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Re: [Phys-L] let's define energy

On 09/26/2015 07:29 AM, Diego Saravia wrote:

if you dont have an adiabatic path, how can you stablish internal
energy?

Let's consider an example: Consider a non-reversible
heat engine. When we transfer energy energy into the
working fluid, rather than doing it isothermally and
reversibly, we do it very irreversibly, using a resistor
in the fluid that gets super-hot. We still know exactly
how much energy was transferred, using the usual formulas
for power dissipated by a resistor.

More generally, keeping track of the energy is the easy
part. Conservation of energy is a powerful tool.

There are plenty of situations (especially dissipative
situations) where it doesn't make sense to talk about
work and heat, but AFAICT it always makes sense to talk
about energy and entropy.

using an adiabatic wall is circular way of defining heat, because is the
way you say "not heat", but I do not see other way.

1) I don't see much value in circular definitions.

2) Very often I find it not worth the trouble to define
"heat" at all. Thermodynamics, despite its name, does
not really revolve around heat.

Just as the idea of phlogiston died out and was replaced
by /two/ ideas (energy and oxygen), heat needs to be
replaced by /two/ ideas (energy and entropy).

https://www.av8n.com/physics/thermo/heat.html