1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy absorption

  1. Oct 7, 2009 #1
    Energy "absorption"

    Hi,

    Apologies if I've misinterpreted / mis-read something here (not a physics student - just a general interest). Just hoping to be put straight on something fairly simple:

    Are atoms / electrons able to interact with waves ie microwave / radio waves? If so, are there waves they are less able to interact with, and is this attributed to the wavelength of the wave vs their EM field size?

    Do the principles used for radio reception apply on an atomic level too? ie for a receiver, would increasing the EM field strength allow it to pick up wider waves? On an atomic level, can you increase the field strength of an electron? If it is possible, can electrons interact with longer wave types?
     
  2. jcsd
  3. Oct 7, 2009 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Re: Energy "absorption"

    There are a number of ways that electromagnetic radiation interacts with matter depending on the wavelength of the radiation.

    Take water for example.
    Short wavelength (high energy) uv and visible light interacts with the electrons in the atom, moving them upto a higher energy level. This is why water absorbs UV.

    Longer wavelength infrared light is about the same energy as the bond between the hydrogen and oxygen atoms in water and makes it vibrate like a spring - and so water absorbs IR.

    At even longer wavelengths microwaves make the entire molecule rotate. Water is polar and so has a slightly more postive and negative end. The changing electric field in a microwave or radiowave attracts and repels each end in turn - giving energy from the EM into the water and so makes a microwave oven work

    In a conductor (eg metal) the electrons aren't as strongly held onto an individual atom but are free to move about the surface (held on average to a bunch of atoms). In the antennea the field from the radio wave pulls the charged electrons one way and then the other. A moving charge creates a voltage difference which the radio receiver measures.
     
  4. Oct 7, 2009 #3
    Re: Energy "absorption"

    Many thanks - just what I was after.

    Regarding the state change of the electron, what happens during this process? Does this include a discharge of energy, or does the size of the EM field increase? (ie once the state change occurs, does the electron react to longer wavelengths than I would have previously?)

    (May relate to the above)
    I thought I heard something somewhere about atoms not absorbing everything.. As far as resonance is concerned, how much energy would be needed to break the coupling bewteen the + and - parts of the water (ie O / H)? If exposed to a strong bi-polar field, and "shaken" in this manner, would much energy be required to split water into O2/ H2?

    In other words, in general terms, how strong is the bond in water?
     
  5. Oct 7, 2009 #4

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Re: Energy "absorption"

    The electron gets promoted up to a higher energy level in the atom absorbing energy from the light in the process - this gives you the dark absorption lines in a spectrum. The electron later falls back to the original level giving off another photon of the same wavelength.
    The electron can only absorb specific wavelengths because there are only a fixed number of energy levels - you can't absorb a photon that is half the energy difference between levels.

    The bond energy in water is around 450kJ/mol - so it takes 900kJ to break all the bonds in 18g of water (2x because there are 2 O-H bonds in each water molecule)
     
  6. Oct 8, 2009 #5
    Re: Energy "absorption"

    Many thanks.

    Re: The quoted figure for bond energy, is this the theoretical minimum, should we be able to achieve it with optimum efficiency, or the typical cost using inefficient methods (ie tank, anode, cathode etc), averaged out?

    How does this compare with the energy achieved by burning the O2 and H2? Your mention of vibration in water earlier reminded me of an amusing thread some time back, relating to splitting water for fuel. Only asking as, at the time, the standard response seemed to be "You can't get more energy out than you put in", which doesn't from a practical standpoint, make any sense. When dealing with a fuel source ie oil, it takes energy to refine it, but we're getting the energy required from the fuel itself.

    On a related note, if successful in breaking the bonds rapidly, will there be a big "pressure increase"?
     
    Last edited: Oct 8, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook