Understanding the Difference Between Heat and Thermal Energy

[hushai1]

There seems to be considerable confusion in the scientific community about thermal energy and heat. I have read several textbooks on thermodynamics and several posts in scientific forums and they certainly don't agree on what is heat and thermal energy. For some: thermal energy is the kinetic energy of the random motion of particles and heat is a transfer of energy between two body that are not in thermal equilibrium. For some thermal energy and heat are the same thing but they are a form of energy associated with the kinetic energy of the random motion of particles and not necessarily associated with transfer.

My questions are these:

Is there an actual agreement in the scientific community about these terms?

If there is, what is the most precise definition given to these terms? What is the mathematics behind it?

If there is not, what would be the most authoritative source that could be used to define them?

Do you know of any book that explain this thoroughly using even advanced mathematics and physics?

Thank you

 

[Andrew Mason]

There seems to be considerable confusion in the scientific community about thermal energy and heat. I have read several textbooks on thermodynamics and several posts in scientific forums and they certainly don't agree on what is heat and thermal energy. For some: thermal energy is the kinetic energy of the random motion of particles and heat is a transfer of energy between two body that are not in thermal equilibrium. For some thermal energy and heat are the same thing but they are a form of energy associated with the kinetic energy of the random motion of particles and not necessarily associated with transfer.

My questions are these:

Is there an actual agreement in the scientific community about these terms?

If there is, what is the most precise definition given to these terms? What is the mathematics behind it?

If there is not, what would be the most authoritative source that could be used to define them?

Do you know of any book that explain this thoroughly using even advanced mathematics and physics?

Thank you

There is general agreement that $\Delta Q$ is "heat flow". But, apart from that, "heat" is a loose term that can be used to refer to different things. The confusing use of the term "heat" may be historical. Heat was originally thought to be some kind of substance that flowed between bodies.

"Heat flow" is embodied in the first law of thermodynamics:

$$\Delta Q = \Delta U + W$$

When applying the first law to a thermodynamic process involving a system, $\Delta Q$ is the heat that flows into the system during the process, $\Delta U$ is the change in internal energy of the system, and W is the work done by the system. If the heat flow is into the system, $\Delta Q$ is positive. If the heat flow is out of the system, $\Delta Q$ is negative. Sometimes scientists use the term "heat" to mean $\Delta Q$.

"Heat" is often used in a different context - as something that a body contains - ie. the quantity of "heat" contained by a body. This is not a good idea to use heat in this way because it causes confusion with $\Delta Q$.

$\Delta Q$ does not refer to anything "contained" by a body. It refers to energy transfer to or from the system during a process. W - work - does not refer to anything contained by a body either. W refers to the mechanical work that is performed by the system during a process. In the first law, the only quantity that represents to something contained by a body is $\Delta U$ - the internal energy of the system.

AM

 

[hushai1]

Thank you very much for your answer.
It seems clear to me now that heat is energy in transfer. What is thermal energy? Is it the same as heat? If a bullet is stopped suddenly, what energy transfer is taking place since it cannot be heat?
Regards,

 

[Andrew Mason]

Thank you very much for your answer.
It seems clear to me now that heat is energy in transfer. What is thermal energy? Is it the same as heat? If a bullet is stopped suddenly, what energy transfer is taking place since it cannot be heat?
Regards,

Thermal energy a form of internal energy. It is the energy due to the kinetic energy of the molecules in a substance which is in thermal equilibrium (ie in which the molecular kinetic energies follow a Maxwell-Boltzmann distribution).

A bullet stopping in a piece of wood is an example of Work: a force being applied through a distance. In that case, the work being done on the wood increases its internal energy. Since dQ = 0, dU = dW (first law of thermodynamics).

AM

 

[sardar]

for bringing up this topic and addressing the confusion surrounding the terms "heat" and "thermal energy." As a scientist, it is important to have a clear understanding of these concepts and how they are related.

To answer your first question, there is a general agreement in the scientific community about the difference between heat and thermal energy. However, there may be some variations in terminology and definitions used by different scientists or in different fields of study. This can lead to confusion and conflicting explanations, as you have observed.

The most precise definition of heat is the transfer of thermal energy between two bodies that are at different temperatures. This transfer occurs due to the difference in temperature between the two bodies, with heat flowing from the hotter body to the cooler body until they reach thermal equilibrium. Heat is measured in units of joules (J) or calories (cal).

On the other hand, thermal energy refers to the total kinetic energy of the particles in a substance. It is a form of internal energy and is related to the temperature of the substance. The higher the temperature, the more thermal energy the substance has. Thermal energy is also measured in units of joules (J) or calories (cal).

The relationship between heat and thermal energy can be described by the first law of thermodynamics, which states that energy cannot be created or destroyed, only transferred or converted from one form to another. In the case of heat and thermal energy, heat is the form of energy being transferred between bodies, while thermal energy is the form of energy that the bodies possess.

As for authoritative sources, textbooks on thermodynamics and heat transfer are excellent resources for understanding these concepts. Some well-known books in this area include "Thermodynamics: An Engineering Approach" by Cengel and Boles, and "Introduction to Heat Transfer" by Incropera and DeWitt. These books use advanced mathematics and physics to explain the principles of heat and thermal energy.

In summary, while there may be some variations in terminology and definitions, there is a general agreement in the scientific community about the difference between heat and thermal energy. Heat is the transfer of thermal energy, while thermal energy refers to the total kinetic energy of particles in a substance. The first law of thermodynamics governs the relationship between these two concepts. For a thorough understanding of these concepts, textbooks on thermodynamics and heat transfer are recommended.