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Thread: Electromagnetism

  1. #1 Electromagnetism 
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    I would like to ask a seemingly daft question. What is the experimental evidence for the traditional magnetic flux, as described by people such as Faraday, Maxwell and Fleming? We should bear in mind that this magnetic flux was based on an ignorance of magnetism - orbiting and spinning electrons were unknown 150 years ago.

    Suppose magnets had been unknown at the time. Experiments with electricity would then have led to a simple law: like currents attract and opposite currents repel. This basic law then explains magnetism, such as the alignment of iron filings around a magnet. Imagine for example that two bar magnets have stuck themselves together side by side, i.e. with north poles touching south poles. The circulating charges in the two magnets are, at their closest, moving in the same direction - a bit like cog wheels meshing together. Hence there is a net force of attraction.

    Using Ockhamís principle, the complication of a circular perpendicular field is then unjustified. (Instead of Biot and Savartís law to predict flux density, the attraction between elements of two current vectors is dF = k I1.I2 ds1 ds2 /r2 where k depends on the permeability.) So magnetic forces just act along the straight lines between moving charges. This is the same simple principle that works for electrostatic forces between stationary charges. We need not assume the universe uses two completely different force mechanisms. When charges move in the same direction their electrostatic repulsion is reduced, but the point Iím making is that the electromagnetic field should be regarded as the sum of two parallel fields. We can then forget the needless complications of Flemingís rule etc. that deter many students from studying science and engineering.

    Traditional magnetic fields are defined as continuous. So the field emanating from the north end of a bar magnet loops round the outside of the magnet to the south pole and returns through the magnetís body back to the north pole. Now imagine a magnet made of a very viscous material that allows a free-moving north pole to drift within it. This internal north pole would be repelled by the magnetís south pole (?) and leave again through its north pole. We are all taught this stuff, but it doesnít make sense to me. We should not view magnets as perpetual motion machines. Force fields begin and end at points: they do not keep going round in circles. There is a measurable energy gradient along a real force field, but there can be no such gradient around a continuous loop.

    Magnetic flux represents the total "lines of force" through a surface, but no force can be detected along these lines. The notion of circular fields perhaps arose when rings of iron filings were seen around a conducting wire, but it was a very odd idea. The circular magnetic field at any point is defined as a vector that is perpendicular to the force it produces. However, if a vector represents something that measurably exists, e.g. a physical force, a wind velocity, a flow of energy or a stampede of hamsters, its perpendicular components are zero. So we can say that a magnetic field having its greatest effect in a perpendicular direction does not exist.

    If a force vector looped round to its starting point then the magnitude of the force would be zero. So even if circular fields existed, they would have no effect.


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    Quote Originally Posted by Andrew? View Post
    Using Ockham’s principle, the complication of a circular perpendicular field is then unjustified.
    Except immediately someone would wind a solenoid and see the same "circular perpendicular field" that we are used to seeing in physics books, and not long after that someone would add a high-u material and get that bar magnet behavior.
    So magnetic forces just act along the straight lines between moving charges. This is the same simple principle that works for electrostatic forces between stationary charges. We need not assume the universe uses two completely different force mechanisms. When charges move in the same direction their electrostatic repulsion is reduced, but the point I’m making is that the electromagnetic field should be regarded as the sum of two parallel fields. We can then forget the needless complications of Fleming’s rule etc. that deter many students from studying science and engineering.
    Moving away from something commonly understood and demonstrated (magnets) to something that most people cannot measure without specialized equipment (i.e. Lorentz force) would seem to make the subject less, not more, approachable.
    Traditional magnetic fields are defined as continuous. So the field emanating from the north end of a bar magnet loops round the outside of the magnet to the south pole and returns through the magnet’s body back to the north pole. Now imagine a magnet made of a very viscous material that allows a free-moving north pole to drift within it. This internal north pole would be repelled by the magnet’s south pole (?) and leave again through its north pole.
    ?? Not really. "North pole" is simply the result of the right hand rule; it's not a single definable location.
    We are all taught this stuff, but it doesn’t make sense to me. We should not view magnets as perpetual motion machines. Force fields begin and end at points: they do not keep going round in circles.
    Magnetism isn't a force field; it simply defines a _potential_ force that can occur in concert with moving charges. Nor is a continuous force a "perpetual motion machine." A spring compressed under a weight generates a force forever, but doesn't move anything.
    Magnetic flux represents the total "lines of force" through a surface, but no force can be detected along these lines.
    Which is why they are referred to as "magnetic field lines" rather than "lines of force."


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    Quote Originally Posted by Andrew? View Post
    I would like to ask a seemingly daft question. What is the experimental evidence for the traditional magnetic flux, as described by people such as Faraday, Maxwell and Fleming? We should bear in mind that this magnetic flux was based on an ignorance of magnetism - orbiting and spinning electrons were unknown 150 years ago....
    Actually, electron spin is not at all fundamental to the development of the flux concept, so ignorance of same is irrelevant. The magnetic field produced by a current-carrying wire is quite blind to spin and, for that matter, orbital motion. So, the short answer to your question is that Faraday's interpretation of Oersted's experiment is what provided the first experimental evidence of flux. Faraday supplemented that with the now-universally familiar patterns formed by iron filings on paper held over a magnet (both permanent and electromagnet types). Faraday treated those patterns as reflective of a substantial reality. He is the one who gave us the language of flux lines, induction being due to the cutting of these lines, etc. Starting with an initially skeptical Thompson (the future Lord Kelvin), other researchers started tentatively to treat these lines as real as Faraday saw them, and were surprised that a self-consistent and experimentally-verified mathematical theory could be constructed from his pictures.

    There's another important bit that your post doesn't acknowledge, and that is that relativity has shown us that electric and magnetic fields are frame-dependent aspects of an electromagnetic field. Again remarkably, Maxwell's equations (which were built on Oersted and Faraday, with a boost from Coulomb/Cavendish, plus the added brilliant stroke of Maxwell's displacement current) continue to be valid relativistically. Even magnetic monopoles, should any ever be found, can be accommodated within the existing formulation. It's remarkable for a Victorian-age formulation!
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    Quote Originally Posted by billvon View Post
    Not really. "North pole" is simply the result of the right hand rule; it's not a single definable location.
    I agree, but the traditional view of magnetism leads people to think of poles as things rather than labels. For example, the direction of a magnetic field is defined in terms of the direction that a free north pole would take. But there is no such thing as a free north pole. People like Faraday were familiar with magnetic poles and this thinking carried on into electromagnetism. Had people understood things better, magnetic poles would be labelled C and A to indicate the direction of current.


    Quote Originally Posted by billvon View Post
    Magnetism isn't a force field; it simply defines a _potential_ force that can occur in concert with moving charges. Nor is a continuous force a "perpetual motion machine." A spring compressed under a weight generates a force forever, but doesn't move anything.
    The traditional view involves free poles going round in perpetual loops. The force fields in a spring begin and end in atoms/particles, i.e. they begin and end at points. The magnetic field goes round in a circle, but real force fields do not.


    Quote Originally Posted by billvon View Post
    Which is why they are referred to as "magnetic field lines" rather than "lines of force."
    I was taught that they were lines of force. I have just looked up a definition of magnetic flux on the internet and it still says lines of force.
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    Quote Originally Posted by tk421 View Post
    Actually, electron spin is not at all fundamental to the development of the flux concept, so ignorance of same is irrelevant. The magnetic field produced by a current-carrying wire is quite blind to spin and, for that matter, orbital motion. So, the short answer to your question is that Faraday's interpretation of Oersted's experiment is what provided the first experimental evidence of flux. Faraday supplemented that with the now-universally familiar patterns formed by iron filings on paper held over a magnet (both permanent and electromagnet types).
    The problem was that Faraday and co didn't know that electrons orbit in atoms and that this current explains permanent magnetism. Had they done so, they would have come up with the simple rule that like currents attract and opposite currents repel. Iron filings align themselves in accordance with this rule. If one assumes that north and south poles are separate things then the alignment suggests a circular force field. But as I've said, such a field does not make sense, and nor does separate poles.


    Quote Originally Posted by tk421 View Post
    There's another important bit that your post doesn't acknowledge, and that is that relativity has shown us that electric and magnetic fields are frame-dependent aspects of an electromagnetic field.
    You probably haven't seen my first post which I unfortunately put in the General Discussion section. Its heading is Equivalence of electric and magnetic fields.
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    Quote Originally Posted by Andrew? View Post
    I agree, but the traditional view of magnetism leads people to think of poles as things rather than labels. For example, the direction of a magnetic field is defined in terms of the direction that a free north pole would take. But there is no such thing as a free north pole. People like Faraday were familiar with magnetic poles and this thinking carried on into electromagnetism. Had people understood things better, magnetic poles would be labelled C and A to indicate the direction of current.
    Yes, and electrons would be considered "positive." But we learn a little at a time, not all at once.


    The traditional view involves free poles going round in perpetual loops. The force fields in a spring begin and end in atoms/particles, i.e. they begin and end at points. The magnetic field goes round in a circle, but real force fields do not.
    ?? What is a "real force field?"
    I was taught that they were lines of force. I have just looked up a definition of magnetic flux on the internet and it still says lines of force.
    I have no doubt that you can find some bad definitions of science on the Net - but still, magnetic fields are not lines of force. Fixing errors on the Internet seems easier than changing how we teach science.
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    Quote Originally Posted by Andrew? View Post
    Quote Originally Posted by billvon View Post
    Which is why they are referred to as "magnetic field lines" rather than "lines of force."
    I was taught that they were lines of force. I have just looked up a definition of magnetic flux on the internet and it still says lines of force.
    In the absence of an electric field, the force on a charge in a magnetic field is:

    F = qv x B

    According to this expression, the direction of the force is perpendicular to the direction of the magnetic field. Thus, in spite of anything you may have learnt, the expression for the force makes it clear that the magnetic field is not lines of force.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by billvon View Post
    Yes, and electrons would be considered "positive." But we learn a little at a time, not all at once.
    I don’t see how it helps to draw attention to another historical accident in physics. Whose side are you on?

    Quote Originally Posted by billvon View Post
    I have no doubt that you can find some bad definitions of science on the Net - but still, magnetic fields are not lines of force. Fixing errors on the Internet seems easier than changing how we teach science.
    I will quote a little from Maxwell’s famous paper "On Faraday's Lines of Force" in 1855/6.

    “I have in the first place to explain and illustrate the idea of ‘lines of force.’

    When a body is electrified in any manner, a small body charged with positive electricity, and placed in any given position, will experience a force urging it in a certain direction. If the small body be now negatively electrified, it will be urged by an equal force in a direction exactly opposite.

    The same relations hold between a magnetic body and the north or south poles of a small magnet. If the north pole is urged in one direction, the south pole is urged in the opposite direction.”

    You are faced with the problem of rewriting history as well as what’s on the internet.

    As physicists don’t like changing how they teach physics, why didn’t they just stick with Newtonian mechanics? The fact remains that Faraday and Maxwell’s magnetic field is completely superfluous mumbo jumbo. It is not even a proper scientific hypothesis that can be tested. Any experiment that is said to support Maxwell's corkscrew can equally be said to contradict it, i.e. Maxwell used the wrong kind of corkscrew and Fleming the wrong hand.

    Physics is undermined by false beliefs. This is because physicists prefer to ignore inconvenient facts – be they historical, logical or experimental.
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    He's gone full crackpot.
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    Quote Originally Posted by Andrew? View Post
    I don’t see how it helps to draw attention to another historical accident in physics.

    Are you following the discussion here? You said that an early assumption led to a tradition that you find hard to understand. I agreed and gave another example. It doesn't mean physics is "wrong" - just that we chose labels that aren't as clear to some people as they otherwise might be.

    I will quote a little from Maxwell’s famous paper "On Faraday's Lines of Force" in 1855/6.
    “I have in the first place to explain and illustrate the idea of ‘lines of force.’

    Another good example of how an early assumption was found to be inaccurate later.

    Physics is undermined by false beliefs. This is because physicists prefer to ignore inconvenient facts – be they historical, logical or experimental.
    And those 'false beliefs' have been rectified - as our accurate understanding of EM demonstrates.

    Notice any electric cars on the road today? They are some of the fastest, most powerful vehicles on the road - because we understand EM and can use Maxwell's equations to build things that work.
    Last edited by billvon; August 23rd, 2017 at 04:59 PM.
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    why didnít they just stick with Newtonian mechanics?
    Because it is based on the notion of "action at a distance", which is little better than proposing magic as the working principle of the universe. Also, Newtonian mechanics is a good approximation for the low-velocity, low-energy regime, but it fails quite miserably when you go outside of that domain, giving wrong predictions. Field theory and the theory of relativity on the other hand very successfully address all these failings, and deliver predictions that are in excellent accord with experiment and observation.
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    Quote Originally Posted by billvon View Post

    Are you following the discussion here? ....
    Another good example of how an early assumption was found to be inaccurate later.


    The discussion involves my criticism of Faraday's perpendicular circular magnetic field. Are you saying this perpendicular circular field was inaccurate?

    Quote Originally Posted by billvon View Post
    Notice any electric cars on the road today? They are some of the fastest, most powerful vehicles on the road - because we understand EM and can use Maxwell's equations to build things that work.
    I know it is difficult to see the simplicity of what I'm saying. I am just saying the magnetic field that you are accustomed to calculating is parallel to the electric field. The magnetic field is simply added to the electric field. It isn't perpendicular to the force, it's parallel - like other fields and forces are. The forces work out the same. The difference is that we avoid the embarrassment of an unverifiable hypothesis and the pantomime of students needlessly poking their fingers in the air.
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    The forces work out the same
    Please provide a mathematical formulation ( i.e. field equations ), so that we can extract numerical predictions from your idea, and test it against known experimental data.
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    For a bystander Andrew's threads are very helpful as the counter explanations flesh out the territory even when one does not have the mathematical tools to follow the arguments "tightly".
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    Quote Originally Posted by geordief View Post
    For a bystander Andrew's threads are very helpful as the counter explanations flesh out the territory even when one does not have the mathematical tools to follow the arguments "tightly".
    Indeed. I have learnt a huge amount from following arguments against mistaken ideas. In part because such people will sometimes ask questions that I wouldn't have thought of. (Sadly, too many of them are not willing to listen to the answers. But if it helps others, that's fine.)
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    Quote Originally Posted by Andrew? View Post
    The problem was that Faraday and co didn't know that electrons orbit in atoms and that this current explains permanent magnetism.
    Electrons orbiting atoms has almost nothing to do with permanent magnetism. Spin is the key.

    Had they done so, they would have come up with the simple rule that like currents attract and opposite currents repel.
    And they did. Or, rather, a contemporary of Faraday -- a fellow named Ampere -- came up with that rule in the 1820s. He also postulated that all magnetism was due to currents. Faraday criticized Ampere's generalisation as having weak experimental foundation and thought it too great a leap. It took the discovery of spin and the working out of quantum mechanics to show that Faraday was correct to be suspicious. Permanent magnetism does not arise from "ordinary" macroscopic currents, contrary to Ampere's (and your) beliefs. As Faraday pointed out, such currents would quickly dissipate, thanks to lossy mechanisms (resistance). Yes, superconducting magnets exist, but they are the exceptions that prove the rule.

    Iron filings align themselves in accordance with this rule. If one assumes that north and south poles are separate things then the alignment suggests a circular force field. But as I've said, such a field does not make sense, and nor does separate poles.
    Nature is indifferent to our personal sense of taste. No one has ruled out the possibility of magnetic monopoles, although there have been no unambiguous detections. There is merely the intriguing observation -- by Dirac -- that if even one magnetic monopole were to exist, electric charge would be quantised. That charge is, in fact, quantised is at least consistent with the existence of monopoles.
    Last edited by tk421; August 25th, 2017 at 12:43 PM.
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    Quote Originally Posted by Andrew?;605864[COLOR=#000000
    ][/COLOR]The discussion involves my criticism of Faraday's perpendicular circular magnetic field. Are you saying this perpendicular circular field was inaccurate?

    Not sure what you mean by "this perpendicular circular field."

    Magnetic fields are not "lines of force." They can INTERACT with charges and GENERATE force, but they are not themselves lines of force, nor is an EM field a force field.

    However, a wire does indeed generate a circular magnetic field. And when a charged particle moves through this field, the force generated (called the Lorentz force) is indeed perpendicular to the field.
    I know it is difficult to see the simplicity of what I'm saying. I am just saying the magnetic field that you are accustomed to calculating is parallel to the electric field.
    In the far field the electrical field is perpendicular to the magnetic field.

    However, in the near field, you could indeed create a static electrical field that is parallel to a magnetic field.
    The magnetic field is simply added to the electric field.
    No, they don't add - they are independent in the near field. You can have one without the other. You can have both. They will have different effects on charged particles. A magnetic field will not affect a charged particle unless it is moving; an electrical field will affect a static charged particle.
    It isn't perpendicular to the force, it's parallel - like other fields and forces are. The forces work out the same.
    No, they don't, and this is easily provable in the lab.
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    Quote Originally Posted by geordief View Post
    For a bystander Andrew's threads are very helpful as the counter explanations flesh out the territory even when one does not have the mathematical tools to follow the arguments "tightly".
    I tend to barely skim or completely skip the Andrew posts whilst I fully read most of the others. I fully agree with the learning stuff part.
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    Quote Originally Posted by tk421 View Post
    And they did. Or, rather, a contemporary of Faraday -- a fellow named Ampere -- came up with that rule in the 1820s. He also postulated that all magnetism was due to currents. Faraday criticized Ampere's generalisation as having weak experimental foundation and thought it too great a leap. It took the discovery of spin and the working out of quantum mechanics to show that Faraday was correct to be suspicious. Permanent magnetism does not arise from "ordinary" macroscopic currents, contrary to Ampere's (and your) beliefs. As Faraday pointed out, such currents would quickly dissipate, thanks to lossy mechanisms (resistance). Yes, superconducting magnets exist, but they are the exceptions that prove the rule.
    OK, thanks for that, but I believe my general point still stands. Faraday/Maxwell magnetic fields were devised before magnetism was understood.

    Quote Originally Posted by tk421 View Post
    Nature is indifferent to our personal sense of taste. No one has ruled out the possibility of magnetic monopoles, although there have been no unambiguous detections. There is merely the intriguing observation -- by Dirac -- that if even one magnetic monopole were to exist, electric charge would be quantised. That charge is, in fact, quantised is at least consistent with the existence of monopoles.
    As you say, Dirac's idea from QM has not been experimentally confirmed - despite considerable effort to do so. I am criticising the logic behind EM theory in which, as I understand it, monopoles cannot exist.
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    Quote Originally Posted by billvon View Post
    No, they don't add - they are independent in the near field. You can have one without the other. You can have both. They will have different effects on charged particles.
    I began the thread by asking about the experimental evidence for the conventional magnetic flux (or field). Everything I had read suggests magnetic fields are only inferred from their effects on electric charges. Hence it is simpler to say that magnetic fields just modify electric fields, i.e. they act in the same direction between point charges. Are you saying a magnetic field in the near field, or anywhere else, can be directly measured in some way other than its effect on electric charges?
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    Quote Originally Posted by Andrew? View Post
    I began the thread by asking about the experimental evidence for the conventional magnetic flux (or field). Everything I had read suggests magnetic fields are only inferred from their effects on electric charges.
    Well, yes - but you could describe large parts of physics in that way.

    Why can you push on a block and have it move? Because of the effect of electric charges (specifically, repulsion once two atoms get close enough.)
    Hence it is simpler to say that magnetic fields just modify electric fields
    It is indeed simpler to say that. However, it is also wrong.
    i.e. they act in the same direction between point charges.
    No, they don't. They act very differently from electric fields.
    Are you saying a magnetic field in the near field, or anywhere else, can be directly measured in some way other than its effect on electric charges?
    All the measurement methods I know of devolve to measuring the interaction of a magnetic field and a charged particle.
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    Quote Originally Posted by Andrew? View Post
    OK, thanks for that, but I believe my general point still stands. Faraday/Maxwell magnetic fields were devised before magnetism was understood.
    It seems that you are making the quite-trivial point that theories are built on evidence, and can become more complete as more evidence accumulates. One would wonder why that is relevant.

    One could argue -- quite effectively -- that Maxwell's equations express an understanding of magnetism. The only "incompleteness" comes from constitutive relationships (what are the particular sources of magnetic fields, and how do they depend on excitations, e.g.).

    As you say, Dirac's idea from QM has not been experimentally confirmed - despite considerable effort to do so. I am criticising the logic behind EM theory in which, as I understand it, monopoles cannot exist.
    Your understanding is incorrect. The existence of monopoles would pose no complications to Maxwell's equations. None. I don't know where you read that monopoles cannot exist. There is no theoretical prohibition against their existence. The correct statement would be that monopoles have not been found, and therefore might not exist. That's a quite different statement.
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    Quote Originally Posted by billvon View Post
    i.e. they act in the same direction between point charges.
    No, they don't. They act very differently from electric fields.
    We agree that we must infer the existence of these fields. We can never directly see them and, like concepts such as energy, they are the products of human minds in order that we can make sense of physical observations. On what basis do you say magnetic fields cannot be thought of as acting between point charges? This seems a better way to make sense of what happens as such a field would be compatible with other relationships between fields and forces. It would not involve the unnecessary assumption that the behaviour of charges involves a gross asymmetry. As I say, any experiment that is said to support Maxwell's corkscrew can equally be used to disprove it.

    For me the argument is about the utility of different ideas rather than about something physical that can never be directly apprehended. Suppose someone said they had developed a theory in which particles have circulating gravitational fields. By using some very neat mathematics, these fields can be turned through a right angle and thereby shown to account for the direct gravitational forces that we observe. I personally would choose a simpler model.

    Quote Originally Posted by billvon View Post
    All the measurement methods I know of devolve to measuring the interaction of a magnetic field and a charged particle.
    OK, thanks again.
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Andrew? View Post
    OK, thanks for that, but I believe my general point still stands. Faraday/Maxwell magnetic fields were devised before magnetism was understood.
    It seems that you are making the quite-trivial point that theories are built on evidence, and can become more complete as more evidence accumulates. One would wonder why that is relevant.
    I think it is relevant because it suggests that the theory may need to be revised. Indeed we seem to agree that it did need to be revised in that their magnetic field did not comprise lines of force as they had described it. Clearly we disagree about the need for further revision.

    Quote Originally Posted by tk421 View Post
    Your understanding is incorrect. The existence of monopoles would pose no complications to Maxwell's equations. None.
    The first reference that came up was in Wikipedia: "Maxwell's equations posit that there is electric charge, but no magnetic charge (also called magnetic monopoles), ". But I see little point in discussing the point further. I did not intend to start a thread about monopoles.
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    Quote Originally Posted by Andrew? View Post
    We agree that we must infer the existence of these fields. We can never directly see them and, like concepts such as energy, they are the products of human minds in order that we can make sense of physical observations. On what basis do you say magnetic fields cannot be thought of as acting between point charges?
    Because a stationary point charge in a magnetic field will feel no force. A stationary point charge in an electric field will feel a force. Therefore they are different.
    For me the argument is about the utility of different ideas rather than about something physical that can never be directly apprehended. Suppose someone said they had developed a theory in which particles have circulating gravitational fields. By using some very neat mathematics, these fields can be turned through a right angle and thereby shown to account for the direct gravitational forces that we observe. I personally would choose a simpler model.
    You could indeed choose a simpler (and inaccurate) model. You would then try to design a motor with your simpler model and fail, while a designer using Maxwell's Equations would succeed.
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    Quote Originally Posted by Andrew? View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Andrew? View Post
    OK, thanks for that, but I believe my general point still stands. Faraday/Maxwell magnetic fields were devised before magnetism was understood.
    It seems that you are making the quite-trivial point that theories are built on evidence, and can become more complete as more evidence accumulates. One would wonder why that is relevant.
    I think it is relevant because it suggests that the theory may need to be revised. Indeed we seem to agree that it did need to be revised in that their magnetic field did not comprise lines of force as they had described it. Clearly we disagree about the need for further revision.

    Quote Originally Posted by tk421 View Post
    Your understanding is incorrect. The existence of monopoles would pose no complications to Maxwell's equations. None.
    The first reference that came up was in Wikipedia: "Maxwell's equations posit that there is electric charge, but no magnetic charge (also called magnetic monopoles), ". But I see little point in discussing the point further. I did not intend to start a thread about monopoles.
    Well, this being a science forum, you will be challenged and errors corrected. You asserted modern theories forbid monopoles. That is wrong. The wikipedia article your offered in refutation of my point does not support your assertion. Maxwell's equations have a zero value for what would be magnetic charge in recognition of the empirical observation that no such charges have been observed. The equations do not forbid such charges from existing. That's an important difference. The equations themselves, as I explained earlier, can easily accommodate any value of magnetic charge. The equations would not need changing at all.
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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by Andrew? View Post
    We agree that we must infer the existence of these fields. We can never directly see them and, like concepts such as energy, they are the products of human minds in order that we can make sense of physical observations. On what basis do you say magnetic fields cannot be thought of as acting between point charges?
    Because a stationary point charge in a magnetic field will feel no force. A stationary point charge in an electric field will feel a force. Therefore they are different.
    I know the magnetic field is different in that it involves moving charges, but why can't it act between point charges?

    Quote Originally Posted by billvon View Post
    You could indeed choose a simpler (and inaccurate) model. You would then try to design a motor with your simpler model and fail, while a designer using Maxwell's Equations would succeed.
    I still don't see what is wrong with the simpler model for magnetism, or gravity.

    It would be very helpful if you could please explain in simple terms why the dot product equation I gave at the beginning (with 4pi in the denominator) predicts incorrect forces between moving charges. Also, do you think it is impossible for any equation to correctly predict the magnetic forces directly between moving charges if it does not involve a perpendicular looped field?
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    Quote Originally Posted by tk421 View Post
    You asserted modern theories forbid monopoles. That is wrong. The wikipedia article your offered in refutation of my point does not support your assertion. Maxwell's equations have a zero value for what would be magnetic charge in recognition of the empirical observation that no such charges have been observed. The equations do not forbid such charges from existing. That's an important difference. The equations themselves, as I explained earlier, can easily accommodate any value of magnetic charge. The equations would not need changing at all.
    I did not assert that modern theories forbid monopoles. I made it clear they are allowed in QM, but I'm trying to discuss an old theory. I thought one of Maxwell's equations, specifically the one based on Gauss's law, used the principle that the total magnetic flux passing through any closed surface is zero. You seem to be saying this equation would not need changing at all if a monopole caused the surface integral to be non-zero. I don't see how this is relevant to my question about Fleming's law being unnecessary.
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    Quote Originally Posted by Andrew? View Post
    Quote Originally Posted by tk421 View Post
    You asserted modern theories forbid monopoles. That is wrong. The wikipedia article your offered in refutation of my point does not support your assertion. Maxwell's equations have a zero value for what would be magnetic charge in recognition of the empirical observation that no such charges have been observed. The equations do not forbid such charges from existing. That's an important difference. The equations themselves, as I explained earlier, can easily accommodate any value of magnetic charge. The equations would not need changing at all.
    I did not assert that modern theories forbid monopoles. I made it clear they are allowed in QM, but I'm trying to discuss an old theory. I thought one of Maxwell's equations, specifically the one based on Gauss's law, used the principle that the total magnetic flux passing through any closed surface is zero. You seem to be saying this equation would not need changing at all if a monopole caused the surface integral to be non-zero. I don't see how this is relevant to my question about Fleming's law being unnecessary.
    I have been extremely consistent that Maxwell's equations allow monopoles. That you continue to struggle tells us that you don't understand Maxwell at all. I recommend that you study them first before pontificating about subjects you don't understand.

    In particular you need to study the conditions under which various "laws" are actually laws. You obsess about the words without understanding the music.

    Study first, overturn physics later. You cannot do it in the reverse order.
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    Quote Originally Posted by tk421 View Post
    The equations would not need changing at all.
    So after nearly two weeks it seems that forum members agree with tk421. Gauss' law would not need to be changed at all to accommodate a monopole. The magnetic flux arising from it would be zero, therefore the mumbo jumbo of Fleming's rule must be necessary after all.

    Seems a good point to close this thread as well.
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    So after nearly two weeks it seems that forum members agree with tk421. Gauss' law would not need to be changed at all to accommodate a monopole.
    I'm afraid I need to correct tk421 on this point. Maxwell's equations do need to be adapted to accommodate magnetic charges; the "magnetic part" - dF=0 - no longer holds if such charges exist, and will pick up a source term that includes magnetic current density and a proportionality constant. When written out in vector form, the equations would look like this:

    https://en.m.wikipedia.org/wiki/Magn....27s_equations
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    Quote Originally Posted by Markus Hanke View Post
    I'm afraid I need to correct tk421 on this point. Maxwell's equations do need to be adapted to accommodate magnetic charges; the "magnetic part" - dF=0 - no longer holds if such charges exist, and will pick up a source term that includes magnetic current density and a proportionality constant. When written out in vector form, the equations would look like this:

    https://en.m.wikipedia.org/wiki/Magn....27s_equations
    Of course, Markus. Here's a fuller context of what I actually said:

    Quote Originally Posted by tk
    Maxwell's equations have a zero value for what would be magnetic charge in recognition of the empirical observation that no such charges have been observed. The equations do not forbid such charges from existing. That's an important difference. The equations themselves, as I explained earlier, can easily accommodate any value of magnetic charge.
    (boldface and italics added).

    The whole discussion concerned whether Maxwell's equations, as presently constituted, forbid a nonzero magnetic charge (as Andrew asserted). I corrected Andrew by pointing out that they do not forbid it, they merely reflect that we have no evidence for a nonzero charge. The equations would readily accommodate a nonzero value of magnetic charge, just as they accommodate any value of electric charge.
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    Noted, tk421
    I hadn't read the entire conversation ( my bad ), and it seems you didn't actually say what Andrew claimed you said. I should have known better.
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    Quote Originally Posted by Markus Hanke View Post
    Noted, tk421
    I hadn't read the entire conversation ( my bad ), and it seems you didn't actually say what Andrew claimed you said. I should have known better.

    No harm, no foul, as they say in the States. I just didn't want to leave the impression of being even more of a doofus than I already am.
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