Sorry for the unclarity. I probably created some bias in your mind having tagged my question with energy-conservation and conservation-laws. I simply consider that, relatively independently of matematical models, energy is everyday changing its "form" both "spontaneously" or triggered by us using our machines. So I ask: why the universe seems to be designed in a way that it "seems" free (at no cost) to change cinetic to thermal energy, for instance? Why we do not need to pay a fee to move energy from "light" to heat? Like exchanging currency without having to pay for that service. I stress once again that I am not referring to the basic consideration of losses. I do not discuss the conservation of energy itself.
$\begingroup$
$\endgroup$
3
-
1$\begingroup$ It appears to assume the invaliditiy of the second law of thermodynamics. As this is a physics forum, and that law is universally recognized, it seems that your question is ill-formed and unanswerable. $\endgroup$– AdamRedwineCommented Oct 14, 2011 at 16:49
-
1$\begingroup$ I think this could be framed in terms of entropy increase in irreversible processes, in which case it's not quite as off-topic as it looks (no offense intended, Giacomo). $\endgroup$– David ZCommented Oct 14, 2011 at 18:11
-
$\begingroup$ A satellite in an eliptical orbit is constantly exchanging potential energy for kinetic energy, and it is very nearly lossless. You could say it loses energy by radiating gravitational waves. Even so, the total energy remains the same, but just the entropy increases, though extremely slowly. $\endgroup$– Mike DunlaveyCommented Oct 14, 2011 at 19:21
Add a comment
|
4 Answers
$\begingroup$
$\endgroup$
3
We have losses in every transformation of energy. If I understood your question and what you meant by "free", transformation of energy is never "free".
E.g: When pushing forward a crate that is on the floor, not all that energy is transformed into movement, some of it is transformed in heat, because of friction.
-
$\begingroup$ I am not discussing about losses and the fact that every machine as an efficiency that is less than 1. I was asking if the "action" of transforming energy from one form to another is really "free" (assumning no losses); why is it possible at all to have energy changing its "appearance"? If we continue to exercise an ideal loss-less machine, that oscillates between two energy forms, are we consuming some resource of the universe ? My be the time? may be we are only playing a little bit? $\endgroup$– GiacomoCommented Oct 14, 2011 at 12:19
-
-
$\begingroup$ the questioner is actually questioning the validity of conservation of energy....even after taking every possible losses into account. $\endgroup$ Commented Oct 25, 2011 at 7:12
$\begingroup$
$\endgroup$
"Consumes" is the wrong word to use, IMO. In any given Physical model, some quantities increase, some quantities decrease, some quantities stay the same. Some quantities change monotonically, some quantities oscillate.
The energy of a given model stays the same if the energy is a conserved quantity of the model dynamics, tautologically, or, slightly less tautologically, if the dynamics is time-translation invariant. The entropy of a model may increase, depending on whether the model satisfies the (usually rather vaguely stated) premises of Boltzmann's H-theorem, say, but the entropy is an extremely coarse-grained quantity that tells us practically nothing about the details of a dynamical state.
Now, you have also to consider what the relationship of whatever Physical model we use might be to the world. How accurate is the model? Does the accuracy (the definition of which is non-trivial) decrease monotonically, or does the accuracy oscillate or improve? If the model is not accurate, then it may be that there are better Physical models in which there is some new quantity that decreases over time. Then again, it may turn out that this better model has to be replaced in the future by another, still more accurate model that doesn't have that particular quantity in it at all.
Finally, it is a prejudice held by some that a perfect Physical model is around the corner, in terms of which we could definitely answer your Question; we could say precisely which quantities go down over time, which increase, which oscillate, ... . Then again, it is a prejudice held by others that every Physical model will always be an approximation, albeit the approximation may improve over time, in which case there cannot be a definite answer to your question. Now, find a way to definitely show that one or the other prejudice is correct.
answered Oct 14, 2011 at 13:27
$\begingroup$
$\endgroup$
No, the transformation of energy from one form to another is never free. You could say that it consumes some amount of work; or, more formally, that it increases the amount of entropy.
Most forms of energy are capable of doing some work. But low-grade heat is capable of doing very little work (for a given amount of energy). Other forms of energy are capable of doing more work. Kinetic energy is capable of doing lots of work. Turning it into low-grade heat, through friction, doesn't change the total amount of energy: the conversion can be (very very close to) 100%. But that will increase the amount of entropy in the system; the kinetic energy was able to do lots of work, and the low-grade heat will be capable of doing very little work.
$\begingroup$
$\endgroup$
Maybe the problem is that you are assuming some ideal process without losses that transforms for example heat energy into kinetic energy without increasing entropy, but that is not the case, as even an ideal combustion engine has a theoretical maximum performance. The decreased value if that is what you are looking for could be exergy, does that make sense?
