Answer to Question #160213 in Molecular Physics | Thermodynamics for Yolande

Question

Explain 3 different common examples of natural processes that involve an increase in entropy

Answer 1

• Mixing two gases that were previously separated. An everyday life example could be opening a can of soda. While being sealed, the can stores $CO_2$ that is generated by soda. When opened, $CO_2$ quickly escapes the can, as its concentration in the can was much higher than the average concentration of $CO_2$ in the atmosphere. This process increases entropy, the process is obviously irreversible (as usually two gases don't separate from each other that easily). From mathematical point of view, the increase of entropy is related to the increasing of the statistical weight.
• Transferring heat from a hot body to a colder one. There is plenty of everyday examples : leaving your hot tea outside (at night, if we want to erase the impact of sun rays) will pretty soon make it cold or warm (depending on the temperature outside). This process is also clearly irreversible, and thus is leading to the increase of entropy. From mathematical point of view, the increase of entropy is related to the Clausius definition of entropy : $dS=\frac{\delta Q}{T}$, by taking into account the heat transferred to the atmosphere by tea, and the heat lost by tea due to the atmosphere, we find that the total change on entropy is positive.
• Converting mechanical energy to the heat energy. The most common example is braking. While we are trying to stop our car, it loses its kinetic energy (as the velocity decreases), but brakes are being heated. This process is also irreversible, as the heat energy can't spontaneously be converted to mechanical energy (to do this type on conversion we use specially built heat engines). From mathematical point of view the increase of the entropy is explained through the heat cycle and the change depends on the manner in which the heat is gained, but the most general calculations still give an increase of entropy.