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Waves (the very basics)

  1. Sep 17, 2015 #1
    Q. Is the following a correct definition of waves?

    “Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas) being transferred. Think of a Mexican wave in a football crowd - the wave moves around the stadium, while each spectator stays in their seat, only moving up then down when it's their turn.”

    http://www.bbc.co.uk/education/guides/z8dp34j/revision/1
     
  2. jcsd
  3. Sep 17, 2015 #2
    "Some waves must travel through a substance. The substance is known as the medium and it can be solid, liquid or gas. Sound waves and seismic waves are like this. They must travel through a medium..."

    Q. When the above (also from bbcbitesize) says, ‘waves must travel through a substance’ (emphasis mine) waves are not something concrete travelling through the substance (as in, for example, fish swimming through a stretch of water) ; the wave is the substance itself, yes (the solid, liquid or gas in question)? The wave is the substance itself vibrating in a uniform manner, yes?
     
  4. Sep 17, 2015 #3
    Other waves do not need to travel through a substance. They may be able to travel through a medium, but they do not have to. Visible light, infrared rays, microwaves, and other types of electromagnetic radiation, are like this. They can travel through empty space. Electrical and magnetic fields vibrate as the waves travel.

    Q. Are the electrical and magnetic fields already there (waiting, as it were, to be vibrated by the waves coming from the Sun), filling the space (between, for example, the Sun and a daffodil that we see as a result of the em radiation reflected off of it)?

    Q. If so, then does that not negate the idea of that space being a vacuum (ie, the space contains electrical and magnetic fields and is not therefore empty)?
     
  5. Sep 17, 2015 #4
    Q. What constitutes a single transverse wave? I’ve looked at several diagrams where an x,y axis is shown - with the wavy line rising to a particular point (from a starting point at the intersection of the axes – marked zero) before falling back to the x axis and below to a particular point before rising towards the x axis again and so on and so forth; and I’ve learned a wee bit about amplitude and wavelength / frequency but I’m unsure as to what one, single wave is.

    Take the first wave in a sequence of waves; it begins at the intersection mentioned above (the zero point of the axes) but where does it end?

    Q. In the Mexican wave analogy which people would make up a single wave?
     
  6. Sep 18, 2015 #5

    Drakkith

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    Your first two posts are pretty much correct.

    Your third: the EM field is typically thought of as existing everywhere. The universe is filled with charged particles, so it would be difficult to imagine it not existing everywhere.

    A vacuum usually means a region of space devoid of matter. In that view, no, the existence of the EM field doesn't change that. However, there are times when thinking of a vacuum as not being completely empty is useful.

    Your fourth: I don't know of a technical definition for a single wave. Perhaps others do.
     
  7. Sep 18, 2015 #6

    davenn

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    I was thinking ... maybe like a single pulse ??
     
  8. Sep 25, 2015 #7
    Drakkith, you said; “…the EM field is typically thought of as existing everywhere. The universe is filled with charged particles, so it would be difficult to imagine it not existing everywhere.”

    Q. Re “the universe is filled with charged particles”; are you talking about atoms with an imbalance of protons/electrons (cations and anions)? And if so, do these fill the universe? Are they everywhere? In front of my face as I type? All around you in the room in which you read what I’ve typed? Throughout regions of the universe where there is no air, regions commonly thought of as a vacuum?

    Q. It’s been said above that EM waves don’t need a medium through which to travel; but isn’t the EM field you’ve just described a medium?

    You (Drakkith) also said, “A vacuum usually means a region of space devoid of matter. In that view, no, the existence of the EM field doesn't change that. However, there are times when thinking of a vacuum as not being completely empty is useful.”

    Q. Is there no such thing as a vacuum? Is the word vacuum a misnomer?

    And, finally, you said, “I don't know of a technical definition for a single wave. Perhaps others do.”

    Q. How can we talk about wavelength if the definition of a single wave is elusive?
     
  9. Sep 25, 2015 #8

    Drakkith

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    I'm talking about both neutral and charged atoms, along with single, subatomic particles, such as protons and electrons. And yes, the universe is essentially filled with them. I don't mean filled as in packed in tightly, but simply that you can't go more than perhaps a few meters anywhere in the universe without encountering at least one charged particle.

    Yes and no. The EM field can be thought of as a non-physical medium, but you run into some issues if you do this. For one, you can't think of the medium as moving anymore.

    No, vacuum isn't a misnomer. You just need to remember that all words have context.

    Because 'single wave' isn't technically useful. Waves don't occur in 'singles'. By its very nature, a wave cannot be easily described in terms of a 'single wave'. Where do you start and end this 'single wave'? One wavelength? From original emission until it stops being emitted? (which is problematic) What about standing waves?

    We have plenty of useful terms to describe waves already without needing to worry about what constitutes a single wave.
     
  10. Oct 23, 2015 #9
    Drakkith, you said,

    “Yes and no [re, ‘but isn’t the EM field you’ve just described a medium?’]. The EM field can be thought of as a non-physical medium, but you run into some issues if you do this. For one, you can't think of the medium as moving anymore.”

    Q. Is the EM field the result of the charged particles that exist practically everywhere in the universe? And why can’t we think of this non-physical medium as moving; doesn’t the field itself move up and down, ie, vibrate/oscillate?

    You also said,

    “No, vacuum isn't a misnomer. You just need to remember that all words have context.”

    Thanks. But I suspect that vacuum on its own may be a misnomer and that to avoid this we should use the terms ‘perfect vacuum’ (an idea that perhaps exists only in theory) and ‘imperfect vacuum’ or ‘partial vacuum’ (which exist in reality). Although even these last two terms still seem to me to be problematic. If an actual vacuum (perfect vacuum) doesn’t really exist why not come up with new terms for what we now refer to as ‘imperfect vacuum’/’partial vacuum’?

    *

    And re a single wave; what do you think of davenn’s reply; “I was thinking ... maybe like a single pulse ??”
     
  11. Oct 23, 2015 #10

    Nidum

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  12. Oct 23, 2015 #11

    Drakkith

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    The field does not move up and down. What happens is that the field vectors oscillate in magnitude and direction. Field vectors describe the direction of the forces exerted by the field and their strength. There is no motion involved in their oscillation.

    There's not really any point. Language is inherently imprecise, which is why context is extremely important in essentially all communication. If you want to describe the details of a vacuum, far better to just use math than try to invent new words that will still be imprecise.
     
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