# How is electricity converted to radio waves?

#### linux kid

I know that electricity can be coverted to microwaves obviously.

But how is it converted to radio waves?

#### jtbell

Mentor
Basically, all you do is produce an alternating current in an antenna, with the desired frequency.The changing current produces a changing magnetic fteld, which produces a changing electric field, which produces a changing magnetic field, and so on...

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#### rbj

linux kid said:
I know that electricity can be coverted to microwaves obviously.

But how is it converted to radio waves?
microwaves are radio waves. what else would you call them? the telephone company uses microwaves to communicate signals just as done with radio waves. other than the frequency parameter (and consequently wavelength), which is only a quantitative difference, what else is different between microwaves and what gets tuned in by your FM receiver?

microwaves, radio, infared, ultraviolet, X-rays, visible light, ... it's all the same thing: electromagnetic radiation.

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 4 x 1014 times per second, you will see it as a blur of visible red light.

that's what E&M radiation is.

how to get that charge moving around with electronic circuits is a good reason to major in electrical engineering. when they hook up that transmitter to an antenna (which could be just a simple metal conducting rod), this charge sloshes back and forth along the "primary element" or "radiating element" (that metal conducting rod or wire that is the part of the antenna connected to the transmitter) which, like that thought experiment above, creates an E&M wave or a "radio wave". at your receiving antenna, when this changing E&M wave arrives, it causes charge to slosh back and forth along that antenna element which creates a very small voltage that the receiver amplifies and does other mathematical magic to it to extract the original signal.

confused now?

#### michaeltorrent

i have a question,
is it possible for us to run very high oscillating current in an antenna as high as 5x10^14 hz?

if we can, then it will produced visible light from the antenna.??

#### rbj

michaeltorrent said:
i have a question,
is it possible for us to run very high oscillating current in an antenna as high as 5x10^14 hz?

if we can, then it will produced visible light from the antenna.??
in my sorta silly thought experiment, i glossed over one important fact: the finite speed of light and that charged particles may not move as fast as c. this defines a physical size for radiating elements (around 1/2 wavelength). for microwaves, the physical radiating element is actually the cavity inside of the vacuum tubes at the back end of the transmitter and the microwave radiation is actually piped out to the big horns for transmission with metal waveguides. for visible light, the radiating elements get closer to the molecular/atomic level.

also, because of the dual particle/wave nature for light, there are other mechanisms for emitting particles of light (photons) that do not appear to be the same as the "sloshing of charge back-and-forth" model that would create an E&M wave. the photoelectric effect would be that.

#### linux kid

Thnaks rbj for that great explanations.

I am much less confused now, but I am left with just one more burning question. What is electromagnetic radiation made of? What are the particles moving back and forth between the two charges?

#### eeka chu

linux kid said:
What is electromagnetic radiation made of? What are the particles moving back and forth between the two charges?
The radiation itself isn't a particle, it's a disturbance in the electromagnetic fields between the transmitter and receiver.

Although, things getting in the way of that disturbance can end up interacting with it. Microwaves for instance. If you put something full of water in the way of a microwave disturbance, the field disturbance causes the water molecules to vibrate. In effect, the water is sucking up some of the energy from the disturbance. That's precisely what's going down when you nuke some dinner in the masterblaster ray gun box. Which is also similar to how they first found out about cooking things with microwaves, or so the story goes... some guy walked infront of a big radio dish and noticed a chocolate bar in his pocket had started to melt.

The way I picture radiotransmission... electrons have a magnetic field around them, like tiny bar magnets. As millions of them rocket into a conductor, they blow a magnetic bubble out around that conductor. If a new pulse rushes in, before the first bubble can collapse, a second bubble pushes the first out. Think of those old TV ads with a pulsing TV attena in them where the waves are pulsing out of the top.

At least, that's according to the radio primer I read. Any radio guys tell me if that's even close to acceptable?

#### rbj

eeka chu said:
The radiation itself isn't a particle, it's a disturbance in the electromagnetic fields between the transmitter and receiver.
there are also particle like properties of E&M radiation as depicted in the photoelectric effect. that's where Planck's constant first appeared.

but, to answer the OP's question, we really don't know what E&M radiatopm os made of, nor everything about it. sometimes it acts like waves and i tried to describe the source of that (and, i think, Maxwell's equations pretty well sum up the wave-like behavior), but sometimes it acts as particles. these are properties of light that just seem to be sorta mutually contradictory to each other which is why we have these two different descriptions.

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