The day after my divorce was finalized I got a tattoo of the T-S diagram for a vapor compression refrigeration cycle.  On the horizontal axis of a T-S diagram is entropy and the vertical axis is temperature.  Entropy is an interesting concept in and of itself, it is a measure of disorder in a molecular/thermodynamic sense.  Another interesting fact is that on the scale of the the Universe, entropy is always increasing and that any physical process can only cause it to increase or stay the same, it can never decrease.  Practically speaking this means that all the energy in the universe is being converted to less useful forms and eventually (and I mean EVENTually… like trillions of years from now) there will be no useful energy in the universe when entropy is at it’s maximum. Anyway… A T-S diagram plots the path of a thermodynamic process against the temperature and entropy content of the working fluid.  The plot for the vapor compression cycle starts with warm, high pressure but low entropy liquid which is throttled across a valve causing its temperature to decrease.  Next, heat is absorbed (from the inside of your refrigerator,  the part of your air conditioner in the attic, some industrial process, etc.) which boils the cold liquid, increasing the entropy in the system but with the temperature remaining relatively constant as the absorbed energy goes to converting the liquid to vapor.  Next the vapor is compressed which increases both the entropy and the temperature.  In the final step the vapor, now at high pressure, is condensed against a cooler heat sink (outside unit of your air conditioner, mesh thing on the back of your refrigerator, the ocean, the atmosphere, etc) and the temperature and entropy decreases.  This doesn’t contradict the “entropy is always increasing”, because although the working fluid entropy goes down the entropy of the heat sink increases and the NET entropy (heat sink and refrigeration cycle) increases as the “high value” energy of the electricity that turned the compressor is lost as lower value heat.  At this point we’re left with high pressure, low entropy liquid and the cycle repeats itself. This was particularly meaningful to me because in addition to the fact that I work with these thermodynamic cycles in my professional life, it seemed then (and now) that my personal life went in cycles of up and down, hot to cold, low pressure to high pressure.  At that time I was just coming out of the ass-end of a compressor and the ink in my skin was a permanent reminder that eventually I’d be back to cool and condensed…