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Help understanding the electromagnetic radiation; mainly near field.

  1. Mar 12, 2014 #1
    I have a solid foundational understanding of far field electromagnetic radiation i.e., two in phase electric and magnetic fields oscilating perpendicular to each other and to the direction of wave propagation. However, I have some ambiguities regarding near field electromagnetic radiation and electromagnetism:

    (i) Basic Electromagnetism: Q1. So if an electrical current flows through a coiled loop of wire a magnetic field will be generated about that coiled loop of wire. However, is there also an electrical FIELD associated with the electrical current flow? So, there is a clear difference between electrical current (i.e., the flow of electrons from an anode to a catode) and an electric field? Does the oscillating electric wave in far field electromagnetic radiation have anything to do with charged particles or are charged particles strictly confined to electronic current flow in wires?

    (ii) Q2. The creation of electromagnetic radiation: Firstly a Capacitor: Is this basically a device which stores up a negative charge (electrons) at one pole and a positive charge (minus electrons) at its opposite pole. The spark is then the mass movement of electrons between the poles?
    Q3. From what I've read; far field electromagnetic radiation is generated by the flow of electrical current (electrons) between oscillating dipoles and the frequency of the generated electromagnetic wave is proportional to the frequency of the dipole oscillation? E.g.s of oscillating dipoles could be changing the anode and catode plates of a Capacitor or the switching of electrical current in a simple wire circuit? Finally, what is the shape/characteristics of the near field electromagnetic fields generated by an oscillating dipole device?

    I hope I've managed to pose my questions clearly and Thankyou in advance if you post a reply :)
     
  2. jcsd
  3. Mar 12, 2014 #2

    Integral

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    An changing electric field implies a changing magnetic field which implies a changing electric field....

    Q2. You need to study caps more. A spark between the plates of a cap means you have just shorted and probably destroyed your cap. Charge does NOT move directly between the plates. The only way to get from one plate to the other is through associated circuitry.

    Basically a cap is considered to be a short to changes in voltage and an open to constant voltage. There is more to it then that ie the "resistance" to changes is related to how fast (ie frequency) the changes are.
     
  4. Mar 13, 2014 #3

    jtbell

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    No, because the wire contains equal amounts of (stationary) positive charge and (moving) negative charge. The net charge is zero, so the wire produces no electric field "directly".

    With an antenna that radiates electromagnetic waves into vacuum, the charge (more precisely the current) is confined to the antenna. The wave at a point distant from the antenna is ultimately caused by the oscillating charge in the antenna, or more immediately by the wave at adjacent points; there is no "extra" or "new" charge at those distant points.

    As an analogy, consider producing a wave in a hanging rope by shaking one end. The oscillation of a part of the rope ten feet away from you is immediately caused by the oscillation of the neighboring part of the rope, because those parts are "stuck together"; but the ultimate cause is you at the distant end.
     
  5. Mar 13, 2014 #4
    Electric current is the movement of charges. An electric field is a force whose magnitude and direction varies at any specific point depending on where the charge was historically.

    When charges are accelerated radiation occurs. Radiation is a self-propagating disturbance of electric field (and magnetic field) values. But remember that movement of charge, rather than acceleration, produces varying fields also, but such a disturbance diminishes quickly in time and space. It is effectively an evanescent wave.

    In the near field, the fields contain a much larger portion of evanescent wave values. The far field consists mostly of the self-propagating wave values.

    Capacitors work because of polarization. That is, the movement of a charge onto one plate (through the external force of the chemical processes within a battery for example) generates an imbalance of EM force. That imbalance is equalized by polarization: the drawing of opposite charges into the region of space to equalize the original disturbance so that opposite charges populate the plate on the other side of the capacitor.
     
    Last edited: Mar 13, 2014
  6. Mar 14, 2014 #5
    There is no electric field associated with the electric current in the coiled loop of wire. However, the wire does have a certain voltage between the windings; usually it is extremely small, depends on the radius of the coil, among other things. This causes a weak electric field between the windings.

    The oscillating electric wave in the far field, after it has left the confines of the wire, has little to do with charged particles. As you say, charged particles are generally confined to the wires and do not 'leave' the wire when a far field EM wave is launched.

    Generally speaking, a capacitor does store negative charge at one plate and positive charge at the opposing plate. However, I find it more meaningful to realize that the energy stored in a capacitor principally relies between the plates in the form of an electric field. Proper capacitors never spark, because this would upset the difference of potential (e.g. the built up charges) between the plates and therefore, it would no longer store energy. In this case, the negative charge on one side would shoot through to the other side... Momentarily the capacitor will not act as a capacitor anymore, instead it would act as a short.

    Well, firstly, statements referring to the 'flow of electrical current' are rather superfluous because current is defined as "the flow of electric charge". Essentially you just said "the flow of electrical flow of electric charge"

    Anyway, what you say is true, electromagnetic radiation is generated by the oscillation of charges between a dipole, the frequency being the rate at which this oscillation occurs. I do not know what you mean by "changing anode and cathode plates of a capacitor", but if by this you mean a spark between the plates, then yes, this does in fact create far field EM waves. So does the switching of electrical current in a simple wire circuit. Any change of current in a circuit causes some sort of far field EM waves, no matter how the circuit is configured.

    The shape of near field electromagnetism is usually related to the geometry of the radiating structure, but in general, the field between two wires is this:

    14182_121_2.jpg
     
  7. Mar 15, 2014 #6
    Thank you all for taking the time to write a response. I think the general consensus is to brush up on my capacitor knowledge which I'll look into.
    @ Menaus thanks for the detailed response and being clear on what iv got right and got wrong, that was very helpful. I especially liked "the flow of electrical flow of electric charge" hehe! Also, the Key of the diagram you've given shows an E field and the H field. Does this mean that their is both an electric field and magnetic field? I ask because you wrote ' There is no electric field associated with the electric current in the coiled loop of wire.' But also 'However, the wire does have a certain voltage between the windings....... This causes a weak electric field between the windings.' Is it the case that the electric field component only comes into play when two wires are put into close proximity of each other? Or perhaps that the generated magnetic field about an active loop of coiled wire will in turn generate an electric field because electric and magnetic fields generate each other?
    @ Menaus and PhilDSP. You've both made it clear that there are no charged particles associated with far field radiation: 'The oscillating electric wave in the far field, after it has left the confines of the wire, has little to do with charged particles' and 'An electric field is a force whose magnitude and direction varies at any specific point depending on where the charge was historically'. However, what then is an electric field!? My understanding of a magnetic field is something which either attracts or repels metal. I can imagine this component of the far field being measured by say the metal head of a compass, moving up and down in tandem with the sine wave pattern. But how would the electrical wave of the far field be measured if there are no charged particles in its vicinity to measure?
     
  8. Mar 15, 2014 #7
    Yes all electric circuits contain both electric and magnetic fields simultaneously. In a simple case, you can define the electric field as the cause of the difference of potential (or voltage) between two arbitrary points on the wire, and the magnetic field as the cause of the movement of charge through an arbitrary point on a wire. The stronger difference of potential, the stronger the electric field, and the more movement of charge, the stronger the magnetic field

    Voltage and current are two fundamentally different things, in this case the voltage causes the electric field, and this is separate from the current, which causes the magnetic field. Both electric and magnetic fields are always in a circuit, as I said above. The electric field component always exists when there is a difference of potential, no matter the distance (although it gets much weaker when wires are not close to each other). Usually the electric field between the windings of a transformer can be ignored because it is very weak. The electric field generated by a magnetic field within the same coil is a bit different than what I am talking about.

    If we look at it in simple terms like you do a magnetic field, an electric field is something which either polarizes dielectrics; usually this is in the form of an insulators. If you take, say, a piece of ceramic like a compass, and expose it to a powerful electric field, then the ceramic is said to be polarized, which means that the part of the ceramic closer to the electric field is oppositely charged from the field it is exposed to, and the opposite end of the ceramic is charged at the same polarity of the field.

    The simplest case of electric fields deal with electrostatics, I suggest you read up on them.

    http://en.wikipedia.org/wiki/Electrostatics
     
  9. Mar 15, 2014 #8

    jtbell

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    I'd better clarify here that when I made the statement above, I was thinking only of the situation outside the wire. Inside the wire, there is an electric field as you describe here.

    Shouldn't this be "electric field" here?
     
  10. Mar 15, 2014 #9
    I'm sure there's also an electric field between the individual turns of wire (outside). I was thinking about the situation outside of the wires as well. It is just negligible.

    Whoops, I think I made a bit of a grammar error in those statements. I meant that the "electric field [is caused by] a difference of potential", and "a magnetic field [is caused by] a flow of charge".
     
  11. Mar 16, 2014 #10
    Thanks, for the replies. I had a read through and I think I'm fairly happy with my level of understanding now especially with the polarizing affect for the electric fields. I'll check out the electrostatics and capacitors when I get round to it and have another read through. Cheers again! :)
     
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