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What's the definition of energy?

  1. Sep 10, 2003 #1
    Today my teacher told me that there is no definition for energy , because according to him it cannot be described. He then went on justfying it by comparing it to Good and Evil , and Big and small. I said that it can and should have a definition because unlike big and small , or good and evil , it exists and is not a comparison of things , so thus can have a definition. I gave him my definition of energy which is "A force that has the ability to move matter from one point in space to another" , he said that I was wrong because it cannot be described and that every other physicists would agree with him.I also went and look in the physics text book and looked up the definition of energy and I got "A non - matter property that is capable of changing matter."

    Is he right? Can energy be defined? If it can be defined what is your definition of it?
  2. jcsd
  3. Sep 10, 2003 #2


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    You have a very smart teacher. This subject came up before. This is how I explained it then. See --

    Tell me something - Do you know what life is? If so then defined the term "life."

    re - "A force that has the ability to move matter from one point in space to another" - That describes one characteristic of energy. But suppose when you said "the ability to move matter" someone else thought "Oh! He means momentum. Yes. Momentum can do that."

    What you've done is to say that energy is a force and then you started to characterize the force.

    re - "Is he right?" - Yes. Absolutely.

    re - "Can energy be defined?" - No.

    Richard Feynman explains this very nicely. See


  4. Sep 10, 2003 #3


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    the root classical definition is "the ability to do work"
    "ergon" is Greek for "work"

    "en" ergon is the potential for work "in" persons or things or situations or systems

    The root classical definiton of energy doesnt fit every situation, you know that there is always some play and flexibility in how people use words----and words are able to GROW in meaning and take on addtional uses.

    One variant of the classical definition would be ability to produce some amount of heat. When you bring a moving car to a stop by applying the brakes the energy of the car's motion is converted to heat in the brake-pads or the disks. The car had some amount of ("kinetic" or motion-) energy---some number of joules---and that got converted to the same number of joules of heat.

    when regenerative braking in an electric car is used the motion energy is used to generate electricity to charge the battery---it is only partly efficient and some gets turned into heat. But some number of joules of motion-energy gets turned into the same number of joules of electricity and gets stored as chemical energy in the battery.

    height is often translatable into gravitational potential energy:
    coast down a hill and the graviational potential is turned into
    energy of motion, which may be enough to carry you partway up the next hill (so grav. potential becomes kinetic which then becomes grav. potential again)

    at any point in these processes you can slam on the brakes and convert the quantity of energy that is in flow from one form to another suddenly into heat. So you can always measure it: say how many joules of heat it is.

    or "calories" if you like, or "footpounds" or "kilowatthours" there are a dozen different units for energy but the metric one is joules.

    Claiming that there is "no definition" can be marginally justified
    because (1) it gets the students' attention and (2) the definition is an open one-----new forms of energy are discovered from time to time, and new processes (like fuel cells) to convert from one form to another, so that one cannot once and for all LIST all the
    various forms of energy and all the various conversion formulas.

    But I would argue that even tho the definition is open and the usage of the work is somewhat flexible there is still a core idea
    which can be clearly explained
  5. Sep 10, 2003 #4


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    I still say that energy's observed conservation and transmutability - which covers the examples you guys are using - are properties that every theory must be consistent with. But it's only with respect to relativity theory that energy can be defined and it's conservation and transmutability understood in a more fundamental way since it's essentially the charge that gravity couples to, which is the definition I've given a number of times.
  6. Sep 10, 2003 #5


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    I looked in a dictionary and there is most certainly a definition for the word "energy" in it.
  7. Sep 10, 2003 #6
    I will not mince words: your
    teacher is insane. The
    word energy is defined in many,
    many, many dictionaries.

    Here's the problem with claiming
    energy cannot be defined:

    In order to make any accurate
    statement about the word energy you have to already believe the word means something,
    and that you understand what it
    means. If you don't then you can't
    claim any statement you make about
    the word to be accurate. You can't
    say "It cannot be defined," and
    claim that statement to be ac-

    If you understand the word energy well enough to make
    any accurate statement about it,
    you could define what you're
    talking about. Some people are too
    inarticulate or lazy. They blame
    language in general, and words in
    particular. Other people think
    being vague is cool. It isn't.
  8. Sep 11, 2003 #7


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    If you look in a dictionary under the word "life" you'll see that it's defined as well. However the scientific community would not go by what the dictionary says. Same with energy, contrary to what you may hear about it being definable in relativity. See Feynman quote - the man really knows what he's talking about.
  9. Sep 11, 2003 #8


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    Re: Re: What's the definition of energy?

    I agree that one can give a touchy-feely definition but it's far from being rigorous. For example: Please define the following term - "life"

    We can use that as an analogy as to why energy can't be defined. A.P. French explains all this very nicely in his text "Newtonian Mechanics." Do you happen to have this text?

  10. Sep 11, 2003 #9


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    There is a difference between being wrong (or lazy) and being insane. For one thing, "insane" is a medical diagnosis and you can be sued for libel for attributing an illness to a person who does not, in fact, have that illness.

    Have you read the other responses? The question about the definition of energy was a physics question. There are several definitions of energy in the dictionary, none of which apply to physics. Historically, energy has been "defined" in physics by repeatedly extending the concept in order to maintain "conservation f energy". Now, we would include mass itself in the concept. The teacher was saying that there is no one, simple, finished, definition of energy. That's quite correct, in my opinion. One might differ with it but it's certainly not "insane".

    Unless, of course, by "insane" you mean "disagrees with me". If you do, that's YOUR problem.
  11. Sep 11, 2003 #10
    "Insane" is no longer used in the
    medical profession. You will not
    find this word in the current
    edition of the Dignostic and
    Statistical Manual of Mental
    Disorders. Doctors now use the
    term mentally ill. My use
    of the word falls in the catagory
    of hyperbole and I'm sure
    you knew that.

    Yes, I've read them. My dictionary
    has four definitions for energy,
    two of which apply to physics.
    According to Wasper his teacher
    was going very much further than
    this. He was saying there is
    no definition of energy
    and no way to describe energy.
    Strong language to assure Wasper
    I view his teacher as someone
    who is indulging in being gratuit-
    ously confusing.
    This is snide and detracts from
    the rest of your argument.
    Last edited: Sep 11, 2003
  12. Sep 11, 2003 #11
    Everywhere that I have read, looked been taught including this forumand this thread energy is attemped to be defined by what is does and some of its properties and relationships with other phenomena. No where have I read,seen or heard what energy IS. It is a quanity of somehing that is todate undefined that has certain properties and characteristics and can do thing or ave effecton material thing and can be transformed into matter and visa versa.
    In short your insane but very smart teacher is right. Energy is not and to date cannot be defined only its properties described.
  13. Sep 11, 2003 #12

    Chi Meson

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    One thing that I always hated about the definiton of energy as "the ability to do work," is that later in the course, when we get to thermodynamics, I have to say that I lied, because there is energy out there that is "unavailable for work."

    Another problem with the definition is that "work" is defined in terms of energy ("work is the transfer of mechanical energy.") So, officially, these two words lead in a cirular logic known as "begging the question."

    I try to have my students think of energy more as "the ability to cause a change in an object or its environment", but that too has pitfalls.

    So in a way I agree with Wasper's teacher that the idea of energy is very hard to grasp and no single definition covers it satisfactorily. But I do believe it does have a definition, it just takes a long time to say the whole thing.
  14. Sep 11, 2003 #13


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    Actually, check it out (dictionary.com cites multiple dictionaries): ENERGY
    They seem pretty reasonable to me.

    Now clearly being an abstract concept and not a noun like "rock" its a little tougher to define than some words. But that doesn't make it undefinable. I wrote a paper in college where my english prof had us define an abstraction. 3 pages later, "music" still boiled down to "Vocal or instrumental sounds possessing a degree of melody, harmony, or rhythm." Like other abstractions, music, energy, love, etc., the definition exists but it can be as simple or complicated as you want it to be.

    RE: Feynman. I haven't read much from him (I really should read more). But it sounds like he's as much a philosopher as a physicist. Thats fine, but part of that is tendancy to overcomplicate definitions. Thats pretty much what philosophy is: the art of making simple concepts complicated.
    Last edited: Sep 11, 2003
  15. Sep 12, 2003 #14


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    So "insane" is not true, but strong language for effect; the same as you are accusing Wasper's teacher of using.

    On energy, I happen to agree with Feynman: there is no complete, consistent definition of energy. I remember learning from a teacher that "energy is the ability to do work". I thought that was pretty lame, but now, years later, I cannot come up with anything better. All we have from physics is a way to assign a number to it, and the knowledge that our technique of assigning a number to it results in its being conserved.
  16. Sep 12, 2003 #15
    Energy - Physics. The capacity of a physical system to do work.
    Work - Physics. The transfer of energy from one physical system to another.

    Substitute work in definition of energy:
    Energy - The capacity of a physical system to do transfer of energy from one physical system to another.

    Or, substitute energy in definition of work:
    Work - The transfer of the capacity of a physical system to do work from one physical system to another.

    It can be described only in terms of what it does, but not what it IS. Sounds circular, but it only tries to put concepts into relationship, escaping need to answer what it really IS, and leaving it open to options.
    Its abit similar with concept of Time. Its hard to say what it IS, much simpler to describe how it manifests and how to measure it.

    As I've understood, question of what energy IS is regarded as metaquestion by science, or one which it doesn't want to deal with, as it will inevitably lead to philosophical speculations. Science tries to dismantle concepts into components that can be dealt with, and concepts of energy and time can't be dismantled any further. They are fundamental concepts without cause, somewhat similar to postulates in theories that do not need proofs.

    thats a layman view.
  17. Sep 12, 2003 #16


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    But energy may not be conserved in general relativity.

  18. Sep 12, 2003 #17


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    Re: Re: What's the definition of energy?

    Was Feynman insane too?
    Thats incorrect. Science is precise whereas normal language isn't. In science one does not always have a rigrous definition of the more basic concepts. Take time for example. This too is another term wich defies a rigorous definition. Same with the term "life." In normal language we go by experience and not quite exact terms. I have a vauge feeling of what the word "life" means but I can't define it.

    In fact even in mathematics this is true. One starts with terms which are undefined. The word "point" is an example of an undefined term in geometry.
    It has nothing to do with being inarticulate or lazy.

  19. Sep 12, 2003 #18


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    My point was that all theories must be consistent with the observed conservation of energy, but that doesn't mean that defining energy in terms of it's observed conservation is the optimal definition. Scientific terms take they're meaning from the theories in which they're couched, so what we're looking for is the most appropriate theory with respect to which energy should be defined. If conservation of energy seems too speculative - and it really isn't too speculative - than find another definition. Since energy's local conservation and transmutability is most directly understood in relativity as a direct result of the geometry of spacetime, relativity seems to be a good choice. So what is the most general definition we can make in terms of relativity? My definition is optimal because in as much as gravity couples always and only to energy, my definition is both completely general and unique.
  20. Sep 14, 2003 #19

    You asked me:

    "Was Feynman insane too?

    What this question means is that you believe Feynman said the same thing Wasper's teacher
    said about energy:

    "Today my teacher told me that there is no definition for energy, because according to him
    it cannot be described."

    The only Feynman book I've read is Six Easy Pieces. I am not sure, therefore, if he specifically addressed the problem of actually defining energy elsewhere, (If he did you will have to tell me where to look) but nothing he said in Six Easy Pieces agrees with Wasper's teacher.

    I have located two sentences that, if read carelessly, could be misinterpreted as saying
    energy cannot be defined:

    "It is important to realize that in physics today, we have no idea what energy is. We do not have a picture that energy comes in little blobs of a definite amount."
    pp.71, 72

    In order not to misinterpret what he is saying it is necessary to notice his italics in the word is. These italics mean the reader is to imagine the sentence spoken with emphasis on the word is (This is a transcription of a lecture, remember.)

    What I understand him to be saying, therefore, is that physics has not, so far, arrived at an understanding of the essence of energy such that we can say anything as specific in general,about it as it all "comes in little blobs of a
    definite amount."

    This is different than saying it cannot be defined. We can define the word "electron" but can anyone say what an electron is? We have gathered an enormous amount of information about the properties and behaviour of electrons, but can you tell me what an electron is?
    That italicized is is a query into the deepest nature of whatever it is applied to. Apply it to anything you want, no matter how obvious and well defined its nature seems to be, and you will find that it automatically pushes the subject into terra incognita. It makes us start pondering what we don't know, instead of being mindful of what we do know.

    Asserting that physicists do not know what energy is should not be misconstrued as meaning it cannot be defined. Did Feynman ever explicitly say energy cannot be defined? I don't know. I will have to rely on you, Pete, to find where he did, if he did, and point me to it.

    Wasper's teacher said energy cannot be defined because it cannot be described. On this point there is no doubt Feynman would vigorously disagree with Wasper's teacher. Feynman spends all of chapter 4 describing energy. He describes Gravi- tational Potential Energy, Kinetic Energy, Elastic Energy, and Heat Energy in detail, and gives thumbnail sketches of other forms (no time for details about these, he says). And he describes al these in reference to their shared obedience to the law of Conservation Of Energy.

    So, back to the question "Was Feynman insane too?" It doesn't look to me like Feynman agreed at all with Wasper's teacher. It is certain he
    completely disagreed with half of what Wasper's teacher asserted. I await your evidence on the other half of the issue.

    I will respond to the rest of your post later.
  21. Sep 15, 2003 #20


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