Is heat energy actually kinetic energy?

Question

According to KMT, heat is just molecules moving/vibrating. By that definition, could heat energy be seen as kinetic energy?

Answer 1

It is true for gases, when the interaction between molecules is small, which is the case of kinetic molecular theory. For liquids and solids the potential energy also play an important role.

Answer 2

No. Heat is not the kinetic energy of molecules and atoms. That energy is properly called the internal kinetic energy of a substance. Heat, on the other hand, is the transport or transfer of energy due solely to temperature difference between substances.

An example of the distinction between an energy transfer mechanism and the energy itself at the macroscopic level is a collision between objects at different speeds (different kinetic energies). The collision is the mechanism for transferring kinetic energy from one object to the other. It is not the kinetic energy itself.

Heat may result in a change in temperature (change in internal molecular kinetic energy) in which case it is referred to as “sensible heat”, or it may result in a change in phase, such as solid to liquid, (change in molecular potential energy), in which case it is referred to as “latent heat”.

For a further discussion of the difference between heat and energy see: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heat.html The article references Mark Zemansky (Co-author of textbook Heat and Thermodynamics) "Plea" about the use and misuse of the term heat. To quote:

"Don't refer to the "heat in a body", or say "this object has twice as much heat as that body". He also objects to the use of the vague term "thermal energy" and to the use of the word "heat" as a verb, because they feed the misconceptions, but it is hard to avoid those terms. He would counsel the introduction and use of the concept of internal energy as quickly as possible.

The emphasis on "hard to avoid those terms" is mine. It acknowledges the common, or colloquial, use of the term heat

Hope this helps.

Answer 3

No. Heat is the microscopic kinetic energy due to random molecular motion. Kinetic energy (as defined in Newtonian physics) occurs when the molecules all have a preferred direction.

Just like the difference between still air and wind. When the air is still, the molecules are all moving but they don't have a preferred direction, and the average is no net movement. However when there is wind, there is a net movement of air molecules.

Answer 4

Yes, heat is kinetic energy of the atoms and molecules.
More precisely: Only the kinetic energy of their disordered movement is called heat energy. But the kinetic energy of their ordered movement (like of a flowing river or a driving car) is not considered heat energy.

Answer 5

Not entirely. For an ideal gas at rest, the thermal energy is indeed the kinetic energy of the gas atoms.

However, real substances are not that simple:

1. When two gas atoms/molecules collide, they first convert some kinetic energy to potential energy and back. Since you always have atoms/molecules colliding, there's always some thermal energy stored in potential energy of colliding particles.

2. When you have molecules instead of atoms, you not only get to store energy in the rotations of the molecules, you also get vibrations that contain thermal energy (if the temperature is high enough). Vibrations constantly convert kinetic energy to potential energy and back, so half the thermal energy stored in the form of vibrations is potential energy.

So, thermal energy is virtually entirely kinetic energy for a low pressure noble gas. But even sufficiently hot nitrogen gas (2-atom molecules that can vibrate along their axis) stores significant amounts of thermal energy in the form of potential energy.

Answer 6

Yes, well, sort of anyway. Rather than "heat", we should probably say thermal energy (internal energy). There was a lot of work done during the latter half of the 19th century to attempt to derive the laws of thermodynamics based on the idea that gases (and matter more generally) is made of a large collection (ensemble) of molecules/particles moving with a certain kinetic energy and interacting with each other through perfectly elastic collisions.

One of the great abstract theories formulated to do this was that of the Canonical Ensemble, which allows one, using statistics, to derive the classical thermodynamic laws from statistical principles, assuming both a large number of particles and all of the possible states that that large number of particles might exist in (all permutations of their velocities and positions).

Introducing the ideas of quantum mechanics and treating them in the same statistical manner extends this idea and allows us to obtain macroscopic explanations from quantum mechanical first principles. This spawned an entire field of physics, now known as condensed matter physics, and it deals with things like Bose-Einstein condensates, superconductivity, Drude-Summerfield models of electrons in metals (including their heat capacity, conductivity, Seekeck effect, etc.) It's the "missing link" from the building blocks of physics to the "real-world" macroscopic picture that we are familiar with in our day-to-day lives. One of the greatest successes of modern physics in my opinion.