Magnetic Fields and EM Radiation

[IssacBinary]

Since a changing current creates a magnetic field, and if there's a wire near by, the magnetic field will induce a current in that wire. What is needed / what does it take to turn that magnetic field into EM radiation such as a radio wave?

Or is it purely just the frequency of the current, so once the current reaches radio frequency it will start radiating EM fields.

So does that mean everything is emitting em radiation but at very low frequencies the magnetic field is stronger than the radiation itself?

...Then it brings me on to say, if you generated a current of visible light frequencies..will light start to emit from the wire?

 

[sophiecentaur]

You are right in saying that you will always get em radiation when alternating currents flow through wires. The amount of energy you can radiate will depend on the size of the structure and how carefully you can match your transmitter impedance to the antenna. Typically, a good choice of size for an em radiator is about a half wavelength. All AC currents will radiate 'a bit' but 50Hz needs an antenna thousands of km in length in order to radiate significant amounts of power. In the other direction, there is a minimum 'length of wire' that you can achieve in practice and that puts an upper limit on the frequency that an antenna can be made to work.

In response to the last part of your post, although all em waves are the same and they can be generated by 'moving charges about', the situation with light is very different from that with radio frequencies.

The radiation of em energy is all about a change in the energy level of a system of charges. For an individual atom, there are discrete energy levels for the electron with respect to the nucleus. The jumps between the energy levels produce photons of visible light and correspond to voltages of a few volts. The photoelectric effect tells us that a voltage of a few volts corresponds to photons of visible light - to remove the electron from the surface of a metal, for instance.
In contrast, think of the situation inside a metal wire. Individual electrons need minuscule amounts of energy to move them from place to place within the wire (a very low resistance involved), the frequency of em photons will be many orders of magnitude less. So, when you suggest that electrons flowing in a wire might generate light frequencies, in fact the electrons would actually be leaving the surface of the wire. We see this happen when there is a high enough voltage to make a spark jump - (which, of course, produces visible light) but this doesn't correspond to an alternating current actually flowing in the wire. There is an upper limit of a few tens of GHZ for the frequency that you can get an actual 'circuit' to behave in a similar way to a radio transmitter / antenna.

 

[IssacBinary]

Ah right I see.

So again in an inductor example. Is the magnetic field that it produces which is used to induce current the magnetic part of EM wave? Or is it a separate magnetic field all together?

So just to clarify then,
Everything is emitting em radiation but at very low frequencies the magnetic field is stronger than the radiation itself, but to get the most power from the EM radiation at a specific frequency the wire / antenna needs to be of specific size etc.

Only at very high frequency (compared to 50hz AC) are the antennas size within reason.

Edit:
And to clarify another point, specifically about sparks. I was wondering why high voltage would create visible light as its not at visible light frequency, its just one jump.

So..the voltage that's needed for a spark to be created (voltage is so high that is able to over come the resistance in the air and use the air as a conductor and go to ground? anyway...), the energies needed for that to happened are so high that they are enough to cause the electron to come free which also creates light.

So then that would mean, its not just "generate AC at a given frequency to create EM radiation of that frequency"?

..But then a test I did was if you un-tune a radio so its just hiss and turn up the volume, when you switch a light switch on/off you can hear a click on the radio. Suggesting that spark is also producing radio frequencies?

 

[sophiecentaur]

Yes, the local fields will contain energy that is not radiating. This is a bit like the high level of energy in a (mechanically) vibrating string which is losing sound to the surroundings at a relatively slow rate. You can also get very high electric fields that don't radiate either. The secret of a good antenna is that it radiates or absorbs as much energy as possible.

Yes, there is a range of operating frequencies for which "reasonable" sized antennas can be used. Of course, the amount of information which can be carried on very low frequency signals is also very low so that's another reason why not to bother with them unless you really need to (like for communicating with submarines deep under water, where normal RF frequencies don't penetrate).

Sparks don't produce light directly because of the ac or dc current passing through them. The light is emitted because the atoms become energised by the electrons flowing through the air / gas and they then emit light at their characteristic frequency as the 'orbiting' electrons fall back into their lower states of energy.

Sparks produce RF interference because the current in a spark is full of many different ac frequencies, some of which lie in the band of your receiver.

 

[jsgruszynski]

The amount of radiation as EM can be thought of as an impedance matching problem. Free space impedance is 377 ohms and the antenna is just a "impedance matching filter" to free space.

When the matching is done well, you get maximum energy transferred from the current to EM radiation.