2
votes
1answer
57 views

Why can we say that $\bar{d}Q=TdS$?

When we introduce entropy we do this by saying that: $$\bar{d}Q=TdS.$$ Now I was wondering why this should be true? I know that by looking at a Carnot cycle, we do get this relation for reversible ...
0
votes
1answer
35 views

Isentropic process and reversibility [duplicate]

I know that every adiabatic reversible process is an isentropic process. Can a process be isentropic but still not reversible adiabatic? Please provide me some examples.
0
votes
1answer
39 views

What is the difference between reversible and irreversible adiabatic expansion?

What is the difference between reversible and irreversible adiabatic expansion? Is it true that the work done by the gas is the same but the pressure applied externally differ between two process? ...
4
votes
2answers
160 views

Difference between irreversible and entropy?

Cedric Villani recently wrote an article on Landau damping, where at least one topic discussed confused me. Besides discussing the issue of how a process can be microscopically reversible and ...
3
votes
3answers
82 views

Definition of Entropy in thermodynamics

In most textbooks, the definition of entropy in reversible processes on a system $S$ is given simply as $$d S=\delta Q/T.$$ It seems to me this definition is insufficient since it does not specify ...
0
votes
1answer
60 views

Efficiency of reversible engines

I'm a little confused about something. All reversible engines have the same efficiency, or one could drive the other to move more heat in the reverse direction. Also, no engine has an efficiency ...
1
vote
0answers
34 views

Gibbs equation and reversibility [duplicate]

The equation $$TdS=dU+PdV$$ is a combination of $$dQ=dU+PdV$$ and $$TdS=dQ.$$ But in some process that is very irreversible, $TdS>dQ$, should we write $$TdS>dU+PdV$$ instead of the first ...
3
votes
0answers
20 views

Is there an alternative metric for isentropic efficiency that remains valid when broken up into multiple segments?

Say that we have an irreversible expansion process which extracts energy, like a turbine. Isentropic efficiency is commonly defined by the following relation, which applies in a similar fashion for ...
1
vote
0answers
70 views

Irreversible heat engines strictly less efficient than reversible ones

I undertand how Canot's theorem implies that irreversible heat engines must be no more efficient than reversible one's, but it isless clear why they need to be less efficient, as I have seen stated in ...
0
votes
2answers
63 views

Not Quasi-static, yet reversible process? Is this a valid example?

Imagine a gas (at room temp. and pressure) enclosed in a thermally insulating spherical container. At some instant, the container instantly expands symmetrically (radially outward) to, say, twenty ...
10
votes
1answer
226 views

Principle of Caratheodory and The Second Law of Thermodynamics

Background Constantin Carathéodory formulated thermodynamics on a purely mathematical axiomatic foundation. His statement of the second law is known as the Principle of Carathéodory, which may be ...
0
votes
1answer
106 views

Do reversible processes exist in Nature?

Classical thermodynamics always discusses entropy in the light of reversible processes, and it lies at the heart of the definition of entropy. But do these reversible processes exist in Nature, or are ...
4
votes
2answers
106 views

Why is adiabatic process isentropic?

I have read that adiabatic process is isentropic because there is no heat exchange in an adiabatic process and thus no change in entropy. But my question is - Even in adiabatic process, work can be ...
1
vote
0answers
38 views

Restrained expansion of a piston/cylinder

Suppose an ideal gas in a piston cylinder has some initial pressure $p_1$, volume $V_1$, and temperature $T_1$. A pin, which holds the piston in place, is suddenly removed, and the gas quickly ...
6
votes
5answers
982 views

How is this process not quasi-static yet reversible?

Consider a (adiabatic) canister with a piston containing some gas kept in a vacuum. There are two weights on the canister which equalize the pressure of the gas on the piston. Assume the system is at ...
0
votes
1answer
82 views

Relation between a Quasistatic and a reversible process

Why is it that if a process is reversible, it is quasi-static? Does it mean that then the process is also non-dissipative if it is quasistatic?
5
votes
3answers
326 views

Is thermodynamic reversibility a function of path?

Question: given a path taken by a system through state space, is it possible to make a statement such as 'that path corresponds to an irreversible process' or 'that path corresponds to a reversible ...
3
votes
1answer
149 views

Hysteresis and dissipation

Hysteretic phenomena are often linked to dissipation. When there is an hysteresis loop, the dissipated energy can usually be computed as the area of the cycle. For example, in ferromagnetic ...
0
votes
1answer
128 views

How do you define a reversible path for general processes?

The equation $dS = \frac{\delta Q}{T}$ is only defined for a reversible path. Given a irreversible path we typically calculate the entropy by choosing a reversible path from the same initial to final ...
4
votes
1answer
394 views

Why is it that a Carnot heat engine will reject no heat to a zero temperature sink?

Looking at the equation for Carnot efficiency, I notice that as the temperature of the heat sink approaches zero, the efficiency approaches unity: $$ \eta_{rev} = 1 - \frac{0}{T_H} = 1 $$ Seeing ...
3
votes
2answers
3k views

Are reversible adiabatic processes always isentropic?

If my understanding is correct, neither reversible nor adiabatic processes are necessarily isentropic. But are reversible adiabatic processes always isentropic?
0
votes
3answers
220 views

Why does time reversibility imply equilibrium in a thermodynamic system? [duplicate]

I'm reading through Kardar's Statistical Mechanics of Particles; in the section 1.5 he says: A reversible process is one that can be run backward in time by simply reversing its inputs and ...
4
votes
2answers
4k views

Efficiency of Stirling engine and Carnot's theorem

I want to calculate the efficiency of this Stirling cycle for an ideal gas $pV = nRT$ The mechanical work is $$ \Delta W_{12} = - \int_{V_1}^{V_2} p(V) \mathrm{d}V = -nRT_2 \ln \frac{V_2}{V_1}\\ ...
4
votes
1answer
210 views

A thermodynamic transformation that can be represented by a continuous quasistatic path in its state space may still be irreversible. Why?

A thermodynamic transformation that has a path (in its state space) that lies on the surface of its equation of state (e.g., $PV=NkT$) is always reversible (right?). However, if the path is a ...
0
votes
2answers
3k views

Isothermal Gas Expansion, a Reversible or Irreversible process?

Consider the following situation. A certain quantity of ideal monatomic gas (say one mole) is confined in a cylinder by a piston and is maintained at constant temperature (say $T_0$) by thermal ...
0
votes
2answers
147 views

What is evidence for an irreversible change?

Knowing some about thermodynamics and reactions, I do understand how it can be shown that a change is reversible. But irreversible? Why can't it be that a change that was deemed irreversible thousands ...
8
votes
3answers
464 views

Thermodynamically reversed black holes, firewalls, Casimir effect, null energy condition violations

Scott Aaronson asked a very deep question at Hawking radiation and reversibility about what happens if black hole evolution is reversed thermodynamically. Most of the commenters missed his point ...
6
votes
3answers
3k views

Why do reversible processes not increase the entropy of the universe infinitesimally?

The book Commonly Asked Questions in Thermodynamics states: When we refer to the passage of the system through a sequence of internal equilibrium states without the establishment of equilibrium ...
3
votes
1answer
502 views

Why is the maximum work achieved in reversible processes?

Let us consider an ideal gas. Let it be present initially in a state $(p_1,v_1,t_1)$. Now let it be driven to another state $(p_2,v_2,t_2)$. Why is it so that during this process the maximum work can ...
13
votes
4answers
1k views

How slow is a reversible adiabatic expansion of an ideal gas?

A truly reversible thermodynamic process needs to be infinitesimally displaced from equilibrium at all times and therefore takes infinite time to complete. However, if I execute the process slowly, I ...
1
vote
4answers
416 views

Irreversible expansion and time reversal symmetry

Suppose there are N non-interacting classical particles in a box, so their state can be described by the $\{\mathbf{x}_i(t), \mathbf{p}_i(t) \}$. If the particles are initially at the left of the box, ...
9
votes
5answers
567 views

Intuitively, why is a reversible process one in which the system is always at equilibrium?

A process is reversible if and only if it's always at equilibrium during the process. Why? I have heard several specific example of this, such as adding weight gradually to a piston to compress the ...