# Magnetic field frequencies?

#### Geuis

Light is found in different frequencies, which accounts for all of the energy types we find such as visible light, x-rays, gamma rays, infrared, radio waves, etc.

Do magnetic fields also exist in different frequency ranges? If so, what are some methods that could be used to manipulate magnetic fields to change them to different frequencies?

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#### chroot

Staff Emeritus
Gold Member
Hi Geuis,

Welcome to physicsforums.

Light is actually a combination of both an electric and a magnetic field, oscillating together. A changing electric field creates a magnetic field, while a changing magnetic field creates an electric field. The two fields oscillate in step, the fields "reinforcing" each other, as the light propagates through space.

It would not be correct to say that the magnetic field "has a frequency;" it would be more correct to say that oscillations of the field have a frequency. Light is one such oscillation of the fields, and, as you are aware, those oscillations can be made at many different frequencies.

- Warren

#### Geuis

So then should it not be possible to make magnetic fields oscillate at the same frequencies as visible light? And if so, could we then *see* magnetic fields?

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#### Ambitwistor

A changing magnetic field produces an electric field, so trying to produce an oscillating magnetic field will end up producing an oscillating electromagnetic field (an electromagnetic wave, like light).

#### Geuis

So I'll take that as a yes?

#### chroot

Staff Emeritus
Gold Member
What Ambitwistor is trying to say is that, yes, we can make magnetic fields oscillate at the same frequencies as visible light. The resulting oscillation, not suprisingly, is visible light.

- Warren

#### Geuis

are there any physical examples of machines built to demonstrate this property? I would be interested to see pictures.

#### chroot

Staff Emeritus
Gold Member
How about lasers? Fluorescent lights? Light bulbs?

- Warren

#### zoobyshoe

Originally posted by chroot How about lasers? Fluorescent lights? Light bulbs?
- Warren
I have wondered about the same thing as Geuis. I think what he may be asking is if it is possible to get visible light from oscillating the polarity of a magnetic field in a coil, as opposed to the above situations where the EM is arising from the oscillation of individual molecules, as I understand it.

To the best of my knowledge no one has been able to produce frequencies above microwave frequencies by alternating the polarities of a magnetic field, or by switching the current on and off. There seems to be an upper limit to this, and I'm not sure if it is an engineering limit or if there is a physical law in the way.

#### Jonathan

I am under the impression that at such high frequencies the vibrating electrons don't really leave there original atoms, which explains why it is molecular vibration.
I have never understood this though: If the magnetic field of a permanent magnet is made up of photons, how are they there? I mean I thought photons had to vibrate, but the permanent magnet's field doesn't, so where would they get this vibration from, and what would determine its quantity? Or do the photons from a magnet (or a static charge for that matter) just not vibrate?

#### Ambitwistor

Originally posted by zoobyshoe
To the best of my knowledge no one has been able to produce frequencies above microwave frequencies by alternating the polarities of a magnetic field, or by switching the current on and off. There seems to be an upper limit to this, and I'm not sure if it is an engineering limit or if there is a physical law in the way.
You'd have to cycle the current about 500 trillion times per second ... that'd be quite an engineering feat.

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You can't have a varying magnetic field without producing an electric field too.

Maxwell wrote four equations (in vector notation), concerning five kinds of things: Electric charge, electric current, electric displacement, the electric field, and the magnetic field.

Maxwell's first equation says the tendency of the elctric field to spread out (or contract) at any point is proportional to the electric charge at that point.

The second equation says the tendency of the magnetic field to expand or contract is zero.

The third equation says that a time change in a magnetic field produces an electric field, and the tendency of the electrical field to twist is proportional to the time rate of change of the magnetic field.

The fourth equation says a magnetic can be produced by two causes, possibly in combination: a changing electric displacement, or a an electric current. And the tendency of the electric field to twist is proportional to a linear combination of those two rates of change.

Electric displacement and current are features of the electric field, so we see that a changing magnetic field can produce a changing electric field and vice versa, and that can produce more magnetic field, and so on, which is electromagnetic radiation (light, x-rays, microwaves, etc) as was said above.

If you get into relativity, all you really need to produce all this is electric charge and relative motion.

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#### Ambitwistor

Originally posted by Jonathan
If the magnetic field of a permanent magnet is made up of photons, how are they there?
A static magnetic field isn't made up of real photons.

I mean I thought photons had to vibrate, but the permanent magnet's field doesn't, so where would they get this vibration from, and what would determine its quantity?
Photons don't themselves don't really "vibrate". (Well, their quantum phase oscillates ...) Photons can be produced by a vibrating charge, if that's what you're getting at ...

But, like I said before, a static magnetic field isn't made up of real photons; if you want to think in terms of photons, you'd describe it in terms of virtual photons, which are always being radiated and absorbed in infinite quantities by anything with charge, and are not observable.

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