What is Magnetism? There is a magnetic force in nature. When your compass needle points north, or when you stick a note onto your refrigerator with a little magnet, you are using one of the fundamental forces of nature, the force we call magnetism to all the ancient civilizations, including the Greeks and Chinese.
Anything capable of exerting a magnetic force (for example, by deflecting a compass needle) is a magnet. The most common magnetic material is iron, and there are many iron ores that have magnetic properties. Indeed, it was these naturally occurring magnets that led Greek scientists to investigate magnetism in the first place.
The Greeks believed that there was an island in the Mediterranean made from naturally occurring magnetic materials.
They warned that ships should not be put together with nails because if such a ship ever sailed near that island the nails would be pulled out and the ship would come apart.
There are, of course, good reasons to build a ship with pegs instead of nails, but the mythical “magnetic island” is not one of them.
Magnets sometimes attract and sometimes repel each other. We say that every magnet has two poles we call them north and south—and that like poles repel each other and unlike poles attract.
Thus, if you bring two north (or two south) poles of two magnets near each other, the magnets will be pushed apart. If on the other hand, you bring the south pole of one magnet near the north pole of another, the magnets will be attracted.
What is Magnetism?
what we really mean is that a magnetic force acts on that compass needle. One end is attracted to the earth’s north pole, the other to the earth’s south pole.
The net result is that no matter where a compass needle points initially, it will be twisted around until it lines itself up in a north/south direction. This, of course, is why a compass is so useful in navigation.
Because the end of the compass labeled “N” points toward the earth’s north pole, that end must actually be a south pole of the needle. To avoid confusion, physicists usually refer to the pole of a compass on which you paint the “N” as the “north-seeking pole.”
What is Magnetism?
The earth is a magnet. The fact that a compass needle responds to forces exerted on it by the earth proves that the earth is capable of exerting a magnetic force and is, therefore, a magnet. In fact, you can think of the earth as being very similar to a giant bar magnet.
There are no isolated magnetic poles in nature. As far as we can tell, every north magnetic pole that exists in nature is accompanied by a south magnetic pole.
If you take an ordinary bar magnet and break it in two, you do not get a north pole and a south pole, but two short magnets, each of which has its own north and south pole.
Where are the magnetic monopoles?
A single isolated north or south magnets pole would be called a “magnetic monopole.” Physicists have expended a great deal of effort in searching for monopoles, but, with one disputed exception, they have not been successful.
This is a great puzzle, because there are many isolated electrical charges in nature (for example, the electron and proton), and because we believe there is as profound symmetry between electricity and magnetism.
To a physics, living in a world with lots of electrical charges and no magnetic monopoles is something like looking at a huge painting which has a piece torn out of it, there is a constant nagging reminder that some-thing is missing.
All magnets are electromagnets. Because there are no free magnetic monopoles in nature, all magnetic fields that we know about have to arise from the effects of moving electrical charges.
For example, an electron in orbit around an atom constitutes an electrical current—a miniature, of course, but an electrical current just the same. It is this electrical current that can make the atom into a small magnet.
In the same way, we believe it is the motion of liquid iron in the earth’s core that gives rise to the earth’s magnetic field (see below) and that the motion of charged particles in the interior of the sun gives rise to the sun’s magnetic field.
The largest manta made magnetic fields are produced at the National Magnet Laboratory in Cam-bridge, Mass., and run to strengths 40,000 or more times that of the earth.
The largest magnetic fields anywhere in the universe are probably those on the surface of pulsars, and may run as high as many billion times stronger than the earth’s field.
Natural magnets, the LP type that is usually made from iron, are called ferromagnetic, or, sometimes, “permanent magnets.”
Here’s how a ferromagnet works: the “magnets” associated with each of the iron atoms tend to line up, as shown in the sketch. This lining up happens because of a force between neighboring atoms. When they reinforce each other, they create a permanent magnet.
The force that causes the atoms to line up creates what is called a “ferromagnetic domain.” This is a block of material about a thousand atoms across in which all of the atomic magnets are pointing in the same direction.
In a normal piece of iron, the domains point in random directions, so there is no net magnetic field outside the iron, even though there is one inside each domain.
How A Piece Of Iron Becomes Magnetized?
The domains are pulled around and made to point in the same direction. In this way, the magnetic fields of all the domains reinforce each other, and the material exerts a large magnetic force on anything near it. The more domains lined up in a given piece of material, the stronger its magnetic field will be.
The existence of ferromagnetic domains explains why a magnet can be demagnetized. A piece of iron or an alloy will be a magnet only so long as its domains are lined up.
If a magnet is heated, however, the domains will be jostled around and will return to their normal random orientations. We say that the iron has been demagnetized.
In order for it to be re magnetized, it has to be placed in a strong magnetic field so that the domains will once again line up.
There are only a few naturally occurring magnetic materials. Iron, of course, is the most common, but nickel and cobalt are also in this class. The most powerful permanent magnets are made from alloys of iron, boron, and neodymium.
The reason these materials and no others are magnetized is that the force that aligns neighboring atoms depends very sensitively on how far apart those atoms are. Only in these elements and some of their alloys is this distance just right for the formation of domains.
What are Some materials are paramagnet?
These are materials that do not produce a magnetic field by themselves, but will do so if they are near another magnet. The way a paramagnet works is this: under normal circumstances, the atomic magnets are pointed in random directions and there is no magnetic field associated with the material.
In the presence of another magnetic field, however, the atomic magnets in the material line up to reinforce and strengthen that external field. Examples of paramagnetic are liquid oxygen and some ions of uranium.
What is Magnetism? The rotation of the earth generates a magnetic field. As the earth rotates, the liquid iron core rotates with it. Liquid iron will conduct electricity, although the fact that it has no net electrical charge means that it doesn’t constitute an electric current in and of itself.
There is, however, a complex process by which such a rotating conductor produces a magnetic field, and it is believed that this is the basic mechanism behind the earth’s magnetic field.
Thus, when you see a compass needle pointing north, you are dealing with a force whose origin is in the very heart of our planet.
The earth’s magnetic field undergoes episodic reversals. Right now, the “north” pole of the earth’s magnet is in the Canadian Arctic. There have been times in the past, however, when the north pole is where Antarctica is now.
We can document at least three hundred such reversals in the last few hundred million years. These reversals are erratic and the complete changeover of the poles seems to require about five thousand years.
It seems to happen by having the field shrink down to zero and then grow again in the opposite direction, rather than having the north pole migrate across the face of the earth.
Why does the earth’s magnetic field reverse?
It’s not too hard to figure out how a planet could have a steady magnetic field. It is very difficult, however, to figure out how it could have a steady field that decides to change directions every once in a while.
The erratic nature of the earth’s magnetic field remains a profound mystery to geophysicists. where the earth’s magnetic field used to be. When the molten rock comes to the surface of the earth, tiny bits of natural magnetic material are floating in it.
These bits of material align themselves by pointing toward wherever the north pole happens to be at that time. Once the rock hardens, this direction is locked in, and the rock retains its memory of where the north pole was when it was formed.
Thus, if we find such frozen “mag-nets” pointing south, we know that the “north” pole of the earth was at what is now the south pole when the rock hardened. The study of old magnetic fields is called paleomagnetism, and this field provides some of the important evidence for plate tectonics.
The sun, too, has a magnetic field. The origin of the sun’s magnetic field is probably similar to that of the earth’s.
The entire sun rotates and is made of a material that will conduct electricity—in this case, the plasma made of loose electrons and the atoms from which they have been torn. The sun’s magnetic field seems to reverse itself every eleven years.
As is the case with the earth, the origin of the sun’s magnetic field (and the reasons for the reversals) is not very well understood.
Sunspots are related to the sun’s magnetic field. The dark spots that are seen on the sun appear to be consequences of magnetic storms and magnetic phenomena under the sun’s surface.
The spots go through an eleven-year cycle, waxing and waning along with the magnetic field. They appear to be most numerous at the end of the cycle and least numerous in the beginning.