High School What Are Reversible Machines and Feynman's Limitations?

[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.

 

[PeterDonis]

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?

 

[fresh_42]

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?

 

[Andrew Mason]

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