Thermal conduction

Thermal conduction is the transfer of heat (internal energy) by microscopic collisions of particles and movement of electrons, that takes place in a solid, liquid, or aeriform medium (inside a single body or two bodies in contact with each other).

Microscopic explanation

Heat transfer occurs naturally from the higher temperature zones to those with lower temperatures. Thermal energy manifests itself at the microscopic level as the oscillation of the molecules of material; this oscillation increases with increasing thermal energy accumulated in the body (i.e., increases with temperature).

The oscillations of each molecule caused by the stored thermal energy occur around the position initially occupied by the molecule, so that in a given interval of time the average position of the molecules is always the same, so that in thermal conduction there is no transport of matter, but the only transmission of molecular collisions.

For example, putting over a flame the end of a metal bar, after some time the other end gets hot and can get burned; this is due to the fact that the heat given by the source raises the energy content of the molecules of the contact part, which oscillate with greater speed; during their oscillation, the molecules collide with the neighboring molecules, exchanging momentum; this exchange of momentum (which takes place at microscopic level) translates at macroscopic level in exchange of thermal energy by conduction; as a result of this exchange of thermal energy after a certain time even the opposite end of the rod in contact with the flame heats up, so the phenomenon of thermal conduction as a consequence makes the temperature of the rod as homogeneous as possible. In other words, the greater is the thermal conductivity of the material, the more easily a body subjected to a local temperature variation distributes this temperature variation along the whole body.

So repeating the experience with a glass tube, it is observed that the point in contact with the flame can become red because of the high temperature, while a few centimeters away there is only a weak increase in temperature; this is due to the fact that glass conducts heat worse than metal.

Oscillations of each molecule caused by thermal energy stored occur around the position initially occupied by the molecule, so in a given time interval the average position of molecules is always the same, so in thermal conduction there is no transport of matter, but only transmission of molecular collisions.

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