You simply cannot continue to extract useful work from something that requires no additional energy input. Energy is a commodity like water or copper or oxygen or anything else. All so-called perpetual motion machines will run down and stop. You can't make a bottle that gives an unlimited supply of water; you can't make an oil well that gives an unlimited supply of gasoline, and you can't make a car that goes forever without adding any additional fuel to the gas tank. It is stupid to think that you can. Even the Sun will burn out some day.How does a perpetual motion machine reverse entropy or time?
Because perpetual motion is a system with infinite motion which is impossible. Those desktop "perpetual motion" displays operate by the transference of motion so they do not violate entropy, but something with infinite motion would never gain disorder of the system which is what entropy is and would in turn violate the 2nd law of thermodynamics, it doesn't happen. They wouldn't reverse time, but the there would be no entropy which is impossible because any time you move something you disturb the system and outside systems as well, increasing entropy. You can't reverse entropy because that would imply reversing time which violates causality. So, the fact that entropy can't be reversed fundamentally boils down to causality.How does a perpetual motion machine reverse entropy or time?Because In theory entropy can only increase in an open system over time.
You can think of "potential energy" also as "potential entropy". Using state functions (you don;t wanna get in to it) you can determine the amount of entropy created by changing a given state (ball on top of a hill) to another state (a ball on the bottom of a hill). The ball at the bottom of the hill will have a higher thermodynamic entropy. (if you wanna get in to here ya go: http://pcp.lanl.gov/ENTRTHER.html )
Anything that can reduce entropy in an open system is... well is NOT.. period. So creating a lower entropy state (adding energy to system) without its corresponding+ increase in entropy someplace else.. is a no go.
You can push the ball back up the hill.. But you need to turn calories in to kinetic motion and heat to do it. Perpetual motion "teleports" that ball back to the top of the hill without using additional energy-entropy.How does a perpetual motion machine reverse entropy or time?
Entropy is a very hard concept to grasp for a lot of people. It is sometimes explained as a measure of disorder in a system. That's a pretty good explanation for most applications, but for thermodynamics, it is insufficient. Entropy is a measure of how much of the energy in a given system is not available to do work.
Let's say you have two balloons, one green that's blown up, and the other red that's not blown up, and the two are joined at the nozzles with a tube. Now keep in mind that we could put a small propeller inside the tube so that when air flows from the filled balloon to the empty one, the propeller spins and does work.
Now, before you release the air in the green balloon, the mass of air that's inside the balloon has an internal energy U. Some of this energy can be used to spin the propeller, and some cannot.
Let's imagine that you release the air in the green balloon, and as the air spins the propeller, the propeller does some amount of work. The important thing to recognize is that you cannot empty out ALL of the air from the green balloon to the red balloon. The air pressures will equalize and that's when the air will stop moving through the tube, and the propeller will stop doing work. The system has gone from a state where there was no equilibrium, to a state where it is balanced, but no further.
In other words, you cannot use all of the internal energy U, to spin that propeller. Only some of the energy can be used. The energy that cannot be used is called entropy, and once you do release the air from the green balloon, and the pressures are equalized, we say that you have generated entropy.
How does this relate to your question? Well, in order to be able to have the propeller to work, you would have to pump all the air out of the red balloon back into the green one (or viceversa) and reintroduce the state of disequilibrium. But to do that, you would need a pump that will also generate entropy. And when you release the air for the second time you will generate more equilibrium, so on and so forth until you run out of energy to operate the pump.
So the important observation here is that entropy only flows one way. You can only generate it. The only way to destroy entropy is to generate more of it somewhere else, such that the net change is still on the side of generating it, rather than destroying it.
So a way to know that time is progressing forward and not backwards, is the observation that entropy is constantly increasing.
A perpetual motion machine, for example, would be one where the propeller that is spinning inside the tube, spins the pump that fills one of the balloons back up. In this case, entropy is not reversed per se, but it is stopped, which is also impossible. In order to be able to make this perpetual motion machine work, you would have to find a way to generate no entropy, and if you want the perpetual motion machine to do work in addition to re-pumping the balloons, you would have to find a way to reverse entropy. If entropy is flowing in the reverse direction (from more entropy to less entropy) then time must also be flowing backwards. This is the single, most absolutely obstacle to time traveling into the past. You would have to make entropy flow in the reverse direction that it does in order for time to also be reversed.How does a perpetual motion machine reverse entropy or time?You are asking how something which you know cannot exist would do something which you know is impossible.
If you are expecting practical schematics, you are going to be disappointed.
No comments:
Post a Comment