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Why do vacuum and the air behave so similarly ?

  1. Feb 28, 2016 #1
    I may be in the dark here but many of the media-dependent constants like permeability, permittivity and even the speed of light are ALMOST identical in both media ie. vacuum and the atmosphere of the Earth. This exists even though the 2 are very different because one is almost empty and the other is filled all kinds of atoms and molecules, has definite volume and mass too.
    Please correct me if I'm wrong.
    your thoughts ?
     
  2. jcsd
  3. Feb 28, 2016 #2

    ZapperZ

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    If you are in NY, and you want to know how long it will take to drive to, say, Schaumburg, IL, which is the suburb of Chicago, will it make any difference that someone tells you the distance between NY and Chicago (which is about 780 miles)? Will the extra few miles from Chicago to Schaumburg, IL makes any significant difference to the overtime travel time of your travel?

    Here's what you can do. Make a table of all the common media, and list out their corresponding permittivitty (and permeability as well if you want to be complete). Now look at the difference between air-vacuum versus, say water-vacuum, glass-vacuum, etc... There are MANY instances where your instrument cannot tell the difference (i.e. they are either not accurate enough, or not sensitive enough) to detect the difference. In such a case, it really doesn't matter which one you use, so why not just assume vaccum values?

    Note that in many physics research, this difference will start to matter, because they are making highly sensitive measurements. In some instances, even variation in air density with height, and even wind movement will come into play (know why stars twinkle?).

    Zz.
     
  4. Feb 28, 2016 #3

    FactChecker

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    It is misleading to say that the atmosphere is "filled with all kinds of atoms". It depends on how light would interact with them. A particle which has no electrical charge, like a neutrino, has very little chance of interacting with anything. 100 trillion neutrinos go unimpeded through your body every second. So the term "filled with atoms" is misleading. There is practically no chance of a particle actually hitting an atom unless it is attracted to it.
     
  5. Feb 28, 2016 #4

    sophiecentaur

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    There are so many significant differences between air and vacuum: blue and red sky depend on air and you have to go hundreds of km above the Earth's surface before the effect of drag on satellites can be ignored.
    The only similarity is that you can 'see' through them both and even that is just a matter of degree.
     
  6. Feb 28, 2016 #5

    Vanadium 50

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    Also, only about 0.1-0.2% of the volume of air is actually filled with molecules. Air acts a lot like empty space because it's a lot like empty space.
     
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