Ok, so I was reading a chapter on thermodynamics (introduction), which built its content on the idea that two systems separated by a diathermic wall will tend to attain thermal equilibrium with each other and the surrounding ( in case they are also separated by surrounding with diathermic wall). I have a simple question in my mind, why do systems tend to attain thermal equilibrium anyway? Any explanation, bulk or microscopic will do, both are also welcome.
If one system, say a fluid, is at a higher temperature, it will have more kinetic energy than the other fluid. Hence it will continue to impart kinetic energy to the other fluid through the diathermal wall, via collisions. Of course the fluid at lower temperature also transfers kinetic energy to the 'hotter' fluid, but the former process is more intense. The result is a net transfer of energy from hotter to cooler, via collisions. This happens until their average thermal energies become equal; the rate of transfer of energies become equal on either side.