How does the kinetic theory of matter define heat?

1 Answer
Mar 22, 2017

Kinetic theory of matter advocates atomism. We have a large number of identical (in some cases non identical as well) gas molecules moving with superb speed in all sorts of directions.

All the thermal energy is associated with these molecules/atoms in their ever existing motion.

Heat can be interpreted as the energy transfered between to systems without mechanical means!

Let me explain below.


In kinetic theory, all the energy of gas molecules is wholly kinetic and no potential energy exists.

These molecules move randomly colliding with one another.

These molecules upon colliding upon the container walls exert pressure by transferring momentum.

All of this can be attributed to the kinetic (thermal energy) of the particles.

So, as defined, heat is the energy transferred between two systems without mechanical work.

When two systems of an ideal gas (to keep the analysis simple) are put to contact heat is transferred from one system (the one at higher temperature) to the one at lower temperature.
In kinetic theory, the molecular motion is responsible for this heat transfer as well. Collision of molecules of one system with another leads to transfer of energy from the more energetic molecule to the less energetic one.

One might ask that when one defines heat transfer to be non mechanically, how can one attribute intermolecular collisions to be responsible for it ?

The answer is straightforward but, subtle and tricky so pay attention.

By mechanical means, we mean heat transfer by one system performing work on the other on the macroscopic scale.
Microscopic analysis is not taken into consideration.

Infact, all heat transfer in matter (conduction and convection) can be attributed to motion of particles. (Close to the kinetic theory idea)

In case of heat convection, the molecular motion of fluid particles makes heat transfer possible while in case of conductors, the motion of the free electrons in the crystal lattice makes heat transfer possible. (Although in either case, heat transfer is taken to be done by no mechanical means)

So far we were talking about only the transfer of heat from one ideal gas system to another when they are kept into direct contact and tried to understand the interpretation of heat (as a transfer of thermal energy).

Let us generalise now. Suppose we take the two systems kept in contact with a conducting wall.

What happens here ?

Considering the system with higher temperature, the molecules are more chaotic and upon having collisions with the conducting wall, they transfer energy to the wall, and hence to the electrons in the wall.

The same thing happens on the other side of the wall. The system at lesser temperature transfers energy to the wall as well.
So, one may ask. Who wins the battle ?

Ofcourse the answer would be, the system at higher temperature would naturally transfer more energy to the wall than the system at lower temperature.

The electrons in the wall conduct heat from one side to another.

So the wall gets hot. How hot? It tries to attain the temperature of the system at higher temperature. As a result, now upon having collisions, the wall transfers heat to the system at lower temperature.

This is how one can explain heat transfer between two systems from the kinetic theory veiw point.

Now let me apologise for my answer being too long. I hope it will help.

Finally, one concluding remark. Even though the transfer of heat is due to Molecular motion, from the kinetic theory veiw point analysis is simple and straightforward.
However, kinetic theory and even classical statistics can't be applied for the case of electrons performing heat transfer.
The statistical analysis of electrons is much more complicated. Further, in the case of electronic motion, one cannot simply put the potential energy to be zero as we did in Kinetic theory. There is always an Electrostatic potential energy due to neighbouring ions and other electrons.