49 votes
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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 ...
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17 votes
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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 ...
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  • 95.5k
16 votes
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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 ...
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  • 57.4k
15 votes
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Why is the Carnot engine the most efficient?

However, isn't any closed loop on a PV diagram reversible? The arrows can simply be drawn in the reverse way to create a refrigerator. If any closed loop is reversible then why does the specific ...
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  • 359
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 ...
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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 ...
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  • 39.6k
11 votes

Why is the Carnot engine the most efficient?

All the answers here are wrong to an extent - or at least very misleading. The Carnot cycle is not the highest efficiency cycle of all possible cycles, it is only one of an infinity of cycles all of ...
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  • 423
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 ...
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9 votes
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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 ...
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  • 53.8k
8 votes
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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 ...
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  • 196
8 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 ...
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7 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 ...
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  • 117k
7 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 ...
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7 votes
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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% ...
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  • 194
6 votes
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Why won't the efficiency of a heat pump increase when $T_\mathrm{cold}$ decreases?

It's pretty much as you say. But it sounds as though you're trying to guess. I'd suggest a couple of little sketches and some first principles reasoning; see my drawing below Both the circles are my ...
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6 votes
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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, ...
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6 votes
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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 ...
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  • 57.4k
5 votes
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Questions on Carnot's theorem

The proof behind Carnot's upper limit posed on the efficiency of heat engines is more robust than this. The quotes you've pasted are among the various statements of the second law of thermodynamics. ...
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  • 3,988
5 votes

Must a reversible engine be a carnot engine?

The Carnot cycle is the unique reversible cycle working between two reservoirs of temperature $T_1$ and $T_2$, such that when the engine is in contact with reservoir 1, evolution is isothermal at ...
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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. ...
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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 ...
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5 votes
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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 ...
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  • 7,702
5 votes
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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 ...
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5 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 ...
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  • 57.4k
5 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 reversible but require more reservoirs. They are less efficient ...
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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 ...
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4 votes
Accepted

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

On the P-V diagram for the Carnot cycle the upper and lower curves are very close together. As a result, this cycle has very little area enclosed within the cycle diagram and very little work is done ...
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  • 423
4 votes

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

The Carnot cycle is so efficient because it minimizes the increase in entropy. This comes at the expense of needing to follow a specific path on the PV diagram that theoretically should take infinite ...
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  • 117k

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