Talk:Energy

Break

Look, nobody has time to go into this with you. Read the article carefully first. The stress-energy tensor only talks about energy-momentum flow through a point, and if you want energy in a volume you need to integrate around the volume of the thing. That's why some energies can't be expressed as dE/dV quantities-- you have to define your volume, integrate around it, and then step away and look at it from flat space. The energy that volume contains is then its invariant mass and the thing that generates that volume's gravitational field. There's your energy.

Gravitational waves are like shock waves (especially shear waves in a solid) but they carry away energy just as shock wave does. They do work (force x distance) on the emitter, and on the receiver. They exert forces on the emitter and the receiver. Example: read the article on the Hulse-Taylor binary system, which is a system of two neutron stars, one of which is a pulsar. This system orbits with a period of only 7.75 hours, coming as close to each other as twice the distance from Earth to moon. The power of the gravitation radiation from this is calculatable in general relativity and is 7.35 trillion trillion watts (10^24 watts). That's almost 2% of the energy that our Sun puts out as light, only this is coming out as gravitational waves. It exerts a force on the system and causes the stars to in-spiral as they lose energy and angular momentum-- they might as well be swimming in some viscous fluid. That's real work, a real force, and a real effect, which has been measured because the rotating pulsar is so great as a clock. Because it's polarized gravitational wave radiation, it carries away the angular momentum from the system, like a polarized light beam would do, but not like any nonpolarized EM radiation from any star (like ours) would do. It only has one possible explanation, and it fits Einstein's prediction over 30 years to within 0.2%. It won a Nobel Prize in 1993 for the guys who discovered and analyzed it (that's from your country, Sweden).

So-- the book raised to the table only increases the potential of the system, but its mass wouldn't change if the force and distance to put it there didn't come from somewhere else in the system (like my muscles, or you could do it with a coiled spring). The mass and gravity field of the whole Earth wouldn't change if you just moved energy from here to there like that, but if you believe energy left the coiled spring, you must believe it went into the system of book+Earth. Just WHERE, you can't say, but from far away, it's still there, even though not in the spring. So where else would it be? In the gravitational potential.

Finally, remember where those atoms heavier than iron and nickel come from. It takes energy to make them and fusion to larger atoms is losing propositon that saps and stores energy, not creates it. So where does this energy come from. It turns out that it's mostly gravitational energy from the collapse of a supernova, so that's stored gravtiational potential also-- except this time in heavy atoms. On a larger scale you can see that a planet like Jupiter still radiates more energy than it gets from the Sun. It's obviously still slowly colapsing, and that potential energy is converted to infrared.

and would you please sign your posts with four tildes: ~~~~. Or pick a username like Yoron? SBHarris 07:56, 12 March 2011 (UTC)

Hm.

Okay, maybe you feel that I'm attacking your article? if so, nope, as for signing every comment, you filled in my original writing with yours, I answered them, staying inside the caption I made originally? Anyway, you raise a question I'm not sure I can answer, with your statement that gravity is energy, as that seems to be the way you look at it? In a way I agree, maybe I don't see it clear enough? Or possibly gravity and its gravitational quadrupole moment are different. Einstein himself seemed to have changed views on gravity waves a couple of times :) so I think I'm excused if so. Gravity is definitely related to 'energy', but it's not a 'force'. If you state that energy contain gravity though, I have no problem agreeing. When it comes to the book I still say you will find no new energy in it. But we seem to agree there? When you define the energy as existing as a gravitational potential, I call it a 'stress energy tensor'. As for defining a arbitrarily chosen system, trusting this to 'define' the energy's boundaries? Then I don't agree, you can always widen this 'system' book-ground, to the whole universe if you like, and still find the same 'energy' released in the final interaction with the book hitting ground, with that 'energy' having been 'somewhere' inside your 'new system' too, as I see it?

This one seems to mirror my confusion, well, slightly :) http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/GR/energy_gr.html

But you've given me a lot to think of, and it still was a pleasure reading you. Yoron. 178.30.69.236 (talk) 23:07, 12 March 2011 (UTC)

Nuclear binding energy is converted

The table in this section looks like it has been vandalized.User:Bleeisme

I don't see where the problem is. Anyway, the place to make this comment is on the TALK page of nuclear binding energy. Please sign your comments with four tildes: (~~~~) SBHarris 21:47, 10 March 2011 (UTC)

The King has no clothes?

The article starts wtih "ἐνέργεια energeia "activity, operation"[1]) is a quantity that is often understood as the ability a physical system has to do work on other physical systems."

Shoudn't the article start wth a definition of what the heck it is talking about? The article says "is often understood as." IMHO, if you are going to launch forth on a topic, at least you should unambiguously define the concept you are talking about. (EnochBethany (talk) 23:59, 4 April 2011 (UTC))

Good point. Our article should begin with a simple explanation of what is meant by energy. Over the years there has been a lot of discussion on this Discussion page about what should be said to define energy. Check threads above, and also the archive. It looks like no consensus was ever reached about how to define it. Dolphin (t) 00:10, 5 April 2011 (UTC)

The article DOES start like that:

In physics, energy (Ancient Greek: ἐνέργεια energeia "activity, operation"[1]) is a quantity that is often understood as the ability a physical system has to do work on other physical systems.[2][3] Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.

What's wrong with the above? What isn't clear? We've even defined the sub-terms for you. A Push/pull exerted over a distance. What is it you don't understand about push/pull or distance? SBHarris 02:18, 5 April 2011 (UTC) == Initial Definition is weak -- why?

"In physics, energy is a quantity that is often understood as the ability a physical system has to do work on other physical systems"

This is just simply awkward bad grammar, in addition to being a very weak statement.

Why does it not read:

"Energy is the measured quantity of a physical system to do work on other physical systems"

Because it's not really what it is. You can have energy which can't do work due to second law thermodynamics. But I agree grammar is awkward and have rewritten. Gerardw (talk) 18:33, 19 May 2011 (UTC)

I don't know what the word "measured" is doing in that sentence, but the rest is correct in the limit of a heat engine with a thermal reservoir at absolute zero. You can get as close to converting heat to work as you like, that way. So in theory, and in limit, energy is the capacity to do work, given the correct circumstances. We have to talk about entropy limits (and do) but can only mention it in passing in the lede. SBHarris 00:21, 11 June 2011 (UTC)

Energy Definition Difficulties

The discussion on the best definition of energy is quite fascinating. The conundrum is considerable. Energy is actually quite an abstract idea in its technical sense. Most of the posts show an awareness of this. The problem is to find a way of presenting a somewhat colloquial description that is easy to understand without being technically misleading or erroneous.

The difficulties sensed with defining energy as "the ability to do work" are well placed. Despite the well meaning efforts to find a simple definition, such a a definition is so erroneous as to be completely misleading, despite the fact that probably over half the world's introductory engineering texts define energy exactly that way. Nevertheless it is wrong. Energy, of itself has no intrinisc ability to do work of any kind whatsoever. Ultimately it is "lack of entropy" which has the ability to do work. Unfortunately, this definition cannot provide the kind of convenient handle for the idea that we need here because it introduces another equally, or even more abstract idea that has not been defined.

I would like to help with the definition of energy here, but have decided not to edit the text because I think it needs approval from everyone concerned before it is changed again, especially considering the mention of the debatge spanning several years.

Instead, I would like to offer some suggestions, phrases and simple sentences that might be suitable for working into an appropriate definition of energy. If these suggestions are rejected, I shall understand why. Energy is diffiult to define simply.

Suggestions: 1. Start with reference to the colloquial use (E.g. A very widespread and colloquial definition of energy is that it is something that has the ability to do work. This is not quite technically correct but it has helped many students start out with an immediate notion that can be very helpful with getting on to the equations and relationships in energy considerations. However, we will eventally have to learn, if we want to get to the bottom of things, that energy itself, in its essence does not have that ability. This distinction is important if we want to try to understand the essence of this thing called energy. It turns out that it is various distributions of energy that provide that ability, not something that is intrinsic to energy itself.

2. Describe Energy in its widest sense, and describe its multitude of forms, briefly. (E.g. When we drive a car, the car's fuel has energy that we convert into a mechanical form that moves the car. The sun's energy is something we see and feel everyday. That energy is a different form altogether, starting with atoms, that, during a process of fusion, produces light and heat, both other forms of energy. So we can see that energy comes to us in a multitude of forms. It is everywhere, in our lives and throughout the universe.

3. Approach the subject of the "usability of energy". (E.g. The first step in coming to understand what energy is, and what it is not, is to think about where and how energy appears "usable" to us. We use energy to heat our homes. We burn fuel to do that. After the fuel is burned and our homes have been heated, is the energy in that fuel available for anything else? It is now in the form of heat. Could we use that heat to propel a car? The answer is, we might, if we found some way of getting it to "flow". So then we have to think about how energy flows, why it flows and under what conditions it flows from one place to another. )

4. Bring it around to the idea that energy in any form, can have a useful part and when we "use" it, there is always a part that cannot be used (Link to entropy). (E.g. All of us are familiar with the idea of friction. It is something that reduces the usability of the energy we are using to get something done. As it turns out, when friction occurs, it is dissipating as heat some of the useful part of the energy we are using, so there is less remaining for doing the useful work. OR We must turn to the notion that energy, to be useful must involve a gradient of some kind. If there is no gradient, that is an unequal distribution of some kind, it will not be useful for doing work. )

5. Then to a more general definition - (E.g. Energy itself can be seen as the medium through which all forces in the universe are transmitted. According to conventional and traditional theory, it is seen as something that can neither be created nor destroyed, nor wasted. For the energy conservationists, energy cannot be NOT conserved. It is always conserved. (Energy conservationists are really concerned with the usable part. Perhaps also add something like - When we have used the "usable" part of energy supplied to us in soem stored form, that usability is gone forever. The energy is still there, but in a less usable form. The universe changes irrevocably every time we use energy.

6. A TECHNICAL DEFINITION OF ENERGY. A technical definition of energy might be most easily done by reference to its dimensions. Energy can be defined as a product of more basic dimensional units representing the basic measurable quantities in the universe such as mass, length, time, charge, etc. For example: we can refer to the E=Mc^2 formula and note the dimensions implicit in this as: MASS multiplied by the dimension of VELOCITY squared. VELOCITY, however, can be further reduced, as a composite dimension to the basic dimensionality of LENGTH/TIME. Thus ENERGY has the dimensions of MASS times DISTANCE squared divided by TIME squared. ( M DISTANCE(or LENGTH) ^2/TIME^2) It can also be pointed out that WORK, having the same dimensions as ENERGY, also can also be defined in those basic dimensions. By way of explaining the various forms in which energy can be represented, WORK appears to have different dimensions from those above for ENERGY, (e.g WORK = FORCE times DISTANCE), but in fact can be seen to transform consistently to the same basic dimensions of ENERGY. (E.g WORK = FORCE times DISTANCE and F = MA implies dimensions: Force = mass multiplied by acceleration, which reduces to MASS multiplied by DISTANCE divided by TIME squared. Since WORK => FORCE X DISTANCE, we can reduce the dimensions of FORCE to its basic dimensions resulting in (MASS X DISTANCE / TIME squared) X DISTANCE which can be reduced to: MASS X DISTANCE squared /TIME squared => MASS X VELOCITY squared, which the same dimensions as the above definition of energy.

In summary: energy is defined dimensionally as MASS * DISTANCE ^2 / TIME ^2.

All forms of energy, whatever they will reduce to this basic dimensionality.

Hope some of these suggestions help.

UpToTheMinute (talk) 22:56, 5 July 2011 (UTC)

It could also be mentioned that since ENERGY and WORK have the same dimensions, saying ENERGY has the ability to DO WORK, would be like saying ENERGY has the ability to DO ENERGY, rather meaningless.

COMMENT. It's not meaningless to say that one thing is not the other, the argument being that they have the same dimensions. Work is measured in units of energy, but that doesn't mean it IS energy. It is not energy, but rather the effect of energy, and the act of transfer of energy. Many kinds of energy are clearly NOT work and won't be for the foreseeable future. All potential energies, for example. And the energy that is trapped as rest mass in particles and as invariant mass in systems. And (for that matter) also kinetic energy. Energies carried by rest-massless particles like photons and gravitons, also are not work. Work is force x distance. It isn't a "thing," but rather an action. Energy can perform work (or not) and work always produces energy of some kind, but they are not the same. One is a noun and the other a verb! SBHarris 18:28, 8 August 2011 (UTC)

The reference 3 doesn't state that energy quantifies "the ability a physical system has to do work on other physical systems" but that "energy is the capacity to produce change" (page 23) and that "work and heat are two of many forms of energy" (same page). The definition of energy is dificult to state, but it isn't really that from the article, even if some books and webs use it. The energy quantifies the capacity of a body to produce changes on other bodies (when the energy is transferred and lost, in the form of work and heat) or on the same body (when the energy is transformed, such as the case of deformation, in the form of work on itself). Using changes instead of work includes those cases when no work is done at all (i.e. full transfer of energy in heat form, such as the collision of a flying metallic ball (at a moderate speed, i.e. not a bullet) against a metallic wall: no deformation at all, no work). LaosLos (talk) 17:22, 10 August 2011 (UTC)

"Change" is far too wishy-washy and general a word to even be useful. I can produce a "change" in a ball of clay by molding it to a different shape, or painting it. None of this has to do with energy. There are infinite ways to change composition of things and keep net energy change at zero. This is just a word to stay away from, SBHarris 17:38, 10 August 2011 (UTC)

This is only because energy implies changes, but not always changes implies energy. We are not talking about changes in general, but about energy, and energy is measured through its effects (the changes).

By the way, your examples are not true: you cannot change the form of anything without force and displacement (no displacement, no change of form), and you cannot do any chemical reaction (composition change) with zero energy (this would be the perpetuum mobile of chemistry, if in equilibrium, or nothing at all, if irreversible).

LaosLos (talk) 18:07, 10 August 2011 (UTC)

I agree... Actually I would think that molding clay involves a complex series of energy transactions in muscles and in the clay and in the surroundings. Similarly for painting the clay. I'm not convinced that any change can occur that does not involve energy transactions. Granted that "change" is not a defined term in a technical sense. However, anything that is colloquially understood as change would seem to involve energy processes. Even a color change (autumn leaves or bleaching) involves chemical processes that in turn involve changes in molecules that necessarily involve energy transactions (reactants to transition states to products, transitions in energy levels of electrons.

If change does not always imply energy then change cannot be used as a definition of energy. When heat spreads out in an object, the energy does not change-- only the distribution changes. But it is a change. A pendullum in a box changes every moment, but that's one sort of energy chaging into another, not a change in energy. Very similarly, vibration of atoms in an ordinary "unchanging" object are just like the pendullum-- as they vibrate back and forth, they trade kinetic for potential energy. Does an object just sitting there, at any real temperature, "change" from moment to moment? The answer depends on what scale you choose to look at. In short, this is a bad definition. SBHarris 19:20, 10 August 2011 (UTC)

"If change does not always imply energy then change cannot be used as a definition of energy", this is really non-sense, energy is not change, energy is measured by the changes it produces, that's all. By the way, the references in the article explains why not to use the word "work". For example, reference 11 does not include any kind of careful definitions of energy (it defines the energy à la Feynmann, with the mathematical expressions of all its forms), but in 1.5 it explains some misunderstandings associated with the use of the concept of "work" in the definition of energy. And in What is the Definition of Energy? the author explains very well the problems related with the definition of energy and he has found some definitions from a variety of books. LaosLos (talk) 19:54, 10 August 2011 (UTC)

Please don't yell. (i.e. cool it with the all caps).

Energy is an abstract quantity that can be measured, but only indirectly. It is useful because it is conserved. I'm gonna argue work is energy is heat. (Joules is joules.) However, most texts follow the historical approach -- it took awhile for science to understand heat is energy, so we treat them as separate things. While "ability to do work" is imperfect, it's probably as good a starting point as any. Regardless of what definition we end up with, it needs to be reliably sourced, not the product of original research. Gerardw (talk) 19:59, 10 August 2011 (UTC)

I aggree with you Gerard, it needs to be reliably sourced, and this is the reason why I'm searching in the references that the article already has. Moreover, I think that the definition of energy should make clear that heat is a form of transfer of energy, in the same way the work is. So, to me, the "ability to do work" sounds like the "ability to produce heat". Then, yes, the energy, when transferred, does work and/or produces heat. If produces changes (a standard definition, reliably sourced) is not a good definition for you, then we can choose the "ability to do work and/or produce heat" option (a very thermodynamical definition) but, as far as I know, this is not a standard one, and I don't know references for this definition. Loosely, we can consider this definition as the outcome of the first principle of thermodynamics (even if the first principle is not actually a definition of energy). LaosLos (talk) 20:32, 10 August 2011 (UTC)

"Produces changes" is NOT a standard definition! Bodies like NIST and ISO produce "standard definitions" in physics. Definitions in physics are not produced by consulting random undergrad college texts, of which there are hundreds, and most don't agree with each other (you should see the argument we had about this on the TALK page of weight; it's gruesome). College texts tell us nothing more than what one random author of one college text thinks, and that is all. White's Geochemistry is just a ridiculous source to draw a definition of "energy" from. Sorry.

Yes, energy is measured by the changes it produces, when it produces a change, and when it's a particular kind of change (one that involves energy units and a form of energy to be defined). That's not very profound, and you might as well back up a step and define the various forms of energy and say that "energy" generic involves any one of them. To address your specific point, many changes really do take no energy, and shape changes consume no energy unless PV work is done, which isn't always the case (the volume change of the system may be zero, and often is, in which case the system shape may change as a result of random thermal motion, but energy does not change). Energy doesn't produce some generic change like color or shape or even entropy content. It produces changes in a system's content of thermal energy or work or mass or something with an equivalent energy value. "Change" isn't measured in joules. SBHarris 21:33, 10 August 2011 (UTC)

Whatever definition is used it needs to be worked out in conjunction with the article on Work (physics). At present this article and the one on work are together faulty, in that this one defines energy in terms of work and the other article defines work in terms of energy. Circular definitions are obviously unsatisfactory. Either this article or the other one need to be changed. Would anyone like to suggest which? One solution, though in my view an unsatisfactory one, would be to merge the articles. 18:00, 24 September 2011 (UTC) — Preceding unsigned comment added by Treesmill (talk • contribs)

Work can be defined as "the work of a force", without reference to energy (by the well-known path integral and its translation into words), which I actually decided to do in the work article. This allows for the the reference to work in the energy article, and I believe it should be kept there, although some refinements would be helpful, following the suggestions by UpToTheMinute.

However, his suggestion to define a physical quantity by its dimension does not reflect standard practice at this elementary/basic level. Also, I agree that the "ability to cause change" is in no way specific to energy.

Finally, I support the suggestions by UpToTheMinute that the lead should illustrate the various forms of energy, and how "useful" it is. But I believe it should end with the classical physics "quantity that is conserved due to the homogenous flow of time" and then with the transition to the mass-energy equivalence.Ilevanat (talk) 00:18, 1 October 2011 (UTC)

Hamiltonian

The article says that the total energy of a system "is sometimes called the Hamiltonian".

It should read:

The Hamiltonian is a function (or in general a functional) of the configuration of a physical system and its values are the total Energy of that system. 91.137.20.132 (talk) 13:43, 8 August 2011 (UTC)

Energy is motivation.

Energy cannot be either created or destroyed therefore energy is a static eternal, unlimited medium. When energy is limited by the Nothingness of the limit of the observer’s I it becomes a unit ‘now’ of consciousness of the observer. The observer can then interact with other such limited units of energy. Two different ‘now’ create a difference which motivates for change. The change can be registered by the observer only as the static difference when he compares two static pictures which exist in two moments ‘now’ of time. Transformation from the static state in the first ‘now’ to the static state in the second ‘now’ is not observed unless the interval of the time of transformation can accommodate the unit ‘now’. The three static elements namely the two pictures and the difference between them is one observation located in the current ‘now’. To transform from the first picture to the second picture requires that the first picture is motivated by an independent cause because static state has no initiative. The cause of the transformation is the energy observed as, and contained within the difference. When the interval of transformation is large and when it can accommodate a limited plurality ‘c’ of the units ‘now’ of the observer’s consciousness, the interval of transformation becomes the sum of the small transformations 1/c. This can be symbolized by (0<u<1) where ‘0’ is the first picture, ‘u’ is the current ‘now’ of the observer, and ‘1’ is the end of the transformation and it is the second picture. The static states of ‘0’, ‘u’ and ‘1’ are one static self-sufficient system if ‘1’ is the cause of change for the picture in ‘0’. The energy contained within the self-sufficient, perfect system is then neither lost nor gained. When the system is imperfect energy is gained from the outside of the system and lost from the inside of the system. If energy is eternal it has no beginning and no end and it neither exists or non-exists. It simply IS as the duality of existence non-existence created by the observer of the energy. It is the duality of existence non-existence that has the beginning ‘0’ and the end ‘1’ together with the dynamism between them created by the observer. KK (78.146.64.106 (talk) 16:09, 23 October 2011 (UTC))

Archimedes Plutonium, is that you? SBHarris 16:36, 23 October 2011 (UTC) No! It is not. Is that you Pythagoras?

Edit request from , 23 November 2011

change "physically impacting it, but that case the energy of motion in an object, called kinetic energy"

to

"physically impacting it, but in that case the energy of motion in an object, called kinetic energy"

173.71.207.63 (talk) 02:30, 23 November 2011 (UTC)

First sentence

It should read:

"...indirectly observed quantity often understood as the ability of physical system to do work on other physical systems."

combine the sentences, and get rid of "has" — Preceding unsigned comment added by 38.98.88.24 (talk) 06:51, 20 January 2012 (UTC)

I don't understand precisely what is being suggested. Nobody Ent 12:33, 26 January 2012 (UTC)

"otheruses"

"otheruses" should be "other uses" in the second line of wiki text. — Preceding unsigned comment added by 76.94.171.77 (talk) 06:38, 26 January 2012 (UTC)  DoneNobody Ent 12:33, 26 January 2012 (UTC)

The international green energy Edit request on 7 March 2012

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