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Questions tagged [carnot-cycle]

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Carnot cycle with varying reservoirs

I was thinking what will happen to the standard Carnot cycle, if instead of - say - the usual cold reservoir, we adopt a smaller refrigerator, whose temperature in fact changes after the heat release ...
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Efficiency of a thermodynamic cycle

We are given a thermodynamic cycle of an ideal gas (but of rigid spheres) composed of a single substance which undergoes a Carnot cycle illustrated below. All processes are assumed to be reversible. ...
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345 views

Irreversible processes don't account in entropy calculations?

Pick a Carnot Cycle (being $T_1<T_2$), it is reversible, therefore $\Delta S_{univ, cycle}=0$. The same result is obtained via the sum of all entropies associated with its transformation, which ...
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3answers
363 views

Where does the energy come from for adiabatic processes of a Carnot cycle?

I'm getting stuck on where this additional energy comes to move the piston further outward/inward during adiabatic processes when the only energy changed is during isothermal processes. Can someone ...
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357 views

Is existence of incompressible bodies prohibited by Carnot's theorem and Clausius' inequality?

Consider an absolutely incompressible body, which does not change its volume $V$ under pressure $p$ but can expand or shrink when its temperature $T$ changes. Then its equation of state has a simple ...
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2answers
257 views

Is “entropy” not a state variable for irreversible process?

Any reversible process can be described as a sum of many infinitesimally small Carnot cycles, so $\oint {dS} = \oint {\frac{{dQ}}{T}} = 0 % MathType!MTEF!2!1!+- % ...
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1answer
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Confusion about forces in the work done on the piston by the gas in Carnot cycle

I was drawing a free body diagram of the 3rd step of the Carnot cycle as shown below. The forces with - sign are reaction forces to the forces without the sign, according to Newton's 3rd law of ...
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How to prove that an arbitrary cycle in the PV diagram has an efficiency less or equal than Carnot's Cycle? [duplicate]

The statement is: Consider an ideal gas with constant number of particles (N is constant), and an arbitrary thermodynamic cycle (closed curve with no self-interesections) in the PV diagram. Show that ...
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1answer
3k views

Calculating the maximum efficiency of an engine cycle

I'm a little confused about calculating the maximum efficiency of an engine cycle. From what I've learned the carnot cycle is connected by two adiabatic processes and two isothermal, and this is the ...
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1answer
141 views

Why does a system expand isothermally?

Considering the first step of the Carnot process, heat is transferred from a bath to the system with both at the same temperature. But how does this process start? Why should the system spontaneously ...
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71 views

Müser engine and solar conversion efficiency

I found in many reference a derivation of the maximal conversion efficiency from solar power to work with the so-called that I found dubious (see for instance here). It usually goes like this. Let us ...
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1answer
501 views

Is Carnot's theorem equivalent to the Second Law of Thermodynamics?

In thermodynamics we have Carnot's Theorem which states that the efficency of any heat engine wich works with only two temperature's source is less or equal to Carnot's engine which works between the ...
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Distributed Workload AS cooling?

To my understanding, many processes require cooling to achieve optimum use. Power stations, for example, require cooling for electricity production. Computer hardware systems, similarly require ...
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1answer
315 views

optimality of the Carnot cycle

It is well-known and is stated always and everywhere that the Carnot cycle is the most efficient thermodynamic cycle. Although stated as such it is rarely proven. One proof is in the article "Engine ...
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997 views

Why is the efficiency of a carnot engine maximum?

I know carnot engine consists of isothermal and adiabatic process. My question is why the efficiency of carnot engine is maximum? Why not it is maximum with other processes. In one book I read ...
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1answer
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Understanding why $\int_{T_1}^{T_2}C_Vd\ln(T) + \int_{V_1}^{V_2} nRd\ln(V) = 0$ is violated when $T_2$ and $V_2$ are picked

In Chapter 4 of Chemical Thermodynamics, by Peter A. Rock published by Oxford University Press, 1983, the author is trying to explain why a new state function is needed, specifically a state function ...
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Difference between reversible and irreversible adiabatic process in PV diagram

let's say we've got a carnot-cycle in a pressure volume diagram with the following processes: 1 -> 2: reversible isothermal 2 -> 3: reversible adiabatic 3 -> 4: reversible isothermal 4 -> 1: ...
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Why is the cyclic volume integral of $P/T$ larger than the one of $P_ext/T$ in an irreversible process?

In order to derive the Clasius inequality we can call the first law of thermodynamics in it's differential form: $$dU={\itđ}q-P_{ext}dV.$$ If the process is reversible: $$dU={\itđ}q_{rev}-PdV.$$ ...
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Why can't we make Carnot heat engine in real life?

Question is obvious: Why can't we make Carnot heat engine in real life? I read Wikipedia and Fundamentals of Physics (Halliday) but I didn't found anything about my question. there we're explanations ...
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1answer
163 views

Question about air-conditioning cycle

All explanations of the air-conditioning cycle are somewhat like this: https://www.swtc.edu/ag_power/air_conditioning/lecture/basic_cycle.htm My question is... how do we ensure the refrigerant is in ...
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83 views

Carnot cycle modelization

In some sources, there's a slight difference in the Carnot cycle stages description. Let's take the "first" - the isothermal expansion. Some say that as a consequence of the contact between the ...
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2answers
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How to make sense of paradoxical heat pump?

Here is the paradoxical situation. Suppose you have two objects at temperatures $T_1$ and $T_2$, where $T_2>T_1$. Also, suppose you have some work at your disposal to power a heat pump whose ...
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1answer
3k views

Final temperature after maximum work output?

Two simple identical blocks with heat capacity $C$ are at temperatures $T_2$ and $T_1$ respectively, with $T_2>T_1$. What is the final temperature of the two blocks if the maximum possible work is ...
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The Carnot engine and entropy?

The Clausius statement of the second law of thermodynamics is as given below: "Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the ...
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Why doesn't the water cycle stop, since it dissipates energy?

In artificial cycles like for example refrigeration cycle, because of irreversibilities extra energy is require to be provided so that the refrigerant is available at the same state as it was at the ...
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2answers
446 views

How is Carnot's theorem valid also for engines that are not heat engines?

I understood the statement and the proof of the Carnot theorem. Carnot's theorem states: All heat engines between two heat reservoirs are less efficient than a Carnot heat engine operating ...
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1answer
549 views

How do you calculate the efficiency of a steam turbine?

Yesterday I saw a comment on an article that claimed a turbine with a steam temperature of 500° F and a room temperature 70 ° F couldn't possibly have an efficiency higher than 45%. He used the carnot ...
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1answer
71 views

What does the ratio of reservoir temperatures have to do with the efficiency of a heat engine?

I am having a conceptual difficulty with heat engine efficiency. I do not understand why having a larger difference in temperature between the hot and cold reservoirs have an effect on the thermal ...
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1answer
774 views

Application of Carnot's theorem to calculate efficiency of thermal engine

I understood Carnot's theorem in theory, but I still have some doubt on the practical use of it. In particular I would like to know: if I have any reversible cycle followed by a gas, then the thermal ...
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5answers
917 views

Does the thermal efficiency depend on the reversibility of the engine when isochoric or isobaric processes are involved?

I'm confused on the efficiency of a thermal engine (with an ideal gas) in the case in which it is reversible or not reversible, in particular where the ideal gas follows isochoric or isobaric ...
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3answers
1k views

Heat Pumps and COP?

Heat Pump COP (Coefficient of Performance) $$COP=1-\frac{T_c}{T_H}$$ With Heat Pumps is the efficiency/COP more dependent on the hot or the cold reservoir and why?
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4answers
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Why is the efficiency of human cells less than the efficiency of an Otto engine?

I always used to think (I don’t know why!) that the efficiency of human (and animal and plant) cells should be equal to or near the efficiency of a Carnot engine or at least should be the highest ...
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1answer
212 views

Entropy and Clausius inequality

From the Clausius inequality we can derive that the efficiency of a Carnot (reversible) cycle is given by: $$e= 1 - \frac{T_c}{T_h}$$ Is this true for every reversible cycle? Is the efficiency of all ...
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4answers
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2nd law of thermodynamics' violation?

According to 2nd law of thermodynamics, it is not possible to convert heat 100% into work. Well, investigating about the Carnot Cycle, I have found out that in the first step, the process is to heat ...
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2answers
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Carnot engine basics?

A line in my textbook says.. ' if we employ any other process that is not adiabatic, say an isochoric process, to take the system from one temperature to another, we shall need a series of ...
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1answer
50 views

A system with temp T is in contact with a reservoir of temp T. Would the system ever take in energy and expand (do work) in this situation?

The context is explain a Carnot cycle and how to get work from a system. Its starts by saying the system is in equilibrium with its surroundings, but then it take in some energy Qin. How does ...
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2answers
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Example of a Carnot machine made of a different physical system than a ideal gas?

Anybody knows an example of a Carnot machine made with any different thing than a gas? For example wire or a magnet. I was wondering that since I read the Kardar's book on Statistical Mechanics. He ...
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0answers
544 views

Carnot Engine- Cold Reservoir is a black body [closed]

My question: A satellite powered by a Carnot engine uses heat from a nuclear reactor at a fixed temperature T0. Heat is released into outer space via thermal radiation emitted by a set of fins at ...
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1answer
497 views

Why is entropy of system same for reversible and irreversible processes? [closed]

I read that entropy change of universe is zero in a reversible process but positive in a irreversible process,then doesn't it mean that entropy change of system of both the processes must be ...
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1answer
3k views

Relation between Pressure and volume in adiabatic process?

Why in adiabatic compression and expansion small volumetric change occurs while in isothermic compression or expansion very small pressure is applied and volume changes very significantly i am ...
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1answer
826 views

Adiabatic compression and expansion and isothermal expansion? [closed]

Please help me in understanding the concepts Why in adiabatic compression and expansion small volumetric change occurs while in isothermic compression or expansion very small pressure is applied and ...
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1answer
402 views

Plotting isothermals and adiabats

first time poster in this site. I am trying to create a small program to make a graph of the Carnot cycle. For isothermals I am simply trying to look for points where P*V would give the same ...
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1answer
94 views

Is there any difference between heat and entropy here?

This screenshot is from the Carnot Cycle page on Wikipedia. In the "Stages" section, it says that: The gas expansion is propelled by absorption of heat energy, $Q_1$, and of entropy, $\Delta S=Q_1/...
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2answers
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Reversibility and isothermal processes

I would like to know why any reversible process has to be isothermal, according to my sources. Why can´t we consider adiabatic processes to be reversible? EDITED https://www.ohio.edu/mechanical/...
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Carnot cycle in steam processes

Our professor told us that in steam processes, such as Clausis-Rankine process, Carnot's cycle is impossible to achieve. Can someone explain why?
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3answers
4k views

Why is the net entropy change of an irreversible engine positive?

In a Carnot engine the net entropy changein a cycle is zero. But in an irreversible engine operating between two temperatures the net entropy change in a cycle is positive. As I have understood, this ...
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1answer
321 views

Carnot cycle equation

How to prove the relation $\frac{Q_{given\space out}}{Q_{absorbed}}=\frac{T_{cold}}{T_{hot}}$ by only writing equations using first law?
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Self-powered heat exchanger w/energy profit? [closed]

It is possible, considering the laws of physics, to build a heat exchanger that runs on the power it gathers from the environment? A typical heat exchanger uses less power to harvest heat for a ...
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2answers
460 views

Why doesn't the separation of particles by mass via gravity decrease entropy?

Let's say we've got a fluid of heavy and light particles inside a cubical flask, which is initially shaken up so that the density of heavy particles is uniform everywhere. Let's also say that these ...
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Carnot cycle's paradox

Carnot cycle is a reversible cycle. By definition a reversible cycle must have an infinitesimal temperature difference. That means ($T_{high}-T_{low}$) is very very small. Any finite temperature ...