A question about heat transfer

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Discussion Overview

The discussion revolves around the concepts of heat transfer and internal energy in thermodynamics, particularly focusing on the conditions under which heat flows between two objects at different temperatures and the forms of energy involved in this process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that if two objects A and B are at the same temperature (TA = TB), no heat flow occurs, but their internal energies may differ.
  • Participants discuss that when TA > TB, heat flows from A to B, and while B's internal energy increases, the change in A's internal energy may involve both kinetic and potential energy contributions.
  • One participant introduces the concept of strain energy, suggesting that heat transfer can also result in changes to strain energy in materials, particularly when different materials are involved.
  • Another participant questions the mechanisms by which non-kinetic energy is transferred as heat, seeking clarification on the nature of energy loss from A during heat transfer.
  • It is noted that elementary thermodynamics focuses on end results rather than intermediate states, which are less defined and subject to ongoing research.

Areas of Agreement / Disagreement

Participants generally agree on the basic principles of heat transfer and internal energy but express differing views on the specifics of energy transfer mechanisms and the role of strain energy. The discussion remains unresolved regarding the precise nature of energy loss from object A during heat transfer.

Contextual Notes

The discussion highlights limitations in understanding intermediate states in thermodynamic processes and the complexities involved in energy transfer, particularly in materials with differing properties.

Who May Find This Useful

This discussion may be of interest to students and professionals in thermodynamics, materials science, and engineering, particularly those exploring heat transfer and energy transformations in different materials.

frensel
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Hi. If there are two objects A and B, the temperature of A is TA, the temperature of B is TB, and my questions are:

1). If TA = TB, their internal energy (which is the kinetic energy plus some other forms of energy) might not be the same. In this case, A and B are in thermal contact. Since TA = TB, no heat flow happens. Am I correct? The internal energies of A and B remain the same although the internal energy of A may be larger than the internal energy of B.

2). If TA > TB, heat flow is from A to B. Here, I know that the internal energy of B will increase (TB might not be increased though in the case of phase transition). However, how about the change of the internal energy of A? Is that all the heat flow from A is from the kinetic energy of A's molecules? or some of the heat flow come from the kinetic energy of A and some of it come from the other forms of energy of A such as the potential energy?

thx.
 
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1) Is completely correct.

2) Has merits and is nearly there but internal energy can also appear as strain energy as well as kinetic energy of the molecules. So if we take two strips of differing metals and fix them rigidly together and then heat one strip. We will find that the heat will be transferred from one to the other until their temperatures equalise. However the resulting compound strip will have bent due to the differing coefficients of expansion of the metals. The resulting bent strip will have a higher stain energy than the unbent strip.
This strain energy can only have come from the supplied heat.

Note for the above strip

Q = \Delta U + W

W = zero since no work is done on or by the strip on the external environment.
 
Studiot said:
1) Is completely correct.

2) Has merits and is nearly there but internal energy can also appear as strain energy as well as kinetic energy of the molecules. So if we take two strips of differing metals and fix them rigidly together and then heat one strip. We will find that the heat will be transferred from one to the other until their temperatures equalise. However the resulting compound strip will have bent due to the differing coefficients of expansion of the metals. The resulting bent strip will have a higher stain energy than the unbent strip.
This strain energy can only have come from the supplied heat.

Note for the above strip

Q = \Delta U + W

W = zero since no work is done on or by the strip on the external environment.

Thank you for your reply. I still have a question for you second point. If you heat strip A, A absorbs heat and some of the energy turn into the kinetic energy of A and some of it turn into the strain energy of A. Since strip A and strip B are in temperature difference now, heat flow from A to B is started. It is clear that the heat flow B absorbed turns into both the kinetic and strain energy of B since B also bent, but how about the energy lost of A? Is that all the energy A transferred to B originated from the kinetic energy of A? If this is not true, what is the microscopic mechanism of transfer the non kinetic energy as heat to other objects?
 
Elementary thermodynamics does not tell us about the intermediate states only the end result. Intermediate states not normally definable and the subject of modern research.
 

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