# Magnetism

Discussion in 'Physics & Math' started by Samidha, Feb 14, 2017.

1. ### SamidhaRegistered Member

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What is the source of magnetism in magnets?

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5. ### James RJust this guy, you know?Staff Member

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The most common permanent magnets contain iron. In iron, the intrinsic magnetic moments of the atoms tend to couple to one another, resulting in relatively large-scale order within the metal in magnetic domains where many individual moments are aligned. Magnetising the metal tends to cause some domains to shrink and others to grow, making the overall alignment even stronger. Moreover, the metal exhibits a kind of memory effect, wherein one the domains are aligned they do not easily unalign again (under ordinary conditions); thus the metal becomes a "permanent" magnet.

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8. ### DaveC426913Valued Senior Member

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And what is atomic magnetic moment? he will ask.
http://physics.bu.edu/~duffy/semester2/c16_atomic.html

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9. ### Magical RealistValued Senior Member

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How does the magnetic field extend out in space? What is the composition of this field? What is the nature of its origin from the magnet itself?

10. ### rpennerFully WiredStaff Member

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The manner in which the magnetic field fills empty space is well-described by Maxwell's equations of the unified theory of electromagnetism(1865). (Physical theories tell you how observable phenomena behave.)
That's in the nature of a philosophical question of what reality is. Maxwell's equations are not very illuminating as to the nature of the magnetic field, except to say that changes in the magnetic field propagate with the speed of exactly one phenomenon with known speed in 1865, light. Other evidence confirmed that light, radio waves, magnetic fields, gamma rays, etc were all part of a continuous spectrum of light-like phemonena, of which our eyes perceive only about 1 octave.
Later, the behavior of light was unified with the behavior of all laboratory-known types of matter into a single theory called The Standard Model of Particle Physics (c. 1970). In this framework, magnetism and light are aspects of a relativistic quantum field of massless bosons that couples to the relativistic quantum fields of other fermions and bosons in proportion to a an associated number associates with every fermion and boson, its electric charge. Since this coupling is relatively small compared with the Planck's constant, $\frac{e^2}{4 \pi \epsilon_0 c} \, \approx \, 7.7 \times 10^{-37} \, \textrm{J} \cdot \textrm{s} \, \approx \, \frac{1}{137} \hbar$, we can meaningfully speak of the theoretical approximation of free electrons and free photons independent of the phenomenon of electromagnetism.
Like all light, the origin of the electromagnetic field at a particular spot is from electrically charged particles, those that have moved or those that have intrinsic angular momentum in the slice of space-time where a signal moving at the speed of light could just reach our particular spot at the time we measure the field.

11. ### Magical RealistValued Senior Member

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If a magnetic field is made of light, what makes it curve in space? Also, if it is made of massless bosons, how does it exert a force? Also, how does the "light" of the magnetic field penetrate matter?

Last edited: Feb 15, 2017
12. ### rpennerFully WiredStaff Member

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It doesn't "curve". It falls off with distance from sources. You are perhaps thinking about magnetic field lines which are a way to visualize the vector field of a field which falls off in magnitude with distance and is directed at right angles to the direction of electric current. That's analogous to the lines on a contour map going around the mountain when the real thing is a falling off of elevation with distance from the peak.

Massless particles in relativistic quantum theory can carry energy and momentum. I would quote you the relations for a free particle from my profile page, but that could be misleading.

Penetrating matter is the default option for electromagnetism due to that above-mentioned small coupling constant. Atomic matter is mostly empty space and due to conservation of angular momentum, even when the momentum of energy of the photon field transfers to the quantum fields of the electrons and protons, it transfers right back with a net effect of a delay, not absorption. When there is a compatible energy structure to an atom, it can delay the light more strongly or even absorb it which explains the rich colors of many gemstones and why pure diamond is transparent but with high dispersion.

13. ### Magical RealistValued Senior Member

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So what is the path of the bosons forming the magnetic field? Do they follow the lines of force? If not what accounts for these lines?

How can a massless particle carry momentum?

So the light of a magnetic field can pass right thru matter but regular light can't. What makes the photons of the magnetic field different from the photons of regular light? And are they also waves?

Last edited: Feb 16, 2017
14. ### dumbest man on earthReal Eyes Realize Real LiesValued Senior Member

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The following is a Link to a brief "primer" of sorts on the nature of Magnetism and Magnetic Flux".
May just sate your queries, Magical Realist...

15. ### geordiefRegistered Senior Member

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Can a magnetic field be necessarily said to "fill empty space"?

Can it be said that it interacts with objects at their respective positions and distances to it and that these intersections can be modeled in a geometric space?

So the field only exists at these points where an interaction occurs.

If so , is there any actual "traveling" or "penetration"of the field through "space" ?

Perhaps my take is wrong (or simply unnecessarily over complicated) ?

16. ### rpennerFully WiredStaff Member

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"Empty space" is the ground state of all quantum fields in the Standard Model description of the behavior of phenomena. I would be critical of the use of the word "fill" and rather say "empty space is equipped with an electromagnetic field." Because of relativity and magnetism's relation to movement, its dangerous to speak of just the magnetic field without a fixed standard of rest and empty space is equipped with no natural standard of rest.

In both Maxwell's equations and the modern Standard Model, the electromagnetic field only has local interactions with charged particles. So no notion of distance is involved, just the charge and motion of the particle and the value of the electromagnetic field at that point.

Distance and geometry come into play when one talks about the value of the field.

Not according to either Maxwell's equations or the Standard Model.

Starlight says yes.

17. ### geordiefRegistered Senior Member

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Thanks.

I am not competent to argue the toss but Starlight would be just an example of photons/light waves which can only be observed when they interact with an object (are detected). The actual passage of the beam of light ,before it is actually detected is unobservable is it not?

Like I say , I am totally unqualified to have a view here so I am not trying to "argue my corner" or anything as ridiculous as that.

18. ### rpennerFully WiredStaff Member

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Starlight is a phenomenon. Detecting it is another phenomenon. We get to choose where one of these happens, which pretty much confirms (given the assumption the universe isn't a perverse simulation of something that is predictable and not predicated on fooling humanity) the other happens even in the places we aren't looking.

Also, light moves slower than 30 cm per nanosecond, so we can literally demonstrate, in the laboratory, that it takes both the path and the elapsed time predicted.

19. ### geordiefRegistered Senior Member

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I am not familiar with that experiment but would I be right to assume that this path is within a medium - and so might be construed as representing a concatenation of different interactions?

I think I may have seen pictures of this process where the shape of the wave is apparent.I not sure how important or significant that is.

Or are you actually referring to a path of light in a vacuum? Is 30 cm per nanosecond the value of c? (It feels terribly slow)

20. ### DaveC426913Valued Senior Member

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It is correct.

30cm/nanosecond (times one billion) = 30 billion cm per second , or 300,000,000 m/s

Yes, we can measure the speed of light and its path on a laboratory tabletop.

21. ### DaveC426913Valued Senior Member

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Different EM waves can pass through different media. We all know this without thinking about it.
• Radio waves can pass right through houses, but can be stopped by a thin layer of tinfoil.
• Microwaves cannot pass through the grill on our oven, even though we can see through it.
• Ultraviolet light can pass through clouds but are blocked by 1/8th inch of window glass.

22. ### James RJust this guy, you know?Staff Member

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The "field lines" that you common see drawn around electric charges or magnets are just a way to help visualise how the field will affect a charged particle that is placed in it.

The magnetic field itself is a vector field, which means that it has a certain direction and magnitude at every point in space. The "field lines", in effect, are just drawn parallel to the field vectors at a selection of chosen spatial points.

Note that this concept of "field lines" is fundamentally a classical one that involves action at a distance. The quantum picture is different. In the quantum picture, (virtual) photons carry information about the field from one charged particle to another.

That's more of a philosophical question than one about physics. All we can say is: this is what is observed.

Einstein's equation relates energy, mass and momentum as follows:

$E^2 = (pc)^2 + (mc^2)^2$
Here, $E, p$ are the relativistic energy and momentum, and $m$ is the rest mass. When $m=0$ we have $E=pc$, so the momentum carried by a massless particle is related to its energy.

Regular light can. Or, it can interact with matter. It depends on the details of the light and the matter. Consider a glass window, for example. It lets visible light through, but blocks some of the UV. The reason has to do with the atomic structure of glass.

Photons have properties of both waves and particles, depending on what experiment you do with them. They are quantum particles.

A photon of regular light is an excitation of the electromagnetic field that persists over a period of time. The virtual photons of quantum field theory only exist for short times. In a very rough way, you can think of them as "borrowing" energy from the vacuum then "paying back" that energy a short time later.

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