The tag has no usage guidance.

learn more… | top users | synonyms

3
votes
5answers
153 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 ...
0
votes
1answer
36 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 ...
2
votes
3answers
72 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?
34
votes
4answers
5k views

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 ...
2
votes
2answers
361 views

Why can't the Carnot cycle be used to operate automobiles, if its the most efficient?

Since the cycle proposed by Carnot (consisting of two isothermal and two adiabatic processes) is the most efficient one on which to run a heat engine, why do we use other, less efficient ones (like ...
1
vote
2answers
200 views

Otto and Carnot efficiency comparison

Lets say both engines operate between same temperature limits. Carnot eff: $$ e_\text{Carnot} = 1-\frac{T_L}{T_H} $$ Otto eff: $$ e_\text{Otto} = 1-\frac{T_4-T_1}{T_3-T_2} $$ assuming ...
1
vote
4answers
1k views

Must a reversible engine be a carnot engine?

I have this homework question: "Show that any reversible engine operating between T1 and T2 is a carnot engine." I think I have a solution, but it feels very hand-wavy. We know that any process that ...
2
votes
0answers
189 views

Is Carnot engine the only reversible engine? [duplicate]

Is Carnot engine the only form of reversible engine? Is it possible to have a different form of reversible engine that goes through different processes? For a standard Otto cycle working with ideal ...
2
votes
3answers
177 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 ...
0
votes
0answers
22 views

Isochoric Kelvin Plank - Clausius Equivalence

As the Clausius statement is underpinned by the assumption that W=P.∆V Therefore ∆V=0 , W=0 Can anyone restate the Kelvin Plank-Clausius Equivalence Proof considering a solely Isochoric process ...
0
votes
2answers
55 views

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 ...
0
votes
1answer
58 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 ...
0
votes
3answers
66 views

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 ...
2
votes
2answers
205 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 ...
0
votes
1answer
32 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 ...
1
vote
1answer
29 views

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 ...
0
votes
0answers
30 views

Efficiency of a Carnot refrigerator without knowledge of the temperatures

We know that in a Carnot refrigerator we can express the efficiency as $$\eta = \dfrac{Q_1}{Q_2-Q_1}=\dfrac{T_1}{T_2-T_1},$$ where $T_1$ and $T_2$ are temperatures of the reservoirs between which ...
2
votes
0answers
58 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 ...
1
vote
1answer
66 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 ...
1
vote
1answer
80 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 ...
1
vote
1answer
63 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 ...
1
vote
1answer
36 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 ...
0
votes
1answer
47 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/...
0
votes
2answers
73 views

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/...
1
vote
0answers
40 views

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?
0
votes
2answers
66 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?
1
vote
0answers
41 views

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 ...
2
votes
3answers
2k views

Questions on Carnot's theorem

This article on Carnot's theorem states that All heat engines between two heat reservoirs are less efficient than a Carnot heat engine operating between the same reservoirs. However, it only ...
3
votes
3answers
176 views

Entropy of loops in the PV plane

The change in entropy of the Carnot and reversible cycles is said to be 0. Several other loops are supposed to have a non-negative change in entropy. This presents 2 problems which I cannot reconcile....
1
vote
0answers
86 views

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 ...
1
vote
0answers
31 views

In a heat pump, why is $Q_{h} - Q_{c} = W_{on}$?

I need help patching up the current holes in my understanding of a heat pump's process. I've explained my understanding and questions that accompany it. The standard explanation as to why $Q_{h} - ...
3
votes
0answers
680 views

Irreversible heat engines strictly less efficient than reversible ones

I understand how Carnot's theorem implies that irreversible heat engines must be no more efficient than reversible one's, but it is less clear why they need to be less efficient, as I have seen stated ...
1
vote
1answer
548 views

Ways to increase efficiency of carnot refrigerator and a four stroke engine? [closed]

What are ways to increase the efficiency of a Carnot refrigerator. I remember something about decreasing the temperature but also about something about the flow rate, and fins? Also, for four stroke ...
2
votes
1answer
86 views

Heating up water with a heat pump

Suppose we wish to heat 1 litre of 0°C water up to 50°C during wintertime by using an ideal heat pump (Carnot cycle in "reverse"). We will place this water amount in an initally empty container, in ...
0
votes
1answer
65 views

Proving the following cycle is equivalent to Carnot cycle

I want to investigate whether the following cycle I'm about to describe is equivalent to Carnot cycle or not. By equivalent I mean, giving two reservoirs($A$,$B$) with temperature $T_1$ and $T_2$ with ...
1
vote
1answer
175 views

How is isothermal expansion/compression physically possible when it assumes heat flow between equally hot bodies?

An ideal Carnot engine is composed of two reservoirs and a working fluid. The hot Reservoir and the cold one have temperatures $T_1$ and $T_2$ respectively, with $T_1>T_2$. The working fluid is in ...
2
votes
2answers
359 views

Carnot cycle: What happens to the rest of the energy?

In a Carnot cycle, the work potential is $$W =Q_\text{in} \left(1 -\frac{T_0}{T_\text{R}}\right)$$ where $T_0$ is the temperature of the sink, $T_\text{R}$ is the temperature of the source heat ...
8
votes
4answers
22k 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}\\ \...
0
votes
1answer
123 views

Understanding the Carnot cycle

My textbook explains the Carnot cycle as follows: Heat is added to the boiler, where the steam inside expands isothermally at high temperature (T(High))--(both valves closed). The intake valve is ...
1
vote
0answers
134 views

Entropy change in a reversible carnot cycle with 2 beakers

Consider 2 beakers each containing 1kg of water. One beaker has initial temperature of 25 degrees celsius and the other at 100 degrees celsius. The beaker are now mixed. Assuming no heat exchanged is ...
1
vote
1answer
115 views

COP of Stirling engine

If we consider the theoretical maximum of COP for a heat pump, we will see that it is possible to make it infinite by approaching the temperatures of each source to each other. Yet, in a real engine ...
4
votes
1answer
302 views

Calculation of the efficiency of a Carnot engine for photon gas

I am working on a problem considering the Carnot cycle for a photon gas. The cycle has two isothermic processes at $T_{H}$ and $T_{C}$ and two adiabatic processes. The entropy of the photon gas is $...
3
votes
2answers
378 views

Why won't the efficiency of a heat pump increase when $T_\mathrm{cold}$ decreases?

Why won't the efficiency of a heat pump increase when $T_\mathrm{cold}$ decreases? In the carnot cycle, efficiency increases when $T_\mathrm{cold}$ gets colder and $T_\mathrm{hot}$ gets hotter. Are ...
0
votes
1answer
15k views

Enthalpy Change in Reversible, Isothermal Expansion of Ideal Gas

For the reversible isothermal expansion of an ideal gas: $${∆H}={∆U}=0 \tag1$$ This is obvious for the case of internal energy because $${∆U} = \frac {3}{2} n R {∆T} = 0 \tag2$$ and $${∆U} = -C_P n {∆...
0
votes
1answer
177 views

How to define the thermodynamic temperature

I've been reading derivations of the thermodynamic temperature scale. I'm assuming these are using Kelvin's method. I follow the math and the conclusion of the argument, but I don't understand how it ...
1
vote
1answer
1k 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 ...
0
votes
1answer
178 views

Energy density and Carnot cycle

For example, diesel has about 40 MJ/kg, what does it mean? Is that if we burn it we will get 40 MJ of energy per kg, but we can not get all of it to mechanical work? If that is the case, then why can'...
6
votes
5answers
7k views

Can I take heat from the air and convert it to electricity?

Its a summer day and the air in my house has been heated up. I could switch on my air conditioning, but then I'd be using energy from the grid in order to reduce the amount of energy in my house. ...
0
votes
2answers
346 views

Inequality of Clausius

I'm having some trouble understanding the inequality of Clausius. My treatment of the inequality applies to an engine operating in a cycle between two temperatures (i.e. like a Carnot engine). The ...
3
votes
1answer
1k views

Net work output of an engine performing Carnot cycle?

My problem gives me a Carnot cycle heat engine with water as its working fluid, with $T_H$, $T_L$, and the fact that it starts from saturated liquid to saturated vapor in the heating process. I need ...