By Nasif Nahle
Scientific Research Director/Biology Cabinet


At present there is confusion with regard to scientific concepts of the physics of heat, much of it emanating from the field of climatology. Some climatologists are applying the concept of heat in an inappropriate and wrong way. The application of the phrase “Heat content of such and such a system” is politically tendentious and is aimed at influencing public opinion in a certain direction. It is deployed rather than the proper term “Energy Content” because the word “heat” has greater utility in terms of “emotionalizing” the subject. What we have here is a complete diversion from the scientific subject of the concept of heat. Hence, the necessity of writing this article for the purpose of clarifying the meaning of the term “heat” and also of the terms “energy”, “internal energy” and “total available energy” from the physical point of view.


Heat is energy in transit due to differences in temperature between two systems. (Engel&Reid. 2006. Page 16)

Heat always flows from the system with a higher temperature towards the system or systems with a lower temperature.

Heat is the quantity of energy which crosses the boundaries of a thermodynamic system (Engel&Reid. 2006. Page 16) that is in a state of high density of kinetic energy and is transferred to another system which is in a State of low density of kinetic energy. This is the equivalent of saying that heat is the energy transferred from a hot system to a cold system.

Before it is transferred, the energy which remains within the boundary of the system is not heat, but internal energy or total available energy.

Once a system absorbs heat, the latter is no longer heat, but internal energy of the system, i.e. it stops being heat because heat is no longer being transferred between two systems at different temperatures.

For it to be heat it must be in the process of being transferred from one system to another system. There is no heat transfer from low kinetic energy density systems to high kinetic energy density systems. The flux of heat photons always moves from maximum to minimum, from high to low. This is the second law of thermodynamics.

Heat cannot be stored nor contained by any system because heat is a process function.

A process function, or process quantity, is a physical quantity which describes the evolution or shift through which a thermodynamic system passes from an initial equilibrium state to another equilibrium state.

It is a category error to use the expression “heat stored” if one does not clarify that it is not heat (a process) which is being stored, but energy which has been transferred from one system to another transposing the boundary of the acceptor system. The proper expression for this is “energy stored by heat transfer”; or simply “energy stored”.


Published: 19 August 2009Update: 29 November 2009 (The definition of thermal energy was eliminated due to copyright claim).
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From the diagram above we now see clearly that the phrase “Heat Content of the Oceans” is terminologically incorrect because heat cannot be contained by the oceans. The energy in transit, or heat, is absorbed by the oceans in the form of internal energy, but it stops being energy in transit, or heat, because it is no longer in transit, forming now instead part of the internal energy of the oceans, which consists of kinetic energy, gravitational potential energy and chemical or nuclear energy.

The energy emitted or released by the system becomes heat the moment it crosses the boundary of the system, i.e. the moment it becomes energy in transit.

Remember, process quantities cannot be stored or contained because they describe the trajectory by which a system acquired an equilibrium state. A process function or process quantity is not a state function.

A state function is a property of a thermodynamic system which depends only on the current state of the system. Internal energy is a state function.

The physical units for heat are Watts (W), Joules/second (J/s) or calories/second (cal/s).



Thermal energy is the total kinetic energy that a system possesses. Thermal energy is an important component of the internal energy of such system. (Engel & Reid. 2006. Page 13, 355)

The kinetic energy is energy of motion. (Wilson. 1994. Page 146 and hyperphysics)

For example, the Sun is the main and fundamental source of energy for the solar system. Incoming energy from the Sun is essential for life on Earth (Suplee. 2009. Page 39). Without solar energy, life would not exist on our planet. (Sutton & Harmon. 2000. Page 49)

The Sun produces energy by nuclear fusion (Suplee. 2009. Page 27). Part of the internal energy of the Sun is kinetic energy or thermal energy which reaches about 27 MeV per 4He nucleon (Maoz. 2007. Pp. 48-49). The thermal energy released from the Sun is transformed into energy in transit or heat (Suplee. 2009. Pp. 28-30 and Maoz. 2007. Page 49). As soon as the transferred energy is absorbed by the Earth or another body of the solar system or other systems beyond our solar system, it is no longer heat and once again becomes kinetic energy, which then forms part of the total internal energy of that body or system. On Earth, the oceans are the main systems where solar thermal energy is stored. (Research News. Science Magazine. Vol. 198)

Another example of the difference between heat and thermal energy is a lit candle. A candle generates thermal energy (particle kinetic energy) as long as it burns. That thermal energy is then released from the candle towards the surroundings (Wilson. 1994. Page 380). As the thermal energy crosses the candle's boundaries, it is no longer thermal energy and becomes heat, that is, energy in transit (Engel & Reid. 2006. Pp.16 and 17). When the energy in transit strikes upon a system with low energy density, for example one's skin, the energy transferred by heat once again becomes thermal energy, i.e. it becomes the molecular kinetic energy which had been transferred to the low energy density system from a higher energy density system. (Wilson. 1994. Page 146)

The following diagram illustrates the differences between heat and thermal energy:
The difference between heat and thermal energy is that thermal energy is not in the process of being transferred; it is not in transit, but remains as part of the internal energy of the system; heat, on the other hand, is energy in transit, i.e. energy in the process of being transferred from a hotter system towards another colder system (Engel & Reid. 2006. Pp.16 and 17). In summary, the thermal energy is energy within the system; heat is energy outside the system.

The thermal energy is continuously converted into gravitational energy and vice verse (Maoz. 2007. Page 48). For example, when we lift an object at rest on the floor, the thermal energy of our body is transferred to the object lifted. As we lift the object, our thermal energy is stored as gravitational energy in the gravity field. There, the energy will remain until the object acquires movement.

Consequently, the thermal energy in the gravity field is always negative, and therefore the gravity field constitutes a stable deposit of thermal energy. (Guth. 1999. Pp. 335-339)

Thermal energy units are Watts*second (W*s), Joules (J) or calories (cal). Notice the difference between the heat units (W, J/s, calories/s) and the thermal energy units (W*s, J, calories).



Suplee, Curt. The Plasma Universe. 2009. Cambridge University Press, New York. Pp. 27-39.

Guth, Alan H. The Inflationary Universe: The Quest for a New Theory of Cosmic Origins. Perseus Books Group, 1999, New York, New York. Pp. 29-31.

Van Ness, Hendrick C. Understanding Thermodynamics. PAGE 17.

Thomas Engel and Philip Reid. Thermodynamics, Statistical, Thermodynamics & Kinetics. 2006. Pearson Education, Inc. Pp. 13, 16, 355.

Maoz, Dan. Astrophysics. 2007. Princeton University Press. New Jersey. Pp. 48-49.

Potter, Merle C. and Somerton, Craig W. Thermodynamics for Engineers. Mc Graw-Hill. 1993. PAGE 40.

Pitts, Donald and Sissom, Leighton. Heat Transfer. 1998. McGraw-Hill.

Sutton, David B., Harmon, N. Paul. Ecology: Selected Concepts. 2000. John Wiley & Sons, Inc. New York. Pp. 49-57.

Wilson, Jerry D. College Physics-2nd Edition; Prentice Hall Inc. 1994.


Peer Reviewed by scientists of the Faculty of Physics and Mathematics of the Autonomous University of Nuevo Leon. August 2009.