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This is the fifth and last part of my "Time and Entropy" series. Why doesn't time run backwards? If you have missed the former ones, don't miss to read them, which are about

• The size of Time
• The problem with clocks. A circular argument?
• The direction of time
• Entropy and Time
  
  
  Enjoy!

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Part 5: Heat Engines and Perpetuum Mobile

How entropy and energy devaluation are related to each other

Some more basically other processes, which preset direction of time, can be found too, but most the time the growth of entropy is discussed in this context, because the processes that make this seen are omnipresent.

Streaming heat

The growth of entropy becomes visible for example, if you bring two containers with water of different temperature into contact with each other. Heat flows from the hotter to the colder reservoir and after a while, both are lukewarm.

This actually never happens the other way round. In this process, the direction of time can be seen clearly.

One might be tempted to build a machine that uses the unlimited heat - and therefor energy - source of the oceans. For this to work, a piece of the thermal energy in the water would have to flow into the other reservoir by itself. The problem: In the moment the second reservoir has reached the same temperature as the ocean, the machine would stop working. Without additional energy supply the thermal gradient can't be maintained, and therefor no energy can be produced in the long run (perpetuum mobile). Not because maintenance of energy would be violated, but rather entropy would decrease.

What energy consumption has to do with entropy

Although in physics we are confident, that energy can neither be produced nor destroyed, in every day life we often speak about energy consumption. From a physical point of view this means, that the amount of energy doesn't change, but rather its usability. That's how energy is delivered from the electricity plant, payed by us and changed into the thermal energy of water with the water cooker.

The energy of the heated water is almost not changeable into another form. This devaluation looks like consumption for us, although it is increase of entropy. This is what often makes physics hard to understand and disliked. It should be the function of teachers to make sure, that no arrow of time towards bad understanding is taking place.

A recommendation for physics lessons is to give entropy as much as significance as we do to energy. Next, we could accomplish enlightenment regarding energy consumption and energy devaluation, to be able to answer the question about the direction of time as well.

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For now this was the last article of my series about the topic time and entropy. I hope you enjoyed it, and it would be a pleasure for me to read some comments.

for much more to come ...