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Magnets and Magnetic Force

But what I'm saying is although we're kind of used to a magnet just like we're used to gravity, just like gravity is also fairly mysterious when you really think about what it is, so is magnetism. So with that said, let's at least try to get some working knowledge of how we can deal with magnetism.

So we're all familiar with a magnet. I didn't want it to be yellow. I could make the boundary yellow. No, I didn't want it to be like that either. So if this is a magnet, we know that a magnet always has two poles. It has a north pole and a south pole. And these were just labeled by convention.


Because when people first discovered these lodestones, or they took a lodestone and they magnetized a needle with that lodestone, and then that needle they put on a cork in a bucket of water, and that needle would point to the Earth's north pole. They said, oh, well the side of the needle that is pointing to the Earth's north, let's call that the north pole. And the point of the needle that's pointing to the south pole-- sorry, the point of the needle that's pointing to the Earth's geographic south, we'll call that the south pole.

Or another way to put it, if we have a magnet, the direction of the magnet or the side of the magnet that orients itself-- if it's allowed to orient freely without friction-- towards our geographic north, we call that the north pole. And the other side is the south pole. And this is actually a little bit-- obviously we call the top of the Earth the north pole. You know, this is the north pole. And we call this the south pole. And there's another notion of magnetic north. And that's where-- I guess, you could kind of say-- that is where a compass, the north point of a compass, will point to.

And actually, magnetic north moves around because we have all of this moving fluid inside of the earth. And a bunch of other interactions. It's a very complex interaction. But magnetic north is actually roughly in northern Canada. So magnetic north might be here.

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So that might be magnetic north. And magnetic south, I don't know exactly where that is.

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But it can kind of move around a little bit. It's not in the same place. So it's a little bit off the axis of the geographic north pole and the south pole. And this is another slightly confusing thing. Magnetic north is the geographic location, where the north pole of a magnet will point to. But that would actually be the south pole, if you viewed the Earth as a magnet. So if the Earth was a big magnet, you would actually view that as a south pole of the magnet. And the geographic south pole is the north pole of the magnet.

You could read more about that on Wikipedia, I know it's a little bit confusing. But in general, when most people refer to magnetic north, or the north pole, they're talking about the geographic north area. And the south pole is the geographic south area. But the reason why I make this distinction is because we know when we deal with magnets, just like electricity, or electrostatics-- but I'll show a key difference very shortly-- is that opposite poles attract.

So if this side of my magnet is attracted to Earth's north pole then Earth's north pole-- or Earth's magnetic north-- actually must be the south pole of that magnet. And vice versa. The south pole of my magnet here is going to be attracted to Earth's magnetic south.

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Which is actually the north pole of the magnet we call Earth. Anyway, I'll take Earth out of the equation because it gets a little bit confusing. And we'll just stick to bars because that tends to be a little bit more consistent. Let me erase this. There you go. I'll erase my Magnesia.

I wonder if the element magnesium was first discovered in Magnesia, as well. And I actually looked up Milk of Magnesia, which is a laxative. And it was not discovered in Magnesia, but it has magnesium in it. So I guess its roots could be in Magnesia if magnesium was discovered in Magnesia. Anyway, enough about Magnesia. Back to the magnets. So if this is a magnet, and let me draw another magnet.

Actually, let me erase all of this. All right. So let me draw two more magnets. We know from experimentation when we were all kids, this is the north pole, this is the south pole. That the north pole is going to be attracted to the south pole of another magnet. And that if I were to flip this magnet around, it would actually repel north-- two north facing magnets would repel each other. And so we have this notion, just like we had in electrostatics, that a magnet generates a field. It generates these vectors around it, that if you put something in that field that can be affected by it, it'll be some net force acting on it.

So actually, before I go into magnetic field, I actually want to make one huge distinction between magnetism and electrostatics. Magnetism always comes in the form of a dipole. What does a dipole mean? It means that we have two poles. A north and a south. In electrostatics, you do have two charges. You have a positive charge and a negative charge. So you do have two charges. But they could be by themselves. An electric current or magnetic dipole creates a magnetic field, and that field, in turn, imparts magnetic forces on other particles that are in the fields.

Maxwell's equations, which simplify to the Biot—Savart law in the case of steady currents, describe the origin and behavior of the fields that govern these forces. Therefore, magnetism is seen whenever electrically charged particles are in motion —for example, from movement of electrons in an electric current , or in certain cases from the orbital motion of electrons around an atom's nucleus.

They also arise from "intrinsic" magnetic dipoles arising from quantum-mechanical spin. The same situations that create magnetic fields—charge moving in a current or in an atom, and intrinsic magnetic dipoles—are also the situations in which a magnetic field has an effect, creating a force.

Following is the formula for moving charge; for the forces on an intrinsic dipole, see magnetic dipole. When a charged particle moves through a magnetic field B , it feels a Lorentz force F given by the cross product : [17]. Because this is a cross product, the force is perpendicular to both the motion of the particle and the magnetic field. It follows that the magnetic force does no work on the particle; it may change the direction of the particle's movement, but it cannot cause it to speed up or slow down.

The magnitude of the force is. One tool for determining the direction of the velocity vector of a moving charge, the magnetic field, and the force exerted is labeling the index finger "V", the middle finger "B", and the thumb "F" with your right hand. When making a gun-like configuration, with the middle finger crossing under the index finger, the fingers represent the velocity vector, magnetic field vector, and force vector, respectively.

See also right-hand rule. A very common source of magnetic field found in nature is a dipole , with a " South pole " and a " North pole ", terms dating back to the use of magnets as compasses, interacting with the Earth's magnetic field to indicate North and South on the globe. Since opposite ends of magnets are attracted, the north pole of a magnet is attracted to the south pole of another magnet. The Earth's North Magnetic Pole currently in the Arctic Ocean, north of Canada is physically a south pole, as it attracts the north pole of a compass.

A magnetic field contains energy , and physical systems move toward configurations with lower energy. When diamagnetic material is placed in a magnetic field, a magnetic dipole tends to align itself in opposed polarity to that field, thereby lowering the net field strength. When ferromagnetic material is placed within a magnetic field, the magnetic dipoles align to the applied field, thus expanding the domain walls of the magnetic domains.

Since a bar magnet gets its ferromagnetism from electrons distributed evenly throughout the bar, when a bar magnet is cut in half, each of the resulting pieces is a smaller bar magnet. Even though a magnet is said to have a north pole and a south pole, these two poles cannot be separated from each other. A monopole—if such a thing exists—would be a new and fundamentally different kind of magnetic object. It would act as an isolated north pole, not attached to a south pole, or vice versa. Monopoles would carry "magnetic charge" analogous to electric charge.

Despite systematic searches since , as of [update] , they have never been observed, and could very well not exist. Nevertheless, some theoretical physics models predict the existence of these magnetic monopoles. Paul Dirac observed in that, because electricity and magnetism show a certain symmetry , just as quantum theory predicts that individual positive or negative electric charges can be observed without the opposing charge, isolated South or North magnetic poles should be observable. Using quantum theory Dirac showed that if magnetic monopoles exist, then one could explain the quantization of electric charge—that is, why the observed elementary particles carry charges that are multiples of the charge of the electron.

Certain grand unified theories predict the existence of monopoles which, unlike elementary particles, are solitons localized energy packets. The initial results of using these models to estimate the number of monopoles created in the Big Bang contradicted cosmological observations—the monopoles would have been so plentiful and massive that they would have long since halted the expansion of the universe. However, the idea of inflation for which this problem served as a partial motivation was successful in solving this problem, creating models in which monopoles existed but were rare enough to be consistent with current observations.

While heuristic explanations based on classical physics can be formulated, diamagnetism, paramagnetism and ferromagnetism can only be fully explained using quantum theory. That this leads to magnetism is not at all obvious, but will be explained in the following. Here the last product means that a first electron, r 1 , is in an atomic hydrogen-orbital centered at the second nucleus, whereas the second electron runs around the first nucleus.

This "exchange" phenomenon is an expression for the quantum-mechanical property that particles with identical properties cannot be distinguished. It is specific not only for the formation of chemical bonds , but as one will see, also for magnetism, i. The " singlet state ", i. The tendency to form a homoeopolar chemical bond this means: the formation of a symmetric molecular orbital, i. In contrast, the Coulomb repulsion of the electrons, i. Thus, now the spins would be parallel ferromagnetism in a solid, paramagnetism in two-atomic gases.

The last-mentioned tendency dominates in the metals iron , cobalt and nickel , and in some rare earths, which are ferromagnetic. Most of the other metals, where the first-mentioned tendency dominates, are nonmagnetic e. Diatomic gases are also almost exclusively diamagnetic, and not paramagnetic. The Heitler-London considerations can be generalized to the Heisenberg model of magnetism Heisenberg The explanation of the phenomena is thus essentially based on all subtleties of quantum mechanics, whereas the electrodynamics covers mainly the phenomenology.

Some organisms can detect magnetic fields, a phenomenon known as magnetoception. In addition to detection, biomagnetic phenomena are utilized by organisms in a number of ways. For instance, chitons , a type of marine mollusk, produce magnetite to harden their teeth, and even humans produce magnetite in bodily tissue. From Wikipedia, the free encyclopedia.

For other uses, see Magnetic disambiguation , Magnetism disambiguation , and Magnetized disambiguation. Electrical network. Covariant formulation. Electromagnetic tensor stress—energy tensor. Main article: History of electromagnetism. See also: Magnetic moment. Main article: Diamagnetism. Main article: Paramagnetism. Main article: Ferromagnetism. Main article: Magnetic domains. Magnetic domains boundaries white lines in ferromagnetic material black rectangle. Main article: Antiferromagnetism. Main article: Ferrimagnetism.

Main article: Superparamagnetism. Main article: Classical electromagnetism and special relativity. Play media. Main article: Magnetic field. Main article: Magnetic dipole. Main article: Magnetic monopole.

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Magnetism - Wikipedia

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