50 votes
Accepted

Why is the efficiency of human cells less than the efficiency of an Otto engine?

Can we compare alive cells with heat engines at all? No, not really, because the living being isn't only a heat engine. There are three main points I want to make here. 1. Homeostasis Requires ...
Selene Routley's user avatar
17 votes
Accepted

Does chaining Carnot heat engines make them more efficient?

If you take out all the heat you put into the intermediate reservoirs, so that heat only flows on net from the hottest to the coldest, then it doesn’t make any difference. That is, the effect of the ...
knzhou's user avatar
  • 102k
16 votes
Accepted

Does isothermal expansion not beat the efficiency of Carnot Heat Engine?

You have to distinguish between the efficiency of a process and the efficiency of a cycle. A reversible isothermal expansion process can be 100% efficient. That does not violate the second law. It’s ...
Bob D's user avatar
  • 71.8k
15 votes

Why doesn't the water cycle stop, since it dissipates energy?

The extra energy comes from the Sun. The energy in the sunlight hitting the Earth evaporates the water, then when the water cools the latent heat goes into heating the the Earth and its atmosphere ...
John Rennie's user avatar
14 votes

Why is the efficiency of human cells less than the efficiency of an Otto engine?

Why is the efficiency of human cells less than efficiency of an Otto engine? It's not. You are comparing two very different things. The low value of 18 to 26% efficiency you found for the human body ...
David Hammen's user avatar
  • 41.4k
11 votes
Accepted

In a Carnot Engine, how does the heat flow from the heat reservoir to the engine if both are at the same temperature?

Allowing the gas to expand a tiny amount (and do a small amount of work on the surroundings) causes the gas temperature to drop a tiny amount relative to the hot reservoir. This then provides the ...
Chet Miller's user avatar
  • 33.6k
10 votes
Accepted

2nd law of thermodynamics' violation?

It is possible to convert heat 100% into work provided that something else happens to your heat machine (i.e. the heat-to-work conversion system). In your case, the gas has expanded, so the state of ...
Leon's user avatar
  • 216
10 votes
Accepted

How to derive the Clausius Inequality?

The inequality for irreversible cycles follows from Carnot's theorem which states that no cyclic process can be more efficient than a Carnot cycle. The basic idea for deriving the equality for ...
Er Jio's user avatar
  • 1,231
9 votes

Optimality of the Carnot cycle

I'm not sure why you say that Carnot's theorem is "rarely proven". Such proofs are given as a matter of course in any undergraduate textbook on classical thermodynamics, and lots and lots of places on ...
Pirx's user avatar
  • 3,763
9 votes

Does chaining Carnot heat engines make them more efficient?

This is common practice in heat engines. For example, in large reciprocating steam engines, you'll have three pistons operating in series: a small, high-pressure piston, a medium-size mid-pressure ...
niels nielsen's user avatar
9 votes
Accepted

Why does there need to be an isothermal compression in a Carnot cycle?

Because what you propose is impossible. You are essentially trying to make a cycle out of only these three steps: 1) Isothermal expansion (A to B) 2) Adiabatic expansion (B to C) 3) Adiabatic ...
BioPhysicist's user avatar
  • 56.4k
9 votes

Generalizing Carnot theorem

The Carnot theorem is very general but we must understand exactly what it means. There is a narrow reading and a broad reading of the theorem: Narrow Reading of Carnot Theorem Think of a temperature ...
Themis's user avatar
  • 5,853
8 votes

Why can't we make Carnot heat engine in real life?

A Carnot engine has to be perfectly reversible. This means zero friction, and perfect thermal conductivity between reservoirs*. In practice neither of these things are possible so you will only ever ...
Floris's user avatar
  • 119k
7 votes
Accepted

How does the Carnot Cycle work?

The typical depiction of the Carnot Cycle is with the use of a closed cylinder containing an ideal gas fitted with a piston with a shaft that extends outside the cylinder to interact with the ...
Bob D's user avatar
  • 71.8k
7 votes

Does the Carnot efficiency apply to photoelectric conversion?

Prism or diffraction grating does not separate the frequencies into parallel beams of rays falling on the same area as before. It increases beam divergence, which increases the area needed to capture ...
Ján Lalinský's user avatar
6 votes
Accepted

Final temperature after maximum work output?

Let $dQ_1$ = heat transferred to hot reservoir = $CdT_1$ Let $dQ_2$ = heat transferred to cold reservoir = $CdT_2$ For the sum of the entropy changes of the system and the surroundings to be zero, ...
Chet Miller's user avatar
  • 33.6k
6 votes
Accepted

Carnot's Engine Proof flawed?

Your flaw is in assuming that you can run an arbitrary engine backwards. In order to reverse the operation of a heat engine the heat engine must be reversible, which the Carnot engine is. The proof ...
By Symmetry's user avatar
  • 9,111
6 votes
Accepted

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

Here is why. To be efficient in this context requires two contradictory properties: 1) that the thermoelectric material have low thermal conductivity (so the temperature difference across the ...
niels nielsen's user avatar
6 votes
Accepted

Efficiency of heat engines

Assuming you mean maximum efficiency when you say maximum energy, then the proof is by contradiction. Essentially, in order for a heat engine to be more efficient than a Carnot engine it would have to ...
Bob D's user avatar
  • 71.8k
6 votes
Accepted

Is Carnot cycle the only "most efficient" cycle?

A nuance is that the Carnot cycle is the only reversible cycle between only two reservoirs. There are other idealized cycles that are also reversible (i.e., that generate no entropy) but require more ...
Chemomechanics's user avatar
6 votes
Accepted

Does the Carnot efficiency apply to photoelectric conversion?

100% quantum efficiency does not translate to 100% efficiency. It simply means 'one electron out per photon in'. A standard single-junction cell has a maximum thermodynamic efficiency of ~33% at 100% ...
TLW's user avatar
  • 213
6 votes

Can IC engines be modeled as Carnot engines?

The Carnot cycle is 4-stage cycle whose stages are (1) isothermal $\to$ (2) isentropic $\to$ (3) isothermal $\to $ (4) isentropic and then start over again. To a first approximation, the Otto-cycle ...
hyportnex's user avatar
  • 18.9k
5 votes

Why doesn't the water cycle stop, since it dissipates energy?

As has already been pointed out, the Sun provides the excess energy for the water cycle to continue indefinitely (or at least for as long as we have the Sun). But let's investigate this a bit further. ...
Andréas Sundström's user avatar
5 votes

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

However, my issue is, how do we know it is a state function? We were only able to say it is a state function when we went through a derivation using the Carnot cycle (a reversible process). It doesn't ...
Ján Lalinský's user avatar
5 votes

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

No. For a cyclic process, the system returns to its original state at the end of the process, so its entropy change is zero. But for the surroundings, that are typically envisioned as being ...
Chet Miller's user avatar
  • 33.6k
5 votes
Accepted

Approximation of any reversible process by carnot cycle

Figure 7.1 answers the question but the $PV$ graph makes it hard to understand the answer. It easier to represent the process on the $TS$ graph because the Carnot cycle on this graph is rectangle. ...
Themis's user avatar
  • 5,853
5 votes

Why adiabatic expansion happens in Carnot cycle?

in $B \rightarrow C$, the working substance is isolated, and thus $Q = > 0$, so $\Delta E_{int} = W$ where $W$ is the work done by the environment to the system, not work done by the environment on ...
Bob D's user avatar
  • 71.8k
5 votes

Does the Carnot efficiency apply to photoelectric conversion?

The carnot limit only applies to heat engines which use thermal energy flows to produce mechanical work. So it does not apply to electrical power generation using moving fields to drive the flow of ...
niels nielsen's user avatar
5 votes
Accepted

How can we say that work done by carnot engine in a cycle equals net heat released into it even when it is operated b/w 2 bodies and not 2 reservoir?

I think the question is asking how—if the Carnot engine's original state was at $T_\mathrm{high}$, corresponding to the initial temperature of a high-temperature finite body—can the engine return to ...
Chemomechanics's user avatar

Only top scored, non community-wiki answers of a minimum length are eligible