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# Physics/thermodynamics

Question
Sir,
How can I defined system, process, cycle and property as appied to thermodynamics.
Also, why is mechanical efficiency of internal combustiom engines less than unity.

You don't need me to define things, that's what a google search is for.  If you just search for "thermodynamic system definition," you'll get a lot of different definitions.  Pick one that makes the most sense to you.  The same for the other terms, I'm not going to rewrite what's out there (sounds like a homework assignment, anyhow, and I don't do those) when you could just google it at your leisure.

All heat engines have efficiency less than unity.  That's one of the laws of thermodynamics (specifically it is the Kelvin-Planck statement of the second law of thermodynamics).  See here:  http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html  Some of the heat energy must be exhausted to the cold reservoir (in this case, hot gas escaping from the tailpipe) in order for a heat engine to work.  The actual derivation of this law from statistical mechanics (microscopic view of thermodynamics) can be quite a bit more complicated, but the law itself is absolutely true.

Physics

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#### Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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