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Electromagnetic radiation

  1. May 16, 2005 #1
    Hi everyone:) I recently began doing some independent studies on electrical engineering, and came across the topic of radios. There was one thing that puzzled me, however. I thought I had a basic understanding of electromagnetic radiation. I had understood that it was a wave that propagated from one source and traveled forever in space in one direction. However I had read from an online article about building radios that radio waves travel back and forth. What does this mean? Do they mean that they travel to and from the source, or are they simply referring to the wavelike motion of electromagnetic radiation?

    Additionally, I am attempting to mentally conceptualize the idea of what electromagnetic radiation exactly is. Is it a disturbance that is generated within a magnetic field only, much like the waves of the ocean? If so, how does electromagnetic radiation travel in the void where no magnetic field exists? Or am I just way off?

    Please help this poor physics noob :)
  2. jcsd
  3. May 16, 2005 #2


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    imagine you're standing there holding a sufficiently negatively charged object and i'm standing here holding an equally, but positively charged object. we both are facing each other and restricting our charged objects from movement along the line connecting the two of us (back and forth motion), but we allow left-right and up-down motion (free movement in a plane at right angle to the imaginary line that connects us). because they are oppositely charged, your charge will move to whatever position gets it closest to my charge.

    now i move my charge up a meter, what does your charge do? then i move it down, then to my left (your right) and then to my right. what does your charge do? it follows my movement. now i move it up and down several times. your charge moves likewise. that is an electromagnetic wave emmited from my moving charge and disturbing your charge. you can think of my charge as a "transmitting antenna" and yours as a "receiving antenna". if i move my charge up and down 1,000,000 times a second, you can tune it in with an AM radio. if i move it up and down 99.9 million times a second, you can tune it in with an FM radio. if i move it up and down fast enough, about [itex] 4 \times 10^{14} [/itex] times per second, you will see it as a blur of visible red light.

    that's what E&M radiation is.

    r b-j
  4. May 16, 2005 #3
    Hey rbj, thanks:)

    I understand your analogy to a certain extent. What I would really like to know is the nature of that wave you are talking about. Why does it exist? Was my analogy of the waves of the ocean a correct conceptualization?
  5. May 16, 2005 #4


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    yer welcome.

    it's not an analogy. i'm trying to describe in brutal simplicity (perhaps overly simplistic for the tastes of some here), the actual mechanism of electromagnetic radiation. EM radiation happens when charged objects or particles move around (strictly speaking, when they are accelerated).

    fundamentally, because like charges repel and unlike charge attract and the intensity of that repellent or attractive force is related to the distance between the charges. when some charge (that affects other charges) is forced to move for some reason, the other charges are affected. that is the fundamental reason for why E&M radiation exists. there are lotsa details needed to get it right (namely special relativity; magnetic forces are just a manifestation of electrostatic forces when special relativity is taken into consideration). but the reason EM radiation is there, is because there is this fundamental force (one of four fundamental interactions of nature) called the "electromagnetic interaction" that exists and, most basically and qualitatively, the electromagnetic interaction is simply that like charges repel and unlike charges attract.

    well, i dunno. waves of the ocean (i guess them's were the original "waves"), geophysical waves, waves on a string, sound waves in a medium, EM waves, gravitational waves, "matter" waves (deBroglie waves) in QM. they all have a "wave equation" that describes them and those wave equations all have the same 2nd order form (some sign changes) when expressed in the simplest manner (ignoring higher order terms that may exist in reality). the physics that generates the 2nd order wave equations may be different and ostensibly unrelated between those different situations.

    there is no sea of magnetism that the EM force disturbs. at one time (about 120 years ago), they believed that there was a sea of some medium (called the "ether") that EM force was propagated in, but the Michaelson-Morley experiment was unable to show that such an ether existed and eventually they stopped believing in it and Einstein nailed the coffin shut on the concept of a medium for EM propagation.

    r b-j
    Last edited: May 16, 2005
  6. May 17, 2005 #5


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    To some extent, but waves on the ocean are actually very complicated. In general, there are two types of waves: transverse and longitudinal. They are distinguished by how the thing that's "waving" or oscillating moves relative to the direction of propagation. In transverse waves, the oscillation moves perpendicular to the direction of propagation and in longitudinal waves, the oscillation is along the direction of propagation. The classic example of a transverse wave is the oscillation of a tight string. The wave moves along the string, but the oscillations are up and down. A classic example of a longitudinal wave is sound. In this case, the waving is in the pressure of the medium. An increase in pressure at one point will be propagated outwards to nearby points (the increase in pressure will create a "push"), each of which push on their neighboring points, and so on.

    Light, it turns out, is a transverse wave because the electric and magnetic fields oscillate perpendicular to the direction of travel. Water is a bit more complicated because there are several things that can oscillate and they don't all act in the same direction. In order to have an oscillation, you need some kind of restoring force to push the oscillating medium back to its original position. On a surface of water, one restoring force is gravity. That is, if you create a "hump" in the water, gravity will try to push it down. It can't go straight down, however, because liquids are mostly incompressible, so the motion of an individual fluid element ends up being more like a circle. Another way in which water can oscillate is through surface tension. If you "stretch" the surface of water, electric forces will try to return it to its original configuration, creating little "ripples" that propagate along with the waves induced by gravity.
    Last edited: May 17, 2005
  7. May 20, 2005 #6
    How does electromagnetic radiation as described by rbj induce an alternating current in a wire?
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