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What is the definition of energy?

  1. Nov 27, 2008 #1
    i want to intiate the discussion on energy?Although its a very basic and general topic, i request u all to participate to study it thoroughly......

    how do u define energy?
    According to common definition given by all, it is the ability to do work. But work is also a form of energy. So i dont think that is the correct way of defining energy. And in one book, i read heat and work are energy in transit and remaining all are stored energy(Ref:thermal engineering by mathur and mehta). In other book, i read all the forms of energy can be grouped into two categories:stored(potential) energy and kinetic(energy in motion)........No Definition is clear....

    So What is the definition of energy? "That definition should perfectly define all forms of energy, say potential, kinetic, heat, thermal,work, internal,nuclear, chemical".
    What is the link between energy, properties of a system? Why should really the energy flows from one body to another?

    need ur valuable points!
  2. jcsd
  3. Nov 27, 2008 #2
    Good question, but you will not get some simple explanation (say a one liner any better than "the ability to do work") covering all aspects of energy.

    For one thing at a fundamental level nobody knows the precise origin and nature of energy. We have lots of explanations for it's different forms as heat, kinetic, potential, and so on, but at it's heart is a complicated area. When the universe originated energy,time,gravity, space, all seemed to pop into existence from either a quantum fluctuation or maybe a big bang...maybe something else. But exacty why all emerge together is not really clear. We do not understand fundamentally how they all relate. But we have many descriptions (formulas) which work well but if they can be combined into some grand unified energy framework, it hasn't been discovered yet.

    Why can't we precisely measure energy/time together?...it's a form of the more well known Heisenberg uncertainty principle regarding momentum/position...It may be that at tiny tiny distances and volumes everything is energy..hence at Planck size, even space and time may vanish in a "quantum foam" of energy...it's perhaps the fundamental constituent of our universe...in string form, maybe we are made of vibrating strands of energy...

    Another thing to consider is that energy is everywhere. Cool a mass to absolute zero....it still has a finite energy called zero point energy. Empty space: same thing!

    Energy manifests iitself in different ways because the universe underwent a phase transition from extremely high energies and temperatures where things were unstable to a lower temperature, lower energy far more stable universe we inhabit. So the initial symmetry was broken and energy disassembled into many pieces as did a unified single force...
  4. Nov 27, 2008 #3


    Staff: Mentor

    No, work is f.d
  5. Nov 30, 2008 #4
    Simply stated, energy is that which must be released and applied in order to perform work. For instance, it requires energy to move an automobile.

    On the other hand, work is the end result of that released and applied energy which produced a force that moved the car a given distance.
  6. Nov 30, 2008 #5

    Obviously we all are familiar with Einstein's equation. The traditional definition of energy being that which can do work is a simple way of describing energy intangibly. However, energy can be expressed without actually doing work. Mass can be converted into energy, and mathematically vise-versa. But it takes work and therefore the expenditure of energy to make these conversions, they are not self starting. What about gravity? Gravity, which as we know, is the result of mass intruding into and bending space thus causing space to push objects toward each other. A falling object is evidence of gravitational force and energy being released and acting on the bodies of mass. So is gravity then a form of energy? It is a result of the effects of mass in space. And thus we have energy, mass and space, and time related to each other. String theory suggests that all matter, energy, and space are composed strings vibrating in various frequencies, differentiating space and the various forms of matter and energy. Time, in string theory, is quantified by the frequencies at which the strings oscillate. So can we deduce that all of existence, that is time, space, matter, are all composed of pure energy?
  7. Nov 30, 2008 #6
    So is energy
  8. Nov 30, 2008 #7


    Staff: Mentor

    No it's not. For example, if an object of mass m is hurtling through deep space at velocity v then it has a kinetic energy of 1/2 m v², meanwhile the force is 0 so f.d is 0. There is no work but there is energy.

    The object has the ability to do work, e.g. by colliding with some other object and thus exerting a force on the other object over some distance, in which case the work done would be equal to the change in energy, but the energy is not work and is therefore not f.d.
    Last edited: Nov 30, 2008
  9. Nov 30, 2008 #8
    Work is energy because it is measured in Joules. Fx and mv^2/2 are both kg m / s^2
  10. Nov 30, 2008 #9


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    They are talking about thermodynamics. But you can see the essential point in Newtonian mechanics already. The work-energy theorem says "work done equals change in kinetic energy", ie. "f.d"="mv2/2". The work done "f.d" is really an integral over a path in space, so it is generally path-dependent, and you cannot tell the amount of work done on a particle by knowing its position and velocity at a particular moment, you need to know its previous history, which is why they say work is energy in transit. In contrast, you can tell the kinetic energy of a particle by its instantaneous velocity without knowing its previous history.
  11. Dec 1, 2008 #10
    thats an amazing response from all of u....thank u...
    Now can i take like this....
    Energy is of two types, energy in transit(work,heat,mass) and stored energy(all other forms). Every body(matter) will have some amount of energy associated with it(stored energy), by which it is capable of doing work. When it does work, energy is said to be transferred from that body to other(energy in transit). So when the energy is transferred, the total energy of one body(source) will reduce and the other body's energy increases(law of conservation). Am i going in the correct way?

    Now,what is the criteria(or factors) that make the energy flow from one body to other? I mean if my above understanding of energy concept is correct, then when will the body do work on the other body?
    Last edited: Dec 1, 2008
  12. Dec 1, 2008 #11
    Here I restrict myself to classical (non-quantum, non-relativistic) mechanics. Sorry to be so abstract, but to me energy is defined to be a quantity that is conserved due to the fact that the laws of physics do not vary over time. Of course, if you're not looking at the entire universe, energy would in general not be conserved in a system where external effects on it vary over time.

    The energy can be divided into two parts: kinetic energy (which arises from the motion of particles) and potential energy (which arises from the position of the particles relative to each other, and perhaps some external effects as well). Kinetic energy includes macroscopic motion of objects, as well as thermal energy (which is typically microscopic vibrations of atoms and molecules).

    Potential energy is that which is stored in fields (gravitational fields, electromagnetic fields); all other forms of potential energy you may have heard of are really special cases of one of these two (usually the latter). Elastic potential energy is really electric potential energy due to separation of electric charges in the material being stretched; chemical energy comes from interactions of charges in the atoms within molecules.

    A proper treatment of chemical energy really involves quantum mechanics, and likewise nuclear energy involves special relativity.

    By that logic, torque is energy because because it has the same units (kg m2 s-2; you forgot a factor of m). But that is absurd.
  13. Dec 1, 2008 #12
    thanks adriank, that was great explanation. now its very clear....
    Now lets move onto next topics...
    what r the factors that make the energy flow from one body to other body?

    As we see, in this universe, there r different bodies with different(both in quantity and type) energies associated with them, like book on a table, moving car etc. Suddenly what makes them to transfer energy to other body and thereby decreasing their energy? what is the criterian needed for the energy to flow?
  14. Dec 1, 2008 #13


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    In mechanics it's the force laws (eg. gravity, electromagnetism, friction etc) that determine how particles interact. It's the same in thermodynamics, but there are now two sorts of energy exchange heat and work, and your ability to get work out depends on the temperature difference, it's much easier to exchange heat and have no work done.
  15. Dec 1, 2008 #14


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    "The first law states that both work and heat are forms of energy, and that the total energy is conserved." Kardar, Lecture 1: http://ocw.mit.edu/OcwWeb/Physics/8-333Fall-2007/LectureNotes/index.htm [Broken]
    Last edited by a moderator: May 3, 2017
  16. Dec 1, 2008 #15
    That's thermodynamics, and I don't like how they say work and heat are energy. I really disagree with that. You can't say "system A has W amount of work and Q amount of heat," because the amount of work done and the amount of heat transferred to a system depend on the path the system takes; work and heat are not state functions. In some special cases (such as when pressure or temperature is held constant), there are state functions (enthalpy for constant pressure, and Helmholtz free energy for constant temperature) whose changes correspond with amount of work done or amount of heat transferred, but in general this is not the case.

    The sentence you quote is probably better reworded to say that work and heat are changes in energy. A system may go from a particular state to another particular state in many ways, and in each of these the amount of work done and the amount of heat added may individually vary. However, the sum of the work done and the heat added will always be the same; this is the change in the internal energy of the system. This is described in the very page of the lecture notes that you quoted.
  17. Dec 1, 2008 #16
    yes atty, this is what i understood from all of ur explanation and i am putting it in proper order below....

    A body has some amount of energy associated with it, which would be either in the form of potential energy or kinetic energy.
    Energy flows from one body to other in the form of work or heat. For work to be done, there should be imbalance in the forces(pressure incase of fluids), thereby the resultant force will cause displacement, which is then work is said to be done. For energy flow in the form of heat, there should be a temperature difference. In both the cases, we will see only the end result i.e increase or decrease of either potential energy or kinetic energy of the body.
    Am i right till now?
  18. Dec 1, 2008 #17
    That sounds good to me. Work and heat quantify energy flows; they are not themselves forms of energy.
  19. Dec 1, 2008 #18


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    Sounds good to me too. I don't think it's a big deal the actual words (I was quoting directly from Kardar's notes). The important thing to remember is heat and work are path dependent (so people often put slashes through "dQ" and "dW" to indicate that, whereas S,T,U are state variables). The one thing I'd correct is that in thermodynamics, a temperature difference is required to extract work (Carnot engine efficiency depends on temperature difference). The difference between thermodynamics and mechanics is that in the former, you don't have control over the individual molecules, you only control macroscopic variables like pressure, volume, temperature. Statistical mechanics is the discipline that tries to relate macroscopic thermodynamics and microscopic mechanics.
  20. Dec 1, 2008 #19
    that was great participation from all of u....i learnt a lot
    we shall continue this discussion further... i mean with related concepts....as of now, i am reading abt energy and laws of motion, properties of matter etc...i will come up with doubts in short time...dont go anywhere..plzz
  21. Jan 1, 2009 #20
    Many of the arguments regarding energy insist that energy is a property of matter, that it is not tangible, nor is it anything but what we define as 'work'. These are skewed and inaccurate definitions.
    Consider a photon. It has no rest mass, but it has energy. This basically means it has no shape or size, but merely kinetic energy. A photon couldn't be an object without energy, so energy is not merely a property of an object, it is sometimes what makes an object an object.
    The quantification of energy insists that energy can travel on it's own anytime, whether it be in the form of photons or gravitational waves. So the question 'What is energy?' is valid. We know what matter is made of (to an extent) but noone has defined energy as anything more than a particle without mass.
  22. Jan 2, 2009 #21

    Ranger Mike

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    Force is the agent of change, Energy is a measure of change. Work is the change in the energy of a system resulting from the application of a force acting over a distance.

    " As we cannot give a general definition of energy, the principle of the conservation of energy simply signifies that there is something which remains constant. Well, whatever new notions of the world future experiments may give us, we know beforehand tha tthere will be something which remains constant and which we shall be able to call energy"
    Henri Poincare (1854-1912)
  23. Jan 2, 2009 #22
    I have a question I would like to raise on a broader philosophical issue of our understanding of energy.

    I have come to a conclusion that their is no evidence solid state particals exist. All energy appears to be formed from fields such as the electro magnetic and strong nuclear force acting around an infinitely small origin. Is this correct? As I find it hard to accept that all forms of matter are fields which compress and expand like waves.

    I have come to this understanding from the following ideas. Please let me know if my understanding is flawed.

    1. No energy (boson or fermion) is ever in a rigid state anywhere in the universe e.g. all objects and fields no matter how dense can be further compressed in all dimensions with sufficiant force.

    2. Electro magnetic force and Pauli repulsion are the reason matter has a rigid like property. However they are both properties based on the idea of fields.

    The reason I am perplexed by the idea of all energy being fields with no rigidity is that it means all objects contract (to some extent) when put in contact with another object and then expand again at varying speeds. For example, tapping a pen on a desk would actualy cause that pen to contract along the axis of applied force then expand again reletive to the force applied. Is this correct?
  24. Jan 2, 2009 #23

    Ranger Mike

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    from what you was saying..

    "I have come to a conclusion that their is no evidence solid state particals exist. All energy appears to be formed from fields such as the electro magnetic and strong nuclear force acting around an infinitely small origin. Is this correct? As I find it hard to accept that all forms of matter are fields which compress and expand like waves."

    you want to know about particals?
    how energy is formed?
    or matter acting like waves?
  25. Feb 28, 2009 #24
    When we say that their are four fundamental forces are we saying that the universe is nothing more than a complex arrangement of those forces. Because if nothing in the entire universe is ridged, then it would seem that the entire universe is one enormouse field built with four component forces. Maybe i'm getting field and force mixed up?

    Any help would be appreciated....
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