Re: Thermo 101 - 2nd Law Clarification

renai wrote:
I've got what is most likely a very simple thermo 101 question or
misunderstanding that I'm hoping someone can clear up regarding the
second law. I do understand the idea that when anything happens a
portion of that activity goes to heat and that it is irreversible. I
understand the idea of heat flowing from a hot reservoir to a system
which can only convert part of that heat into work it will do on its
surroundings the rest of which will be lost as heat to a colder
reservoir, i.e. no engine is 100% efficient. Then I see the equation
that says that change in entropy is equal to the heat content of a
system divided by the temperature of the system at equilibrium. But I
don't completely understand how it corresponds to the example that I
did understand above. Is this for an isolated system (i.e. no heat
transfer). If so, does that mean that the delta Q that I see in the
equation is simply the internal kinetic energy of the system.
Furthermore, if temperature is basically average internal energy, then
isn't the entropy equation effectively dividing internal kinetic energy
by internal kinetic energy? Lastly I keep getting confused about the
symbol Q. I though Q was the heat flowing in or out of a system. But
if a system is isolated there is no heat flow so why do they use Q in
the entropy equation

Any help or guidance would be much appreciated.

Sincerely,

Renai


Maybe a sample problem will help?

Two blocks with initial temperatures T1 and T2, and heat capacities C1 and C2, are thermally connected and come to a final temperature Tf. What is the change of entropy of block 1, block 2, and the two-block system?

dS = dQ/T, dQ = C dT

dS = C dT/T

Integrate both sides,

delta S1 = C1 ln(Tf/T1)

delta S2 = C2 ln(Tf/T2)

delta S = delta S1 + delta S2

All of thermodynamics is nominally for an isolated system. But sometimes that means "the rest of the universe" is a reservoir. Draw a boundary around your system of interest and track heat, work, entropy, etc. coming in and going out. If heat goes out, it will carry entropy away with it. The heat and entropy will go somewhere and the laws of thermodynamics will be satisfied, but you don't always care where they go. E.g. if you're analyzing a jet engine you don't really care what happens to the exhaust once it's clear of the nozzle.
.

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