Why does heat naturally flow from hot to cold?

Answer 1

Warmer regions have faster moving molecules. Cooler regions have slower moving molecules. If you put a warmer and colder region together, the overall movement of all the molecules of the combined regions tend to equalize so the formerly warmer region will get cooler and the formerly colder region will get warmer.

Answer 2

Three phenomenons are involved: conduction, convection and radiation.

Heat (thermal energy in transit) travels through material bodies from high

temperature zones to low temperature zones. If we limit the case to an opaque solid

body, and we heat up a side of it, the particles that conform the body (atoms,

molecules) in the hot spots (high temperature) will be vibrating at a higher intensity

(such particle vibrations are part of the solids "internal energy") than the particles in

the cooler spots (lower temperature). Since in a solid this particles are very close

together, the vibration energy is passed to neighboring particles and a cascade of this

vibrating energy is formed. The energy flows from high intensity vibrating zones (high

temperature) to low intensity vibrating zones (low temperature).

Answer 3

That is related to probability (or statistics). Atoms move randomly at different speeds; it is likely that some of the fast-moving atoms (in the hotter body) will collide with the slower-moving atoms (in the colder body), transferring part of their movement energy, i.e., their heat. Not all atoms move at the same speed; some randomly move faster than others. But that's where probability enters the picture - on average, more collissions between atoms will transfer energy from the hot body to the cold body than vice versa.

Answer 4

Because that's how the universe works. There isn't really a "why".