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I'm mostly wondering about radio frequencies. I understand that voltage is the movement of electrons, and that the antenna acts as a light bulb, emitting at radio frequencies, following the reverse square law, some materials are opaque, some are transparent. Yet, at the receiver end, it's almost the same as having the two antennas connected, except with a voltage drop. Are photons and electrons the same thing? (It is called the electromagnetic spectrum). It's obviously not quite the same as electrons moving through the air, as this creates lightning, or something similar.

Is anything I've said incorrect? What happens when an electron goes through an antenna?

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3 Answers 3

Electrons and photons definitely are not the same. For instance electrons have rest mass and photons do not. Also, electrons have charge, while photons do not. I could go on about their very different statistical behavior in quantum mechanics (electrons have a quantum mechanical property called "spin" that is half-integer, photons have integer "spin" ) but I'll keep it qualitative -suffice it to say that electrons and photons are quite different particles.

What happens with radio frequency is that electromagnetic waves are generated and radiated, not electrons. This is a consequence of current -i.e. the movement of electrons in a conductor (the antenna) driven by a time varying voltage. The current in the antenna generates both electric and magnetic fields and this field is radiated.

At the receiving antenna, the time varying field induces a current in the receiving antenna,and as a result, a detectable voltage.

So the electron flow is confined to within the transmitting and receiving antennas, and what is flowing between the antennas is electromagnetic fields, not electrons.

Where photons come into play is that electromagnetic fields are quantized - that is, the energy of an electromagnetic wave depends on its frequency - and the photon is the unit of quantization.

Hope this helps, it's a qualitative treatment and just scratches the surface.

For a quantitative description of what is going on one must turn to Maxwell's equations, which describe how electric and magnetic fields are related to electric current and charge distributions.

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I think the treatment of where photons some into play is a bit lacking. Photons are generated when an electron goes from a high energy level to a lower energy level; an electron can absorb a photon, bringing it from a lower energy level to a higher one. That's the important part of where radio waves come into play. –  Gabe Aug 4 at 5:47
    
@Gabe, sounds like you are alluding to energy level transitions in atoms - no, these are typically far from radio frequency (for example visible or ultraviolet hydrogen atom levels). There is the 21cm line in hydrogen but that's due to hyperfine structure and doesn't contribute to antenna transmission. Radio frequency emission from an antenna is due to the acceleration of charge within a conductor. –  paisanco Aug 4 at 22:37

I understand that voltage is the movement of electrons

No, the movement of electrons is (one type of) electric current. There can be a voltage without the movement of charge.

and that the antenna acts as a light bulb

No, an antenna is a resonant system that, ideally, has zero resistance while an incandescent light bulb has a resistive element that is heated by an electric current to the point that it visibly glows. They're not remotely similar.

Are photons and electrons the same thing?

No, photons are the massless spin 1 quanta of the electromagnetic potential and have zero electric charge. Electrons are the massive spin 1/2 quanta of an 'electron field' and have electric charge. They're not remotely similar.

What happens when an electron goes through an antenna?

Accelerated electrons within the antenna radiate photons that propagate away a the speed of light.

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To get to the heart of the matter, you need to understand two things:

  1. Maxwell's equations
  2. Fourier analysis

Maxwell's equations tell us the relationship between electricity and magnetism. In particular, they tell us that a change in an electric field causes a magnetic field. Similarly, they tell us that a change in a magnetic field causes a change in an electric field. Maxwell's equations are also differential equations that we can solve (in some circumstances). If an electric field is sinusoidal, then the associated magnetic field is sinusoidal as well, and timed in such a way that they reinforce each other. This means that energy gets transferred between the electric field and magnetic field and back again. This is what we call an electromagnetic field. It is a self-perpetuating wave.

Now, the other side of the story is that a stationary charge will produce a static field. But if the charge moves towards you, the field strength will increase. In particular, this means that a magnetic field is created as well. The movement of the charge does not have to be sinusoidal. Basically, we can use Fourier analysis to add up any changing electric field into a sum of sine waves. And the associated magnetic field will be a sum of cosine waves in the way we expect. The point is, every motion of an electric charge will create some waves.

An antenna works in reverse. There are basically two types. Those that absorb magnetic energy and those that absorb electric energy.

A pure wave will have an associated wavelength, and if an antenna is exactly half that wavelength, then the difference between the field at one end and the field at the other end will be maximal (for the given wave). When the wave passes over the antenna, it will effectively set up a voltage difference between the "high point" and the "low point" of the wave.

A magnetic antenna works in a similar way, but it captures magnetic energy. And it's harder to visualize. But basically, a magnetic antenna is a fancy tuned inductor.

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