3/refrigerators.html

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

 

[mech-eng]

As a relevant question, are there any other energy types related to "temperature difference"? Is it only heat that is related to temperature difference?

 

[256bits]

As a relevant question, are there any other energy types related to "temperature difference"? Is it only heat that is related to temperature difference?

You must have something in mind to have asked the question.
the temperature difference is the part of the second law of thermodynamics - hotter things cool down, cooler things do not heat up - spontaneously.

 

[hutchphd]

The OP book quote says:
Heat is defined as the form of energy that is transferred across the boundary of a system at a given temperature to another system (or the surroundings) at a lower temperature by virtue of the temperature difference between the two systems

What is the problem? The heat pump itself is not "a system at a given temperature" and so there is no violation. as has been said in different ways here. The internal energy transfers involve other than heat.

.

 

[mech-eng]

What is the problem?

I was confused a lot by the situation. It was the problem. Now I can see the question was ridiculous. I thought the definition implies "the heat flows from higher temperature to lower" and then thought such devices violated that rule implied by the definition.

 

[mech-eng]

You must have something in mind to have asked the question.

Many thoughts are coming into and going from my mind. Yes, I wondered about "temperature difference" as "What would be the results other than "heat transfer?" Or "After heat transfer, can there be something different, for example a "mechanical result?" "Temperature difference" causes "heat transfer" and "heat transfer can cause "expansion" or "contracting" and those "expansions" and "contractions" can cause the change of "potential energy", which is a mechanical?

 

[256bits]

Many thoughts are coming into and going from my mind. Yes, I wondered about "temperature difference" as "What would be the results other than "heat transfer?" Or "After heat transfer, can there be something different, for example a "mechanical result?" "Temperature difference" causes "heat transfer" and "heat transfer can cause "expansion" or "contracting" and those "expansions" and "contractions" can cause the change of "potential energy", which is a mechanical?

You mean what can the results be from a heat transfer from a warm body to a cooler body, vice versa.
Phase change ie solid, liquid, gas
chemical species change H⁺ + OH^- ⇔ H₂O
molecular structure change
change in pressure in a container ( larger for a gas than a liquid or solid )
change in volume of the substance
electrical currents
magnetic properties of the substance.
...

Transfer of heat to or from a substance changes its state, and this would be reflected by the mechanical properties of the substance.

 

[mech-eng]

change in volume of the substance

Does this mean "potantial energy change", since the level of a liquid in a tank may considerably change?

 

[sophiecentaur]

Does this mean "potantial energy change", since the level of a liquid in a tank may considerably change?

I think you are falling into a bit of a trap here. You are finding that Classification tends to add problems and not to solve them. Temperature and Heat are two different concepts (we know that really, of course). The definition of Temperature is based on the Kinetic Energy of particles in a substance. For a so-called Ideal Gas, held in a fixed volume and in an insulated container the only effect of adding heat energy will be to increase the temperature - and that is the average kinetic energy of the particles. For any other process (like everything real) some of that added heat energy will make other changes and will not result in just a temperature change; mechanical work, change of state or intra-molecular forces will take things out of your hoped-for classification system.

This doesn't mean it's a waste of time to discuss ideal gas behaviour because it gives you a way into the basics of the problem. But you then move on to practical considerations. If the level in your tank increases (and the CM of the fluid rises) then work will have been done and the simple formula for heat in and temperature rise will not apply. Let's face it, all Heat engines rely on this happening; you put heat in and you take heat out and what's been lost can be in the form of useful work.

 

[sophiecentaur]

Does this mean "potantial energy change", since the level of a liquid in a tank may considerably change?

There's a simple answer to that. As the increasing (thermal) vibrations of the liquid cause an increase in pressure as it heats up, then the liquid will 'flow upwards' in the container but a small amount. That will constitute work done to increase the total gravitational potential energy. So you will need to 'add more heat' for a similar temperature rise.

 

[Andrew Mason]

"Heat" is used in a variety of ways, most of which are not consistent with the thermodynamic definition of "heat". The definition in the OP is the correct thermodynamic definition: energy transfer resulting from a temperature difference.

The term "heat" had been in use long before its physical nature was fully understood with the development of kinetic theory in the late 19th Century. Before Joule, heat was thought to be some kind of a mysterious fluid that a body contained. That concept still lingers.

We still talk about "adding more heat" but what we mean is "increasing its temperature" by increasing internal energy. That can occur without any heat flow at all (ie. by doing mechanical work).

"Heat pump" is one of those examples where the use of the term misleading. A heat pump does not transfer or pump "heat" from one body to another. A body does not contain heat. It contains internal energy. The heat pump transfers internal energy.

AM