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melech
2010-Jul-17, 03:57 PM
Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet? For example, will it attract a piece of iron? If so, why is this not apparent to an observer e.g., holding a flashlight near a piece of iron?

Nereid
2010-Jul-18, 12:28 AM
Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet? For example, will it attract a piece of iron? If so, why is this not apparent to an observer e.g., holding a flashlight near a piece of iron?
Quick answer: the magnetic field in electromagnetic radiation (EMR) - as it is understood in classical physics - does not directly interact with stuff like iron. In EMR, the electric and magnetic fields self-propagate; the electric field varies in such a way as to create a (the) magnetic field, which varies in such a way as to create an (the) electric field, and so on.

The way EMR interacts with matter is complicated. For example, it is relatively straight-forward to explain radio wave-metal dipole interactions, using classical physics (Maxwell's equations), but not the photoelectric effect (for that you need the quantum theory of EMR).

melech
2010-Jul-18, 02:22 AM
You said the magnetic field in EMR does not directly interact with iron. Is this different from the interaction of the magnetic field surrounding a magnet? I think that's the crux of my question: how does an EMR magnetic field differ from the magnetic field surrounding a magnet? And if an EMR field acts like a magnet, why do we not notice the attraction of a beam of light with a piece of iron?

astromark
2010-Jul-18, 03:55 AM
Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet? For example, will it attract a piece of iron? If so, why is this not apparent to an observer e.g., holding a flashlight near a piece of iron?

No. Not at all.

We use the term 'Electro Magnet Radiation...' and it is misleading..

A photon stream is way along the frequency from electrical energy.

Light does not have a electrical component.

A handful of iron fillings will be illuminated by the flashlights output... thats all.

korjik
2010-Jul-18, 07:07 AM
Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet? For example, will it attract a piece of iron? If so, why is this not apparent to an observer e.g., holding a flashlight near a piece of iron?

Yes, it is just like the magnetic field surrounding a magnet. The difference is that the intensity of the field in a beam is really really small compared to a magnet, and the field oscillates trillions (or more) of times per second.

Hornblower
2010-Jul-18, 01:04 PM
In my opinion astromark is mistaken. What we call light is just as much electromagnetic radiation (EMR) as is what we call radio-frequency radiation. See post #11 in the following thread,

http://www.bautforum.com/showthread.php/106037-Radio-waves-amp-electromagnetic-field-questions

as well as some other posts in this thread.

In all fairness to astromark, the quantum mechanical stuff that Maxwell could not have anticipated is a mind blower. I found plenty of things that bumfuzzled me at times while studying physics in college.

Len Moran
2010-Jul-18, 01:25 PM
Yes, it is just like the magnetic field surrounding a magnet. The difference is that the intensity of the field in a beam is really really small compared to a magnet, and the field oscillates trillions (or more) of times per second.

I think it is a bit misleading to say that the beam, between a source and sink has a magnetic field and an electric field - we derive those notions in terms of measurements at the source and sink (or sinks) and at lower EM frequencies (as opposed to light) we can, as you imply, measure these components at a sink. Without multiple macroscopic sinks (such as transmission through a dielectric), there does not appear to be any such properties as an electric and magnetic field in vacuum between the source and sink. For example, as far as I understand, no measured interference ever occurs between two EM waves crossing in vacuum.

Apparently however, a predicted property of quantum fluctuations opens up the possibility of considering vacuum itself as a material medium with dielectric properties. One such predicted property is that vacuum becomes birefringent in the presence of a magnetic field due to photon to photon interactions occurring (in vacuum) between a source and sink. An experiment by the PVLAS consortium in Italy (PRL, 110406 (2006)) outlined a means of testing this prediction involving a very strong super conducting magnet rotating around the axis of a laser beam (in vacuum) with the magnet isolated from the source and measurement instruments.

The initial results indicated a positive result, but this was subsequently traced to flux leakage from the magnet affecting the instrumentation and the claims of the original paper have been withdrawn. So, as far as I know, there is no experimental evidence to show that light in a vacuum is composed of fields or particles that show up (macroscopically) in terms of an interaction with similar fields or particles within vacuum.

I think that your explanation is only really valid in terms of the point at which measurement takes place, and then, as you say, the magnetic component (and the electric component) are of such a frequency that cannot be measured in terms of a fluctuating wave.

So in terms of the original question, the beam of light does not have the same properties as a magnetic field such that some kind of interaction could be observed between them in familiar terms (say in terms of the beam of light being deflected by a magnet in close proximity to it). In fact even in the very unfamilar terms of quantum mechanics it appears that a beam of light in vacuum is not affected by a magnetic field (in terms of the controlled, properly known vacuum of the PVLAS experiment).

Jeff Root
2010-Jul-18, 01:31 PM
The trillions (or more) oscillations per second that korjik mentioned
are of visible light. Radio oscillations are much slower. The higher
the energy of the individual photons, the faster the oscillations.
The energy and the frequency are different measures of the same
property.

Sorry, Mark, but I need to correct what you said.

There are electromagnets and there is electromagnetic radiation.

An electromagnet is a coil of wire through which electric current
flows, causing a magnetic field around the coil.

"Electromagnetic radiation" refers to the the entire spectrum of
light, from long-wave radio, through microwave, infrared, visible,
ultraviolet, X-ray, and gamma ray. These different parts of the
spectrum are characterized by how light in those parts of the
spectrum interact with matter. The only differences in the light
itself are the energy/wavelength/frequency, which again are just
different measures of the same property.

A photon of light has an electric field and a magnetic field, which
varies in strength as it moves past an observer. However, one
of the strange quantum behaviors of light is that you have to see
the entire photon in order to see it at all: Either you see all of it,
or you don't detect that it even exists. So you can't detect that
the electric or magnetic field of a single photon varies. That is
inferred from the observation of large numbers of photons acting
together.

Although a photon has an electric field, it has no electric charge.
So a beam of light does not generate a detectible magnetic field,
and is not itself affected by a magnetic field.

But the varying electric field in each photon (varying because it
moves past the observer) does generate a very tiny magnetic
field which varies in synch with the variation of the electric field.
That variation of the electric and magnetic fields *is* the photon,
and embodies the photon's energy.

I think the main reason that the magnetic fields of photons are
not directly detectible is... too complex for me to understand
well enough for me to be able to explain. But the tiny energy
of each photon, each having a very low-intensity magnetic field,
as korjik said, is an important factor, and may be a sufficient
reason even if it isn't the primary reason.

One thing to note is that the shape of the magnetic field in a
photon is nothing like the shape of the magnetic field emanating
from a magnet. The field of a magnet extends outward around
it. I'm not sure that the field of a photon has *any* extension
in space at all. It is like a mathematical point. Although the
location of that point is made fuzzy by uncertainty.

-- Jeff, in Minneapolis
.

korjik
2010-Jul-18, 04:29 PM
Can we all please answer the question posed in the OP and only the question in the OP?


Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet? For example, will it attract a piece of iron? If so, why is this not apparent to an observer e.g., holding a flashlight near a piece of iron?

Since he asks about a 'beam of light', this falls very squarely into the realm of classical electrodynamics. There is no need to mention photons at all in this case.

All you need is Maxwells equations, which show that the magnetic component of the beam is a magnetic component of the form Bcos(kx-wt) or so. If you then treat this as the field involved with the iron filings, you can quite easily find that the forces involved for a realistic beam are small and very rapidly varying. Even down at the AM radio bands, there are around a million oscillations per second. At that sort of frequency, there isnt enough time for the magnetic force to do anything before the field changes sign and pushes the other direction.

That is really all that there is to this one

Len Moran
2010-Jul-18, 04:53 PM
........So a beam of light does not generate a detectible magnetic field.....


A EM wave (a beam if you like) interacting with a coil generates a voltage - the only way a voltage can be generated in a stationary coil is through the interaction of that coil with a changing magnetic field. So in terms of the EM wave, its changing magnetic field (which corresponds to the frequency of the transmitter oscillator) is certainly detectable. We can't do such a measurement with light because we have no means in which to measure the high frequency magnetic (or electric) oscillations, but in terms of the (accepted) EM spectrum as consisting of an electric and magnetic field, it is a difference only of degree. So in terms of a "beam" of light and a "beam" of radio waves, the mathematical formulation of such beams in terms of changing electric and magnetic fields are identical, but the means of detecting those changing fields are subject to the physical limitations of measurement apparatus rather than a beam of light not "generating" a magnetic field.

What is of real interest is to distinguish between a magnet inducing a voltage in a coil and a EM wave (or light wave, but we simply can't physically measure the fluctuating induced voltage) doing exactly the same thing. Yet a magnet will deflect an identical magnet, but cross two beams of light in vacuum and nothing whatsoever happens. So the observed end result of an induced voltage from an EM wave and a magnet seem to be derived from a changing magnetic component that is of a different "form" in terms of the magnetic properties of a moving magnet and a EM field. That I think is the core of the original question asked - for this is what melech asked:

"Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet?"

astromark
2010-Jul-18, 08:02 PM
What a bunch of foolish nonsense... For goodness sake.

Even I said that light is part of the 'Spectrum' of Electro Magnetic Force. Look at the 'QUESTION'... that was asked.

Wittering on about what light is... is not what this OP is asking. That subject is understood.

The other issue as to if light can or has electro magnetic energy... You all might recall the high school experiment where

a light is shone on a small vain... it spins... SO, Yes there's energy at work. Wave or particle... both.

It is electromagnetic energy... BUT. Its so far up the scale that it does not have a signature that you could call electrical,

magnetic force 0.

Answer the question with anything other than a NO and you are misleading the OP... Light has NO magnetic properties.

Complicating that answer with all of the electromagnet knowledge of humanity does not make me wrong.... The answer is NO.

and its interesting that after telling me I am wrong... I then get told I am right...

Who was it from history that stood in the bunker and said. "We can stand up now. We are out of range." .........................................

In a vacuum you can shine a light and not disturb the pile of iron fillings....

At 200 km / hr Turning the lights on does not slow the car...

but then I could argue that all the moths mass that the light has drawn into my path does... sigh.

pzkpfw
2010-Jul-18, 08:07 PM
Tone it down, please, astromark - you are verging on "rude". While we do sometimes have a problem with answers that get far too deep with respect to the question, or go off-track, this question needed more than your "no".

astromark
2010-Jul-18, 08:26 PM
The trillions (or more) oscillations per second that korjik mentioned
are of visible light. Radio oscillations are much slower. The higher
the energy of the individual photons, the faster the oscillations.
The energy and the frequency are different measures of the same
property.

Sorry, Mark, but I need to correct what you said.

There are electromagnets and there is electromagnetic radiation.

An electromagnet is a coil of wire through which electric current
flows, causing a magnetic field around the coil.

"Electromagnetic radiation" refers to the the entire spectrum of
light, from long-wave radio, through microwave, infrared, visible,
ultraviolet, X-ray, and gamma ray. These different parts of the
spectrum are characterized by how light in those parts of the
spectrum interact with matter. The only differences in the light
itself are the energy/wavelength/frequency, which again are just
different measures of the same property.

A photon of light has an electric field and a magnetic field, which
varies in strength as it moves past an observer. However, one
of the strange quantum behaviors of light is that you have to see
the entire photon in order to see it at all: Either you see all of it,
or do don't detect that it even exists. So you can't detect that
the electric or magnetic field of a single photon varies. That is
inferred from the observation of large numbers of photons acting
together.

Although a photon has an electric field, it has no electric charge.
So a beam of light does not generate a detectible magnetic field,
and is not itself affected by a magnetic field.

But the varying electric field in each photon (varying because it
moves past the observer) does generate a very tiny magnetic
field which varies in synch with the variation of the electric field.
That variation of the electric and magnetic fields *is* the photon,
and embodies the photon's energy.

I think the main reason that the magnetic fields of photons are
not directly detectible is... too complex for me to understand
well enough for me to be able to explain. But the tiny energy
of each photon, each having a very low-intensity magnetic field,
as korjik said, is an important factor, and may be a sufficient
reason even if it isn't the primary reason.

One thing to note is that the shape of the magnetic field in a
photon is nothing like the shape of the magnetic field emanating
from a magnet. The field of a magnet extends outward around
it. I'm not sure that the field of a photon has *any* extension
in space at all. It is like a mathematical point. Although the
location of that point is made fuzzy by uncertainty.

-- Jeff, in Minneapolis


Thank you Jeff... you have nailed it... the answer is No... but with a explanation that says you might yet be proven to be what... ?

It might be wrong to be so blunt... so for that... " Oops sorry." But I meant every misunderstood word of it.

The POINT must be made that NO useful energy is available from a stream of passing photons in the visible light section of the spectrum...

korjik
2010-Jul-18, 08:55 PM
What a bunch of foolish nonsense... For goodness sake.

Even I said that light is part of the 'Spectrum' of Electro Magnetic Force. Look at the 'QUESTION'... that was asked.

Wittering on about what light is... is not what this OP is asking. That subject is understood.

The other issue as to if light can or has electro magnetic energy... You all might recall the high school experiment where

a light is shone on a small vain... it spins... SO, Yes there's energy at work. Wave or particle... both.

It is electromagnetic energy... BUT. Its so far up the scale that it does not have a signature that you could call electrical,

magnetic force 0.

Answer the question with anything other than a NO and you are misleading the OP... Light has NO magnetic properties.

Complicating that answer with all of the electromagnet knowledge of humanity does not make me wrong.... The answer is NO.

and its interesting that after telling me I am wrong... I then get told I am right...

Who was it from history that stood in the bunker and said. "We can stand up now. We are out of range." .........................................

In a vacuum you can shine a light and not disturb the pile of iron fillings....

At 200 km / hr Turning the lights on does not slow the car...

but then I could argue that all the moths mass that the light has drawn into my path does... sigh.

You are wrong Astromark.

A beam of visible light has the same properties as a radio wave, it is just a higher frequency. It has a magnetic component and an electric component and both function like any other magnetic and electric field there is. The only reason you dont see a macroscopic effect is that the amount of evergy in a single cycle is so small that you dont get much of an effect.

korjik
2010-Jul-18, 08:56 PM
Thank you Jeff... you have nailed it... the answer is No... but with a explanation that says you might yet be proven to be what... ?

It might be wrong to be so blunt... so for that... " Oops sorry." But I meant every misunderstood word of it.

The POINT must be made that NO useful energy is available from a stream of passing photons in the visible light section of the spectrum...

funny, photoelectric cells seem to work.....

Jeff Root
2010-Jul-18, 09:09 PM
Mark,

Sorry to say, but I think I missed while korjik and Len Moran hit
the nail on the head with the observation that photons or light
waves of any frequency oscillate too rapidly to induce detectible
motion in macroscopic bits of iron. Radio-frequency waves are
able to induce motion in the free electrons in conductors, and
higher-frequency waves are able to induce motion of individual
electrons in atoms. The motions are due to the electromagnetic
interactions between the photons and the electrons. We don't
see light beams attracting or repelling bits of iron because the
fields change direction too rapidly to produce a detectible net
effect.... Plus being very weak!

-- Jeff, in Minneapolis

melech
2010-Jul-18, 10:06 PM
Perhaps (as the OP) I contributed to the disagreement between astromark and most of the other posters by loosely referring to a "light beam". I was really thinking of the magnetic effects at the level of the individual electric/magnetic fields of each light wave (or photon) as it passed an observer. Therefore, astromark emphasized the lack of measurable or useful energy coming from the beam as a whole. The sense I am getting from most posters is that, while in theory light waves or photons generate magnetic fields, the strength and duration of the field is so limited that it is beyond current measurement capabilities.

Is this an accurate summary?

Nereid
2010-Jul-18, 10:38 PM
Perhaps (as the OP) I contributed to the disagreement between astromark and most of the other posters by loosely referring to a "light beam". I was really thinking of the magnetic effects at the level of the individual electric/magnetic fields of each light wave (or photon) as it passed an observer. Therefore, astromark emphasized the lack of measurable or useful energy coming from the beam as a whole. The sense I am getting from most posters is that, while in theory light waves or photons generate magnetic fields, the strength and duration of the field is so limited that it is beyond current measurement capabilities.

Is this an accurate summary?
First, I assume by 'light' you mean 'visible light', rather than 'electromagnetic radiation', or EMR ('light' is often used as a shorthand for EMR, in general).

If so, then you can find ways to describe the effect of light, on some substances, in terms of magnetic or electric field interactions (or both), but I think such descriptions are (generally) pretty esoteric.

One example is the inverse Faraday effect, in which circularly polarised light can induce magnetisation.

In general, however, the interactions of light with matter - that somehow involve magnetic properties - are best described in terms of quantum mechanics, rather than classical electrodynamics. For example, the analog of spintronics (where photons play the role of electrons).

Further, mixing quantum and classical descriptions can be extremely misleading ... I'm not saying that you won't find such descriptions (you certainly will), but that you have to be very very careful with them, to not leave all manner of inaccurate or misleading (obvious and not-so-obvious) implications lying around for those with inquiring minds to cut themselves on.

korjik
2010-Jul-18, 11:33 PM
Perhaps (as the OP) I contributed to the disagreement between astromark and most of the other posters by loosely referring to a "light beam". I was really thinking of the magnetic effects at the level of the individual electric/magnetic fields of each light wave (or photon) as it passed an observer. Therefore, astromark emphasized the lack of measurable or useful energy coming from the beam as a whole. The sense I am getting from most posters is that, while in theory light waves or photons generate magnetic fields, the strength and duration of the field is so limited that it is beyond current measurement capabilities.

Is this an accurate summary?

Thomson scattering with sufficient resolution would allow you to directly measure the B field of a photon, but that would not be a trivial task. You would prolly have to look back 50-60 years to see if anyone has done it.

One thing: A wave and a photon are very different things. A photon is a purely quantum beastie and has to be treated using QM. A wave means you have a very large number of photons and you can treat them classically. The wave is pretty easy to measure, as the huge numbers of photons give a decent signal. Photons are so low energy that measuring pretty much anything about them directly is not easy.

astromark
2010-Jul-19, 12:52 AM
Perhaps (as the OP) I contributed to the disagreement between astromark and most of the other posters ...... ( snip )
The sense I am getting from most posters is that, while in theory light waves or photons generate magnetic fields, the strength and duration of the field is so limited that it is beyond current measurement capabilities.

Is this an accurate summary?

And Thank you for coming back with the understanding appropriate for the question you asked ...

and yes you are correct with a little amend um... Its almost useless but is measurable... Its visible light. We see it.

I trust all the rest is clearly understood...

mugaliens
2010-Jul-19, 07:06 AM
The most common phenomenon of the interaction between electromagnetic radiation and matter is in the form of microwaves at 2.54 GHz, which heat dielectric materials (those which contain polar molecules) in food, including water. The rapidly alternating electric field component of the electromagnetic microwaves causes the polar molecules to spin, which in turn bump into other molecules, and raise the overall level of energy in the food.

So, to answer the OP's questions directly:

"Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet?"

Yes, in that it's a magnetic field, but no in that the magnetic field surrounding a magnetic is static, while the magnetic field component of an EM wave is oscillating.

"For example, will it attract a piece of iron?"

Yes, but only on the scale of its wavelength.

"If so, why is this not apparent to an observer e.g., holding a flashlight near a piece of iron?"

The Earth's magnetic field is very weak, ranging from less than 30 microteslas throughout most of the inhabited world, to over 60 microteslas near the magnetic poles. Meanwhile, the strongest magnetic field created in the laboratory was 2,800 teslas.

The smallest magnetic field ever measured was on the order of attoteslas (10-18 tesla). The magnetic field of an EM wave is dependant on its amplitude, not its frequency, but as korjik mentioned, the wave characteristics are a function of the combined activity of its quantum components. In our most powerful lasers, such as those in the high TW and low PW range, the magnetic field is extraordinarialy large, except for one thing: The laser light may be coherent, but it's not polarized. That is, both the electric and magnetic fields of the waves are in step in terms of peak and min power (crest and trough of the wave), but they're not in step with respect to the orientation of the wave.

While lasers can be polarized, I'm not familiar with how this is done (short of a polarization filter). I'm also unfamiliar with how to calculate the peak strength of a polarized laser beam based on its wattage and cross-sectional area.

Korjik? Do you know?

IsaacKuo
2010-Jul-19, 02:44 PM
In our most powerful lasers, such as those in the high TW and low PW range, the magnetic field is extraordinarialy large, except for one thing: The laser light may be coherent, but it's not polarized. That is, both the electric and magnetic fields of the waves are in step in terms of peak and min power (crest and trough of the wave), but they're not in step with respect to the orientation of the wave.

While lasers can be polarized, I'm not familiar with how this is done (short of a polarization filter).
All lasers are all polarized. All lasers are, by definition, coherent. And all coherent light beams are, by definition, polarized in unison.

If a light beam isn't polarized, then it isn't coherent, and therefore isn't a laser.

IsaacKuo
2010-Jul-19, 02:57 PM
Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet? For example, will it attract a piece of iron? If so, why is this not apparent to an observer e.g., holding a flashlight near a piece of iron?
Answering the original question--the magnetic field portion of a beam of light does not have the same properties that are relevant to attracting a piece of iron. The most important differences are:

1) Extent. The magnetic field of a magnet extends outward in all directions, but the magnetic field of a beam of light only extends to where the light actually travels. It only "spreads out" to the extent that the light beam itself "spreads out". So, for a piece of iron to be affected by a beam of light's magnetic field, the light must directly shine on the iron.

and

2) Oscillation. Unlike the magnetic field of a magnet, the magnetic field of a beam of light is oscillating. This affects iron in a different way than a static magnetic field. A static magnetic field has an attactive effect because of how ferromagnetism works. But an oscillating magnetic field works differently because of how it interacts with induced currents.

neilzero
2010-Jul-19, 03:56 PM
In my opinion, it isn't so much that Mark is wrong, but the term electromagnetic radiation is miss leading or wrong. Apparently we are afraid to offend the guy that coined the term two centuries ago. Please, let's quit using archaic terminology, and stupid explanations to try to justify our reluctance to change. We can produce laser light concentrated enough to quickly burn though a foot of solid steel, but iron filings are not attracted. Neil

melech
2010-Jul-19, 04:20 PM
In my opinion, it isn't so much that Mark is wrong, but the term electromagnetic radiation is miss leading or wrong. Neil

Why do you think this term is misleading? Doesn't the beam or ray of light consist of alternating electric and magnetic fields? BTW, who did coin the term? Maxwell?

Jeff Root
2010-Jul-19, 05:24 PM
I don't see anything wrong or misleading about the term
"electromagnetic radiation". That term is completely apt.

-- Jeff, in Minneapolis

Geo Kaplan
2010-Jul-19, 05:40 PM
In my opinion, it isn't so much that Mark is wrong, but the term electromagnetic radiation is miss leading or wrong. Apparently we are afraid to offend the guy that coined the term two centuries ago. Please, let's quit using archaic terminology, and stupid explanations to try to justify our reluctance to change. We can produce laser light concentrated enough to quickly burn though a foot of solid steel, but iron filings are not attracted. Neil

Electromagnetic radiation is a perfectly fine term (the radiation has both an electric and magnetic field). It is neither misleading nor wrong. We don't keep using it out of fear of giving offense -- we use it because it succinctly captures the essentials. If you have a contrary notion, perhaps you'd like to defend it in the ATM forum?

trinitree88
2010-Jul-19, 06:42 PM
I don't see anything wrong or misleading about the term
"electromagnetic radiation". That term is completely apt.

-- Jeff, in Minneapolis

Jeff. Agreed. In the teaching of electromagnetism, it is essential to remind students of Maxwell's first unification of fields. The phenomena of electricity, magnetism, and optics which once were considered entirely separate areas of physics, under Maxwell's equations could be seen to be different aspects of a common phenomenon. It was a precedent leading to the partial unification of forces, by Salam Glashow and Weinberg, incorporating the weak force, too, under electroweak unification about a century later, and leading to the present attempts to unify them all. pete

SEE:http://en.wikipedia.org/wiki/Electroweak_interaction

astromark
2010-Jul-19, 07:56 PM
There is nothing to this.... Yes its properly called 'Electro Magnetic Radiation.'... There is no disagreement.

The problem is understanding the energy. Wave like particle stream... and then the argument begins...

From one end of the scale truly massive antenna are required. while at the other end of the scale less than a millimeter of band width...

From massive arrays of radio telescopes to the detection of a gamma ray or neutrino's... Microscopic. Its all the Electro Magnetic Scale...

Visible light is just a small band of output that we have become reliant on for vision...

Things would have been different if our atmosphere was not transparent.

melech
2010-Jul-20, 02:00 AM
[QUOTE=IsaacKuo;1764550]Answering the original question--the magnetic field portion of a beam of light does not have the same properties that are relevant to attracting a piece of iron. The most important differences are:

1) Extent. The magnetic field of a magnet extends outward in all directions, but the magnetic field of a beam of light only extends to where the light actually travels. It only "spreads out" to the extent that the light beam itself "spreads out". So, for a piece of iron to be affected by a beam of light's magnetic field, the light must directly shine on the iron.

IsaacKuo, you raise a point that I had not considered. Can you explain WHY the magnetic field in a light beam does not spread out infinitely?

IsaacKuo
2010-Jul-20, 02:17 AM
Can you explain WHY the magnetic field in a light beam does not spread out infinitely?
It's because very common materials interact with light waves in ways that block, absorb, and/or reflect them.

We are used to the idea of obstacles "blocking" other things. If you try to throw a baseball through a brick wall, it doesn't work--the brick wall blocks it. Same thing with a flashlight beam. We're so used to this, that we think that this is the normal way of things.

But in reality, the normal way of things is that particles and fields just keep on going and have no problem passing through each other unless there's some particular reason for them to interact.

So the question of why electromagnetic waves can be reflected and focused by a flashlight mirror is actually quite non-trivial. It's true that static magnetic fields and electric fields will pass right through a flashlight mirror. If you place a permanent magnetic in the place of a flashlight bulb, the magnetic field penetrate right through the mirror almost as if it weren't there at all. But electromagnetic waves will interact with the electrons of the mirror's atoms in a way which induce new electromagnetic waves. These induced waves interact with the original waves in such a way to perfectly cancel out the ones which would have penetrated the mirror while also creating new waves going outward through the front of the flashlight.

The mathematics and physics involved is pretty complex. The bottom line is that interactions with electrons cause electromagnetic waves to be unable to penetrate through opaque objects.

publius
2010-Jul-20, 02:24 AM
A standard exercise in E&M texts is to calculate the electric and magnetic field strengths of incident sunlight on the earth. You may be suprised that the (RMS) E field strength is around 800V/m or so, IIRC.


-Richard

Sir Knots A Lot
2010-Jul-20, 03:41 AM
I just thought I'd throw in that the magnetic fields around magnetars (http://en.wikipedia.org/wiki/Magnetar) are thought to polarize the vacuum, becoming strongly birefringent.

The end result of this vacuum polarization is that light passing through the intense magnetic field of the magnetar is split into polarized components, one of which is affected and refracted at higher angle than the other which passes through the field relatively unaltered.

So in a sense, strong magnetic fields can 'bend' light similar to how gravity bends light, but gravity doesn't play favorites with polarization.

mugaliens
2010-Jul-20, 06:55 AM
All lasers are all polarized. All lasers are, by definition, coherent. And all coherent light beams are, by definition, polarized in unison.

If a light beam isn't polarized, then it isn't coherent, and therefore isn't a laser.

Negative (http://www.rp-photonics.com/polarization_of_laser_emission.html). This "axiom" is often-repeated, but is not entirely correct. The reason most lasers are polarized is two-fold. First, it's a highly desirable characteristic of laser light. Second, it's the natural result of the physics of creating laser light in a mirrored lasing cavity.

In short, people often assume the two terms are synonomous, but they're not. Coherence is simply when crests and troughs are aligned (constructive interference with a constant relative phase), while polarization simply involves the direction of the electric field oscillation.

Light (indeed, all EM) can be found in all four of the following states:

1. non-coherent, non-polarized (sunlight)

2. coherent, non-polarized (a few lasers)

3. non-coherent, polarized (Rayleigh scattering, reflection off non-mirrored surfaces, view through Raybans (linear) or Disneyvision 3D (circular), etc.)

4. coherent, polarized (most lasers)

More (http://www.rp-photonics.com/laser_light.html).

IsaacKuo
2010-Jul-20, 09:55 AM
In short, people often assume the two terms are synonomous, but they're not. Coherence is simply when crests and troughs are aligned (constructive interference with a constant relative phase), while polarization simply involves the direction of the electric field oscillation.
This doesn't make sense, because the electric field is a vector, not a scalar. In order to constructively or destructively interfere, the direction of the electric field matters. For example, if the polarizations are perpendicular, then there won't be any constructive or destructive interference at all.

But it seems that the language used by laser scientists is not so cut and dry. When they talk of a non-polarized laser, it apparently doesn't mean that the light is actually not polarized. The polarization may be either unstable or purposefully randomized (http://www.adaptifphotonics.com/Products/Synchronous-Scrambler.htm) (rapidly switched randomly every few microseconds).

Len Moran
2010-Jul-20, 10:07 AM
I just thought I'd throw in that the magnetic fields around magnetars (http://en.wikipedia.org/wiki/Magnetar) are thought to polarize the vacuum, becoming strongly birefringent.

The end result of this vacuum polarization is that light passing through the intense magnetic field of the magnetar is split into polarized components, one of which is affected and refracted at higher angle than the other which passes through the field relatively unaltered.

So in a sense, strong magnetic fields can 'bend' light similar to how gravity bends light, but gravity doesn't play favorites with polarization.

Just for the record, an experiment by the PVLAS group in Italy (PRL 110406 (2006)) did not find any evidence of birefringence in vacuum resulting from magnetisation. They surrounded an optical resonator cavity with a rotating 5T superconducting magnet and measured any interaction of the magnetic field and the resonating laser within the cavity (which was in vacuum). The initial results (mentioned in the PRL) were positive, but this was subsequently traced to flux leakage affecting the instruments. As far as I know, no rotation due to the vacuum becoming birefringent has so far been observed in this experiment.

I realise that you are talking about "magnetars" which are thought to have massive field intensities, but even so, the measuring instruments in the PVLAS experiment were extremely sensitive, being able to measure any beam rotation resulting from the vacuum becoming birefringent and dichroic down to 10E-7 rad.

Len Moran
2010-Jul-20, 11:13 AM
The mathematics and physics involved is pretty complex. The bottom line is that interactions with electrons cause electromagnetic waves to be unable to penetrate through opaque objects.


Do we know why two light beams "interacting" in vacuum produce no change whatsoever in the properties of either beam as observed at the end of each of the beams? The standard explanation says that no interaction takes place in a linear medium (and I suppose a vacuum is such a linear "medium" (for want of a better word)). A beam of electrons can be deflected by a magnet, so what is considered to be the difference between electrons with magnetic "properties" and photons with magnetic "properties"? We can certainly say that EMR consists of alternating magnetic fields, and that a static field adjacent to a light beam is a different scenario to that of an electron beam, but interaction between such alternating fields can produce addition and subtraction within a linear medium, so I'm not talking about new frequencies being generated which do require a non linear medium.

Clearly there is a difference between the magnetic "properties" of EM waves and magnetic "properties" of electron beams that is not simply associated with the fact that the fields in the former are alternating and those in the latter are not. I can think of the beam of electrons as a current fowing through a solid wire and the wire being deflected as a result of interaction with a magnet, so the movement of the electron with respect to the magnet induces a magnetic field around the electron. But a photon does no such thing - I know a photon has no mass and no charge, but still, what exactly is the nature of the magnetic component of a photon that does not respond to an external magnetic field?, unlike a single electron that can be deflected in flight in vacuum (which is a linear "medium").

If we prefer to think of EMR as consisting of "physical" magnetic and electric waves then where is the medium in which the wave can travel? If there were such a medium, then interaction should occur with an external magnetic field. If we think of EMR as travelling photons, and the photons having magnetic "properties", then why can't we deflect them in flight?

So I do think there is a substantial difference between a magnetic field that attracts iron filings and the magnetic field associated with EMR as "existing" (in a familiar physical sense) between the source and sink. The only time (it seems to me) we get to apply familiar notions of magnetic fields to EMR is at the detector.

Nereid
2010-Jul-20, 12:34 PM
The mathematics and physics involved is pretty complex. The bottom line is that interactions with electrons cause electromagnetic waves to be unable to penetrate through opaque objects.Do we know why two light beams "interacting" in vacuum produce no change whatsoever in the properties of either beam as observed at the end of each of the beams? The standard explanation says that no interaction takes place in a linear medium (and I suppose a vacuum is such a linear "medium" (for want of a better word)). A beam of electrons can be deflected by a magnet, so what is considered to be the difference between electrons with magnetic "properties" and photons with magnetic "properties"? We can certainly say that EMR consists of alternating magnetic fields, and that a static field adjacent to a light beam is a different scenario to that of an electron beam, but interaction between such alternating fields can produce addition and subtraction within a linear medium, so I'm not talking about new frequencies being generated which do require a non linear medium.

Clearly there is a difference between the magnetic "properties" of EM waves and magnetic "properties" of electron beams that is not simply associated with the fact that the fields in the former are alternating and those in the latter are not. I can think of the beam of electrons as a current fowing through a solid wire and the wire being deflected as a result of interaction with a magnet, so the movement of the electron with respect to the magnet induces a magnetic field around the electron. But a photon does no such thing - I know a photon has no mass and no charge, but still, what exactly is the nature of the magnetic component of a photon that does not respond to an external magnetic field?, unlike a single electron that can be deflected in flight in vacuum (which is a linear "medium").

If we prefer to think of EMR as consisting of "physical" magnetic and electric waves then where is the medium in which the wave can travel? If there were such a medium, then interaction should occur with an external magnetic field. If we think of EMR as travelling photons, and the photons having magnetic "properties", then why can't we deflect them in flight?

So I do think there is a substantial difference between a magnetic field that attracts iron filings and the magnetic field associated with EMR as "existing" (in a familiar physical sense) between the source and sink. The only time (it seems to me) we get to apply familiar notions of magnetic fields to EMR is at the detector.
I think this post is a good illustration of exactly what I mentioned earlier; namely, if you mix classical electrodynamics and QED explanations, you end with a confusing mess, and the conclusions you draw from it are unreliable (at best).

The only way to model the behaviour of a stream of photons in a magnetic field is to use QED, and QED only ... and that is a dauntingly difficult thing to do. Further, when you do do that work, translating the math into a description that makes sense, in some intuitive way, is even more difficult.

Nereid
2010-Jul-20, 12:50 PM
Just for the record, an experiment by the PVLAS group in Italy (PRL 110406 (2006)) did not find any evidence of birefringence in vacuum resulting from magnetisation. They surrounded an optical resonator cavity with a rotating 5T superconducting magnet and measured any interaction of the magnetic field and the resonating laser within the cavity (which was in vacuum). The initial results (mentioned in the PRL) were positive, but this was subsequently traced to flux leakage affecting the instruments. As far as I know, no rotation due to the vacuum becoming birefringent has so far been observed in this experiment.

I realise that you are talking about "magnetars" which are thought to have massive field intensities, but even so, the measuring instruments in the PVLAS experiment were extremely sensitive, being able to measure any beam rotation resulting from the vacuum becoming birefringent and dichroic down to 10E-7 rad.
Even if the effect were to be linear, the PVLAS experiment would say nothing about vacuum birefringence near a magnetar ... magnetar magnetic fields may be as strong as 10^10 T, fully several billion times stronger than the magnet used in the PVLAS experiment. So, if linear, a beam rotation of 10^-7 rad, at 5T, would correspond to one of ~200 rad, at 10^10 T!

A 200 rad beam rotation would correspond to pretzelling light to an extreme extent.

Len Moran
2010-Jul-20, 01:07 PM
Even if the effect were to be linear, the PVLAS experiment would say nothing about vacuum birefringence near a magnetar ... magnetar magnetic fields may be as strong as 10^10 T, fully several billion times stronger than the magnet used in the PVLAS experiment. So, if linear, a beam rotation of 10^-7 rad, at 5T, would correspond to one of ~200 rad, at 10^10 T!

A 200 rad beam rotation would correspond to pretzelling light to an extreme extent.

Thanks for putting that experiment in context. I must admit, I didn't think to try and extrapolate the results (or lack of them) in the way you have, the sensitivity seemed so incredibly good for detecting rotation in the lab that I assumed those results were relevant to the wider view.

Jeff Root
2010-Jul-20, 01:20 PM
Can you explain WHY the magnetic field in a light beam does not
spread out infinitely?
It's because very common materials interact with light waves in
ways that block, absorb, and/or reflect them.
Isaac,

You seem to be answering some question other than what
melech asked.

I can't give a good answer to melech's question, but my generic
answer would be that there simply is no reason for the magnetic
field to extend outside the beam. That is in contrastrast to your
answer, which seems to say that matter blocks the magnetic
field from escaping the beam.

I suspect that you were instead answering the question, "Why
don't all light beams spread out in all directions?"

-- Jeff, in Minneapolis

Sir Knots A Lot
2010-Jul-20, 01:22 PM
Even if the effect were to be linear, the PVLAS experiment would say nothing about vacuum birefringence near a magnetar ... magnetar magnetic fields may be as strong as 10^10 T, fully several billion times stronger than the magnet used in the PVLAS experiment. So, if linear, a beam rotation of 10^-7 rad, at 5T, would correspond to one of ~200 rad, at 10^10 T!

A 200 rad beam rotation would correspond to pretzelling light to an extreme extent.


A magnetic field of 10 gigateslas is enormous relative to magnetic fields typically encountered on Earth. Earth has a geomagnetic field of 30–60 microteslas, and a neodymium based rare earth magnet has a field of about 1 tesla, with a magnetic energy density of 4.0105 J/m3. A 10 gigatesla field, by contrast, has an energy density of 4.01025 J/m3, with an E/c2 mass density >104 times that of lead. The magnetic field of a magnetar would be lethal even at a distance of 1000 km, tearing tissues due to the diamagnetism of water. At a distance halfway to the moon, a magnetar could strip information from all credit cards on Earth.

For a larger idea of the field strength comparison...

IsaacKuo
2010-Jul-20, 01:43 PM
Isaac,

You seem to be answering some question other than what
melech asked.

I can't give a good answer to melech's question, but my generic
answer would be that there simply is no reason for the magnetic
field to extend outside the beam. That is in contrastrast to your
answer, which seems to say that matter blocks the magnetic
field from escaping the beam.

I suspect that you were instead answering the question, "Why
don't all light beams spread out in all directions?"
If common materials weren't able to block, absorb, and reflect light beams, then light beams would indeed spread out in all directions, and they would then spread out indefinitely (since they aren't blocked by common materials).

But because common materials are able to block and reflect light beams, a flashlight beam can be focused to only go "forward" rather than spreading out everywhere.

Jeff Root
2010-Jul-20, 01:53 PM
Isaac,

Then it does look like you misunderstood melech's question,
and answered a different question, instead. Melech asked
why the magnetic field in a light beam does not extend
beyond the light beam.

As I said, my only answer to that question is that there isn't
any reason for the magnetic field to extend beyond the beam,
which isn't much of an answer.

-- Jeff, in Minneapolis

IsaacKuo
2010-Jul-20, 02:12 PM
Isaac,

Then it does look like you misunderstood melech's question,
and answered a different question, instead. Melech asked
why the magnetic field in a light beam does not extend
beyond the light beam.
No, I understood it perfectly. And I gave the ultimate reason that matters.

You can't read my mind, don't tell me what I'm thinking.

As I said, my only answer to that question is that there isn't
any reason for the magnetic field to extend beyond the beam,
which isn't much of an answer.
This is actually incorrect. There is a reason for the magnetic field to extend beyond the beam--it's called diffraction. But the amount of the beam which spills out due to diffraction is very low for a focused beam (even a poorly focused beam like a flashlight). The ultimate reason for this is because common materials are able to reflect and focus this beam.

As I described, fields don't need an excuse to go in all directions and penetrate everything. What's needed is a reason for them to behave the way we're used to--with opaque obstacles blocking things and such.

Jeff Root
2010-Jul-20, 03:15 PM
Then it does look like you misunderstood melech's question,
and answered a different question, instead. Melech asked
why the magnetic field in a light beam does not extend
beyond the light beam.
No, I understood it perfectly. And I gave the ultimate reason
that matters.

You can't read my mind, don't tell me what I'm thinking.
I can read your mind, but I agree not to tell you what
you are thinking.

Even if I couldn't read your mind, I can read your words, and
I can read that the answer you gave is not an answer to the
question melech asked. It is an answer to a different question.
Since, as you say, you understood the question perfectly, it is
curious that you gave an answer to a different question.
It looks exactly as if you misunderstood it.




As I said, my only answer to that question is that there isn't
any reason for the magnetic field to extend beyond the beam,
which isn't much of an answer.
This is actually incorrect.
It is incorrect as an answer to the question that you answered.
It is correct as an answer to the question melech asked.
Yours is not.

-- Jeff, in Minneapolis

IsaacKuo
2010-Jul-20, 03:31 PM
It is incorrect as an answer to the question that you answered.
It is correct as an answer to the question melech asked.
No. Your answer is incorrect, period. To recap, your claim is that "there isn't any reason for the magnetic field to extend beyond the beam". This claim is wrong, regardless of what question you're responding to.

You're confused about the physics involved, and this would seem to be the ultimate reason why you're confused about my explanations.

mugaliens
2010-Jul-20, 10:04 PM
"Is too.." "Is not..." "Is too!" "Is..."

Isaac, why don't you explain why it's incorrect, and explain why you think your statement is correct. Or at least provide links to reputable sources.

Jeff Root
2010-Jul-20, 10:35 PM
The problem is that what Isaac is saying about the behavior of
light is obviously correct-- It's elementary school level stuff -- but
it isn't the answer to the question melech asked. Isaac doesn't
need to support the correctness of his answer, he needs to support
its relevance.

And that he is having trouble with it is weird because he's the
one who prompted melech's question, with a correct description
of the relation between the light beam and its magnetic field.
(Post #23 and post #30)

-- Jeff, in Minneapolis

IsaacKuo
2010-Jul-21, 12:04 AM
The problem is that what Isaac is saying about the behavior of light is obviously correct-- It's elementary school level stuff -- but it isn't the answer to the question melech asked.
Elementary school level explanations of how light works are wrong. They're okay for describing the geometry of simple light reflection, but they can't explain diffraction or the electric fields or the magnetic fields. Elementary school level explanations can't explain beam spread or diffraction.

In order to answer melech's question, it is necessary to get beyond the elementary school level and get into the electromagnetic wave nature of light. You have to discard your elementary school level intuitions and accept the fact that seemingly obvious features of how light behaves are actually complex emergent behaviors resulting from Maxwell's equations. If you understand those equations, you see that the natural thing is for electric and magnetic fields to simply permeate everywhere. The fact that light beams don't simply go everywhere is actually an emergent behavior of how electromagnetic waves interact with familiar things.

Jeff Root
2010-Jul-21, 12:58 AM
Isaac,

I said that the answer you gave is elementary, not that an
answer to melech's question is elementary. I agree that a
good answer to melech's question would not be elementary,
and it could involve some or all of the ideas you just raised.
I can't give a good answer to his question-- maybe you can.
But so far you haven't addressed melech's question, much
less answered it.

He didn't ask why light beams don't go everywhere; he asked
why the beam's magnetic field doesn't go beyond the beam,
as you correctly told him in post #23.

-- Jeff, in Minneapolis

cjameshuff
2010-Jul-21, 01:39 AM
In my opinion, it isn't so much that Mark is wrong, but the term electromagnetic radiation is miss leading or wrong. Apparently we are afraid to offend the guy that coined the term two centuries ago. Please, let's quit using archaic terminology, and stupid explanations to try to justify our reluctance to change. We can produce laser light concentrated enough to quickly burn though a foot of solid steel, but iron filings are not attracted. Neil

The term "electromagnetic radiation" is not at all wrong or misleading. It describes exactly what the radiation is. The argument about light not attracting magnetic materials simply displays a lack of understanding of electromagnetism. There are magnetic and electric fields involved, and the electrons and atoms of the material do respond to them. This is why a laser can deposit energy into a block of steel and cut through it. There is no net attraction because a laser beam isn't a constant magnetic field, it is waves of electromagnetic radiation. The magnetic fields involved vary on femtosecond timescales, in waves a few hundred nanometers long, and under such conditions the ferromagnetic properties of iron are rather unimportant.



IsaacKuo, you raise a point that I had not considered. Can you explain WHY the magnetic field in a light beam does not spread out infinitely?

In a way, it actually does. Shine light through a small enough pinhole, and it'll "go around the corner", spreading out in a hemisphere on the far side. Light can be modeled as a sum of spherical waves constructively and destructively interfering with each other...it effectively propagates to infinity in all directions, but cancels itself out everywhere outside the beam. By passing a portion of it through a pinhole, you exclude the parts that would cancel out the field to the sides and reinforce the field in the center, and you get light spreading out after going through the pinhole.

If you've got a reasonably fast computer, these applets might be interesting:
http://www.falstad.com/ripple/

This one's a little easier on the CPU:
http://www.falstad.com/wave2d/

publius
2010-Jul-21, 02:46 AM
I was trying to remember the term and it finally hit me:

http://en.wikipedia.org/wiki/Gaussian_beam

A Gaussian beam is a pretty good model for a laser beam and demostrates how such a beam is a valid solution of Maxwell.


-Richard

mugaliens
2010-Jul-21, 05:58 AM
By passing a portion of it through a pinhole, you exclude the parts that would cancel out the field to the sides and reinforce the field in the center, and you get light spreading out after going through the pinhole.

This is the most succinct description of edge diffraction effects I've read yet - nice! By the way, the optimum diameter of the pinhole is 1.9*sqrt(f*lambda), where f is the focal length (distance from pinhole to the focal plane) and lambda is the wavelength of the light used in the camera. If you're using a range of wavelengths, use the longest of them to reduce the diffraction, but be prepared for a slightly fuzzier image.

peterg7lyq
2010-Sep-09, 06:51 AM
wow its nice to see there is someone else asking these sort of questions
heres a link to another site i use where i aske a simular question

-----------------------------------------------------------------------------------------------
What is magnetism

http://www.sciencedaily.com/releases/2010/09/100903121418.htm

I know its the magnetic attraction of ferrous metal to a magnet, although non ferrous metal is also attracted to magnectic fields ie as in electricity meters.

and if frozen in liquid nitrogen even some ceramic materials.

so if the magnetic field can attract so many items what exactly is it ?

this question goes there with " what exactly is Gravity" other than the pull or attraction of a lither body to a larger body or mass, But what is the pull ?

--------------------------------------------------------------------------------------------------------------

Any source of magnetism, such as a magnet or electromagnet, is surrounded by a magnetic field. That field can be detected by various devices, which can also give information about the direction of the field and even its strength.

A simple compass can detect a magnetic field and demonstrate its direction. Iron filings can be used to show the shape of a magnetic field. At the sophisticated level, a gaussmeter can detect a field and indicate its strength, as measured in gauss units.

Questions you may have include:

How does a compass detect a magnetic field?
How do iron filings demonstrate a magnetic field?
What is a gaussmeter?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

------------------------------------------------------------------------------------------------------------------------------

Compass
A compass is simply a thin magnet or magnetized iron needle balanced on a pivot. It can be used to detect small magnetic fields. The needle will rotate to point toward the opposite pole of a magnet. It can be very sensitive to small magnetic fields.

Using a compass to show the magnetic field

When you bring a compass near an item suspected of being magnetized or having a magnetic field, the compass will turn and point toward the appropriate pole of the object.

Compass needle attracted to magnet's N pole

A famous experiment showed that a wire with DC electric current running through it created a magnetic field. When the electricity was turned on, a nearby compass moved to indicate a magnetic field was present.

Earth is a huge magnet

The compass was used to discover that the Earth is a huge magnet. The North-seeking pole of the compass needle will always point toward the Earth's North magnetic pole.

Iron filings
By spreading fine iron filings or dust on a piece of paper laid on top of a magnet, you can see the outline of the magnetic lines of force or the magnetic field. The picture below

Iron filings and compasses show the shape and direction of the magnetic field

This experiment also shows that magnetism will act through many materials, such as paper. Would the experiment work if a sheet of iron were used to sprinkle the filings? What about aluminum foil?

Gaussmeter
Gaussmeters are used to measure the strength of a magnetic field. They use a electronic chip called a Hall effect device, which gives off a tiny electrical current when exposed to a magnetic field. The current is amplified with electronic circuitry and a meter shows the number of gauss (the units of magnetic field strength).

These devices are used to detect and measure magnetic fields in scientific experiments, in industry and even in people's homes.

Summary
Magnetic objects are surrounded by a magnetic field. Devices can detected the field and also give information about the direction of the field and even its strength. A compass can detect a magnetic field and show its direction. Iron filings can show the shape of a magnetic field. A gaussmeter can detect a field and indicate its strength.

------------------------------------------------------------------------------------------------------------------------------------------------

Build your own Gaussmeter - At-home instructions

http://www.google.co.uk/search?q=magnetic+lines+of+force&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7GZEU_en&redir_esc=&ei=ZXOITISECMOQjAe9rZyXBA

Magnetic Fields in and around Horseshoe and Ring Magnets

Magnets come in a variety of shapes and one of the more common is the horseshoe (U) magnet. The horseshoe magnet has north and south poles just like a bar magnet but the magnet is curved so the poles lie in the same plane. The magnetic lines of force flow from pole to pole just like in the bar magnet. However, since the poles are located closer together and a more direct path exists for the lines of flux to travel between the poles, the magnetic field is concentrated between the poles.

If a bar magnet was placed across the end of a horseshoe magnet or if a magnet was formed in the shape of a ring, the lines of magnetic force would not even need to enter the air. The value of such a magnet where the magnetic field is completely contained with the material probably has limited use. However, it is important to understand that the magnetic field can flow in loop within a material. (See section on circular magnetism for more information).

General Properties of Magnetic Lines of Force

Magnetic lines of force have a number of important properties, which include:

They seek the path of least resistance between opposite magnetic poles. In a single bar magnet as shown to the right, they attempt to form closed loops from pole to pole.
They never cross one another.
They all have the same strength.
Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability.
Their density decreases with increasing distance from the poles.
They are considered to have direction as if flowing, though no actual movement occurs.
They flow from the south pole to the north pole within a material and north pole to south pole in air.
even the flow of electricity is wrong most people think elctricity flows from posative to negative well it dont

A word of caution about the right-hand clasp rule
For the right-hand rule to work, one important thing that must be remembered about the direction of current flow. Standard convention has current flowing from the positive terminal to the negative terminal. This convention is credited to Benjamin Franklin who theorized that electric current was due to a positive charge moving from the positive terminal to the negative terminal. However, it was later discovered that it is the movement of the negatively charged electron that is responsible for electrical current. Rather than changing several centuries of theory and equations, Franklin's convention is still used today.

the so called scientist have a lot to answer for

http://www.google.co.uk/imgres?imgurl=http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Graphics/BarMagField3(small).jpg&imgrefurl=http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/MagneticFieldChar.htm&h=474&w=300&sz=38&tbnid=iq-Kuz2STn5y-M:&tbnh=129&tbnw=82&prev=/images%3Fq%3Dmagnetic%2Blines%2Bof%2Bforce&zoom=1&q=magnetic+lines+of+force&usg=__IdyA2unHuG6Uw6vdtdYDfsDCyKI=&sa=X&ei=ZXOITNCiDpD-4Aac9LzSBA&ved=0CC0Q9QEwBw

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So a beam of light does not generate a detectible magnetic field.....A EM wave (a beam if you like) interacting with a coil generates a voltage - the only way a voltage can be generated in a stationary coil is through the interaction of that coil with a changing magnetic field. So in terms of the EM wave, its changing magnetic field (which corresponds to the frequency of the transmitter oscillator) is certainly detectable. We can't do such a measurement with light because we have no means in which to measure the high frequency magnetic (or electric) oscillations, but in terms of the (accepted) EM spectrum as consisting of an electric and magnetic field, it is a difference only of degree. So in terms of a "beam" of light and a "beam" of radio waves, the mathematical formulation of such beams in terms of changing electric and magnetic fields are identical, but the means of detecting those changing fields are subject to the physical limitations of measurement apparatus rather than a beam of light not "generating" a magnetic field.

What is of real interest is to distinguish between a magnet inducing a voltage in a coil and a EM wave (or light wave, but we simply can't physically measure the fluctuating induced voltage) doing exactly the same thing. Yet a magnet will deflect an identical magnet, but cross two beams of light in vacuum and nothing whatsoever happens. So the observed end result of an induced voltage from an EM wave and a magnet seem to be derived from a changing magnetic component that is of a different "form" in terms of the magnetic properties of a moving magnet and a EM field. That I think is the core of the original question asked - for this is what melech asked:

"Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet?"

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Eliminate electro magnetism it may make it easier to explain.

Electro magnetism comes about by the passage of electricity in wires thus artificially creating the magnetic force.

So i suppose the question should be what is naturally occurring magnetism ?

And what is the magnetism in permanents magnets that gives the material the ability to attract ferrous metals and to some degree non ferrous materials

As in permanent magnets


Post edited by Tusenfem. (taken out lots of useless information)
peterl7lyq, please do not copy-paste verbatim webpages onto BAUT. If you think that the discussion on another forum is important, then please give a quicklook summary and the link to the forum in question.
I bolded where the same question was put as here on BAUT.

TrAI
2010-Sep-12, 11:56 PM
...

I think the main reason that the magnetic fields of photons are
not directly detectible is... too complex for me to understand
well enough for me to be able to explain. But the tiny energy
of each photon, each having a very low-intensity magnetic field,
as korjik said, is an important factor, and may be a sufficient
reason even if it isn't the primary reason.
...
-- Jeff, in Minneapolis
.

Hmmm... Perhaps it is just that the net magnetic and electric charge of a photon is 0?.

I can't think of an exact analogue, but perhaps it is similar to how you may have a line coding that employs a signal changing between positive and negative, but to prevent the cable being charged, it is designed to keep the average charge of a cycle to 0. If you had such a cycle, where the signal is positive and negative for 50% each, but only could detect the average charge of an entire cycle, you would find it neutral.


A EM wave (a beam if you like) interacting with a coil generates a voltage - the only way a voltage can be generated in a stationary coil is through the interaction of that coil with a changing magnetic field. So in terms of the EM wave, its changing magnetic field (which corresponds to the frequency of the transmitter oscillator) is certainly detectable. We can't do such a measurement with light because we have no means in which to measure the high frequency magnetic (or electric) oscillations, but in terms of the (accepted) EM spectrum as consisting of an electric and magnetic field, it is a difference only of degree. So in terms of a "beam" of light and a "beam" of radio waves, the mathematical formulation of such beams in terms of changing electric and magnetic fields are identical, but the means of detecting those changing fields are subject to the physical limitations of measurement apparatus rather than a beam of light not "generating" a magnetic field.

What is of real interest is to distinguish between a magnet inducing a voltage in a coil and a EM wave (or light wave, but we simply can't physically measure the fluctuating induced voltage) doing exactly the same thing. Yet a magnet will deflect an identical magnet, but cross two beams of light in vacuum and nothing whatsoever happens. So the observed end result of an induced voltage from an EM wave and a magnet seem to be derived from a changing magnetic component that is of a different "form" in terms of the magnetic properties of a moving magnet and a EM field. That I think is the core of the original question asked - for this is what melech asked:

"Does the magnetic field portion of a beam of light have the same properties as the magnetic field surrounding a magnet?"

Seems to me that radio does not use the wave of a single photon, but rather the wave of a group of photons. I should think the bandwidth of a single photon may be somewhat limited. I suppose, the average charge of a radio wave could also be 0, but unlike a single photon, this is spread over a certain reception time...

Jeff Root
2010-Sep-13, 09:54 AM
TrAI,

Your description seems okay. After my post, korjik gave what
appears to be the correct (and very simple!) answer in post #14.
The reason that the magnetic fields of photons are not directly
detectible is that the combination of small amplitude and short
wavelength means the amount of energy in a single half-cycle
is too small to have a macroscopic effect before the next half of
the cycle reverses the direction of the non-effect.

A single photon of radio frequency doesn't have enough energy
to be detected by *any* means, as far as I know. Vast numbers
of photons are required by ordinary radio receivers.

-- Jeff, in Minneapolis