Post by: PmbPhy on 12/06/2016 01:14:19
My goal is that we all come to have the same understanding of what is meant by the term energy as its correctly used in physics so that when we discuss something dealing with it we won't have to worry about its meaning. Thanks for your contribution.
Post by: guest39538 on 12/06/2016 07:43:59
In many discussions over the years its come to my attention that most physics enthusiasts, and even some professional physicists, don't know what energy is. That's why I'm asking the question What do you think energy is and why do you believe that? I'm asking so that I can learn what people here think it is so that we can come to understand the correct meaning of the term energy. By the correct meaning I'm talking about how I describe it on my website. Please understand that I'm not being arrogant by any means. I say this because I've studied the subject in depth, read several articles from physics texts and journals and from prominent physicists such as Richard Feynman, Sheldon Glashow, A.P. French, John Stachel, Alan Guth, etc. A solid study of the subject shows exactly what is meant by the term energy. That's how I came to write my webpage on the subject. All I did was take what has been described and explained by these brilliant well-experienced first rate physicists and put it in my own words.
My goal is that we all come to have the same understanding of energy so that when we discuss something dealing with it we won't have to worry about its meaning like what happened recently. Thanks for your contribution. Please take note of whose posts I can't see.
I think energy is just a word, a word we use because we don't actually understand what energy is, so we give it this group name of energy. Energy is a group name that contains sub groups.
Post by: PmbPhy on 12/06/2016 09:50:56
Quote from: Thebox
I think energy is just a word, a word we use because we don't actually understand what energy is, so we give it this group name of energy. Energy is a group name that contains sub groups.Thanks for answering half the question. However, it's not at all helpful to me if you only say what you think energy is. It's more important to me to explain why you think its that way. In fact that's what I really want to know, i.e. why people think energy is what they believe it to be. Many people have very different beliefs about what energy is. My goal is to learn why. Therefore stating only what you think it means is totally useless to me.
Note: I modified the question so as to extract unhelpful and irrelevant comments. Also, I found a wonderful book online all about energy for those who are interested: Energy: A Beginner's Guide by Vaclav Smil. It can be downloaded for free at: http://book4you.org/book/697501/3fd733
All you have to do is register with the site. Registration is free and does not require a credit card.
Post by: JohnDuffield on 12/06/2016 11:08:17
I think energy is the fundamental thing from which matter is made.
I believe that because in his E=mc² paper (https://www.fourmilab.ch/etexts/einstein/E_mc2/www/) Einstein said "If a body gives off the energy L in the form of radiation" and "The mass of a body is a measure of its energy-content".
Light is a form of energy, and energy is something that exists inside a body. For example the Poynting vector "means that there is a flow of energy going around the electron". Energy is something real rather than some bookkeeping abstraction.
Post by: acsinuk on 12/06/2016 13:14:39
Post by: PmbPhy on 12/06/2016 16:53:06
Light is fundamental magnetic energy represented by a vibration of an area of flux/current that can move through space effortlessly. If it interacts with neutron matter it can transfer the vibration energy by increasing the matters temperature.I don't know why this came up but I'm guessing that JD posted something having to do with light. He's on my ignore list so I'm unable to see his posts.
Light is an electromagnetic (EM) wave whose wavelength lies in the range 390 to 700 nm. Most of the energy in an EM wave is in the electric field component. An EM wave cannot interact with neutrons because their charge is zero and are therefore unable to interact via the electromagnetic interaction. I.e. neutrons cannot couple to an EM field.
I noticed that you chose not to address the subject of this thread. Care to take a shot at it?
Post by: guest39538 on 12/06/2016 17:23:50
Quote from: TheboxI think energy is just a word, a word we use because we don't actually understand what energy is, so we give it this group name of energy. Energy is a group name that contains sub groups.Thanks for answering half the question. However, it's not at all helpful to me if you only say what you think energy is. It's more important to me to explain why you think its that way. In fact that's what I really want to know, i.e. why people think energy is what they believe it to be. Many people have very different beliefs about what energy is. My goal is to learn why. Therefore stating only what you think it means is totally useless to me.
Note: I modified the question so as to extract unhelpful and irrelevant comments. Also, I found a wonderful book online all about energy for those who are interested: Energy: A Beginner's Guide by Vaclav Smil. It can be downloaded for free at: http://book4you.org/book/697501/3fd733
All you have to do is register with the site. Registration is free and does not require a credit card.
I never explained what I thought energy was, I explained what the word energy means, energy being a generalised group term with subgroups rather than something specific.
Elastic ''energy'', kinetic ''energy'', nuclear ''energy'' etc.
If I was to define energy as an entity , I would define energy as the positive nature of positive work and expansion. My reason would be because:
+E=>4/3 pi³ because ''things'' expand when ''energy'' is gained in a ''things'' independent entropy (S).
Post by: Bill S on 12/06/2016 20:10:27
I’m with The Box in saying that energy is in some ways a blanket term, but I’m not sure his examples are of the best. I would go for: Mass Energy, Kinetic Energy and Potential Energy.
Why? Because they manifest clear differences. That said; I think it could be argued that there is only kinetic energy.
Why? Because, as JD quoted from Einstein: "The mass of a body is a measure of its energy-content". This energy reflects atomic movement, so is, arguably, kinetic energy.
In the case of potential energy; the word “potential” implies that it is not actually energy, until it moves. Thus, it is potential kinetic energy.
Acsinuk says: “Light is fundamental magnetic energy…” This sounds like saying that a photon is pure energy, but perhaps that is my misinterpretation. In any event; experts seem to disagree on this point.
Matt Strassler; https://profmattstrassler.com/articles-and-posts/particle-physics-basics/mass-energy-matter-etc/matter-and-energy-a-false-dichotomy/ says:
“Energy is something that photons have; it is not what photons are.”
“ Photons are particles just as electrons are particles; they both are ripples in a corresponding field, and they both have energy.”
On the other hand, Sascha Vongehr http://www.science20.com/alpha_meme/quantum_perspective_nonexistence_light-76047
argues that the energy of a photon = ħ, and that this is exhausted in a single reaction, after which the photon no longer exists. This could be interpreted as saying that the photon is “pure energy”.
So, what do I think energy is?
I think it is a manifestation of the interaction between massive objects.
I equate that with the statement that "The mass of a body is a measure of its energy-content" by suggesting that the interaction involved is that between atoms within the mass.
Post by: guest39538 on 12/06/2016 20:22:24
Matt Strassler; https://profmattstrassler.com/articles-and-posts/particle-physics-basics/mass-energy-matter-etc/matter-and-energy-a-false-dichotomy/ says:
“Energy is something that photons have; it is not what photons are.”
I personally would not agree with this, I do not think photons have ''energy'', I think they have potential energy and the ''energy'' is made/released when a photon interacts with ''matter'' or a medium.
I do not think energy exists unless there is a minimum of two components ''interacting''.
Post by: PmbPhy on 13/06/2016 01:28:26
Post by: agyejy on 13/06/2016 02:57:35
Light is an electromagnetic (EM) wave whose wavelength lies in the range 390 to 700 nm. Most of the energy in an EM wave is in the electric field component. An EM wave cannot interact with neutrons because their charge is zero and are therefore unable to interact via the electromagnetic interaction. I.e. neutrons cannot couple to an EM field.
The energy and the electric and magnetic field components of a photon in free space are equal. Here is a reference:
http://physics.bu.edu/~duffy/PY106/EMWaves.html <- Lecture notes on EM waves
Quote from: The notes
The energy in an electromagnetic wave is tied up in the electric and magnetic fields. In general, the energy per unit volume in an electric field is given by:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fbuphy.bu.edu%2F%7Eduffy%2FPY106%2F20b.GIF&hash=49f37c14f3e85ca77b975db0fa072839)
In a magnetic field, the energy per unit volume is:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fbuphy.bu.edu%2F%7Eduffy%2FPY106%2F20c.GIF&hash=bf90e373b201f10ebc68e37974ce56a8)
An electromagnetic wave has both electric and magnetic fields, so the total energy density associated with an electromagnetic wave is:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fbuphy.bu.edu%2F%7Eduffy%2FPY106%2F20d.GIF&hash=02ee4c2bace48b2527d3c46cce7dd234)
It turns out that for an electromagnetic wave, the energy associated with the electric field is equal to the energy associated with the magnetic field, so the energy density can be written in terms of just one or the other:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fbuphy.bu.edu%2F%7Eduffy%2FPY106%2F20e.GIF&hash=e1f2a0a8f4c676c46ff0bd1450f3955f)
http://physics.info/em-waves/ <- Here is another source
Quote from: Another Source
For an electromagnetic wave in free space, half the energy is in the electric field and half is in the magnetic field
η =
η =+
This gives us this very compact equation for the total energy density of an electromagnetic wave…
η =
or this one, if you prefer to state things in terms of the magnetic field instead…
η =
This is an interesting and simple set of relations, but keep in mind that it only works for electromagnetic waves in free space. Things are different in a media and the electric and magnetic fields can have any values they want if they're static.
http://web.mit.edu/8.02t/www/materials/StudyGuide/guide13.pdf <- Here is an example from MIT
Quote from: The MIT link
To relate intensity to the energy density, we first note the equality between the electric
and the magnetic energy densities ...
So except from some pretty specific cases in fairly specific materials the energies of the electric component and magnetic component of an EM wave are either identical or nearly equal.
Additionally, while the neutron has no net electrical charge it does have a magnetic field which means it can interact with an EM wave.
http://adsabs.harvard.edu/full/1993ApJ...417...12G <- Neutron-Photon scattering in the early Universe
Now the cross section for neutron-photon scattering is much lower than for proton-photon or electron-photon scattering but it isn't zero.
Post by: JohnDuffield on 13/06/2016 08:29:50
Post by: jeffreyH on 13/06/2016 08:42:34
Post by: acsinuk on 15/06/2016 17:55:37
Post by: IAMREALITY on 15/06/2016 20:32:23
Why do I believe this? Cause the very wise physicist Feynman believes that, and trust me; that dude knows what he's talking about! Have you read or watched his lectures? I'd point you to chapter 4-1 if you haven't. It'll give you a good sense of what I'm talking about!!!
[:P]
Post by: PmbPhy on 16/06/2016 06:44:32
Quote from: IAMREALITY
Why do I believe this? Cause the very wise physicist Feynman believes that, and trust me; that dude knows what he's talking about!Thank you very much for directly answering that part of the question. I greatly appreciate it. And it's nice to see someone who has the same take on the subject that I do! [:)]
Quote from: IAMREALITY
Have you read or watched his lectures?Yep.
Quote from: IAMREALITY
I'd point you to chapter 4-1 if you haven't. It'll give you a good sense of what I'm talking about!!!No need. I actually used it as a reference in the webpage where I address the question What is Energy? If you'd like to read it then just go to http://www.newenglandphysics.org/physics_world/cm/what_is_energy.htm
The reference at the bottom of the page reads
Quote
[2] The Feynman Lectures on Physics, Vol I, Feynman, Leighton, and Sands, Addison Wesley, (1963)(1989). Section 4-1 What is energy? page 4-1.
The other references read as follows. From reference [1] by H.A. Kramers who was talking about some of the more difficult concepts which need to be defined, he wrote
Quote
My own pet notion is that in the world of human thought generally, and in physical science particularly, the most fruitful concepts are those to which it is impossible to attach a well-defined meaning.
From reference [3] Newtonian Mechanics by A.P. French,
Quote
Of all the physical concepts, that of energy is perhaps the most far-reaching. Everyone, whether a scientist or not, has an awareness of energy and what it means. Energy is what we have to pay for in order to get things done. The word itself may remain in the background, but we recognize that each gallon of gasoline, each Btu of heating gas, each kilowatt-hour of electricity, each car battery, the wherewithal for doing what we call work. We do not think in terms of paying for force, or acceleration, or momentum. Energy is the universal currency that exists in apparently countless denominations.
The above remarks do not really define energy. No matter. It is worth recalling once more the opinion that H.A. Krammers expressed: “The most important and fruitful concepts are those to which it is impossible to attach a well-defined meaning.” The clue to the immense value of energy as a concept lies in its transformation. It is conserved – that is the point. Although we may not be able to define energy in general, that does not mean that it is only a vague, qualitative idea. We have set up quantitative measures of various specific kinds of energy: gravitational, electrical, magnetic, elastic, kinetic, and so on. And whenever a situation has arisen in which it seemed that energy disappeared, it has always been possible to recognize and define a new form of energy that permits us to save the conservation law. And conservation laws, as we remarked at the beginning of Chapter 9, represent one of the physicist’s most powerful tools for organizing his description of nature.
From reference [4],l i.e. Thermal Physics Daniel V. Schroeder wrote
Quote
To further clarify matters, I should really give you a precise definition of energy. Unfortunately, I can't do this. Energy is the most fundamental dynamical concept in all of physics, and for this reason, I can't tell you what it is in terms of something more fundamental. I can, however, list the various forms of energy - kinetic, electrostatic, gravitational, chemical, nuclear - and add the statement that, while energy can often be converted from one form to another, the total amount of energy in the universe never changes. This is the famous law of conservation of energy. I sometimes picture energy as a perfectly indestructible (and unmakable) fluid, which moves about from place to place but whose total never changes. (This image is convenient but wrong - there simply isn't any such fluid).
Since I wrote that I've been wrestling with the common definition where most physicists claim that energy is defined as the ability/capacity to do work.
Post by: Colin2B on 16/06/2016 12:37:47
Since I wrote that I've been wrestling with the common definition where most physicists claim that energy is defined as the ability/capacity to do work.Just a few thoughts rather than a definitive reply:
I think there are 2 levels to energy. At the top level I agree with majority of physicists who describe it as a property of objects and systems. I also view it is an abstract, mathematical concept that allows us compare disparate properties in a consistent manner. In this way we can talk about forms of energy, transfer of energy, energy content, storage etc and convert between different forms, almost like transferring between bank accounts and undergoing a currency conversion. Why do I think this, because this is the way I was taught to think about it. Also, it makes sense when you do the experiments to establish the relationship between, say, work and heat - you put a paddle in water, do work with the paddle, measure the heat and effectively calculate the conversion factor. It also makes sense when you consider what I think of as the next layer down.
At the next level down we have to ask what is actually happening, because there is no substance or thing being transferred or stored. In most cases the mechanics appear to involve either activity or the potential for activity. Heat being activity of molecules, light the activity of the em fields, stored chemical energy the amount of activity necessary to separate compounds, etc. In each case there is a root cause, rather than a mystical component.
Post by: JohnDuffield on 16/06/2016 14:12:48
agyejy posted some interesting observations. Now John tell me where in the post the supporting evidence for the assertion that the electron is a knotted photon was made. I can't see it.See Pete's website article: http://www.newenglandphysics.org/physics_world/cm/what_is_energy.htm
"An interesting example of energy flow is the energy flowing around an electron. An electron has a charge as well as a magnetic moment. It therefore has a magnetic field. The cross product of the electric field E and magnetic field H gives the Poynting vector S = ExH which is the energy flow per unit volume. See Figure 4 below:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.thenakedscientists.com%2Fforum%2Findex.php%3Faction%3Ddlattach%3Btopic%3D67135.0%3Battach%3D21714%3Bimage&hash=8a4f8cd72d5f581f60cb640821a268a5)
This means that there is a flow of energy going around the electron. What is it that's actually flowing? All we can do at this point is give it a name. And the name we give it is energy."
The Poynting vector for the electron goes round and round, the Poynting vector for a photon is linear:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi.stack.imgur.com%2FZZWTl.gif&hash=002a1f159a4ad3083539b03608c3ed0f)
Post by: jeffreyH on 16/06/2016 16:57:50
Show me where in this article where the Poynting vector is spinning round and round an electron.
https://en.m.wikipedia.org/wiki/Poynting_vector (https://en.m.wikipedia.org/wiki/Poynting_vector)
You try to make it look like you know what you are talking about by misrepresenting the work of others. This may be because you have no original work of your own to present.
Post by: jeffreyH on 16/06/2016 17:34:51
"How seriously must we take the physical existence of this energy? No more and no less than any other bookkeeping practices. The physical quantities are those such as the EM field."
To neglect this paragraph is to misrepresent Pete's intention for your own ends. That is not an honest way to promote your case.
Post by: PmbPhy on 16/06/2016 18:27:12
Quote from: agyejy
The energy and the electric and magnetic field components of a photon in free space are equal. Here is a reference:I'm a firm believer of admitting ones mistake as soon as possible and in this case I appear to be quite wrong. I must have remembered that wrong. So thank you for correcting my mistake. It's greatly appreciated. [:)]
Post by: Colin2B on 17/06/2016 14:52:15
Tellingly John what you neglected to post was the next paragraph from Pete's page.
"How seriously must we take the physical existence of this energy? No more and no less than any other bookkeeping practices. The physical quantities are those such as the EM field."
To neglect this paragraph is to misrepresent Pete's intention for your own ends. That is not an honest way to promote your case.
An important paragraph which aligns with light not being energy, but the oscillation of the EM fields just like any other wave where a physical property oscillates.
Post by: garth john greiner on 21/06/2016 12:16:05
I feel that energy and purpose work together to deliberately form life. ( im a strong believer in the anthropic principle). The reason i feel this way about energy is i observe how my own body produces energy or indeed struggles to as the case maybe
and i spend alot of time observing nature. My answer to this question is limited to a post big bang understanding. A little look at pre-big bang energy production might be interesting ;-)
Post by: JohnDuffield on 21/06/2016 14:55:29
Show me where in this article where the Poynting vector is spinning round and round an electron.It doesn't mention the electron, but see the static field section (https://en.wikipedia.org/wiki/Poynting_vector#Static_fields): "Although there are only static electric and magnetic fields, the calculation of the Poynting vector produces a clockwise circular flow of electromagnetic energy, with no beginning or end. While the circulating energy flow may seem nonsensical or paradoxical, it is necessary to maintain conservation of momentum".
You try to make it look like you know what you are talking about by misrepresenting the work of others. This may be because you have no original work of your own to present.I'm not misrepresenting anything. The article on pmb's website says what it says, and so does the Wikipedia article.
Tellingly John what you neglected to post was the next paragraph from Pete's page.Don't blame me if the article says What is it that's actually flowing? All we can do at this point is give it a name. And the name we give it is energy" and then contradicts itself by claiming this thing flowing round and round is some book-keeping abstraction.
"How seriously must we take the physical existence of this energy? No more and no less than any other bookkeeping practices. The physical quantities are those such as the EM field."
Post by: jeffreyH on 21/06/2016 15:15:13
Post by: PmbPhy on 21/06/2016 15:42:08
Quote from: garth john greiner
I believe that energy is a fundamental part of universal mechanics. The production of Energy in my opinion is a direct result of the behavior of chemicals interacting with each other to form anything from star systems to intelligent life forms.garth - This post doesn't address the question that I posed in the opening post
I feel that energy and purpose work together to deliberately form life. ( im a strong believer in the anthropic principle). The reason i feel this way about energy is i observe how my own body produces energy or indeed struggles to as the case maybe
and i spend alot of time observing nature. My answer to this question is limited to a post big bang understanding. A little look at pre-big bang energy production might be interesting ;-)
Post by: dlorde on 23/06/2016 13:04:21
I think is is a reasonable summary because energy is not 'stuff' that can be directly observed, it is a conserved attribute of the configuration of systems, indirectly observable in that configuration, and it is measured in terms of potential for action.
Post by: PmbPhy on 23/06/2016 16:24:03
Quote from: dlorde
Energy is an equivalence relation between different forms of potential for action.That doesn't make any sense to me. An equivalence relation is a mathematical binary relation. Please see:
https://en.wikipedia.org/wiki/Equivalence_relation#Definition
and tell me how it applies to what you think energy is.
Quote from: dlorde
I think is is a reasonable summary because energy is not 'stuff' that can be directly observed, it is a conserved attribute of the configuration of systems, indirectly observable in that configuration, and it is measured in terms of potential for action.It sounds like you mean "The capacity to do work" which is how energy is most often defined in physics texts. That definition has been shown to be wrong. In any case you've left out the defining quality of energy and that's it's property of being conserved.
Post by: alancalverd on 24/06/2016 00:32:16
Why do I believe that? For the same reason that I believe a hammer is a device for hitting nails. We invent some tools to beat the universe into the shape we want, and others for describing how it works.
Post by: Ethos_ on 24/06/2016 01:55:53
While matter contains mass/energy, it can not be defined as concentrated energy as has been falsely stated by those who are less well informed.
Matter only contains mass/energy, it is not "Mass/Energy"
Post by: PmbPhy on 24/06/2016 10:54:34
Quote from: alancalverd
Why do I believe that? For the same reason that I believe a hammer is a device for hitting nails. We invent some tools to beat the universe into the shape we want, and others for describing how it works.It appears that nobody understood what I was aiming at when I asked that question. I was attempting to ascertain where you learned it from, i.e. a text on thermodynamics, chemistry, journal articles, a combination of all of those, did you make it up yourself from your own genius, etc?
Post by: alancalverd on 25/06/2016 02:02:14
It appears that nobody understood what I was aiming at when I asked that question. I was attempting to ascertain where you learned it from, i.e. a text on thermodynamics, chemistry, journal articles, a combination of all of those, did you make it up yourself from your own genius, etc?
OK, it is what we were taught in school and have used ever since, because it is the definition used by every other scientist, just like engineers use the term "hammer" to denote a thing you use for bashing other things. And it turns out that every serious scientific text (as distinct from woo-woo, journalese or political mumbojumbo) uses the term energy to mean exactly the same thing. The fact that energy turns up in all sorts of forms is no problem - everything from the aural malleus of a weasel to the machine that turns a sheet of steel into a car body, is a hammer!
Post by: PmbPhy on 25/06/2016 06:43:02
Quote from: alancalverd
OK, it is what we were taught in school and have used ever since, because it is the definition used by every other scientist, just like engineers use the term "hammer" to denote a thing you use for bashing other things. And it turns out that every serious scientific text (as distinct from woo-woo, journalese or political mumbojumbo) uses the term energy to mean exactly the same thing. The fact that energy turns up in all sorts of forms is no problem - everything from the aural malleus of a weasel to the machine that turns a sheet of steel into a car body, is a hammer!I don't disagree with you that this is how people come to learn what energy is. However I've never seen a physics text define energy by stating that its conserved and stating its dimensions. That definition doesn't even work in all instances. For example: it doesn't work for non-conservative systems. In such systems energy is defined but not conserved. An example is a charged particle in a time varying electromagnetic field.
Nearly all textbooks define energy as the ability/capacity to do work. Two notable exceptions are Feynman and Glashow, two well-known Nobel Laureates. The explain that energy can't be defined and give the reasons why. They then explain what it is. If that's confusing to some members then note that defining and explaining are not the same thing. In math the term set is an undefined term. Its explained by giving examples of it and rely on our common sense to understand it.
Recall what Feynman wrote in his lectures on energy
Quote
It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and we add it all together it gives “28” - always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.
Let's
Post by: agyejy on 25/06/2016 08:25:12
For example: it doesn't work for non-conservative systems. In such systems energy is defined but not conserved. An example is a charged particle in a time varying electromagnetic field.
Any non-conservative system can be made into a conservative system by an extension of the boundaries of said system. Since all systems are defined relatively arbitrarily by some human that is studying some process or another such redefinitions of the boundaries are always possible. In short as far as we have been able to observe energy is always conserved humans just sometimes draw arbitrary boundaries or forget to account for certain types of energy. For example, a system of a charged particle in a time varying electromagnetic field is conservative if you consider the source of the time varying field and any radiation the charged particle might emit (assuming there is nothing for it to collide with but if there is you just include that too).
Too bad you're apparently ignoring me for no real reason I can discern besides the fact that I didn't agree with you on something.
Post by: alancalverd on 25/06/2016 08:43:44
Ultimately all scientific definitions tend towards "that which...." because science begins with observation. You could define hydrogen as "any number of atoms each of which consists of a single electron and a single proton" but some brilliant philosopher will point out that you haven't defined a proton, so chemists define hydrogen as "the element which..." and for most purposes it turns out that adding one or two neutrons doesn't alter its chemical characteristics, so it's a good definition - and applicable to all other elements too.
Your nonconservative system is excluded from the definition I gave because it is not a closed system. Friction is a nonconservative force but in a closed system the kinetic energy of your car is entirely converted by brake friction into heat energy. If the energy of a closed (i.e. complete) system were not conserved, we couldn't calculate the temperature rise of the brake disc (or re-entry module), which is kinda important when you want to build a machine that can stop without melting. You might argue that the definition now becomes circular as I've effectively defined a closed system as one in which energy is conserved, but it isn't because we can in principle measure every element of that parameter.
Post by: guest39538 on 25/06/2016 11:26:45
If you want to play linguistics, you could start by claiming that momentum isn't defined - or even mass, for that matter. But we physicists try to build useful mathematical models of the universe rather than muck about with Aristotelian definitions, so if ∑mv turns out to be conserved, we give it a name because it is useful.
Ultimately all scientific definitions tend towards "that which...." because science begins with observation. You could define hydrogen as "any number of atoms each of which consists of a single electron and a single proton" but some brilliant philosopher will point out that you haven't defined a proton, so chemists define hydrogen as "the element which..." and for most purposes it turns out that adding one or two neutrons doesn't alter its chemical characteristics, so it's a good definition - and applicable to all other elements too.
Your nonconservative system is excluded from the definition I gave because it is not a closed system. Friction is a nonconservative force but in a closed system the kinetic energy of your car is entirely converted by brake friction into heat energy. If the energy of a closed (i.e. complete) system were not conserved, we couldn't calculate the temperature rise of the brake disc (or re-entry module), which is kinda important when you want to build a machine that can stop without melting. You might argue that the definition now becomes circular as I've effectively defined a closed system as one in which energy is conserved, but it isn't because we can in principle measure every element of that parameter.
Sorry this may be a little off topic but I have a question and will not reply to the answer.
Is ''light'' passing through space a conserved ''system''?
Post by: dlorde on 25/06/2016 19:20:26
... tell me how it applies to what you think energy is.When I said 'equivalence relation', I meant it allows us to compare or relate different forms of energy using a common measure. Double-entendre...
Quote
It sounds like you mean "The capacity to do work".Action isn't work [;)]
Quote
I think you haven't read what I posted carefully enough (I've bolded the relevant part) [:)]Quote from: dlorde... energy is not 'stuff' that can be directly observed, it is a conserved attribute of the configuration of systemsIn any case you've left out the defining quality of energy and that's it's property of being conserved.