About entropy...

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From what I have heard, entropy is the amount of energy that is unavailable to do work. What exactly does it mean by "unavailable energy", and can someone give some examples of energy being unavailable to do work in real life?
 
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  • #4
Simon Bridge
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You want an example of a thermodynamic system that does work yet is unrelated to a heat engine?

Perhaps you'll get it if you see the PV-diagram representation:
http://home.iitk.ac.in/~suller/lectures/lec27.htm
... then look for the PV diagram for anything you can name, and compare.
 
  • #5
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You want an example of a thermodynamic system that does work yet is unrelated to a heat engine?
Depends. Is there not much else that is a thermodynamic system?
 
  • #6
Simon Bridge
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A heat engine is a thermodynamic system that does work ... by definition.
So there is nothing else.

Perhaps you just need a concrete example of a heat engine?
A steam engine? A diesel motor? Stirling engine?

The heat in a gas can push a piston, but you notice that not all the heat is used up doing this?
 
  • #7
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A heat engine is a thermodynamic system that does work ... by definition.
So there is nothing else.

Perhaps you just need a concrete example of a heat engine?
A steam engine? A diesel motor? Stirling engine?

The heat in a gas can push a piston, but you notice that not all the heat is used up doing this?
So it is just engines and nothing else? I guess the link you provided above will be enough. Maybe I was thinking of something that does not use heat to do work.
 
  • #8
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One more thing about entropy; if I have this down right, if there was zero or minimal energy (or work) and all or maximum entropy in something, that something would not be able to even move, or affect something else, or think and act in the case of a living being? After all, entropy is how much energy is not able to be used for work, right?
 
  • #9
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Maximum entropy is one of the more reasonable ideas of how the Universe ends - heat death.
In gazzillions of years particles will be too spread out for any significant amount of interaction to be occurring.
All forms of life by any sane definition become impossible well before that happens.
 
  • #10
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Maximum entropy is one of the more reasonable ideas of how the Universe ends - heat death.
In gazzillions of years particles will be too spread out for any significant amount of interaction to be occurring.
All forms of life by any sane definition become impossible well before that happens.
I didn't mean maximum entropy in that sense, I believe. I was talking about something having more entropy and less energy. I may have misspoke. Let me ask again; if something has much more entropy and much less energy, would it be very hard for something to affect something or even move, or in the case of a living being, act and think?
 
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Entropy is not a force which negates energy.
It is a measure of order/disorder within a system.
A more orderly system is more effective at doing work, because less randomness.
 
  • #12
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Entropy is not a force which negates energy.
It is a measure of order/disorder within a system.
A more orderly system is more effective at doing work, because less randomness.
I don't think I said that, or meant to imply that. (bolded part)

So, a more orderly system would be more effective at moving and affecting other things through actions, living or non living? And with a less orderly system, not so much?
 
  • #13
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I don't think I said that, or meant to imply that. (bolded part)

So, a more orderly system would be more effective at moving and affecting other things through actions, living or non living? And with a less orderly system, not so much?
That's pretty much my take on it, but let's see what anybody else might say.
 
  • #14
Here is a thought experiment that more closely ties the ideas of entropy and work, and extends to the real world. Consider a box containing an inert gas at some pressure volume and temperature. Extending from the top bottom and sides of the box are thin sheets of paper that will be pushed if the gas moves. If the box is insulated so that all sides are at the same temperature, this closed system will reach equilibrium and there will be no movement detectable. The gas has reached a state of maximum entropy or randomness. The density of the gas is evenly distributed, with a slight pressure gradient top to bottom if we want to consider gravity. If one surface is made hotter or colder from outside the box, it expands or cools the gas at that surface, creating order (lower entropy): the gas density is different at that location. This creates an air flow that does work by moving the paper. With the temperature difference removed, the gas movements return the system to an evenly distributed density, returning to maximum entropy. The real world example of this is the outdoor winds we feel every day. Note that there is energy in the closed system, in the form of pressure and heat which at the atomic level is gas atoms whizzing around. But because it is so randomized and evenly distributed, it can't move the paper in a detectable way.

If you want to think in terms of living things, think of the delegates of a political party unable to decide on a leader because the candidates all have basically the same platform.
 

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