Talk:Energy

ENERGY, "ENERGY", energy, and "energy"

The problem with this page seems to be that it discusses an entity which is very abstract. ENERGY is the collection of phenomena under study, each form forever beyond the reach of our direct knowledge, while energy is but one of these phenomena. "ENERGY" is the sum of our models concerning um, maybeeee.....energy? While "energy" is the particular model under discussion in any singular way.
So, ENERGY cannot be defined, nor can energy, and "ENERGY" is too big to handle except in one of its specific instances, e.g., E=mc^2, I ran out of _energy_ for more examples...etc. So, therefore, Q.E.D., only "energy" can be defined fairly well. Link this page to these other uses, and stop trying to stretch the Sierpinski Carpet.

Or in other words, "HEY YOU KIDS! GET OFF MY LAWN!"

Now go wash your hands and have a cup of tea and a sit-down.

Thanks. --TheLastWordSword (talk) 22:09, 15 November 2010 (UTC)

If you use a single dimensional (S versus T) diagram to depict the physics of motion, But supplement it with a force vector, which sticks perpendicularly out of the paper to depict the magnitude of any force that causes a motion in the S-T diagram, Then you have a way of relating the force to the indicated motion in the indicated manner. An integration of the force vector during the applied time interval (Delta T)will give you the value of the impact that was applied to the impelled particle, and therefor, if you know its mass, you can calculate its change in momentum (M x delta V). Also if you integrate the force vector over the distance traveled (Delta S) by the particle during a time interval, you can calculate its change in kinetic energy of motion, which is M/2 times the integral of F squared. And since you soon note that to give additional energy to a moving particle you first have to catch up to it with your impelling force, it makes apparent the difficulty in causing a particle to achieve a velocity limit by sending out an impelling force.WFPM (talk) 13:20, 16 November 2010 (UTC)

Energy is described not defined.

Energy is described via its manifestations upoin matter. So what is enery? Bcuratolo (talk) 16:58, 24 January 2011 (UTC)

I agree, this is one of the worst opening paragraphs on wikipedia. [comment by 92.17.89.69]

Okay, a bold edit needed

Okay, agreeing with these many complaints and seeing not much done, I've been WP:BOLD and rewritten the LEDE to define energy as the ability to do work, which work exerts pushes and pulls through distances. It's also equivalent to mass, and never appears without mass. Potential energy appears as trapped energy, when pushes and pulls through forces are made, and the new configuration is locked so it cannot relax (like a spring). Heat is resolved to EM or kinetic energy, and thermal energy to kinetic and potential. The last part of the lede in which entropy, which conservation, conversion, and so on are discussed, is not changed as much. I've pointed out that if you transfer energy to another sytem by any means than just adding some matter to it, you're going to change it, because you've done work on it. SBHarris 01:04, 10 February 2011 (UTC)

Lots of weirdness

Energy, momentum, potential energy, speed, relative mass etc are all definitions created in their relation. When you use them you 'lend from time', that is define a coming 'moment in time' as a possible interaction, and then define whatever property you use from looking at that possible interaction. You have invariant mass defined as invariant in all frames and motions, not relativistic as that is a definition of a relative mass (relation), and that goes for momentum too as far as I know. If you don't get the basics right you will stare yourself blind at equations made from flawed premises. It's strange, you guys should really know this? Do you have any General relativity in your courses, or is it all 'quanta'?

What goes for momenta "as far as you know"? There is no "invariant momenta". The invariant quantity is the E,p,p,p 4-vector that includes energy. And the length of which is invariant mass. But this is not an article about mass, or rest energy. It's an article about energy so we are stuck dealing with the fact that it is conserved but relative to the observer. SBHarris 18:53, 9 March 2011 (UTC)

==

Momentum in a photon is a relation to a invariant speed 'c', from any frame measured, expressed differently from any of those frames as 'energy', when measured from whatever frame, depending on its speed relative that 'photon/wave'. And so it is 'relative'. Seen as a 'lightquanta' we express it differently. But a photon have no 'rest frame' as I know? Am I wrong there?

No. you're correct. The momentum a single photon is anything you like (down to something approaching zero, or large wihtout bound), since your observer of the photon can be in any frame you like. All of them see the photon moving at c, but each sees a different photon E and p. Kinetic energy for massive objects (the photon has no mass = rest mass) is the same, in a way. For any single particle kinetic energy can be anything you like, down to zero (rest frame of the moving object). Rest energy = rest mass is the minumum energy for massive objects. For systems of particles where we cannot find a frame where the KE of every particle is zero, the minimum total energy is in the center of momentum (COM) frame where system p is zero. Im that frame, the residual kinetic energy of the system contributes to its invariant mass (as do the various rest energies and potentials). See systems section in kinetic energy. That is sort of the "rest frame" of the system, even though parts of it are moving. Systems of photons also have an invariant mass, which is their mass in their COM frame (which doesn't change in a particle annihillation that makes photons, for example, so invariant mass is conserved).

=

"For systems of particles where we cannot find a frame where the KE of every particle is zero, the minimum total energy is in the center of momentum (COM) frame where system p is zero. Im that frame, the residual kinetic energy of the system contributes to its invariant mass (as do the various rest energies and potentials). See systems section in kinetic energy."

Thanks for your answer SB :) and, I have no problems with your statement, that energy is measurable after all. That is, you are referring to the system 'jiggling', as I read it? And that's also my point :) 'Energy' needs to be able to be measured if you want to refer it as belonging to a single object. And there we have 'jiggling' and compression as the telltales I know off. Compression as the spring still have a added 'invariant mass' even after the dissipating kinetic energy, produced in the compression, is gone.

==

"Potential energy appears as trapped energy, when pushes and pulls through forces are made." is terribly wrong. Where the he* do you get the idea that 'potential energy' pushes and pulls?

That's not what the sentence says. It says "potential energy appears as trapped energy, when pushes and pulls through forces are made." Which it does. There may be other ways to store potential energy besides letting a force act through a distance where the energy doesn't go into some other form of energy like kinetic E, but this method is the most common one. How do you make potential energy except by doing work? SBHarris 18:53, 9 March 2011 (UTC)

==

'Trapped energy' Can you prove that experimentally? Except in a compression? Are you thinking of 'relative mass' too? I'm sorry, maybe I'm not getting your idea right? Energy is interactions to me, or as expressed in a compression. Can you show me any proof for a speeding spaceship for example, storing 'energy' in its relative motion? If you mean the 'stress energy tensor', the warping of SpaceTime by 'relative speed' I might agree, although I have trouble defining its speed even so, maybe SpaceTime hasn't though? But as far as I know there is no 'energy' stored in that Spaceship I mentioned here? It makes me head hurt assuming that we have all kinds of 'secret, invisible, and unmeasurable, energy stored in that Spaceship :) Or? Can you prove my assumption wrong? If so I'm very interested. You just need to link me to the experiment proving it.

You can see energy stored when mass changes. In fusion, you bring two charged nuclei together and they are compressed against their EM fields like springs, until they reach a point that the nuclear force draws them in where they bind. That is a process that stores energy, if you are a supernova making (say) atoms of uranium. Each uranium nucleus now sits like a coiled spring, with more mass than the nuclei it was made from, and the extra is the mass that you trapped when you did the work on it (the temperature and kinetic energy did the work, and was trapped). When the uranium is split later, the opposite process happens. All these energies are stored as nuclear and EM potential energies, which trade off (move of one is made than the other is destroyed). Experimentally, this stuff is called nuclear fission and fusion. Potentials in both (fields which have mass) are converted to kinetic energies and EM radiation. There is nothing special about your coiled spring-- it's just another system that has stored potential. It's like a book that you picked up and put on a table. You store energy in that system. When the book falls, it's converted to kinetic energy, then heat (which is purely kinetic energy in monatomic gases, but half EM potential energy and half kinetic energy in solids). SBHarris 21:28, 10 March 2011 (UTC)

The Book is a interesting example. I can see some of your thoughts on the subject there but as far as I know there is no measurable new energy stored in that book, although there is different views on it :) simply expressed I would formulate it as 'gravity' is no force, therefore you won't get any extra energy stored in the book, that it will interact differently when falling is not a result of its 'intrinsic energy' being changed, to me it's a result of a different position in SpaceTime relative the object it may interact with, and that I see as a result of 'distortion/potential gravity/stress energy tensor'. As for why I call the spring the only example I know of? Well, it's not true, as it to me is the exact same principle as the one making a Black Hole, compression but there's my reason for accepting it. "You can see energy stored when mass changes. In fusion, you bring two charged nuclei together and they are compressed against their EM fields like springs, until they reach a point that the nuclear force draws them in where they bind. That is a process that stores energy." That one I will need to think off, I'm not sure. But I enjoy your views and I will get back to you when I sorted my thoughts out. What I can say though is that as long as we're discussing 'energy' as a concept I have no problem with adding different 'energies' interacting into a greater amount of 'stored energy', as long as we are discussing the same principle that, in time :), fill up a Black hole with more 'energy', coming from the infalling 'debris' well, sort off :)

A pleasure reading you. Yoron.

==

To make my point clearer, consider that spaceship crashing at three possible locations simultaneously, ala Feynman 'paths' :), delivering you three different 'energies' in those interactions. So, which one had it 'stored'? That one isn't that clear though as you can define it as a relation relative those objects, although when in a uniform motion you are free to define all motion to only one of those objects and if we have three (same exact invariant mass) uniformly moving at different speeds relative you, giving you three different energies? Still, better to consider how you define that 'stored energy' right :) You do it through using your inertial frame, don't you? Like Earth. So when you speak of that stored energy you mean 'relative Earth as a 'inertial frame' '. Or do you know any other way? And as all uniform speeds are the same in a black box, so your definition becomes not only relative, but also indefinite as I see it.

Here you are talking about kinetic energies, which are not stored in single objects (for reasons discussed above) but are stored in a dispersed and non-locatable way, in SYSTEMS of objects (and such minimal kinetic energies, easily seen in the system COM frame, are invariant). Potential energies don't involve motion, and because of that, they store energy in a way that is invariant from the beginning, but that's natural because they always involve doing work against some field/force, so a system of two objects (at minimum) is always involved anyway. Compress a spring and its increased mass is the same in all frames, since the mass increase shows up in the COM frame, and is invariant mass. However, like the book on the table, there's no motion storing the energy. The field and configuration of objects does it. Pull two objects appart gravitationally and that system stores the energy without storing it kinetically. This storage is also invariant and is seen by all observers, even though you cannot locate it precisely in space. Gravitational waves are one more interesting system where the energy is stored as a potential, but not in any location smaller than the wavelength of the wave. You have to "stand back" and look at the thing from a distance to "see" the effect of the energy (which is that the wave carries off energy and mass from systems, just must contain energy in itself somewhere, albeit diffusely). SBHarris 21:28, 10 March 2011 (UTC)

Yes I agree, by defining a arbitrarily chosen 'system' you can define a 'potential energy', or just 'energy', as a relation existing between the objects in that 'system'. What I don't like is when it sounds as if this 'energy' actually 'exists'. It doesn't, not until the interaction. I differ between measurable 'energy' (compressed spring) and conceptual 'energy' as in a 'system' where you want to light up the possible interactions and relations existing. Gravitational waves is to me 'vibrations' in the 'Jello field of gravity/SpaceTime', not 'energy' per se and the reason is that there is no 'force' involved. To me SpaceTime is somewhat like a Jello :) You can send 'chock waves' through it that 'distorts' it, propagating, but there is no 'energy' involved, that is when you're inside the distortion I don't expect you to weight/invariant mass more (as long as we're not talking a compression). Still, I see why it's seen as a very useful concept when manipulating mathematics, and I better add that I didn't react on your article as such, just on some comments I found unclear in the talk session. But, to me a added 'energy' should also be measurable, as 'jiggling' or as an added invariant mass (greater gravitational potential). I'm afraid this talk page may grow :)

==

Entropy is very simple to understand if you look at it as 'energy'. Not that we can lift up a ounce of 'energy' but it is a very useful concept. Then entropy will be that 'energy' interacting and so doing lose some of its 'energy' falling into a lower state. That why you will find our universe to equalize out in the end, all energy states being at that level where none can be used anymore, also called 'work done'. Why we don't do that spontaneously is because you need to add some 'energy' to any system you want to start interacting losing 'energy'. And that saves us all from instantly decaying. If you want to understand your equations you need to look behind them, to the concepts they manipulate. In chemistry entropy is expressed as heat, but the real state that change is 'energy', even though not defined by itself. Maybe you could use the word radiation instead of heat, I don't know, but 'energy' is the proper one for it I think as even radiation has a end state, as in a photon interacting annihilating itself.

"Energy is described via its manifestations upoin matter" If you by that mean relations interacting, losing energy by it and falling into lower energy states, as seen for the whole system? But there are no manifestations, only transformations. Some of them may end in a higher energy for part of those relations but always losing energy as a whole 'system'. The only thing 'defining' energy that I know of is the compression of a spring. After the kinetic energy has 'clung out' there will still be an added 'invariant mass' to that compressed spring as compared to it before getting compressed. And that's the only proof I know for the idea of 'energy'. But it's perfectly sufficient too :)

If you look at the stress energy tensor you will see that it uses property's only defined in a relation, like momentum. The energy that transforms into 'oblivion/SpaceTime' is expected to add to that tensor as I see it. And why it has to do so is because all interactions not only transforms, but also loses some of that 'energy'. As we have a definition of 'conservation of energy' we still need it to stay trapped. So it has to add to 'SpaceTime', and then the stress energy tensor is what you have left, as I know that is. The universe is weird :)

Yoron. User:178.30.6.228 12:52, 9 March 2011 (UTC)

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)

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))