What Are Reversible Machines and Feynman's Limitations?

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Discussion Overview

The discussion revolves around the concept of reversible machines as described by Feynman, particularly focusing on the reasoning behind the assertion that no machine can lift a weight higher than a reversible machine. Participants seek clarification on the definitions and implications of reversible machines and the associated thermodynamic principles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants express confusion regarding Feynman's reasoning about the impossibility of building a machine that lifts a weight higher than a reversible machine.
  • There is a request for clarification on what constitutes a reversible machine, with some participants suggesting that it involves mechanical work derived from heat flow between reservoirs.
  • One participant describes a reversible machine as one that operates with heat flow occurring very slowly, with temperature differences being arbitrarily close to zero, and emphasizes the need for work to maintain heat flow under certain conditions.
  • Another participant mentions that the impossibility of surpassing the performance of a reversible machine relates to the laws of thermodynamics, prompting inquiries about participants' existing knowledge on these laws.

Areas of Agreement / Disagreement

Participants generally express confusion and seek clarification, indicating that there is no consensus on the understanding of reversible machines or Feynman's reasoning. Multiple viewpoints and interpretations are present.

Contextual Notes

Participants have not yet provided specific scenarios or examples to clarify their questions, and there is an indication of missing assumptions regarding the definitions and implications of reversible machines in thermodynamics.

Ankush07
I am no able to understand the reasoning of Feynman in deducing that it is impossible to build a machine that will lift a weight higher than it will be lifted by a reversible machine. I am also not able to understand what reversible machine is. So, please help me.
 
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Ankush07 said:
please help me.

These questions are too general. Can you narrow things down to a specific scenario that Feynman describes, involving a reversible machine, and then explain, in terms of that scenario, what you are having trouble understanding?
 
Ankush07 said:
I am no able to understand the reasoning of Feynman in deducing that it is impossible to build a machine that will lift a weight higher than it will be lifted by a reversible machine. I am also not able to understand what reversible machine is. So, please help me.
What do you know so far or what have you found on the attempt to answer this question? It is impossible because it violates the laws of thermodynamics, so what do you know about them?
 
Ankush07 said:
I am not able to understand the reasoning of Feynman in deducing that it is impossible to build a machine that will lift a weight higher than it will be lifted by a reversible machine. I am also not able to understand what reversible machine is. So, please help me.
Hi Ankush. Welcome to PF!

The general type of machine that Feynman is referring to is a machine that does useful mechanical work from heat flow between a hot reservoir and a cold reservoir. A reversible machine of this kind is one that operates with all heat flow occurring very slowly due to temperature differences that are arbitrarily close to zero (ie the temperature difference between the system and the hot reservoir or between the system and the cold reservoir). As heat flows from the hot to the system and then from the system to the cold reservoir the machine does mechanical work W (i.e. lifts a weight a change in height from 0 to H) . But with an infinitesimal change in conditions (an infinitesimal reduction of the temperature of the system) heat flow starts to occur in the other direction. In order to keep it flowing, work has to be done on the system. The same amount of work, W, done on the system will return the system and surroundings (the reservoirs) back to their original state (ie. the same amount of heat flow out of the hot reservoir when the engine was doing work W is returned to the hot reservoir). That is a reversible machine.

AM
 
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