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Re: [Phys-L] ?conservation of _internal_ energy



In the context of:
https://www.av8n.com/physics/thermo/state-func.html#sec-internal-energy

On 01/14/2016 08:23 AM, Herbert Schulz wrote:
Yes, imagine a gas instead of the spring. As the gas expands it
pushes the piston and does work.

I say it does /not/ do work. No work is done by (or on) the blue
system. There is no F·dx: At the place where there is a nonzero
force, there is no displacement. At places where there is a
nonzero displacement, there is no force.

The Temperature of the gas will go
down and the Internal energy of the gas will go down. So yes, the
Internal energy is not constant. What is your argument that the
Internal energy SHOULD be constant?

I'm not arguing that it is constant or "SHOULD" be constant.
I'm arguing quite the opposite.

I suggest the discussion is worth having because this stuff is
evidently quite non-obvious. The wrong ideas are widely believed.
I cite Wikipedia, which is a cornucopia of things that are widely
believed without actually being true.
https://en.wikipedia.org/w/index.php?title=Internal_energy&oldid=697674365

If the containing walls pass neither matter nor energy, the system
is said to be isolated. Then its internal energy cannot change.

Furthermore, even if we replaced that by a similar-sounding
true statement, it would still be highly misleading, for
reasons discussed in the vicinity of
https://www.av8n.com/physics/thermo/state-func.html#tab-internal-energy

Just because some variable is constant in an isolated system
does not mean it is conserved.