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  #1  
Old Wednesday, August 13, 2008
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Default Magnetism

Magnetism


Magnetism is a force that acts at a distance due to a magnetic field. This field is caused by moving electrically charged particles or is inherent in magnetic objects such as a magnet. A magnet is an object that exhibits a strong magnetic field and will attract materials like iron to it. Magnets have two poles, called the north (N) and south (S) poles. Two magnets will be attacted by their opposite poles, and each will repel the like pole of the other magnet. Magnetism has many uses in modern life.

Questions you may have include:
  • What is a magnetic field?
  • What are magnets?
  • How is magnetism used?




Magnetic field
A magnetic field consists of imaginary lines of flux coming from moving or spinning electrically charged particles. Examples include the spin of a proton and the motion of electrons through a wire in an electric circuit.

What a magnetic field actually consists of is somewhat of a mystery, but we do know it is a special property of space.







Names of poles
The lines of magnetic flux flow from one end of the object to the other. By convention, we call one end of a magnetic object the N or North-seeking pole and the other the S or South-seeking pole, as related to the Earth's North and South magnetic poles. The magnetic flux is defined as moving from N to S.

Magnets
Although individual particles such as electrons can have magnetic fields, larger objects such as a piece of iron can also have a magnetic field, as a sum of the fields of its particles. If a larger object exhibits a sufficiently great magnetic field, it is called a magnet.



Magnetic force
The magnetic field of an object can create a magnetic force on other objects with magnetic fields. That force is what we call magnetism.

When a magnetic field is applied to a moving electric charge, such as a moving proton or the electrical current in a wire, the force on the charge is called a Lorentz force.




Attraction
When two magnets or magnetic objects are close to each other, there is a force that attracts the poles together.






Magnets also strongly attract ferromagnetic materials such as iron, nickel and cobalt.



Repulsion
When two magnetic objects have like poles facing each other, the magnetic force pushes them apart.






Magnetic and electric fields
The magnetic and electric fields are both similar and different. They are also inter-related.

Electric charges and magnetism similar
Just as the positive (+) and negative (−) electrical charges attract each other, the N and S poles of a magnet attract each other.

In electricity like charges repel, and in magnetism like poles repel.

Electric charges and magnetism different
The magnetic field is a dipole field. That means that every magnet must have two poles.

On the other hand, a positive (+) or negative (−) electrical charge can stand alone. Electrical charges are called monopoles, since they can exist without the opposite charge.

Summary
Magnetism is a force that acts at a distance and is caused by a magnetic field. The magnetic force strongly attracts an opposite pole of another magnet and repels a like pole. The magnetic field is both similar and different than an electric field.







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  #2  
Old Saturday, October 04, 2008
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Magnets


A magnet is an object or material that attracts certain metals, such as iron, nickel and cobalt. It can also attract or repel another magnet. All magnets have North-seeking (N) and South-seeking (S) poles. When magnets are placed near each other, opposite poles attract and like poles repel each other. Various electrical devices make use of magnets.

Questions you may have include:
  • What types of magnets are there?
  • What are some common properties of magnets?
  • Where are magnets used?


Types of magnets
There are permanent magnets, temporary magnets and electromagnets.

Permanent magnets
A permanent magnet is one that will hold its magnetic properties over a long period of time.

Magnetite
Magnetite is a magnetic material found in nature. It is a permanent magnet, but it is relatively weak.

Alloys
Most permanent magnets we use are manufactured and are a combination or alloy of iron, nickel and cobalt. Rare-earth permanent magnets are a special type of magnet that can have extreme strength.

Temporary magnets
A temporary magnet is one that will lose its magnetism. For example, soft iron can be made into a temporary magnet, but it will lose its magnetic power in a short while.

Electromagnet
By wrapping a wire around an iron or steel core and running an electrical current through the wire, you can magnetize the metal and make an electromagnet. If the core is soft iron, the magnetism will diminish as soon as the current is turned off. This feature makes electromagnets good for picking up and dropping objects. Typically DC electricity is used, but AC current will also result in an electromagnet.

Properties of magnets
Magnets always have two poles, come in various shapes, and attract or repel other magnets.

Names of poles
All magnets have a North-seeking pole (N) and South-seeking pole (S). In a compass, the side marked (N) will point toward the Earth's North magnetic pole. Thus, it is called the "North-seeking pole." Also note that the Earth's North magnetic pole is not the same thing as the North Pole. They are actually several hundred miles apart.

NOTE: To avoid confusion, you should try to be exact in what you are describing, especially concerning magnets.

Various shapes
The magnet can be made into various shapes. The bar magnet is the most common configuration.



Magnets also can be square, spherical, shaped like a horseshoe, and even shaped like a donut.



If you put an iron plate across the N and S poles of a horseshoe magnet, that would essentially "short circuit" the effect of the magnetism, such that its strength would not be very great. As soon as the plate was removed, the magnet would regain its full strength. That method is sometimes used in magnets that are temporary to help keep their magnetic properties for a longer time.

Cutting a magnet
An interesting characteristic of magnets is that when you cut a magnet into parts, each part will have both N and S poles.

Attraction and repulsion
Magnets strongly attract iron, nickel and cobalt, as well as combinations or alloys of these metals.

Also, unlike poles of two magnets will attract, but like poles will repel. Thus, N and S attract, while S and S will repel each other.

Applications
There are numerous applications of magnets.

Creating a magnet
You can magnetize a piece of steel by rubbing a magnet in one direction along the steel. This lines up the many of the domains or sections of aligned atoms in the steel, such that it acts like a magnet. The steel often won't remain magnetized for a very long time, while the true magnet is "permanently" magnetized and retains its strength for a long time.

If you use soft iron or steel, such as a paper clip, it will lose its magnetism quickly. Also, you can disorient the atoms in a magnetized needle by heating it or by dropping the needle on a hard object.

Compass
The first true application of a magnet was the compass, which not only helps in navigation by pointing toward the North magnetic pole, but it is also useful in detecting small magnetic fields. A compass is simply a thin magnet or magnetized iron needle balanced on a pivot. The needle will rotate to point toward the opposite pole of a magnet. It can be very sensitive to small magnetic fields.

Other uses
Magnets are found in loudspeakers, electrical motors and electrical generators.

A very common application of magnets is to stick things to the refrigerator. Since the outer shell of most refrigerators is made of steel, a magnet will readily stick to it. The type of magnets used often consists of a thin sheet of a magnetic material.

As a novelty, magnetic disks can be stacked on a pencil to show magnetic levitation.



Summary
A magnet attracts iron, nickel, cobalt and combinations of those metals. All magnets have North-seeking (N) and South-seeking (S) poles. When magnets are placed near each other, opposite poles attract and similar poles repel each other. Magnets are found in many of our electrical appliances.
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  #3  
Old Saturday, October 04, 2008
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Detection of a Magnetic Field


Any source of magnetism such as a magnet or electromagnet is surrounded by a magnetic field. Information about this field can be retrieved from various devices. A compass can detect a magnetic field. The shape of the magnetic field can be demonstrated with iron filings. A gaussmeter will detect a field and indicate its strength, measured in gauss units.

Questions you may have include:
  • How does a compass detect a magnetic field?
  • How do iron filings demonstrate a magnetic field?
  • What is a gaussmeter?

Compass
A compass is simply a thin magnet or magnetized iron needle balanced on a pivot. It can be used to detect small magnetic fields. The needle will rotate to point toward the opposite pole of a magnet. It can be very sensitive to small magnetic fields.

Using a compass to show the magnetic field
When you bring a compass near an item suspected of being magnetized or having a magnetic field, the compass will turn and point toward the appropriate pole of the object.



A famous experiment showed that a wire with DC electric current running through it created a magnetic field. When the electricity was turned on, a nearby compass moved to indicate a magnetic field was present.


Earth is a huge magnet
The compass was used to discover that the Earth is a huge magnet. The North-seeking pole of the compass needle will always point toward the Earth's North magnetic pole.

Iron filings
By spreading fine iron filings or dust on a piece of paper laid on top of a magnet, you can see the outline of the magnetic lines of force or the magnetic field. The picture below



This experiment also shows that magnetism will act through many materials, such as paper. Would the experiment work if a sheet of iron were used to sprinkle the filings? What about aluminum foil?

Gaussmeter
A gaussmeter is used to measure the strength of a magnetic field. They use a electronic chip called a Hall effect device, which gives off a tiny electrical current when exposed to a magnetic field. The current is amplified with electronic circuitry and a meter shows the number of gauss (the units of magnetic field strength).

Summary
You can detect a magnetic field with a compass, iron filings, or a gaussmeter. The strength of a magnetic field is measured in gauss units.
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  #4  
Old Saturday, October 04, 2008
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Factors Determining Magnetic Properties


The factors that determine the magnetic property of a material are the configuration of the electrons in the material, the ability of the atoms or molecules in the material to align magnetically, and the alignment of domains or sections in the object. Since alignment is so important in the magnetic properties of materials, liquids and gases are typically not magnetic because their molecules aren't held in place as they are in solids. An exception is in rotating fluids.

Questions you may have include:
  • What are electron orientation factors?
  • What are molecule factors?
  • What are domain factors?


Electron orientation
Electrons can behave as tiny magnets, each with north (N) and south (S) poles. When an atom's electrons are lined up in the same orientation, with most having their N pole facing one direction, the atom becomes like a magnet, with N and S poles. It is also possible for the electrons to be in various directions, making the atom not magnetic.

Moving electrons create magnetic field
The reason that electrons can behave like tiny magnets is the fact that when electrons move, they create a magnetic field. Placing a compass near a wire carrying DC electrical current can show that a magnetic field is created due to the electrons moving through the wire.

A magnetic field is also created when electrons rotate around a nucleus and when they spin while in orbit.

(Note that modern theories of the atom no longer accept the Bohr or solar system model. In the new theories, electrons are thought of as clouds or strings. You should be aware that there are new explanations, but for the sake of understanding we will still follow the Bohr model of spinning electrons rotating around a nucleus, similar to planets rotating around the Sun.)

Spinning electrons
Electrons have a property called spin. This spinning creates a magnetic field with N and S poles, just as the spinning Earth has magnetic poles. Note that the N pole on an electron is really a North-seeking pole, just as in a magnet.

If electrons in the shells of an atom spin in the same direction, the atom will exhibit a magnetic field and will respond to the forces of a magnet. If half of the electrons spin one way and the rest spin the other way, they will neutralize each other and the material will not be affected by a magnetic field



Strong and weak electron alignments
Atoms such as iron have most of their electrons aligned in the same direction. Thus, iron or nickel would be attracted to a magnet. Aluminum only has a few electrons aligned, and thus it is only weakly magnetic. An element with half of its electrons oriented one way would not be attracted to a magnet.

Atomic and molecular alignment
Although some atoms may be highly magnetic, they really need to be aligned to make a material magnetic. If magnetic atoms are facing different directions, their fields will cancel out each other.

Solids and fluids
Since the atoms or molecules in a solid are fixed in place, most magnetic materials are solids. This is because once the atoms or molecules become aligned, they tend to stay in place. An example is seen when you magnetize a piece of iron.

As a material becomes heated or when it is in its liquid or gaseous state, the atoms or molecules are in rapid motion and are not aligned. Thus, fluids are seldom magnetic.

An exception is when a magnetic material such as iron is in its liquid state and is continually rotating around an axis. In such a situation, the atoms can be aligned in one direction, even though they are in rapid motion.

For example, the core of the Earth is made of liquid iron. Since the Earth rotates on its axis, the liquid iron is rotating, thus creating the Earth's magnetic field. Also, the Sun rotates on its axis, and the material in its plasma state creates the Sun's magnetic field.

Molecules
If two or more elements are chemically combined to form a molecule, it is quite possible that the compound is not very magnetic because the orientations of the atoms in the molecule work against each other.

A good example of this is to compare the magnetic properties of iron as compared to its compounds if iron oxide (rust) and iron sulfide. A piece of iron is highly magnetic, but a hunk of rusty iron is not.

Alloys
Metals of different elements can be mixed when they are in the molten or liquid state to form alloys. These combinations result in materials with slightly different physical and chemical properties than the elements by themselves.

If the metals typically respond well to a magnetic field--such as iron and nickel--then their alloy has even a stronger reaction to magnetism. On the other hand, there are some alloys of iron--such as forms of stainless steel--that do not respond well at all to a magnet.

Domains
The final factor in a material being magnetic concern the orientation of its domains in a solid. A group of atoms in a metal may become aligned, but the various groups may be misaligned. These groups are called domains.

It is necessary to line up many of the domains in a material like iron in order for it to become a magnet.



Summary
Alignment of electrons, atoms and domains are important in determining the magnetic response of a material and whether it is a magnet. Since the atoms or molecules need to be aligned, gases and liquids are typically not magnetic, and most magnets are solid metals. An exception is in the rotating liquid iron core of the Earth and the rotating plasmas of the Sun.
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Old Saturday, October 04, 2008
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Magnetic Materials


Materials respond differently to the force of a magnetic field. A magnet will strongly attract ferromagnetic materials, weakly attract paramagnetic materials, and weakly repel diamagnetic materials. The orientation of the spin of the electrons in an atom, the orientation of the atoms in a molecule or alloy, and the ability of domains of atoms or molecules to line up are the factors that determine how a material responds to a magnetic field. Ferromagnetic materials have the most magnetic uses. Diamagnetic materials are used in magnetic levitation and MRI.

Questions you may have include:
  • What are ferromagnetic materials?
  • What are paramagnetic materials?
  • What are diamagnetic materials?


Ferromagnetic materials
Ferromagnetic materials are strongly attracted by a magnetic force. The elements iron (Fe), nickel (Ni), cobalt (Co) and gadolinium (Gd) are such materials. (See the Periodic Table in the Chemistry section for more information.)

The reasons these metals are strongly attracted are because their individual atoms have a slightly higher degree of magnetism due to their configuration of electrons, their atoms readily line up in the same magnetic direction, and the magnetic domains or groups of atoms line up more readily. (See Factors Determining Magnetic Response for more information.)

Iron and steel
Iron is the most common element associated with being attracted to to a magnet. Steel is also a ferromagnetic material. It is an alloy or combination of iron and several other metals, giving it greater hardness than iron, as well as other specialized properties. Because of its hardness, steel retains magnetism longer than iron.

Permanent magnets
Alloys of iron, nickel, cobalt, gadolinium and certain ceramic materials can become "permanent" magnets, such that they retain their magnetism for a long time.

Temperature effect
Strongly magnetic ferromagnetic materials like nickel or steel lose all their magnetic properties if they are heated to a high enough temperature. The atoms become too excited by the heat to remain pointing in one direction for long.

The temperature at which a metal loses its magnetism is called the Curie temperature, and it is different for every metal. The Curie temperature for nickel, for example, is about 350°C.

Paramagnetic materials
Paramagnetic materials are metals that are weakly attracted to magnets. Aluminum and copper are such metals. These materials can become very weak magnets, but their attractive force can only be measured with sensitive instruments.

Temperature can affect the magnetic properties of a material. Paramagnetic materials like aluminum, uranium and platinum become more magnetic when they are very cold.

The force of a ferromagnetic magnet is about a million times that of a magnet made with a paramagnetic material. Since the attractive force is so small, paramagnetic materials are typically considered nonmagnetic.

Diamagnetic materials
Certain materials are diamagnetic, which means that when they are exposed to a strong magnetic field, they induce a weak magnetic field in the opposite direction. In other words, they weakly repel a strong magnet. Some have been used in simple levitation demonstrations.

Strongest
Bismuth and carbon graphite are the strongest diamagnetic materials. They are about eight times stronger than mercury and silver. Other weaker diamagnetic materials include water, diamonds, wood and living tissue. Note that the last three items are carbon-based.

The electrons in a diamagnetic material rearrange their orbits slightly creating small persistent currents, which oppose the external magnetic field.

Uses
Although the forces created by diamagnetism are extremely weak--millions of times smaller than the forces between magnets and ferromagnetic materials like iron, there are some interesting uses of those materials.

Levitation
The most popular application of diamagnetic materials is magnetic levitation, where an object will be made to float in are above a strong magnet. Although most experiments use inert objects, researchers as the University of Nijmegen in the Netherlands demonstrated levitating a small frog in a powerful magnetic field.

MRI
Another important application of diamagnetic materials is magnetic resonance imaging (MRI). In this useful diagnostic tool in medicine. The way it works is that when carbon-based atoms in the body are exposed to a strong magnetic field, they are slightly repelled by the field. This movement of the atoms can be detected and used for analysis.

Summary
Magnets will strongly attract ferromagnetic materials, weakly attract paramagnetic materials, a nd weakly repel diamagnetic materials. Ferromagnetic materials have the most magnetic uses. Diamagnetic materials are used in magnetic levitation and MRI.




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