Sha31 Posted November 30, 2009 Share Posted November 30, 2009 http://en.wikipedia.org/wiki/Magnetic_charge - "monopole detection is an open problem in experimental physics" - "magnetic monopole particle has never been conclusively observed" - "has never been evidence for the existence of magnetic monopoles" How come magnetic monopoles supposedly do not exist, and why would it be hard to find evidence and conclusively observe magnetic monopole fields if all we need is a battery, wire, permanent magnet and know about Ampere's law? http://en.wikipedia.org/wiki/Ampere%27s_law So, obviously magnetic field of a moving charge has only one pole, right? Magnetic dipoles look like this: http://en.wikipedia.org/wiki/Magnetic_dipole Link to comment Share on other sites More sharing options...
swansont Posted November 30, 2009 Share Posted November 30, 2009 The divergence of the magnetic field is zero. This implies no monopoles. So, obviously magnetic field of a moving charge has only one pole, right? No; what you've shown is the field of an infinite straight wire. A real wire has to complete a circuit, and when you look at the complete system it will no longer look like a monopole. The field of a single charge does not look like this. Link to comment Share on other sites More sharing options...
ajb Posted November 30, 2009 Share Posted November 30, 2009 The divergence of the magnetic field is zero. This implies no monopoles. But it is so easy to add a term to Maxwell's equations to include magnetic monopoles. In fact this term makes Maxwell's equations truly symmetric in electric and magnetic fields. From a theoretical perspective it seems more natural to have magnetic monopoles than not to have them! However, there is no direct evidence of magnetic monopoles in nature. Link to comment Share on other sites More sharing options...
Sha31 Posted December 1, 2009 Author Share Posted December 1, 2009 (edited) The divergence of the magnetic field is zero. This implies no monopoles. No; what you've shown is the field of an infinite straight wire. A real wire has to complete a circuit, and when you look at the complete system it will no longer look like a monopole. The field of a single charge does not look like this. No. What is shown there is a magnetic field (potential) of a single electron (moving charge) in a single point in time. Therefore, monopole electric charge is in the same time monopole magnetic charge. The field of a single charge does look like that. How do you think it looks like? Can you provide some image, some reference? How many poles do you see there, one or two? Real wires do not need to have circular circuit, all you need is lack of electrons on one end, be it circular or straight line that has nothing to do with this. For simplicity you can imagine electron beam - no circuit, just the beam and magnetic field still has only one pole, which you ought to call "monopole", yes? Merged post follows: Consecutive posts mergedBut it is so easy to add a term to Maxwell's equations to include magnetic monopoles. In fact this term makes Maxwell's equations truly symmetric in electric and magnetic fields. From a theoretical perspective it seems more natural to have magnetic monopoles than not to have them! However, there is no direct evidence of magnetic monopoles in nature. That's right. And yes Wikipedia also says: "there is no direct evidence of magnetic monopoles in nature." But now, I'm asking _you, to tell me, based on your reason, logic and deduction - how many poles does magnetic field of a moving charge have, one or two? Is then magnetic field of a moving charge monopole or dipole, why? Edited December 1, 2009 by Sha31 Consecutive posts merged. Link to comment Share on other sites More sharing options...
Mr Skeptic Posted December 1, 2009 Share Posted December 1, 2009 But now, I'm asking _you, to tell me, based on your reason, logic and deduction - how many poles does magnetic field of a moving charge have, one or two? None, or infinite. As it is a circular field, you could consider every point a dipole as it has both magnetic field lines entering and leaving it. The magnetic field of a magnetic monopole would look like the electric field of an electric monopole -- acting as only a source or a sink. If all you have is closed loops for your magnetic field lines you do not have a monopole. They need to either start or end at your supposed monopole. Link to comment Share on other sites More sharing options...
Sha31 Posted December 1, 2009 Author Share Posted December 1, 2009 None, or infinite. None?! ...infinite??!! Both of those answers are awful, especially since you actually gave two oppositely contradicting answers... that's just terrible. As it is a circular field' date=' you could consider every point a dipole as it has both magnetic field lines entering and leaving it. [/quote'] That's awfully false. There is only one point in the origin of both of these electric and magnetic fields, and you can not consider just "any point", but that one 'point charge' i.e. moving electron. Would you please take a battery, wire and permanent magnet for a moment and convince yourself of this reality fact? The magnetic field of a magnetic monopole would look like the electric field of an electric monopole -- acting as only a source or a sink. If all you have is closed loops for your magnetic field lines you do not have a monopole. They need to either start or end at your supposed monopole. Nonsense. To distinguish between monopole and dipole you only have to know how to count up to two. Monopole fields have one pole' date=' dipole fields have two. If all you have is closed loops for your magnetic field lines you do not have a monopole. How did you come up with that? Where did you get an idea to dismiss experimental facts with your imaginary assumptions? "Monopole" means "one pole", "dipole" means "two poles", ok? Link to comment Share on other sites More sharing options...
Mr Skeptic Posted December 1, 2009 Share Posted December 1, 2009 None?!...infinite??!! Both of those answers are awful, especially since you actually gave two oppositely contradicting answers... that's just terrible. Have at it then. Where does a circle start? Where does it end? That's awfully false. There is only one point in the origin of both of these electric and magnetic fields, and you can not consider just "any point", but that one 'point charge' i.e. moving electron. Would you please take a battery, wire and permanent magnet for a moment and convince yourself of this reality fact? How can the magnetic field lines start at the electron when they don't connect to it? Nonsense. To distinguish between monopole and dipole you only have to know how to count up to two. Monopole fields have one pole, dipole fields have two. Nope. The field of a dipole has circular loops entering one pole and leaving the other. The field of two monopoles placed together has the field lines leaving one monopole and entering the other monopole. This looks very similar to that of a dipole except that the field lines between the two monopoles are very strong in the opposite direction. Two monopoles of opposite charge placed together are usually referred to as a dipole, but they are different from a true dipole. How did you come up with that? Where did you get an idea to dismiss experimental facts with your imaginary assumptions? "Monopole" means "one pole", "dipole" means "two poles", ok? I didn't come up with it, it is a fact. A dipole looks very similar to two monopoles, except for the difference I described. Link to comment Share on other sites More sharing options...
Sha31 Posted December 1, 2009 Author Share Posted December 1, 2009 Where does a circle start? Where does it end? How can the magnetic field lines start at the electron when they don't connect to it? Why are you talking about "field lines"? What does that have to do with the number of poles? Field lines are only 2D topological representation of 3D fields' date=' it is only a two-dimensional plane intersection of field magnitude-gradient aka "slope". This is more like horizontally equidistant contour lines describing elevation gradient on topographic maps, where the direction of the slope and magnitude has to do with the spacing of the lines, not the direction. [img']http://upload.wikimedia.org/wikipedia/commons/thumb/d/d1/Courbe_niveau.svg/180px-Courbe_niveau.svg.png[/img] Nope. The field of a dipole has circular loops entering one pole and leaving the other. The field of two monopoles placed together has the field lines leaving one monopole and entering the other monopole. This looks very similar to that of a dipole except that the field lines between the two monopoles are very strong in the opposite direction. Two monopoles of opposite charge placed together are usually referred to as a dipole' date=' but they are different from a true dipole. [/quote'] I said: "Monopole fields have one pole, dipole fields have two." "DI" - two, pair --> di-pole = two poles "MONO" - single, one --> mono-pole = one pole No filed lines... why field lines?!? What definition are you referring to? What "physics" are you talking about? What text-books? What reference? I didn't come up with it' date=' it is a fact. A dipole looks very similar to two monopoles, except for the difference I described.[/quote'] Fact? According to whom? Can you show some reference? DIPOLE (left) and MONOPOLE (right) Link to comment Share on other sites More sharing options...
ajb Posted December 1, 2009 Share Posted December 1, 2009 That's right. And yes Wikipedia also says: "there is no direct evidence of magnetic monopoles in nature." Thank goodness that Wikipedia agree with me. Merged post follows: Consecutive posts mergedBut now, I'm asking _you, to tell me, based on your reason, logic and deduction - how many poles does magnetic field of a moving charge have, one or two? Is then magnetic field of a moving charge monopole or dipole, why? Without magnetic monopoles, the field lines are either circular or extend to infinity. The cross-section of the magnetic field of a moving charge is basically what you have given us in your picture of a wire. No monopoles. Link to comment Share on other sites More sharing options...
Mr Skeptic Posted December 1, 2009 Share Posted December 1, 2009 Sha31, you have to stop making stuff up. Your monopole is not a monopole since it does not act as a source nor sink of either electric nor magnetic field. The field lines of a positive monopole, a negative monopole, and a neutral particle. The field of two monopoles of opposite charge. Note that as I told you the field lines between the two monopoles are the opposite direction of what the field lines in a dipole would be. The field lines of a dipole. The simple fact is that Maxwell's Equations require that a monopole and only a monopole acts as a source/sink of the field (which is shown by field lines starting or ending). Specifically, Gauss's Law. If you were to modify Gauss's Law for Magnetism to allow for magnetic monopoles, it would say the same thing. Link to comment Share on other sites More sharing options...
Sha31 Posted December 1, 2009 Author Share Posted December 1, 2009 Without magnetic monopoles' date=' the field lines are either circular or extend to infinity.[/quote'] You basically said: - "Magnetic dipoles are circular or extend to infinity." So? How does that tell you this is not a monopole? How do you imagine magnetic monopole looks like when you are so sure this magnetic moment of a moving charge, as shown above, is not one? The cross-section of the magnetic field of a moving charge is basically what you have given us in your picture of a wire. No monopoles. "No monopoles."? What in the world? Is it a dipole then? Please count how many poles do you see on that image above? Link to comment Share on other sites More sharing options...
ajb Posted December 1, 2009 Share Posted December 1, 2009 (edited) I said the field lines are either circular or extend to infinity. Please, just pick up a book on classical electromagnetism. I am sure that would help you more than we can via this medium. Merged post follows: Consecutive posts mergedI refer you back to swansont The divergence of the magnetic field is zero. This implies no monopoles. This may not be obvious at first glance, but I can assure you it is correct. Edited December 1, 2009 by ajb Consecutive posts merged. Link to comment Share on other sites More sharing options...
Sha31 Posted December 1, 2009 Author Share Posted December 1, 2009 Your monopole is not a monopole since it does not act as a source nor sink of either electric nor magnetic field. According to whom? Can you show some reference? The field lines of a positive monopole' date=' a negative monopole, and a neutral particle. The field of two monopoles of opposite charge. Note that as I told you the field lines between the two monopoles are the opposite direction of what the field lines in a dipole would be. [/quote'] Why are you talking about electric charges and electric field-lines? You obviously think magnetic monopoles would have field-lines like electric monopoles, yes? Based on what? Can you count the number of poles, please? Magnetic fields are different than electric fields. They have different TOPOLOGY, different field lines. Sha31' date=' you have to stop making stuff up. [/quote'] "DI" = two, pair --> di-pole = two poles "MONO" = single, one --> mono-pole = one pole That? What exactly is the part you are not aware is true? Link to comment Share on other sites More sharing options...
swansont Posted December 1, 2009 Share Posted December 1, 2009 How does that tell you this is not a monopole? The field lines do neither originate nor end on the particle; they form a closed loop. (and if you transformed into the rest frame of the particle, whey would not exist at all) Link to comment Share on other sites More sharing options...
Sha31 Posted December 1, 2009 Author Share Posted December 1, 2009 I said the field lines are either circular or extend to infinity. What field lines? Magnetic' date=' electric, dipole, monopole? [img']http://upload.wikimedia.org/wikipedia/commons/thumb/9/91/Electromagnetism.svg/180px-Electromagnetism.svg.png[/img] What are you trying to say and how does that tell you this is not a monopole? How do you imagine magnetic monopole looks like when you are so sure this magnetic moment of a moving charge, as shown above, is not one? Please' date=' just pick up a book on classical electromagnetism. I am sure that would help you more than we can via this medium. [/quote'] I write books, my dear. It's about me helping you, but you have to start thinking. Just quoting some sources without any reason or explanation given makes for no knowledge or argument. Can you not say anything for yourself? Do you think you can at least count the number of poles in the image above? This may not be obvious at first glance' date=' but I can assure you it is correct.[/quote'] No, what you are trying to do is to re-assure yourself. Have you ever read "The Emperor's New Clothes" by Hans Christian Andersen? Merged post follows: Consecutive posts mergedThe field lines do neither originate nor end on the particle; they form a closed loop. (and if you transformed into the rest frame of the particle, whey would not exist at all) How does that tell you that's not a monopole? According to whom? Why are you talking about field lines when all we have to do is count the poles? Field lines are just 2D topological visualization of 3D fields. Magnetic fields are obviously different to electric fields, so they have different topology, which has nothing to do with the number of poles. Why do you expect magnetic monopole should look like electric monopole? Link to comment Share on other sites More sharing options...
ajb Posted December 1, 2009 Share Posted December 1, 2009 (edited) What field lines? Magnetic, electric, dipole, monopole? The magnetic field lines. What are you trying to say and how does that tell you this is not a monopole? How do you imagine magnetic monopole looks like when you are so sure this magnetic moment of a moving charge, as shown above, is not one? It is not a delta-like source, which is what we usually mean by monopole. I write books, my dear. It's about me helping you, but you have to start thinking. Just quoting some sources without any reason or explanation given makes for no knowledge or argument. Can you not say anything for yourself? Do you think you can at least count the number of poles in the image above? Not physics books I hope. Please, lets not let this devolve into a personal attack. I hate it when that happens and it usually results from not listening to what people have to say. Especially when things are "text book". We could go through Maxwell's equations and try to find monopole solutions. You will find that the absence of monopoles is equivalent to the second law. There is one magnetic dipole in the diagram. No, what you are trying to do is to re-assure yourself. No, I am just stating well accepted and known physics. Any book on EM will say something about monopoles. I expect that they do exist, they seem unavoidable in certain supersymmetric theories. And as we know EM can be easily modified to cope with them. The example you give of an infinity long wire is not considered to be a monopole by the physics community. We could move on and discuss indirect evidence of magnetic monopoles and how to explain why they are not cosmologically significant. Edited December 1, 2009 by ajb Link to comment Share on other sites More sharing options...
Mr Skeptic Posted December 1, 2009 Share Posted December 1, 2009 Based on what? Based on the part of my post that you ignored. What exactly is the part you are not aware is true? Now that's funny. Go learn some electromagnetism. --- Now about those poles: A monopole has one pole, and the field lines as I showed. Two monopoles has two monopoles, and the field lines as I showed. A dipole has no monopoles, and the field lines as I showed. However, it does look like the field of two monopoles, except between the two monopoles. So long as you are only outside of this area, you can consider two monopoles to be a dipole. Link to comment Share on other sites More sharing options...
ajb Posted December 1, 2009 Share Posted December 1, 2009 Specifically , have a think about Gauss' law of magnetism. In integral form [math]\oint_{S}B.dS= 0 [/math] That is consider the total flux of magnetic field leaving a surface surrounding your system. It will always be zero, unless there are monopoles in which case the above will not be true. Now, take your wire and enclose it with cylinder. You see that the magnetic field is always tangential to this surface. There is no flux cutting the surface. Thus, we have Gauss' law and no monopole. Ok, so you could try and argue that this is due to my choice in surface. I tell you it is not and claim you will not find a surface such that Gauss' law is violated. You can violate Gauss' law explicitly and construct magnetic charges. But you will not find magnetic charges if you keep Gauss' law as it is. Link to comment Share on other sites More sharing options...
Mr Skeptic Posted December 1, 2009 Share Posted December 1, 2009 I expect that they [magnetic monopoles] do exist, they seem unavoidable in certain supersymmetric theories. And as we know EM can be easily modified to cope with them. ajb, wouldn't the presence of both magnetic monopoles and magnetic dipoles violate the law of conservation of energy? For example, if you were to move a magnetic monopole in a closed loop following a magnetic field line, it would continuously gain energy. Where would this energy come from? Link to comment Share on other sites More sharing options...
swansont Posted December 1, 2009 Share Posted December 1, 2009 How does that tell you that's not a monopole? According to whom? Why are you talking about field lines when all we have to do is count the poles? According to physicists. This is our sandbox, and we get to make the rules. What we physicists have defined as a monopole has the behavior described by myself, ajb and Mr Skeptic. You're making an argument based on semantics and equivocation. You're looking at the field after you've transformed it into a moving frame. What does the field look like from an observer who is off-axis? It goes into and out of the screen. How many poles does that have? Link to comment Share on other sites More sharing options...
Sha31 Posted December 2, 2009 Author Share Posted December 2, 2009 For some reason, all of you expect field-lines of magnetic monopole to be like the field-lines of electric monopole, even though it's very well known, from what we do certainly know, how these fields differ and produce very different field-lines. -- All you have to do is take a battery, wire and some magnet, then pass the current and move magnet around, see how many poles can you find. Link to comment Share on other sites More sharing options...
ajb Posted December 2, 2009 Share Posted December 2, 2009 (edited) A bar magnet has two poles, we all agree on that. These two poles cannot exist by themselves in isolation. I would define a monopole (maybe this is not the most sophisticated) as a violation of Gauss' law of magnetism. How would you define a monopole? Merged post follows: Consecutive posts mergedajb, wouldn't the presence of both magnetic monopoles and magnetic dipoles violate the law of conservation of energy? For example, if you were to move a magnetic monopole in a closed loop following a magnetic field line, it would continuously gain energy. Where would this energy come from? I will have to think about this, but I am sure we have conservation of energy. I am sure (due to electromagnetic duality) that moving a magnetic charge would create an electric field. Same as a moving electric charge produces a magnetic field. Edited December 2, 2009 by ajb Consecutive posts merged. Link to comment Share on other sites More sharing options...
bigbadjesus Posted December 2, 2009 Share Posted December 2, 2009 ajb, wouldn't the presence of both magnetic monopoles and magnetic dipoles violate the law of conservation of energy? For example, if you were to move a magnetic monopole in a closed loop following a magnetic field line, it would continuously gain energy. Where would this energy come from? I wouldn't go so far as to say it would continuously 'gain energy', but it would continuously follow the path of least resistance, which would cause it to continuously accelerate from our perspective. No energy is gained or lost in such a behavior. Does a magnet 'lose energy' by interacting with another magnet? Do electrons 'gain energy'? That is the problem with most people's perception on the conservation of energy.. they refuse to consider that available energy is conserved while in a nonequilibrium steady state system. Link to comment Share on other sites More sharing options...
Sha31 Posted December 2, 2009 Author Share Posted December 2, 2009 A bar magnet has two poles' date=' we all agree on that. [/quote'] Yes, it looks something like this: These two poles cannot exist by themselves in isolation. Yes' date=' that's what you still think even if I keep showing you one, like this: [img']http://upload.wikimedia.org/wikipedia/commons/thumb/9/91/Electromagnetism.svg/180px-Electromagnetism.svg.png[/img] I would define a monopole (maybe this is not the most sophisticated) as a violation of Gauss' law of magnetism. How would you define a monopole? I would define monopole normally' date=' like they are defined: a.) monopole has only one pole, only one center of origin b.) strength of monopole decreases with distance by inverse-square law c.) finally, to avoid confusion, strength of dipole decreases with distance by inverse-cube law Simply said, monopole is what you get when you split dipole in half, like this: [img']http://upload.wikimedia.org/wikipedia/commons/thumb/3/3f/Magnetic_ring_dipole_field_lines.svg/180px-Magnetic_ring_dipole_field_lines.svg.png[/img] Link to comment Share on other sites More sharing options...
ajb Posted December 2, 2009 Share Posted December 2, 2009 Simply said, monopole is what you get when you split dipole in half, like this: That is not really a monopole. All you have in reality is a loop of wire with a current passing through it. -> dipole. Cutting them in half does not give you two monopoles. There is no "magnetic charge" here, which is what we are really talking about here. In Maxwell's equations there is no magnetic charge, this is Gauss' law. Other common magnetic dipoles are current carrying coils, bar magnets and the Earth. Most subatomic particles including the electron, proton and neutron also have magnetic moments. Accept you have misinterpreted what is going on and progress. We just seem to be repeating ourselves. Link to comment Share on other sites More sharing options...
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