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Homework Help: Electromagnetic Energy & Oxygen

  1. Apr 26, 2008 #1
    First off, I know nothing about physics so if this question seems bizarre, that's probably why!

    In theory, are there conditions that would permit electromagnetic energy to block or limit the passage of oxygen (normal, everyday oxygen levels)? Is electromagnetic energy denser than oxygen under some circumstances?

    I've found that the density of oxygen is 1.429 kg/m(to the power of)3 at 0 C, but the density of electromagnetic energy is more complex (if it is even comparable to that of oxygen). I've discovered that the total electromagnetic energy (EME) density is the sum of u(subscript)e and u(subscript)m. These specify at any time how much electromagnetic field energy there is at any point -- but what if it was over a certain amount of space (i.e. many points)?

    Does the density of EME depend on the size of the wavelength?

    I'm at a loss... Any help would be appreciated, even if it's just to say "This doesn't make any sense!"
    Last edited: Apr 26, 2008
  2. jcsd
  3. Apr 26, 2008 #2
    Not to be rude, but your entire situation makes absolutely no sense.

    Block or limit the passage of oxygen where?

    EM energy density isn't the same as the mass density of particles (like oxygen). E&M fields / energy do not have mass. E&M fields/energy can't "block"/"impede" anything, in the macroscopic, physical sense of the term, like a road block etc.
    What exactly are you thinking about / looking for?
  4. Apr 26, 2008 #3
    No worries, you're not being rude. As I said, I know nothing about physics and so am trying to understand.

    I thought that every mass has an associated energy and vice versa. In my non-physics mind, that means that an electromagnetic field, which is a form of energy, would have a mass.

    However, I do understand how it might not have "density" in the same way that oxygen does -- which is why I added "(if it is even comparable to that of oxygen)."

    Basically, I am trying to understand the properties of an EM field/energy. If it does have a mass and/or density, to what other substance/energy do they compare? And can such an EM field/energy physically "block" another substance/energy? Or would it merely "interfere" with another substance/energy?

    Another question is: is an EM field/energy always a frequency and wavelength? Does a frequency or wavelength have a mass (even a microscopic one)?
  5. Apr 27, 2008 #4
    If you have mass you have energy, you are correct, but not the visa versa.
    The EM field most certainly has energy, but no "mass" in the classical sense. I probably should just say it doesn't have mass at all, but at the same time it does have momentum which one might use to define mass... so it becomes somewhat semantical.
    Anyway, effectively; and EM field does not have mass (i.e. it is not effected by gravity).
    Similarly the fields don't have a mass density (the way you're probably using the term), but at the same time, because they have energy and occupy space they do have an energy density.
    EM fields interact with anything that has charge (everything made of matter). Often that interaction doesn't happen at all on the macroscopic scale (because opposite charges cancel out in effect).
    The only way and EM field can "block" something (although i'm still not completely sure what you mean by that), or interfere with it, is by repulsing a charged object... like not being able to push 2 magnets together.
    I think you might be confusing the nature of light, with that of EM fields in general.
    Light (electromagnetic waves) is a complex electromagnetic field (numerous fields), that does have a periodic nature and therein a frequency (how fast it's oscillating), and a wavelength (the distance between oscillations). EM fields do not necessarily have any fundamental / innate frequency or wavelength (both just properties / descriptions of waves), as they may be chaotic, constant, or in any way aperiodic (although non-periodic fields can result from an infinite number of periodic ones -> different issue).
    Does that help / answer your question?
  6. Apr 27, 2008 #5

    Thank you for taking the time to explain. It's much appreciated. You've clarified quite a few things for me, at least enough to keep my brain waves busy for a bit while they work it all out. If you don't mind, I might ask more questions later.
  7. Apr 27, 2008 #6
    And, by the way, someone may want to correct the Wikipedia entry for "Mass-energy equivalence" from which I copied the statement "In physics, mass–energy equivalence is the concept that any mass has an associated energy and vice versa." (My bolding.) Unless I misunderstood the statement. (And I realize that Wikipedia isn't the most reliable source of information...)
  8. Apr 27, 2008 #7
    I stand by what i said, and i feel that it is the best answer to your questions and what you're looking for... but i also understand what the article is saying, and its not incorrect...
    This is the troublesome regime of physics where the macroscopic and the microscopic are of course intrinsically linked - but not always quite the way you would think.

    When we're talking about subatomic particles (electrons, protons etc), "mass" isn't extremely well defined. This is actually a big question in modern physics; if you've heard of the new LHC (particle collider) to open in Switzerland, one of its main purposes is to discover the "higg's boson" which might be what gives things mass.
    Anyway, the most practical definition of mass is 'that which interacts with gravity.' And thats not really sufficient for subatomic situations.

    Momentum is however very well defined, measured etc. For instance we can calculate and measure (quite accurately) the momentum of an electric or magnetic field. Because they do have momentum one can technically say they have mass (in the macroscopic world, momentum is velocity multiplied by the mass, suggesting that zero mass always means zero momentum, and vice versa).
    But again, EM fields still have no physical extent, and to my knowledge they are not effected by gravity (i'm going to look into that). Further, "fields" are really only a model. If there is an EM field in the forest, and theres nothing there to feel it (no charges), then theres really nothing there except an energy and momentum place-holder.

    Anyway, this stuff is weird... and i definitely can't be called an expert on the subject; not that anyone perfectly understands it of course.
    Hopefully i've been helpful, my pleasure to help.
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