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Questions tagged [carnot-cycle]

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Why is the Peltier / Seebeck Effect's efficiency so low in practical devices?

Usually solid state devices are able to attain excellent efficiency. However, currently existing Peltier coolers and Seebeck generators have horrible efficiency -- far worse than Carnot, and also far ...
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Thermodynamics problems with a cycle (insufficient data?)

I am having some troubles in solving the following problem a professor gave us, and in my opinion I think that either there are insufficient data or the solutions are infinite. We have a perfect gas,...
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Why a reversible engine's efficiency must be equal to that of a Carnot engine

Apparently, in order to prove that an engine must be as efficient as a Carnot engine if reversible is because apparently for its efficiency in either direction: $\eta_E \le (1-T_2/T_1)$ $\eta_R \ge (...
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1answer
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Clausius's Carnot Cycle Equation Derivation

In his paper clausius describes a Carnot cycle and he continue arguing like below: In order to make an analytical application of the method...we will assume that all the changes which the gas ...
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Calculating efficiency of a Engine

Given are four points in a PV-diagram: $(V_1, 3/2P_1)$ is the leftmost point, $(V_2,3/2P_1)$ is the rightmost point, $(\frac{V_1+V_2}{2},P_1)$ is the lowest point and $(\frac{V_1+V_2}{2},2P_1)$ is the ...
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Why is there Net Work Done on a Pressure/Volume Cycle?

The area enclosed by the rectangle is said to be the Net Work Done. My question is why would there be any net workdone if it returns to its initial position (W). Shouldn't it act like a force-...
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1answer
35 views

Can you calculate the supplied heat given a p(V) - Diagram of a isobar process?

You are given an p(V) - Diagram like this: Source: How to calculate the efficiency from a $p$-$v$ diagram? There you see two isothermal processes and two isochor processes. For every given ...
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Why does there need to be an isothermal compression in a Carnot cycle?

This is the part that allows the Carnot engine to not violate the 2nd law of thermodynamics, but, hypothetically, why can't we just adiabatically compress the working substance to get it back to State ...
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1answer
134 views

Why is there a cold reservoir in a Carnot engine?

I'm trying to wrap my head around the idea behind a Carnot engine. The main question I'm trying to figure out is how a Carnot engine actually works in producing work. I'll provide a picture for ...
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1answer
34 views

Deriving the second law of thermodynamics from an irreversible carnot process

I have studied the ideal carnot cycle extensively where we assume that $$\Delta S_{\mathrm{total}}=\sum_i \frac{Q_i}{T_i} =0$$ Now I was wondering whether it is possible to derive basic properties ...
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1answer
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Limitations of converting pressure into kinetic energy

Bernoulli's principle states that an increase in the kinetic energy of a fluid occurs simultaneously with a decrease in pressure for isentropic processes. What then limits us from converting all of ...
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How do I prove that the Carnot cycle is the most efficient graphically? [closed]

I have to prove that the Carnot cycle is the most efficient by comparing it with a arbitrary cycle working within the temperature range of $T_L$ to $T_H$ in a ST-diagram. How do I do this?
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1answer
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Entropy generation in an endoreversible engine

An endoreversible engine is one that is internally reversible (i.e., a perfect Carnot engine) but the heat transfers occur across imperfect conductors (resistances), thus rendering the engine ...
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2answers
110 views

How is the work done by Carnot Engine obtained or converted to usable form?

I read about the Carnot Cycle and Carnot engine where it was said that the work done by this engine is equal to the area enclosed in the PV diagram. I understood the proof and everything but I am not ...
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1answer
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Efficiency of a Carnot machine, in which, some of its extreme points are in the region of liquid transition to steam

I need to calculate the efficiency of a Carnot machine, in which, some of its extreme points are in the region of liquid transition to steam with the works, differences of energy and heats of each ...
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1answer
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Carnot's Engine Proof flawed?

https://www.youtube.com/watch?v=LUoUb4hGMH8 I was looking at the reason as to why a Carnot Engine and watched this video on KhanAcademy. He used a very clever method of proving this by using the ...
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Thermodynamic cycles

Consider a system that takes excess air (assume it to be an ideal gas, $Cp = \frac{7}{2} R$) from a power plant stack at $370 \rm K$ (typo in figure) and $2$ bar and compresses it to 20 bar ...
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Does chaining Carnot heat engines make them more efficient?

Since the efficiency of a Carnot cycle depends on the difference between the hot and cold sides of the engine, could you put multiple heat engines in a series to maximize overall efficiency? A $3000 \,...
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Total Reversibility of Carnot Cycle

I was reading about Carnot cycle in "Thermodynamics by Cengel & Boles", now it is stated that Carnot cycle is a totally reversible cycle, i.e it is an internally as well as externally reversible ...
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322 views

Could a Carnot engine linked to a Carnot “refrigerator” create a perpetual heat flow between two reservoirs?

Here is a diagram of the system in question (adapted from the diagram on this page): All of the work produced by the Carnot engine is used to drive the Carnot refrigerator, which is simply a Carnot ...
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1answer
195 views

Actual sequence of refrigeration cycle

Whenever i have studied refrigeration cycle on books, i have found that the diagrams are always explained starting from the compressor*, then condenser then throttle and finally evaporator. I find it ...
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1answer
359 views

Validity of Carnot's theorem when operating between finite reservoirs

Carnot’s theorem tells us that "the most efficient heat engines operating between 2 reservoirs have the same efficiency as a Carnot engine". This theorem assumes that the engines are operating ...
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1answer
784 views

Reversible and irreversible cycles: Changes in entropy of the system and the surrounding

Entropy is a state function. Is it true that at the end of a cyclic process, the change in entropy of the of the system and that of its surrounding are both separately zero irrespective of whether ...
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3answers
133 views

Is there an implemented example of an almost reversible heat engine?

I'm reading Feynman's discussion of Reversible engines (Vol I 44.3) in The Feynman Lectures on Physics. His discussion is very abstract, and leaves a lot of practical questions unanswered. Rather ...
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2answers
201 views

Heat transfer in the isothermal expansion in the Carnot cycle

Problem In the Carnot cycle there are two adiabatic and two isothermal processes. This question focuses on the isothermal expansion. In the isothermal expansion books often writes that the system is ...
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1answer
252 views

Why does the Carnot cycle require adiabatic processes specifically?

Why does the Carnot cycle need two isentropic adiabatic processes to move between hot and cold reservoirs? Surely as the only requirement for the Carnot cycle is that it is constructed out entirely ...
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1answer
304 views

Carnot Engine - Maximum Efficiency Proof?

As an introductory physics student (independently studying before my return to official classes this summer), I am given a "proof" that the Carnot engine has the maximum efficiency possible by way of ...
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1answer
179 views

How Much Heat Is Absorbed/Rejected By The Heat Engine?

If a heat engine undergoes a reversible cycle consisting of two isobaric processes and two adiabatic processes, what methodology is used to determine how much heat is absorbed or rejected by the heat ...
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1answer
334 views

How do we know that entropy is a state function given by $\delta Q_{\mathrm{rev}}/T$ for any arbitrary (not just reversible process)?

In this video, the professor goes through a "derivation" for entropy using the Carnot cycle, in which he establishes that entropy is a state function for that process. However, he then continues the ...
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2answers
671 views

Prove carnot engine is most efficient

According to Carnot's theorem, Carnot engine is the most efficient engine. I wanted to know its proof. I looked up and I found a proof on Khan Academy in which he used a reverse carnot engine to prove ...
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2answers
648 views

Why is the entropy of a Carnot engine always constant?

I am 99% sure that I am wrong here, but here is my reasoning. The efficiency of a Carnot engine can be written as: $$ \eta_{carnot} = 1 - \frac{T_f}{T_i} $$ But since Effency's definition says $\...
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0answers
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Why can't a Carnot engine heat sink be at absolute zero? [duplicate]

We know that for a Carnot engine the efficiency is given by $1-(T_2/T_1)$, but why can't a real engine have $T_2=0\,{\rm K}$? I know that the efficiency cannot be $1$, but why can't we make the heat ...
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335 views

Is the efficiency of a reversible heat engine independent of the processes involved?

Does the following question conflict that fact that efficiency of a reversible engine is independent of processes involved?
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201 views

What makes a process reversible?

I just want someone to verify what I think. I originally thought that for a reversible process to occur, no irreversibilities like friction can occur. Nevertheless, if we consider the Carnot ...
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224 views

Why doesn't the heat in the two isochoric processes count towards efficiency calculation in Stirling cycle?

I'm trying to understand Stirling Cycle that consists of two isochoric processes and two isothermal processes. The heat transfer are given as $$Q_1 = \nu RT_1 \mathrm{ln} \frac{V_1}{V_2}$$ $$Q_2 = ...
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1answer
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Carnot cycle restoration of original system state

In the Carnot cycle the original state is restored so there is no overall change in entropy. Why is it not possible to accomplish this simply with two reversible isothermal steps, the first an ...
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1answer
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Derive the Carnot Efficiency without using the Carnot Cycle? [closed]

How can you derive the Carnot efficiency using only properties of reversible cycles?
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Is there only one Carnot cycle for every thermodynamic system?

So, Carnot engine is THE reversible engine operating on two reservoirs. No other engine can beat its efficiency on two reservoirs (eg. Otto cycle is not Carnot as it requires infinite number of ...
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Are there actual technical realizations of the Carnot cycle?

The Carnot cycle is composed of four steps: isothermal expansion, adiabatic expansion, isothermal compression and adiabatic compression. In the case of combustion engines, we have the idealized Otto ...
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Is my interpretation of how a Carnot Cycle works correct?

I'm trying to understand how the Carnot cycle works in my learning of it. Here's what I so far believe is what is happening: (A $\to$ B) A cylinder of gas is connected to a frictionless piston, and ...
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1answer
677 views

Does Piston moves faster when we accelerate the car?

I am learning about car basics. I found that engine is nothing but a system where heat energy is converted into up/down motion of the piston in the cylinder. Finally, this up/down motion is converted ...
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About the 2nd Carnot Corollary

I have a question about the 2nd Carnot Corollary. According to Moran, Fundamentals of Engineering Thermodynamics, the 2nd Carnot Corollary stated that "all reversible power cycles operating between ...
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1answer
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Does a Carnot-efficient heat engine recover all work that was initially done?

There are two adiabatic containers filled with an ideal gas. In container A work is done which eventually turns completely into heat, leading to a temperature difference between container A and ...
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Cascade Carnot cycle

In a Carnot cycle about 50% of the energy is converted in work whereas the other is dissipated in heat to the heat sources. Is it possible to reuse the heat in another cycle to reconvert 50% of it ...
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Confusion about Clausius inequality

I am trying to learn the derivation of Clausius inequality in the following manner. Consider the following diagram "Reversible cyclic device" is working in Carnot's cycle. The combined system gets ...
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1answer
85 views

Reverse the combustion engine

All you know what a combustion engine is doing: Transform chemical energy into kinetic energy. What about the opposite? I know when I park a car at a slope and lose the breaks it will drive ...
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1answer
99 views

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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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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340 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 ...