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Author Topic: Can someone explain the neutron magnetic moment?  (Read 14950 times)

Online jeffreyH

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Can someone explain the neutron magnetic moment?
« on: 30/11/2014 18:33:11 »
I am wondering about the magnetic field produced by the neutron. Does it have one and how does it work?


 

Offline chiralSPO

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Re: Can someone explain the neutron magnetic moment?
« Reply #1 on: 30/11/2014 18:42:47 »
A neutron definitely has a magnetic moment. It has a spin of 1/2. I'm no physicist, so I don't know the exact details about how it manifests this, but I can tell you from experimental experience that nuclei have magnetic fields that are determined by the spins of the constituent nucleons--both protons and neutrons are spin 1/2 species.
 

Offline PmbPhy

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Re: Can someone explain the neutron magnetic moment?
« Reply #2 on: 01/12/2014 01:12:41 »
I am wondering about the magnetic field produced by the neutron. Does it have one and how does it work?
Yes. The neutron has a magnetic moment. The fact that it does is an indication that it has substructure. It is now known that the neutron is made of one up quark (charge of +2/3 e) and two down quarks (charge of −1/3 e). The magnetic field comes from those particles.
 

Offline acsinuk

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Re: Can someone explain the neutron magnetic moment?
« Reply #3 on: 03/12/2014 09:58:56 »
What I want to know is what the inside of a molecule will look like magnetically.  Charge wise; what is stopping the positive charges in the protons from touching the neutrons turning the nucleus into a lump which then cancels the charges on the electron enclosure?   The only way I can think of is to have a magnetic force moment half on each bundle of charge pushing them apart.
 

Offline evan_au

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Re: Can someone explain the neutron magnetic moment?
« Reply #4 on: 03/12/2014 11:05:40 »
Quote from: acsinuk
What I want to know is what the inside of a molecule will look like magnetically.
You get a really strong magnetic effect (ferromagnetism) if you have an element with unpaired inner electrons, like iron or cobalt.

For organic chemicals, there is a magnetic field from the nucleus of atoms with an odd number of nucleons, including hydrogen and some isotopes of sodium and phosphorus. This is how Magnetic Resonance Imaging works.
 
Quote
Charge wise; what is stopping the positive charges in the protons from touching the neutrons?
The atomic nucleus is a pretty dense droplet of nucleons, with protons and neutrons continually jostling with each other. So yes, they do touch each other.
 
The average distance is set by the operation of the Strong Nuclear force, which has a minimum energy at about 1 femtometer.
Since the proton is charged, and the neutron is not charged (overall), there is no charge that keeps them apart. However, they are exchanging pions, which keep them together.

Quote
turning the nucleus into a lump which then cancels the charges on the electron enclosure?
The nucleus is a lump, and it basically does balance the charges of the surrounding electrons. (In the case of ionic solids or dissolved ions, you just need to look a little further afield to find the surrounding electrons.)

Quote
   The only way I can think of is to have a magnetic force moment half on each bundle of charge pushing them apart.
It is the wave nature of electrons which prevents them from collapsing into the protons and neutrons of the nucleus.

If you place two bar magnets in close proximity, they will spin around and line up North-South, pretty much canceling the external magnetic field.

The Helium nucleus is very stable, perhaps because with two protons and two neutrons, it allows all these magnetic fields to cancel. The common isotopes of Carbon and oxygen are multiples of 4 nucleons, and don't show up in an MRI.

[Wikipedia mentions that deuterium has a magnetic moment that is detectable by MRI, despite having an even number of nucleons - I am interested in the reason, alancalvard.]
 

Offline acsinuk

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Re: Can someone explain the neutron magnetic moment?
« Reply #5 on: 03/12/2014 17:10:58 »
Evan
Thank you for confining that molecules with single unpaired electrons in an inner shell are magnetic.  The point is that the only thing inside the molecule is electric charges; a neutron that is neutral to the outer touchable negative shell and the electrons which are charged and also magnetic.
So all the inside binding forces must also be electrical or gravitational. Now to compress the similar charges together into bundles requires a strong nuclear force for sure although it is very short range. We then need a magnetic moment force to push the positive and negative charges apart or they will annihilate. Only other force we require is weak repelling electrostatic one to stop the different positive charge bundles from colliding.  No other mechanical force or boson is needed to stabilise a simple molecule and gravity force is of course totally insignificant
 

Offline PmbPhy

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Re: Can someone explain the neutron magnetic moment?
« Reply #6 on: 04/12/2014 07:00:48 »
Quote from: acsinuk
Thank you for confining that molecules with single unpaired electrons in an inner shell are magnetic.
That is not quite correct. Something is typically called magnetic when it maintains a magnetic field. E.g. a magnet is said to be magnetic. The situation you described here, i.e. more generally, when the basic atomic unit of the material has a net angular momentum from unpaired electrons the substance is called paramagnetic. However, unlike magnetic substances, paramagnetic substances only have a magnetic field when an external magnetic field is applied and induces a magnetic field in the substance. When the external field is removed the magnetic field created by the paramagnetic substance disappears. This is quite unlike ferromagnetic substances.

The definition of paramagnetism I gave above is taken word for word from the EM textbook by a distinguished author. The textbook is Classical Electrodynamics - Third Edition by John D. Jackson, (1999), page 15.

For more information on Paramagnetism please see http://en.wikipedia.org/wiki/Paramagnetism

For example:
Quote
Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field because thermal motion randomizes the spin orientations.
 

Offline lightarrow

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Re: Can someone explain the neutron magnetic moment?
« Reply #7 on: 04/12/2014 13:20:32 »
Yes. The neutron has a magnetic moment. The fact that it does is an indication that it has substructure.
Why? Is the same true for the neutrino?
(I am asking because I don't know).

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Offline PmbPhy

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Re: Can someone explain the neutron magnetic moment?
« Reply #8 on: 04/12/2014 14:06:39 »
Yes. The neutron has a magnetic moment. The fact that it does is an indication that it has substructure.
Why? Is the same true for the neutrino?
(I am asking because I don't know).

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lightarrow
I read it in a Wikiepedia. I assume that the author stated it because a magnetic moment implies the presence of charge and since the neutron is uncharged it must be made of charges. The author may have been wrong though but I don't see how.

http://en.wikipedia.org/wiki/Neutron_magnetic_moment
Quote
The neutron magnetic moment is the magnetic moment of the neutron. It is of particular interest, as magnetic moments are created by the movement of electric charges. Since the neutron is a neutral particle, the magnetic moment is an indication of substructure. For a time, the neutron was thought to be made of a proton, with a charge of +1 e and an electron, with a charge of −1 e, whose charge would cancel out. However, since the advent of the quark model, it is now known that the neutron is made of one up quark (charge of +2/3 e) and two down quarks (charge of −1/3 e).
« Last Edit: 04/12/2014 14:13:25 by PmbPhy »
 

Offline PmbPhy

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Re: Can someone explain the neutron magnetic moment?
« Reply #9 on: 04/12/2014 19:30:30 »
lightarrow - I asked a friend of mine, who's an authority/expert on the subject and he wrote back and said the following (he gave me explicitly permission to quote him so long as I don't mention his name):
Quote
Well, the language is cautious: "is an indication" is probably OK---it does SUGGEST substructure---especially since the neutron's magnetic moment is huge (comparable to the proton's).  But by itself the mere existence of a magnetic moment does not prove substructure.  In the case of the electron (say) interactions (QED, in that case) MODIFY the pre-existing magnetic moment (giving it an "anomalous" magnetic moment), but they constitute a tiny correction.  In the case of the neutrino, interactions (electro-weak) are responsible for the whole thing (the "bare" magnetic moment is zero), and it is miniscule.  I say you have a valid point, but Wikipedia is not incorrect.
« Last Edit: 04/12/2014 20:07:05 by PmbPhy »
 

Offline evan_au

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Re: Can someone explain the neutron magnetic moment?
« Reply #10 on: 04/12/2014 20:17:07 »
Quote from: lightarrow
The neutron has a magnetic moment... Is the same true for the neutrino?

The neutron is fairly easy to detect because it is massive, and responds to the strong force - but even without this, it would be possible to detect its magnetic moment.

Neutrinos have very low mass, and don't respond to the strong force. If the neutrino had a magnetic moment of similar magnitude to the neutron, the neutrino would be much easier to detect than it is found to be. 
 

Offline lightarrow

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Re: Can someone explain the neutron magnetic moment?
« Reply #11 on: 05/12/2014 17:16:03 »
Yes. The neutron has a magnetic moment. The fact that it does is an indication that it has substructure.
Why? Is the same true for the neutrino?
(I am asking because I don't know).

--
lightarrow
I read it in a Wikiepedia. I assume that the author stated it because a magnetic moment implies the presence of charge and since the neutron is uncharged it must be made of charges. The author may have been wrong though but I don't see how.

http://en.wikipedia.org/wiki/Neutron_magnetic_moment
Quote
The neutron magnetic moment is the magnetic moment of the neutron. It is of particular interest, as magnetic moments are created by the movement of electric charges. Since the neutron is a neutral particle, the magnetic moment is an indication of substructure. For a time, the neutron was thought to be made of a proton, with a charge of +1 e and an electron, with a charge of −1 e, whose charge would cancel out. However, since the advent of the quark model, it is now known that the neutron is made of one up quark (charge of +2/3 e) and two down quarks (charge of −1/3 e).
ightarrow - I asked a friend of mine, who's an authority/expert on the subject and he wrote back and said the following (he gave me explicitly permission to quote him so long as I don't mention his name):
Quote
    Well, the language is cautious: "is an indication" is probably OK---it does SUGGEST substructure---especially since the neutron's magnetic moment is huge (comparable to the proton's).  But by itself the mere existence of a magnetic moment does not prove substructure.  In the case of the electron (say) interactions (QED, in that case) MODIFY the pre-existing magnetic moment (giving it an "anomalous" magnetic moment), but they constitute a tiny correction.  In the case of the neutrino, interactions (electro-weak) are responsible for the whole thing (the "bare" magnetic moment is zero), and it is miniscule.  I say you have a valid point, but Wikipedia is not incorrect.
Thank you, interesting.

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lightarrow
« Last Edit: 05/12/2014 17:24:50 by lightarrow »
 

Offline lightarrow

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Re: Can someone explain the neutron magnetic moment?
« Reply #12 on: 05/12/2014 17:18:40 »
Quote from: lightarrow
The neutron has a magnetic moment... Is the same true for the neutrino?

The neutron is fairly easy to detect because it is massive, and responds to the strong force - but even without this, it would be possible to detect its magnetic moment.

Neutrinos have very low mass, and don't respond to the strong force. If the neutrino had a magnetic moment of similar magnitude to the neutron, the neutrino would be much easier to detect than it is found to be. 
Thank you.
Do you believe that maybe the wiki voice would have better stated that "...the relatively high value of neutron's magnetic moment is an indication that it has substructure..." ?

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Offline acsinuk

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Re: Can someone explain the neutron magnetic moment?
« Reply #13 on: 08/12/2014 14:08:00 »
The contents of the molecule is opposing electric charges.  Thus there has to be a voltage between the parts which is an internal EMF.  Now if that electro motive force is a curling force i.e. it acts 1/3 attractive or repel and 2/3 around either down or up wouldn't that be recognisable as a magnetic spin?  Something to ponder on over Christmas!       CliveS
 

Offline PmbPhy

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Re: Can someone explain the neutron magnetic moment?
« Reply #14 on: 08/12/2014 14:28:20 »
Quote from: acsinuk
The contents of the molecule is opposing electric charges.  Thus there has to be a voltage between the parts which is an internal EMF.
Most people confuse EMF with a difference in potential. An EMF is the voltage developed by any source of electrical energy such as a battery or dynamo. The inside of a molecule is not a source of electrical energy.

Quote from: acsinuk
Now if that electro motive force is a curling force i.e. it acts 1/3 attractive or repel and 2/3 around either down or up wouldn't that be recognisable as a magnetic spin?
You didn't make much sense here. What do you mean by a "curling force"? The term as you used it is not the same as you'd find in a math or physics text. What you said is very confusing.
 

Offline acsinuk

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Re: Can someone explain the neutron magnetic moment?
« Reply #15 on: 09/12/2014 17:21:05 »
No, it is not confusing.  The international unit for magnetic moment is measured in Joule per turn. So how are we to get a neutron and proton to turn or spin together unless we have a curling force inside a molecule?
 

Offline PmbPhy

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Re: Can someone explain the neutron magnetic moment?
« Reply #16 on: 09/12/2014 18:11:05 »
No, it is not confusing.  The international unit for magnetic moment is measured in Joule per turn. So how are we to get a neutron and proton to turn or spin together unless we have a curling force inside a molecule?
Why should I care about international units? I actually can't find that but it makes no difference since the magnetic moment of a neutron has nothing to do with turns. The turns you mentioned in the units refers to the turn of a wire in a solenoid which can create a magnetic moment. It has nothing to do with curl.

And your comment most certainly is confusing since "curl" means something very specific in physics (and mathematical physics) and its not how you used it. See http://mathworld.wolfram.com/Curl.html

What you were referring to is torque and not curl.

It you want to learn more about magnetic moments then see:
http://en.wikipedia.org/wiki/Magnetic_moment

The magnetic moment is not created by anything which has a classical analogy since it's quantum in nature.

You claimed that the EMF is a curling force when in fact EMF is not a force at all!

See http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elevol.html
Quote
emf represents energy per unit charge (voltage) which has been made available by the generating mechanism and is not a "force".
A fact known to all physicists and electrical engineers.

To learn what an EMF is please read:
http://en.wikipedia.org/wiki/Electromotive_force
« Last Edit: 09/12/2014 18:15:44 by PmbPhy »
 

Offline acsinuk

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Re: Can someone explain the neutron magnetic moment?
« Reply #17 on: 15/12/2014 15:20:30 »
Surely Faraday knew what he was talking about!  If he says that it is an electro-motive force to do with cutting lines of flux then he must be correct.  Probably, Wikipedia is looking only at DC electrics based on electron particle movements
 

Offline PmbPhy

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Re: Can someone explain the neutron magnetic moment?
« Reply #18 on: 17/12/2014 01:11:22 »
Dear acsinuk,

I don't know anything at all about the level of math that you know so please don't be offended if you know all of what I'm about to describe like the back of your hand, okay? :)

Below I'm going to refer to a Cartesian coordinate system which is defined here:
http://mathworld.wolfram.com/CartesianCoordinates.html

Quote from: acsinuk
Surely Faraday knew what he was talking about!
Sure. Nobody said that he wasn't. I'm sorry my friend, but I don't understand where you got the idea that either I or someone else said otherwise. Can you help clarify that for me please? Thank you.

Quote from: acsinuk
If he says that it is an electro-motive force to do with cutting lines of flux then he must be correct.
You didn't respond to my explanation in my last post so it's not clear to me whether you understand what the term electromotive force means.

Just in case you don't know the term as it's used in the field of electrodynamics then the term Electromotive Force, simply put, is a potential difference between two points. It could be between two terminals of a battery or it could be between the ends of a straight copper wire which is moving through a magnetic field where the wire is oriented so as to "cut the magnetic field lines." That's a qualitative description. The phrase word cut was defined so as to be able to describe the physics in terms of conductors "cut across a magnetic field." It's much easier to speak about the physics when we use this term even though there's no physical quantity known as a magnetic field line. These "lines" are merely for the sake of visualizing the physics.

The quantitative description, i.e. how to calculate it to find an expression for the value that you'd measure in an experiment, is difficult to describe if you don't know calculus. However we're lucky in that we can chose a simple example such that it's easy to describe when we use such examples. Suppose we have a uniform magnetic field where the field lines are pointing in the direction of the z-axis of a Cartesian coordinate system. Let the wire be parallel to the y-axis and be moving at a constant speed in the x-direction. Then the EMF is simply the product of the length of the conductor, the speed of the wire and the magnitude of the magnetic field. The result is a voltage and that voltage is referred to as an EMF. But you have to keep in mind that the EMF is not a force.

If this doesn't convince you then look it up in a physics textbook. For example

Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (2nd Edition) by Randall D. Knight
http://bookzz.org/book/1111788/8edbe0

Classical Electrodynamics by Hans C. Ohanian
http://bookzz.org/dl/540517/7db1a3

Quote from: acsinuk
Probably, Wikipedia is looking only at DC electrics based on electron particle movements
No. How I just described it is exactly what it is both defined and what's found in experiments.
 

Offline acsinuk

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Re: Can someone explain the neutron magnetic moment?
« Reply #19 on: 20/12/2014 13:18:16 »
"Then the EMF is simply the product of the length of the conductor, the speed of the wire and the magnitude of the magnetic field. The result is a voltage and that voltage is referred to as an EMF. But you have to keep in mind that the EMF is not a force."
I am not into maths but I can assure you that electricity and magnetism are 3 dimensional energy states.  Your explanation of voltage appears to be scalar magnitude only but in practice a voltage produces an EMF sideways force that spins an area of current flux
 

Offline A Davis

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Re: Can someone explain the neutron magnetic moment?
« Reply #20 on: 28/12/2014 01:43:40 »
Back to the post a neutron does have a magnetic field but apparently no charge. Looking at it a different way the average value of sin squared is 1/2 but the average value of sin times cos is zero. If the phase of each solution is different they won,t interact but two neutron will interact electrically (charge). Unfortunately science can,t produce a neutron field it is unstable.
 

Online jeffreyH

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Re: Can someone explain the neutron magnetic moment?
« Reply #21 on: 28/12/2014 02:33:46 »
Back to the post a neutron does have a magnetic field but apparently no charge. Looking at it a different way the average value of sin squared is 1/2 but the average value of sin times cos is zero. If the phase of each solution is different they won,t interact but two neutron will interact electrically (charge). Unfortunately science can,t produce a neutron field it is unstable.

So in a neutron star free electrons must be rare if any at all. With only a magnetic field would you still have production of photons? How is this likely to affect gravity? I presume the electrical interactions are extremely localised.
 

Offline acsinuk

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Re: Can someone explain the neutron magnetic moment?
« Reply #22 on: 28/12/2014 16:59:03 »
The neutron is touching the negative electron enclosure so although at full minus charge appears to be neutral.  The inside of stars are exactly opposite with all molecules enclosed in positron shells so there will be no free electrons inside stars.
 

Offline lightarrow

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Re: Can someone explain the neutron magnetic moment?
« Reply #23 on: 29/12/2014 08:55:02 »
"Then the EMF is simply the product of the length of the conductor, the speed of the wire and the magnitude of the magnetic field. The result is a voltage and that voltage is referred to as an EMF. But you have to keep in mind that the EMF is not a force."
I am not into maths but I can assure you that electricity and magnetism are 3 dimensional energy states.  Your explanation of voltage appears to be scalar magnitude only but in practice a voltage produces an EMF sideways force that spins an area of current flux
What about to open a college book of physics? You seem to have never done it...
Regards.

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Offline acsinuk

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Re: Can someone explain the neutron magnetic moment?
« Reply #24 on: 29/12/2014 15:29:01 »
I am only interested in facts and not text book quotes.  The fact is scientists do not understand that magnetism is a massless flux current area that can exist in space vacuum permanently without energy loss provided there are no charged particles to react with.
 

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Re: Can someone explain the neutron magnetic moment?
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