Thermodynamics: Heat - Understanding Energy Transfer & Internal Energy

In summary: Correct. The internal energy (U) is the sum of the kinetic energy (K) and the potential energy (E) of all molecules composing the system. The internal energy is an extensive property while the K and E are intensive properties. and a kinetic energy part and a potential energy part; this kinetic energy part, also called "thermal energy", corresponds exactly to the second meaning of heat.No, don't confuse the kinetic energy (K) and the potential energy (E) of an individual molecule with the internal energy (U) of the system. The internal energy is the sum of all the K and E of all molecules. The internal energy may be used to do work (W) and heat flow (Q).In
  • #1
Hari_Seldon
3
0
Hello,

I am a Math student trying to learn some Thermodynamics and Statistical Mechanics on my own. This has been quite hard, because many concepts are confusing and the terminology is a mess. I have zero background in Physics, so forgive my complete ignorance.

My main problem at the moment is with the definition of heat. From what I understand, the word heat is used with two different meanings. The first meaning is: heat is a kind of energy transfer, as is work, but heat and work are different (and I cannot really understand the difference).

The second meaning is that heat is an amount of energy contained in a system. Correct me if I am mistaken: the sum of all energy contained in a system is called "internal energy"; it may be divided in a potential energy part and a kinetic energy part; this kinetic energy part, also called "thermal energy", corresponds exactly to the second meaning of heat.

In summary the first meaning corresponds to a flow of energy and the second, which should be replaced by "thermal energy", corresponds to a stock of energy. This is somewhat confusing but from the context, it is usually easy to understand which of the two meanings is being employed.

However, I have found the following in my http://www.google.com/url?sa=t&sour...521Bw&usg=AFQjCNFxzInRHyxwpoz2JR4f9yBdaeAJLg"

The principle qualitative difference between work and heat is very simply explained in the microscopic picture. According to that picture, heat is energy which is statistically distributed over all particles. For instance, let us consider some particles with parallel (ordered) momenta which move in one direction. The kinetic energy of these particles can be completely regained at any moment and can be converted into other forms of energy, e.g., by decelerating the particles through a force. However, if the particles move in a completely disorder and statistical manner, it is obviously not possible to extract all the kinetic energy by a simple device. (...) It is therefore considerably simpler to change work into heat, which practically always happenx by itself than to gain utilizable work from heat.

Now, this confuses me completely. I cannot say in which of the two meanings "heat" is being used above, or if it is actually in a third, new meaning I don't know of. Heat is being compared to work, which suggests that it should be understood as an energy transfer, but at the same time I see the idea of something contained in the system that we are trying to extract.

I know I haven't really asked any precise question, but maybe someone can see the source of my confusion and tell me something that clarifies my mind. If so, I would be really thankful.
 
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  • #2
Hari_Seldon said:
From what I understand, the word heat is used with two different meanings. The first meaning is: heat is a kind of energy transfer, as is work, but heat and work are different (and I cannot really understand the difference).

That is correct- heat refers to the flow or transfer of a certain amount of energy. Work also involves the transfer of energy. They are different becasue they have different effects: 'work' is taken to refer to a change in pressure, volume- a mechanical change in the object. It's possible to extend the definition of work to include chemical reactions as well. 'Heat' is taken to be changes in energy that have no mechanical component- a change in temperature, a change of phase, etc.

Mathematically, work can usually be written as an exact differential, while the heat cannot generally be written as an exact differential.


Hari_Seldon said:
The second meaning is that heat is an amount of energy contained in a system. Correct me if I am mistaken: the sum of all energy contained in a system is called "internal energy"; it may be divided in a potential energy part and a kinetic energy part; this kinetic energy part, also called "thermal energy", corresponds exactly to the second meaning of heat.

This is not correct. It is true that the total energy is a system may be divided into potential energy and kinetic energy; but the kinetic energy does *not* generally correspond in any way to thermal energy. If your first definition divided the energy into thermodynamic quantities (heat and work),your second definition divided the energy into *mechanical* quantities (potential and kinetic).

Does this help? An excellent introductory text is Fermi's "Thermodynamics".
 
  • #3
As a mathematician you might enjoy to read Caratheodory's (maybe better known among mathematicians for his theorems on set theory etc.) intent of an axiomatic phenomenological thermodynamics which is very clear:
Untersuchungen ueber die Grundlagen der Thermodynamik, Math. Ann., 67 (1909) p. 355-386
I am not sure whether there is an english translation available.

Caratheodory goes as far as abandoning at least the term "heat" from his considerations for reasons that should now be too obvious to you.
 
  • #4
In order to understand heat is work we can take example of electric heater in which whenever electricity passes through, then an electric field establishes on it and causes some work to heat. i.e., compensation of work is nothing but heat. This my explanation about the difference between heat and work. We can take the example of electric bulb also to understand the difference between the heat and the work.
 
  • #5
Hari_Seldon said:
My main problem at the moment is with the definition of heat. From what I understand, the word heat is used with two different meanings. The first meaning is: heat is a kind of energy transfer, as is work, but heat and work are different (and I cannot really understand the difference).
In classical thermodynamics, "heat" means heat flow or thermal energy flow: [itex]Q[/itex]. Work is mechanical energy (Force acting through a distance) performed on or by the thermodynamic system. Do not confuse heat flow (Q) with internal energy (U).

The second meaning is that heat is an amount of energy contained in a system. Correct me if I am mistaken: the sum of all energy contained in a system is called "internal energy"; it may be divided in a potential energy part and a kinetic energy part; this kinetic energy part, also called "thermal energy", corresponds exactly to the second meaning of heat.
If you refer only to "heat flow" and not just "heat", you will avoid the confusion. The thermal energy contained by a substance is its internal energy. It is a function of temperature. Do not refer to this as heat. Refer to it as thermal energy. Change in the internal thermal energy can be accomplished by doing work on it without heat flow - ie compressing a gas adiabatically.

In summary the first meaning corresponds to a flow of energy and the second, which should be replaced by "thermal energy", corresponds to a stock of energy. This is somewhat confusing but from the context, it is usually easy to understand which of the two meanings is being employed.
Your summary is correct.

However, I have found the following in my http://www.google.com/url?sa=t&sour...521Bw&usg=AFQjCNFxzInRHyxwpoz2JR4f9yBdaeAJLg"

The principle qualitative difference between work and heat is very simply explained in the microscopic picture. According to that picture, heat is energy which is statistically distributed over all particles. For instance, let us consider some particles with parallel (ordered) momenta which move in one direction. The kinetic energy of these particles can be completely regained at any moment and can be converted into other forms of energy, e.g., by decelerating the particles through a force. However, if the particles move in a completely disorder and statistical manner, it is obviously not possible to extract all the kinetic energy by a simple device. (...) It is therefore considerably simpler to change work into heat, which practically always happen by itself than to gain utilizable work from heat.
This passage uses incorrect terminology. The author is using "heat" to mean "internal thermal energy" not heat flow: ie. U and not Q.

At the microscopic level, heat flow or transfer of thermal energy to/from a substance is the transfer of kinetic energy of the molecules that comprise the substance to/from its surroundings. So, at the microscopic level, it is mechanical energy flow. But, as the author points out, one cannot extract all of that kinetic energy to perform useful work at the macroscopic level. One can only extract some of it as useful work. So we distinguish between thermal energy flow and mechanical work. Using conservation of energy, the difference between heat flow into the system and work done by the system must be equal to the change in internal energy of the system.

Hope that helps.

AM
 
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  • #6
It should be noted that interactions involved in work are not strictly different from those involved in heat (they may come down to the same thing -- if you were to look at the individual interactions in a heat process, you could evaluate them as work processes). The difference primarily arrives in the many-body problem, in which interactions become a statistical measure, and so Q is a statistical dissipation (or the dissipation of the internal energy of a macroscopic system). When we separate the terms for work and heat, we're essentially separating a macroscopic measure from a microscopic measure.
 
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  • #7
Andrew Mason said:
At the microscopic level, heat flow or transfer of thermal energy to/from a substance is the transfer of kinetic energy of the molecules that comprise the substance to/from its surroundings. So, at the microscopic level, it is mechanical energy flow.

But this cannot be true in general- blackbody radiation is an electromagnetic field at thermal equilibrium. Where is the mechanical energy flow in blackbody radiation impinging onto matter?
 
  • #8
Andy Resnick said:
... An excellent introductory text is Fermi's "Thermodynamics".

I just re-read this myself a couple of months ago. Very easy to follow. It's a slim book, and it's available cheap as a Dover paperback edition.
 
  • #9
Andy Resnick said:
But this cannot be true in general- blackbody radiation is an electromagnetic field at thermal equilibrium. Where is the mechanical energy flow in blackbody radiation impinging onto matter?
That's true. A black body, theoretically, can be just a resonant cavity that is not associated with kinetic energy of particles of matter.

AM
 
  • #10
Thermodynamics is NOT the easiest topic within physics to study and understand on one's own. Try wikipedia ...the first few paragraphs address some of your questions:

http://en.wikipedia.org/wiki/Heat

It's also useful to keep in mind that at the most fundamental level, nobody understands heat nor spin nor space nor time nor the four fources, for example. Why those entities manifest themselves as they do is unknown..the best we can do is model observed behavior and see if our models match observations;hopefully we can make some new predictions which can then be experimentally verified.
 
  • #11
Thanks all, it has been tremendously helpful.
 
  • #12
Some small clarifications: "Heat" could have two meanings: 1) the transfer of energy between two bodies at different temperatures; 2) the temperature (almost equivalent to internal energy) itself.

In the second definition, heat is actually equivalent to kinetic energy - temperature is defined as the average kinetic energy of molecular motion of an ideal gas.

Good explanation found here: http://video.google.com/videoplay?docid=949035002599580195# [Broken]
 
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  • #13
thopsy said:
Some small clarifications: "Heat" could have two meanings: 1) the transfer of energy between two bodies at different temperatures; 2) the temperature (almost equivalent to internal energy) itself.

In the second definition, heat is actually equivalent to kinetic energy - temperature is defined as the average kinetic energy of molecular motion of an ideal gas.

Good explanation found here: http://video.google.com/videoplay?docid=949035002599580195# [Broken]
You are quite correct. Heat can have two meanings. But in thermodynamics we distinguish between Q - "heat flow" and U - internal energy. If we call them both "heat" we can get confused. So in thermodynamics we refer to Q as heat flow and U as internal energy. See: heat.

AM
 
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  • #14
Andrew Mason said:
You are quite correct. Heat can have two meanings. But in thermodynamics we distinguish between Q - "heat flow" and U - internal energy. If we call them both "heat" we can get confused. So in thermodynamics we refer to Q as heat flow and U as internal energy. See: http://en.wikipedia.org/wiki/Heat" [Broken].

AM

Hi Andrew,
Perhaps this is where I am misunderstanding, or perhaps being misunderstood.

I have proposed a thought of providing work (Q) to an air compressor that is fully enclosed in a volume of liquid propane (or some safe refrigerant) the amount of Q that is normally discarded to the surroundings as waste heat is taken in by the liquid and becomes U, The 99% (?) of Q is now residing as U in both air and liquid.
There should now be a potential of work performance in both parts of the system.

Do I have this wrong ? if so why.

Thanks
Ron
 
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  • #15
RonL said:
Hi Andrew,
Perhaps this is where I am misunderstanding, or perhaps being misunderstood.

I have proposed a thought of providing work (Q) to an air compressor that is fully enclosed in a volume of liquid propane (or some safe refrigerant) the amount of Q that is normally discarded to the surroundings as waste heat is taken in by the liquid and becomes U, The 99% (?) of Q is now residing as U in both air and liquid.
There should now be a potential of work performance in both parts of the system.

Do I have this wrong ? if so why.

Thanks
Ron
It is just a matter of sticking to conventions to avoid confusion. First of all, if you do work on the air, you are adding W, not Q. Assuming the compression is adiabatic (it occurs much more rapidly than the heat flow to the surroundings), the compression increases the internal energy U by the amount W (this is just a result of the first law when Q=0). Since U increases, air temperature increases. Because the temperature of the air increases, heat now flows from the air to the surrounding liquid propane until the temperatures of air and propane are the same. This heat flow would be called Q. I am assuming that the propane is in an insulated container so there is no heat flow out of the propane. In the end, by doing W work on the system, the internal energy of the liquid propane and air increases by the amount W.

However, the potential of the system to do work is not increased by W. Because some of that work has been used to create heat flow, only a portion of that increased internal energy can be recovered as useable work.

AM
 
  • #16
Andrew Mason said:
It is just a matter of sticking to conventions to avoid confusion. First of all, if you do work on the air, you are adding W, not Q. Assuming the compression is adiabatic (it occurs much more rapidly than the heat flow to the surroundings), the compression increases the internal energy U by the amount W (this is just a result of the first law when Q=0). Since U increases, air temperature increases. Because the temperature of the air increases, heat now flows from the air to the surrounding liquid propane until the temperatures of air and propane are the same. This heat flow would be called Q. I am assuming that the propane is in an insulated container so there is no heat flow out of the propane. In the end, by doing W work on the system, the internal energy of the liquid propane and air increases by the amount W.

However, the potential of the system to do work is not increased by W. Because some of that work has been used to create heat flow, only a portion of that increased internal energy can be recovered as useable work.

AM

Thanks Andrew,
I need to take care so as to not ruin the thread, the tank would not be insulated, the goal is to bring heat in from outside as well as from the compressor. Finding the balance and maintaining heat flow and resulting pressure inside the tank is the mechanical challenge (I see several options).
That much being said, I will continue to study your wording.

Thanks
Ron
 

1. What is thermodynamics?

Thermodynamics is a branch of physics that deals with the study of energy and its transformation from one form to another. It also focuses on how energy affects matter and its properties.

2. What is heat in thermodynamics?

Heat is a form of energy that is transferred from one system to another due to a difference in temperature. It is a result of the random motion of molecules within a system.

3. What is the difference between internal energy and heat?

Internal energy refers to the total energy of a system, including the kinetic and potential energies of its particles. Heat, on the other hand, is the transfer of energy from one system to another due to a difference in temperature.

4. How is energy transferred in thermodynamics?

Energy can be transferred in thermodynamics through three main processes: conduction, convection, and radiation. Conduction is the transfer of heat through direct contact, convection is the transfer of heat through the movement of fluids, and radiation is the transfer of heat through electromagnetic waves.

5. What are the laws of thermodynamics?

The laws of thermodynamics are fundamental principles that govern energy and its transfer in a system. The first law states that energy cannot be created or destroyed, only transferred or converted. The second law states that energy will always flow from areas of high concentration to low concentration, and the third law states that the entropy of a perfect crystal at absolute zero is zero.

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