- #1
wowowo2006
- 13
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Homework Statement
Hi everyone
In an isochoric process ( a solid heat body with T1 temp. connected with heat bath with T2 temp)
we could calculate its entropy change(heat body) from dS = Cv/T dT
and entropy change of heat bath from dS = dQ / T2, using dQ = Cv dT in 1st equation
( i read these from notes actually but i am confused with the reversibility in it)
Homework Equations
but I remember that we have proved before
dS >= dQ/T for all process, and equality hold when process is reversible(like isotherm and adiabats)
The Attempt at a Solution
Then i compute the entropy change of heat body in isochoric system = nR ln(T2/T1)
and entropy change of heat bath = - Cv(T2-T1)/T2
so total entropy is not zero when T2 and T1 has a finite change.
There comes my question, we find out that this isochoric process is irreversible
but when we use the equation dS = dQ/T, we could only use it in reversible process.
I try to solve this problem by constructing 1 isotherm and 1 adiabat to replace original isochoric path and find out that dS actually = dQ / T = Cv/T dT ,
although i know where this come from ( dS = dQ /T) from maths way
but I think i misunderstand some physical concept
can someone explain why dS = dQ/T , even when this process is not reversible in some physical way?