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2
votes
2answers
56 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 ...
1
vote
1answer
65 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
60 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 ...
0
votes
0answers
61 views

Heat engine efficiency, compression ratio vs temperatures

I want to understand the role or even existence of a cold reservoir in the Otto cycle gasoline engine. The efficiency of the Otto engine is given as a function of compression ratio $$ ...
0
votes
1answer
31 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 ...
1
vote
0answers
76 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
0answers
26 views

If maximal work is extracted, in what manner does the heat flow between bodies?

There are two bodies starting with different temperatures and specific heat capacities (for simplicity assume that the bodies are solid to the volumes are constant). Let the rate of heat flow from ...
0
votes
2answers
105 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 ...
0
votes
1answer
1k 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 ...
1
vote
1answer
573 views

Adiabatic proccess and Carnot cycle in a photon gas

I am making a comparation between the photon gas and the ideal classic gas for my Thermodynamics class. The photon gas is defined by the equations: $$U=aVT^4 $$ $$P=\dfrac{1}{3}aT^4$$ I found this ...
2
votes
1answer
215 views

Volume quotient in Carnot-cycle

Problem: One kilomole of an ideal, monatomic gas undergoes a reversible Carnot-processes between temperatures 300 °C and 20 °C. The work done during one cycle is 1500 kJ. a) Find the ...
1
vote
1answer
247 views

What is the importance of state functions in physics?

I'm currently reading about the Carnot cycle and its significance on the formulation of entropy (because I want to try to understand the concept better), but I can't seem to answer the following ...
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}\\ ...
1
vote
2answers
751 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 ...
3
votes
1answer
249 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 ...
2
votes
1answer
101 views

Where does the energy flow differ between a forward and reverse process?

Mathematically ($W=\int PdV$) and by the First Law, I understand that $1 \rightarrow 4 \rightarrow 3 \rightarrow 2 \rightarrow 1$ and $1 \rightarrow 2 \rightarrow 3 \rightarrow 4 \rightarrow 1$ are ...
2
votes
1answer
200 views

Are Carnot theorem and Carnot Cycle related?

This has been in my head for a while, and I think I finally have the answer. I was wondering what was so special about the Carnot cycle so it was used to demonstrate the Carnot theorem; the ...
6
votes
5answers
2k 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. ...
1
vote
2answers
225 views

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

In a Carnot cycle, the work potential is $W =Q_{in}( 1-\frac{T_0}{T_R})$ where $T_0$ is the temperature of the sink, $T_{R}$ is the temperature of the source heat reservoir, and $Q_{in}$ is the heat ...