# How is heat transferred (microscopically)?

Microscopically talking, how is heat transferred and the temperature of bodies changed?

For example, if a hot material touches a colder piece of conductive metal, how does the heat and energy transfer from one body to another? I think the result is that the velocities of the molecules of the colder body raise ( $T \propto v_{th}^2)$ until they reach the thermal velocity of the hotter one and establish a thermic equilibrium. What is the mean through which this happens? Collisions between the molecules of the two surfaces in contact? Phonons interaction?

• This is well document topic and so I am surprised that you have asked such a general question about this phenomena. Commented Sep 12, 2018 at 14:21
• I saw some things online about collisions with molecules (for example gasses) but I was thinking also about rotational kinetic energy (is that exchanged also by collisions?) about some characteristic potentials (are attractive fields involved in this exchange between two bodies?), collisions between electrons and phonons to thermalize, I thought I couldn't be that simple, microscopically. Commented Sep 12, 2018 at 14:27
• If I thought about energy exchange, also an electromagnetic wave, for example, could raise the temperature modifying the velocity of the polar molecules Commented Sep 12, 2018 at 14:28
• There are three heat transference types. I don't think the phenomena is very well explained on the web. In an imaginary experiment, where there are no molecules around, the piece of metal will emit radiation. So part of the heat will be transfered in terms of collisions, part of it in terms of radiation.
– user153036
Commented Sep 12, 2018 at 14:47
• Phonons are not necessarily involved. You may define phonons only on crystals (periodic lattice systems). Heat transfer by contact is more general than that, it can be between any two bodies (of matter). Commented Sep 12, 2018 at 15:00

The molecules having higher average translational kinetic energy (higher kinetic temperature) will transfer kinetic energy to those molecules having lower average translational kinetic energy (lower kinetic temperature) by means of collisions. The final temperature of the two bodies will generally be somewhere between the two original temperatures, unless one body is a thermal reservoir (having heat capacity so high that its temperature change is negligible), in which case the final temperature of both will be that of the thermal reservoir body.

Hope this helps

• What about, for example, a microwave heated water (where, if I'm not wrong, the rotational energy levels are raised) is in contact with another body? It makes me think that the exchanging mode could be a little different Commented Sep 12, 2018 at 14:31
• @Costantino: You are wrong. While the whole of the water may not be in perfect equilibrium, water is so dense that energy will not be locked in rotational energy levels. Water molecules interact often enough to distribute energy. Commented Sep 12, 2018 at 14:35
• So the rotational energy is diffused into other energy levels where the medium is dense, like water. I guess this interaction between water molecules is one the ways in which rotational kinetic energy is transferred from particle to particle... Commented Sep 12, 2018 at 14:41
• @Costantino.The interaction between microwaves and water occur at such energies that contribute only to rotational and vibrational energy levels. The orderly rotational or vibrational energies are then randomized my means of collisions with molecules contributing to their translational kinetic energies. Check out "interaction of radiation with matter" on the Hyperphysics web site. Hope this helps. Commented Sep 12, 2018 at 14:53
• Yes it is helping indeed, thank you. Just I wanted to demand if all the three energy modes (translational, rotational and vibrational) of the molecules are involved when the heat is transferred between two bodies, or if only the translational one is involved, after the energy is diffused from rotational and vibrational into the remaining one with previous inside-the-body interactions in an appropriately dense material. Commented Sep 12, 2018 at 15:22