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What is Energy?

  1. May 18, 2009 #1

    r16

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    I've been throwing this term energy around for a while now, and thinking about it I have absolutely no idea what it is. Is it something that actually exists in the universe, or just a construct that we use to simplify problems?

    Terms like kinetic energy, and even gravitational potential energy (from a Newtonian standpoint) are a little bit easier to understand because they are exist in everyday life, but when you get E&M (I've only studied classical E&M) you have this idea of a field, the field has energy and you need to calculate it? How does a field have energy?

    Consider a conducting bar, moving on rails with a large resistor at one end bathed in a strong, uniform magnetic field. As the bar moves, the magnetic flux increases, inducing a current in the circuit and thus energy is lost in the resistor as heat, so the bar must be slowing down (it never actually stops even though it moves a finite distance, as i've calculated). I can ascertain what happens in this situation because I know about E&M, but it bothers me that I don't feel like I understand what is actually happening. If you showed me that and I didn't know physics, I would say the bar moves an infinite distance. How can some invisible field (with energy) stop a real, moving object? This is the whole idea of transfer of this energy which I don't know what it is?

    Is there a good model of a physical interpretation of this phenomenon? Is it just a mathematical construct?
     
  2. jcsd
  3. May 18, 2009 #2
    Here is a real physical example of a conducting bar moving in a non-uniform magnetic field. I was working in a group using Enrico Fermi's cyclotron magnet for bending a very high energy beam of muons. The pole tip diameter was about 13 feet. We had opened up the gap so it was roughly 40 inches. The field in the center was about 1.5 Tesla, but we needed to measure it exactly. I was chosen to crouch/walk into the gap and place a NMR (nuclear magnetic resonance) probe in the center with the magnet ON. After placing the probe in the center, we needed to put the probe on a non-magnetic support. The only one we could find locally was a piece of 12 inch high. 6 inch wide, 24 inch long aluminum I beam. I started to pull the I beam into the magnet, and was met with resistance-lots of it. It was like pulling the I beam through molasses (honey). This was a problem only in the edge field, not in the center of the magnet (uniform field), as long as I rotated the I-beam only about a vertical axis (parallel to magnetic field). Rotating about any horizontal axis met with resistance. See eddy current explanation at:
    http://en.wikipedia.org/wiki/Eddy_current
     
  4. May 18, 2009 #3

    Andy Resnick

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    Energy is something I need more of.
     
  5. May 18, 2009 #4

    r16

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    wow that story sounds pretty hardcore.

    I was thinking about it, and you could define energy simply as the ability to do work, but then your stuck with the question again, what is work
     
  6. May 19, 2009 #5
    Energy is anything that can be converted into mass , or that is already mass.
    this may not be 100% accurate but i am trying to relate it to E=mc^2
    like fore example a photon which is energy can decay into an electron positron pair.
     
  7. May 19, 2009 #6
    If you could find out what energy is as a force, then i think you may find the answer to life itself.

    Perhaps.
     
  8. May 20, 2009 #7
    Photons above roughly 5-10 MeV produce e-p pairs, which then produce heat (ionization). Photons under this energy produce heat via photoelectric effect or Compton scattering.
    The end result of nearly all energy sources (including fissionable and fusionable materials)produce heat as an end product. If you want to watch the last throes of energy wasted as heat, put an ice cube in a glass of water.
     
  9. May 20, 2009 #8
     
    Last edited by a moderator: Sep 25, 2014
  10. May 20, 2009 #9

    Dale

    Staff: Mentor

    Work is force times distance (W=f.d)
     
  11. May 20, 2009 #10
    Is there a difference?
     
  12. May 21, 2009 #11
    How would you explain light using this.
     
  13. May 21, 2009 #12
    If it is moving or if it can make things move it has energy.We can calculate energy from the work formula or the variations of it such as the K.E. formulae. Energy is conserved(mass itself being a form of energy) and everything has energy.But what is energy and why is it as we observe it to be?At the most fundamental level we don't know.One of the books I read on energy had the following as the opening sentence;
    Energy does not make sense.
     
  14. May 21, 2009 #13

    russ_watters

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    You wouldn't.

    Perhaps you could rephrase the question...?
     
  15. May 21, 2009 #14

    russ_watters

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    Yeah, we do - "the most fundamental level" is e=w=f*d. People who are looking for some deeper meaning to the word beyond the physics definition are attempting to attach a significance to it that simply doesn't exist and is completely unnecessary. IMO, this results from confusion about a possible link between the scientific and mystical definitions for the word "energy". In physics, "energy" is simply a name given to a useful mathematical relationship. That's it.

    Why don't people look for similar "most fundamental level"/deeper meanings for the word "speed"? If there should be more to energy, there should be more to speed too.
     
  16. May 21, 2009 #15
    E=hf
     
  17. May 21, 2009 #16
    people do look for deeper meanings in speed , relativity , time dialtion .
    the faster you go time slows down and your mass becomes bigger , as i'm sure you are aware.
     
  18. May 21, 2009 #17
    Just one of the things we observe energy to be is a conserved quantity.At the most fundamental level we don't know why this is the case,it is just that our definitions and observation show that it is so.Are you saying that we do know why energy is conserved?
     
  19. May 21, 2009 #18

    Dale

    Staff: Mentor

    Light can exert a force on an object. When that object moves the light does work according to W=f.d. Therefore light has the ability to do work (energy).
     
  20. May 21, 2009 #19
    are you talking about radiation pressure , so would W=(F)(d) come up with the same
    answer as E=hf

    Does light have kinetic energy?
     
  21. May 21, 2009 #20

    Dale

    Staff: Mentor

    Yes.
    Yes.
    I would say that a photon's energy is purely kinetic energy.
     
    Last edited: May 21, 2009
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