Entropy example, Need explanation

[DecSpec]

Hi all,

I had a thermodynamics lecture a few week ago where our lecturer showed a video of an egg falling and smashing into the floor, which was then replayed in reverse showing the egg come back together, he then asked us can anyone explain why that isnt possible... The answers of "because it cant" or "because that dosnt make sence" were not really suffice haha. I know the 2nd law of thermodynamics is involved, but leaving the law aside, what is a good way of explaining this??? Please help!


Thanks

 

[ione]

An isolated system always evolves in a way its entropy increases. Now the sound egg is in a state of lower entropy (more “ordered”) than the cracked egg and so it is not possible the system evolves in the reverse way, that is from a more disordered to a more ordered state (second law of thermodynamics).

 

[DecSpec]

Ok thats making more sence to me now, thanks ione, so it would be the exact same for say a car crashing into a wall, or the explosion in the combustion cycle in an engine or even the universe expanding!?

 

[olixus]

^ yes

 

[DecSpec]

What if i turned around and said "why is it impossible to go from a higher state to a lower state" or "less ordered too more ordered" or " why is it impossible to evlove in a reverse way",

Sorry if asking the same question again, its just our lecturer wouldnt even accept the 2nd law as our answer that day, he just smiled and kept saying " but why not?"

 

[Latexman]

1st law of thermodynamics - "you can't get something for nothing."

2nd law of thermodynamics - "not only can't we get something for nothing, but we always lose in the process."

Good luck,
Latexman

 

[25362]

IMHO, the underlying reason for the obvious asymmetry, or irreversibility of all events, whether macro or microscopic, is the impossible (invariant) time arrow reversal.

Am I right?

 

[DecSpec]

Yeah 25363 I do also agree, but why? Haha i know some might say Im just going round in circles with my why's, but there must be a easy way of explaining thus without using a law type answer

 

[25362]

It appears to me that the impossibility of reverting the time arrow or, if you wish, cause and effect processes, is the irrefutable answer to all those why's.

 

[ione]

In real life you don't have a remote control with rewind button

 

[DRWeig]

No need to have a philosophical discussion. You don't need to reverse the egg drop. Chickens make them every day. Get a new one.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Latexman]

Thanks for that diversion, Dave. I thought we were heading towards a religious discussion or something. If one could reverse the egg drop, what would happen to all those freshman Engineering students and their egg protector projects?

Good luck,
Latexman

 

[25362]

DeecSpec, back to thermodynamics, you are probably trying to compare a spontaneous (egg breaking) process with a non spontaneous (egg reassembling) one.

 

[MikeClay]

It is a first, not second law issue. The egg falling changes its potential energy to kinetic. The kinetic energy is then absorbed into the breaking of the egg. There is no energy left to reassemble the egg and launch it back to its original position. Even if the heat of breaking was recovered, the second law states that not all of it can be recovered.

I suppose enough work can be done to reassemble the egg and get it back to its original condition, but then, we are back to the philosophy discussion. But the Heisenberg uncertainty law would suggest that it can never go back to the initial condition.

--Mike--

 

[davefitz]

A lot of examples can be recited. The bottom line is , the tendency toward randomness is inextricably tied up with the concept of time ; as time moves forward, processes move from ordered toward random. To reverse the order for specific isolated process requires an input of work, but when all process are considered, there is yet a net increase in randomness.

One example is the process of operating an automobile- the gasoline flows from the tank to the fuel injectors, is combusted in the cylinders, exhausted thru the tailpipe, and some heat is lost thru the radiator to the ambient air. As far as the first law is concerned, all those processes can be reversed and maintain conservation of mass and energy; the exhaust gas would get sucked up by the tail pipe flowing from a low pressure zone to a higher pressure zone , heat from the colder ambient air would flow back thur the hotter radiator to the antifreee mixture and into the hotter cylinder,the exhasut gases would from a mixture of gasoline droplets and ambient air in the engine cylinders, and the droplets form a liquid stream back into the gas tank. But, in my limted experience, that never has happened.

 

[telecomguy]

Wow! You just solved our energy problems. No more imported oil, we'll just create our own gasoline using the Fitzilator.

 

[TPL]

You need to think about what entropy really is. Basically, the universe is running down (think of it as a giant watch spring that is slowly unwinding, with no means of rewinding it). The second law of thermodynamics says that whenever energy is exchanged or converted from one form to another, the potential for energy to do work gets less. This really is what entropy is all about. It is a measure of the lack of potential or quality of energy; and once that energy has been exchanged or converted, it cannot revert back to a higher state. Ultimately, everything throughout the entire Universe will end up at the same temperature, so the entropy of the Universe is always increasing.

Reversible adiabatic processes are good enough describing for some practical processes, but there is always some loss of energy (friction) in the real world, even if it is quite small.

Let's replace the egg with a simpler example, say a brick wall. Suppose the brick wall is knocked down - it can be rebuild so that it looks just the same as before but, in mixing the mortar adding the water and moving the bricks the universe has not actually lost any energy (because that cannot be destroyed or created) but the ability of the universe to do useful work with that energy has been ever so slightly diminished.

I've just re-read this and I'm not sure if it helps or not

 

[rmw]

It's simple. It's because the chicken came first.

rmw

 

[davefitz]

rmw,

We all know the egg Mcmuffin came before the chicken Mcnugget.

 

[rmw]

And that perfectly explains entropy. At the time when one would be munching down on an egg McMuffin, at that point the perspective is that the day appears so fresh and so hopeful. By Chicken Nuggets time, it is a total loss.

rmw