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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18 votes
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Why do we have to dump the extra entropy created in a heat engine?

We are talking about cycles here. After a complete cycle the system must be right back where it started. Since entropy is a state variable, it must then be that after one complete cycle the entropy is ...
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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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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
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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12 votes
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Why does entropy allow heat to be converted to work?

From what I learned, a heat engine can take energy in and convert it into work. However, heat can be defined as more "disordered" than work (because in heat, particles are more "random", but in ...
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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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8 votes
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What's the efficiency of real heat engines?

The most efficient heat engines are invariably the biggest and slowest working. For a steam turbine, "slowest working" means having many turbine stages, so that work is extracted from the ...
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8 votes
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Heat pump intuition

You describe the process. For an example, you might think of how a gas refrigerator works. You take a gas and expose it to the cold area, which cools it to the cold temperature. Then you isolate it ...
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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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7 votes

Why do we have to dump the extra entropy created in a heat engine?

The reason you need to dump the heat is because engines, by their definition, operate in a cycle. They return to a previous configuration before continuing on. So your solution of just using an ...
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6 votes

How much heat from a fire actually warms your home?

This is such a complicated question! The worst part is that as heat leaves the chimney, it draws air from the room with it - air that needs to be replaced from outside. This actually makes fires quite ...
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6 votes
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How is the $2^\mathrm{nd}$ law of thermodynamics valid?

You are doing a common misconception of the second law. The Kelvin statement of the Second Law says it impossible to have a process whose only effect is convert heat into work. The term "only effect" ...
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6 votes

How heat flow through finite temperature drop is an irreversible process?

A heat flow through a finite difference of temperature is irreversible because it does not satisfy the definition of reversibility. A thermodynamic process is reversible if an infinitesimal change of ...
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6 votes
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Power generation by extracting heat from the environment?

Which makes me wonder: why isn't it practicable to generate electricity by drawing thermal energy from the environment? You can do it but it may not be practicable because you will always need to ...
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6 votes
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Can you please explain cut off ratio as described in Heat and Thermodynamics by Mark W Zemansky and Richard H Dittman

It is a misprint in Dittman and Zeamnsky's book. Even the best books, however often one reads and rereads the proofs, still contain misprints. Comparing (6.6) with the first definition of $\eta$ one ...
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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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5 votes
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How is the efficiency of a heat engine related to the entropy produced during the process?

The Short Answer How is the efficiency of a heat engine related to the entropy produced during the process? The maximum efficiency for any heat engine operating between two temperature $T_H$ and $...
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5 votes

Why does heat engine need to reject energy to environment?

1)You must have clear on your mind that engines do not work on any thermodynamic cycle.Per example, a diesel engine "consumes" fuel and air and expels an equal amount of exhaust gas. Nothing returns ...
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5 votes
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How would a Stirling Engine's throttle work?

Controlling the speed of a Stirling engine is problematic and poor throttle response is one of the key reasons this engine type has not seen use in automotive applications. In general, the Stirling ...
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  • 423
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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5 votes
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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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5 votes

Reverse refrigeration and 2nd law of thermodynamics

The process you describe doesn't have heat transfer as its sole result. The machine did work, which means electrical energy was consumed as part of the result.
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4 votes
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Why does a cooling car engine crackle?

I was told a long time ago that the sound is from "twinning" - this is where a metal under large stress experiences a reorientation of grains to relieve stress. However I am not convinced this is ...
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4 votes

Intuitive explaination for why higher engine compression ratio is more efficient?

You are missing the fact that when I have a lot of air in a tight space, and I then heat it up, I get really high pressure. You have to draw yourself a diagram of pressure vs volume - compressing the ...
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4 votes

How much heat from a fire actually warms your home?

This is an excellent question, but not readily answered, as discussed in Floris's Answer. Here is a stab at how to get an estimate of efficiency. It is the method which I believe is reasonably ...
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4 votes
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Efficiency larger than one?

It is a matter of definition. In the case of the heat engine, you are seeing how much of the heat can be converted into useful work - since something will be lost in the conversion process, a number ...
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