Question #6d6f8

1 Answer
Aug 11, 2017

Answer:

Well, in terms of Thermal Engines, you cannot even reach #100%#!!!

Explanation:

The reason for this impossibility is a bit complicated and you can find it well explained in many textbooks on thermodynamics.
Here we can say that (again) in terms of a thermal engine the possibility of reaching #100%# efficiency would mean that all the energy/heat absorbed from a reservoir could be converted entirely into work without rejecting even the smallest amount of energy in the environment. Although difficult to "see" this kind of situation (if possible) would produce a system that "self-feed" itself producing a system where energy is in a kind of loop (energy in -> work out to feed another engine that restore energy into the initial reservoir and the process continues without interruption and losses).

Intuitively this cannot be: if you consider a steam engine..well...the smoke has to go somewhere! There must be a tube, an aperture or something to let it go out! The system cannot be closed.

A way to see this impossibility is also to consider Ideal Engines or Carnot Engines; they are ideal meaning NOT that they are #100%# efficient; instead they have the best possible efficiency when operating in between two given temperatures.
For these kind of engines the efficiency is:
#eta=1-T_"cold"/T_"hot"#
This efficiency could be #100%# if we had #T_"cold"=0K# but....we never got this cold!

Now a bit that it is even weirder....energy degrades in quality when we use it! So when we use energy to do work we reject energy that has a low "quality" meaning that we cannot use it again in an efficient way. Again, the smoke from our steam engine; it is hot, it is a gas....we could use it as a reservoir of energy for an engine, BUT it has a low quaity compared to the energy it came from (stored in coal and freed during combustion).