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Can someone help me to better understand energy

  1. Nov 12, 2009 #1
    I think that until this point I have always held the misconception that energy was the transfer of electrons, but I have come to realize my mistake. I have tried using wikipedia's definition of energy to better understand, but I don't think I am really getting it.

    When you take a measure of energy what exactly are you measuring? Is energy a term that describes several different phenomena?
     
  2. jcsd
  3. Nov 12, 2009 #2

    mgb_phys

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    The normal definition is that "energy is the capacity to do work" which is completely useless, since the only way of defining work is the same statement.

    The technical definition of energy is do with entropy - but gets a bit complicated.
    A good way to think about it is that energy is what you need to put into a system to put it into a less stable or more disordered state
     
  4. Nov 12, 2009 #3
    :smile: Don't worry, I really want to get into the actual definitions of energy. If you end up going over my head I'll do the reading I need to better understand. I want to be an engineer and I want to specialize in physics so I will have to really understand this eventually.

    Complicated is good, if the eventual result is my understanding how energy operates this thread will be a success.
     
  5. Nov 12, 2009 #4
    It's not complicated luckily. Energy is the thing that never goes up or down in a closed system, no matter what happens. It can 'convert' between many forms, but when you add all the values for the different forms up (kinetic, potential, electrical, chemical) it should always be the same in a closed system.
     
  6. Nov 12, 2009 #5
    As an aside, most engineers do not need to know what energy "is". I assume you are an early undergraduate student in engineering, and I am basing the following statements on that assumption:

    Take it from someone who spent way too much time trying to understand the minutia of physics in his undergraduate engineering classes. Unless you are going to be in a very specialized electrical engineer, it will not matter what "energy is". Indeed, if your job depends on this understanding, most likely you are a scientist rather than an engineer. As an engineer, you absolutely must know what sources of energy you have, and how that energy behaves (ie: how it is transferred from one state and/or object to another), but you do not need to know what energy is truly based on.

    While I am warning not to get obsessed with this matter at the expense of learning other more applicable knowledge, I do applaud your curiosity, and hope you find the answer you are searching for.
     
  7. Nov 12, 2009 #6

    mgb_phys

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    So how is that different from angular momentum, spin, parity or charge?
     
  8. Nov 12, 2009 #7
    Could 0 Kelvin be described as the absence of energy?
     
  9. Nov 12, 2009 #8
    Well, damn...
    Angular momentum is kinetic so I guess that fits in pretty well. As for spin and charge, they could be lumped into conservation of energy right? Since those values are also conserved it would make no difference I think. And parity is a symmetry relationship, and I don't know how it would fit in but I'm guessing it could similarly.

    Seems like there is some arbitrary-ness in the distinction IMO. But I'm probably profoundly wrong.
     
  10. Nov 12, 2009 #9
    I don't think you can do that, there is something called the 'zero-point' energy, which is simply the minimum energy that the system can have. But assuming you could get to zero kelvin, yeah, it would be pretty much the absence of energy... you could freeze stuff out of existence! (once again warning on profound wrongesses)
     
  11. Nov 12, 2009 #10
    For all practical purposes all energies that we know are one of the following
    • kinetic energy due to speed of some massive object (or atoms or electrons)
    • potential energy due to a force (gravitation between masses, electrostatic between charged objects, magnetic dipoles in magnetic field) between each pair of objects
    • energy from electromagnetic waves (light, radio waves, x-rays)
    • mass energy according to [itex]E=mc^2[/itex]
    There is no other.
    Not all energy is extractable though, since the second law of thermodynamics can constrain which energy you will be able to extract by any real process.
    And energy is conserved (this can achieved by fiddling with the definition of potential energy)
     
    Last edited: Nov 12, 2009
  12. Nov 12, 2009 #11
    Apart from quantum mechanics ideas about zero point energy, classically 0K could be taken as the statement, that there is no "kinetic energy" (see my first point). You still can have "potential energy" and "mass energy".

    And btw, chemical energy is a combination of mass energy and potential energy.
     
  13. Nov 13, 2009 #12
    "What is energy" is kind of a fundamental question, which I'm not sure can be answered in a way that would satisfy you. It's kind of like..."what is mass". "What is space". "What is existance".

    At least that's my take on it. Maybe there is some finite answer rooted deep in quantum physics, but either way I don't think you or me will find a true understanding of it. At least yet.
     
  14. Nov 13, 2009 #13

    Dale

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    Why do you say that? Work is force times distance. It is easy to define non-circularly.
     
  15. Nov 13, 2009 #14

    A.T.

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    You never measure energy directly. Energy is an abstract concept, just like force. It is an quantity that you calculate from measurable quantities. It is useful because it is constant for a closed system.
     
  16. Nov 13, 2009 #15
    If it moves or if it can make things move it has energy.Look around you,everything you can see and things which you can't see but which you know are there has energy.The mouse on your desk has energy,the full stop at the end of this sentence has energy,everything has energy.Matter is the stuff and energy the thing that can move the stuff. Even when it's stationary matter has stored energy because it is able to make things move.
    It's more complicated than this and to get a fuller understanding you need to look at the definition of work,the different forms of energy and how they are calculated and,most interestingly,the conservation of energy.
     
  17. Nov 13, 2009 #16
    So energy is not any single thing/substance. It is a term used to describe anything that has movement or the ability to move. Is this a correct statement? Does energy stop being a factor when you remove time from the equation?
     
  18. Nov 13, 2009 #17
    Like my description above I would say it's a reasonable statement but to define energy properly you need to get mathematical.I don't know how to remove time from the equation but ,and this is a total guess,I am assuming that for many engineering applications a highly detailed knowledge of energy may not be necessary.
     
  19. Nov 13, 2009 #18
    If you remove time from the equation then you remove physics from the equation.
    I do not understand why so many people come up with this "time removal" idea.Why not space removal? It is as essential to physics as time but nobody asks "What happens when we remove space from the equation?"
     
  20. Nov 13, 2009 #19
    Not a single at least. Movement (kinetic) is OK. Ability to move is very vague. Let's say is the contribution from all forces pull that object. Now you should also include that every mass has an energy, and that electromagnetic waves have an energy. Then you are indeed correct.

    It's rather hypothetical, philosophical but true: the energy concept is useless if you have only one instant of time. Energy was invented to have a new parameter which is independent of time and can help you to solve problems.
     
  21. Nov 13, 2009 #20
    Its not so much that I feel that I would need to have a firm grasp of energy to be an engineer, but I want to have a firm grasp of energy. I don't know if I can express how stupid I felt at having though energy always had to do with the transfer of electrons from one thing to another. That was a misconception that made Chemistry rather difficult.

    Energy is not something that can be put into a bucket and weighed. We only know of its existence from its effects from one object to another. Right?
     
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