The term magnet is typically reserved for objects that produce their own persistent magnetic field even in the absence of an applied magnetic field. Only certain classes of materials can do this. Most materials, however, produce a magnetic field in response to an applied magnetic field; a phenomenon known as magnetism. There are several types of magnetism, and all materials exhibit at least one of them.
The overall magnetic behavior of a material can vary widely, depending on the structure of the material, and particularly on its electron configuration. Several forms of magnetic behavior have been observed in different materials, including:
Ferromagnetic and ferrimagnetic materials are the ones normally thought of as magnetic; they are attracted to a magnet strongly enough that the attraction can be felt. These materials are the only ones that can retain magnetization and become magnets; a common example is a traditional refrigerator magnet. Ferrimagnetic materials, which include ferrites and the oldest magnetic materials magnetite and lodestone, are similar to but weaker than ferromagnetics. The difference between ferro- and ferrimagnetic materials is related to their microscopic structure, as explained below.
Paramagnetic substances such as platinum, aluminium, and oxygen are weakly attracted to a magnet. This effect is hundreds of thousands of times weaker than ferromagnetic materials attraction, so it can only be detected by using sensitive instruments, or using extremely strong magnets. Magnetic ferrofluids, although they are made of tiny ferromagnetic particles suspended in liquid, are sometimes considered paramagnetic since they cannot be magnetized.
Diamagnetic means repelled by both poles. Compared to paramagnetic and ferromagnetic substances, diamagnetic substances such as carbon, copper, water, and plastic are even more weakly repelled by a magnet. The permeability of diamagnetic materials is less than the permeability of a vacuum. All substances not possessing one of the other types of magnetism are diamagnetic; this includes most substances. Although force on a diamagnetic object from an ordinary magnet is far too weak to be felt, using extremely strong superconducting magnets diamagnetic objects such as pieces of lead and even mice [7] can be levitated so they float in mid-air. Superconductors repel magnetic fields from their interior and are strongly diamagnetic.
There are various other types of magnetism, such as spin glass, superparamagnetism, superdiamagnetism, and metamagnetism
Tuesday, 20 July 2010
Magnetic moment
The magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M, with units A/m. It is a vector field, rather than just a vector (like the magnetic moment), because different areas in a magnet can be magnetized with different directions and strengths (for example, because of domains, see below). A good bar magnet may have a magnetic moment of magnitude 0.1 A·m2 and a volume of 1 cm3, or 1×10−6 m3, and therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter. Such a large value explains why magnets are so effective at producing magnetic fields.
Magnetic field
Main article: Magnetic field
The magnetic field (usually denoted B) is a vector field. The magnetic field vector at a given point in space is specified by two properties:
1.Its direction, which is along the orientation of a compass needle.
2.Its magnitude (also called strength), which is proportional to how strongly the compass needle orients along that direction.
In SI units, the strength of the magnetic field is given in teslas.
The magnetic field (usually denoted B) is a vector field. The magnetic field vector at a given point in space is specified by two properties:
1.Its direction, which is along the orientation of a compass needle.
2.Its magnitude (also called strength), which is proportional to how strongly the compass needle orients along that direction.
In SI units, the strength of the magnetic field is given in teslas.
Magnet
This article is about objects and devices that produce magnetic fields. For a description of magnetic materials, see magnetism. For other uses, see Magnet (disambiguation).
Iron filings that have oriented in the magnetic field produced by a bar magnet
Magnetic field lines of a solenoid which are similar to a bar magnet as illustrated above with the iron filingsA magnet (from Greek μαγνήτις λίθος magnḗtis líthos, Magnesian stone) is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials like iron and attracts or repels other magnets.
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include iron, nickel, cobalt, some alloys of rare earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.
Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron which can be magnetized but don't tend to stay magnetized, and magnetically "hard" materials, which do. Permanent magnets are made from "hard" ferromagnetic materials which are subjected to special processing in a powerful magnetic field during manufacture, to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity whereas "soft" materials have low coercivity.
An electromagnet is made from a coil of wire which acts as a magnet when an electric current passes through it, but stops being a magnet when the current stops. Often an electromagnet is wrapped around a core of ferromagnetic material like steel, which enhances the magnetic field produced by the coil.
The overall strength of a magnet is measured by its magnetic moment, while the local strength of the magnetism in a material is measured by its magnetization.
Iron filings that have oriented in the magnetic field produced by a bar magnet
Magnetic field lines of a solenoid which are similar to a bar magnet as illustrated above with the iron filingsA magnet (from Greek μαγνήτις λίθος magnḗtis líthos, Magnesian stone) is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials like iron and attracts or repels other magnets.
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include iron, nickel, cobalt, some alloys of rare earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.
Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron which can be magnetized but don't tend to stay magnetized, and magnetically "hard" materials, which do. Permanent magnets are made from "hard" ferromagnetic materials which are subjected to special processing in a powerful magnetic field during manufacture, to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity whereas "soft" materials have low coercivity.
An electromagnet is made from a coil of wire which acts as a magnet when an electric current passes through it, but stops being a magnet when the current stops. Often an electromagnet is wrapped around a core of ferromagnetic material like steel, which enhances the magnetic field produced by the coil.
The overall strength of a magnet is measured by its magnetic moment, while the local strength of the magnetism in a material is measured by its magnetization.
Hertz
This article is about the unit of frequency. For other uses, see Hertz (disambiguation).
"MHZ" redirects here. MHZ is also the IATA airport code for RAF Mildenhall in Suffolk, England.
"MHz" redirects here. MHz may also refer to MHz Networks.
"Megahertz" redirects here. For the German NDH industrial metal band, see Megaherz.
Hertz
Standard: SI derived unit
Quantity: Frequency
Symbol: Hz
Named after: Heinrich Hertz
Expressed in: 1 Hz =
SI base units 1/s
The hertz (symbol: Hz) is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon.[1] One of its most common uses is the description of sine wave, particularly those used in radio and audio applications.
"MHZ" redirects here. MHZ is also the IATA airport code for RAF Mildenhall in Suffolk, England.
"MHz" redirects here. MHz may also refer to MHz Networks.
"Megahertz" redirects here. For the German NDH industrial metal band, see Megaherz.
Hertz
Standard: SI derived unit
Quantity: Frequency
Symbol: Hz
Named after: Heinrich Hertz
Expressed in: 1 Hz =
SI base units 1/s
The hertz (symbol: Hz) is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon.[1] One of its most common uses is the description of sine wave, particularly those used in radio and audio applications.
X-ray laser active media
Most often used media include highly ionized plasma created in a capillary discharge or when a linearly focused optical pulse hits a solid target. In accordance to the Sah equation, the most stable electron configurations are neon-like with 10 electrons remaining and nickel-like with 28 electrons remaining. The electron transitions in highly ionized plasma usually correspond to energies in the order of 100s eV.
• Capillary plasma discharge medium: In this setup, a several centimeters long capillary made of resistant material (e. g. alumina) confines a high current, sub microsecond electrical pulse in low-pressure gas. The Lorentz force causes further compression of the plasma discharge (see pinch). A pre-ionisation electric pulse and/or optical pulse is often used. An example is the capillary neon-like Ar8+ laser (generating at 47 nm).
• Solid slab target medium: After being hit by optical pulse, the target emits highly excited plasma. Again, a longer prepulse is often used for the plasma creation and a second, shorter and more energetic pulse is used for further excitation in the plasma volume. For short lifetimes, a sheared excitation pulse may be needed (GRIP - grazing incidence pump). The refractive index gradient causes the amplified pulse to bend from the target surface. This can be compensated using curved target or multiple targets in series.
• Plasma excited by optical field: At optical densities high enough to cause effective electron tunelling or even to suppress the potential barrier (> 1016 W/cm2), it is possible to highly ionize the gas without contact with any capillary or target. Usually a collinear setup is used, enabling to synchronize pump and signal pulses.
Alternative amplifying medium is the relativistic electron beam in free electron laser.
A completely different approach to X-ray generation is the high-harmonic generation.
• Capillary plasma discharge medium: In this setup, a several centimeters long capillary made of resistant material (e. g. alumina) confines a high current, sub microsecond electrical pulse in low-pressure gas. The Lorentz force causes further compression of the plasma discharge (see pinch). A pre-ionisation electric pulse and/or optical pulse is often used. An example is the capillary neon-like Ar8+ laser (generating at 47 nm).
• Solid slab target medium: After being hit by optical pulse, the target emits highly excited plasma. Again, a longer prepulse is often used for the plasma creation and a second, shorter and more energetic pulse is used for further excitation in the plasma volume. For short lifetimes, a sheared excitation pulse may be needed (GRIP - grazing incidence pump). The refractive index gradient causes the amplified pulse to bend from the target surface. This can be compensated using curved target or multiple targets in series.
• Plasma excited by optical field: At optical densities high enough to cause effective electron tunelling or even to suppress the potential barrier (> 1016 W/cm2), it is possible to highly ionize the gas without contact with any capillary or target. Usually a collinear setup is used, enabling to synchronize pump and signal pulses.
Alternative amplifying medium is the relativistic electron beam in free electron laser.
A completely different approach to X-ray generation is the high-harmonic generation.
X-ray laser
X-ray laser is a device that uses stimulated emission to generate or amplify the electromagnetic radiation in the near X-ray or extreme ultraviolet region, usually in the order of several nanometer or tens of nm.
Because of high gain in the medium, short upper state lifetime (1 - 100 ps) and problems associated with construction of X-ray mirrors, the X-ray laser usually operates without any resonator. The emitted radiation based on amplified spontaneous emission has relatively low spatial coherence. The line is mostly Doppler broadened (which depends on the ion temperature).
As the common laser transitions between electronic or vibrational states correspond to energies up to 10 eV, a different active medium is needed
Because of high gain in the medium, short upper state lifetime (1 - 100 ps) and problems associated with construction of X-ray mirrors, the X-ray laser usually operates without any resonator. The emitted radiation based on amplified spontaneous emission has relatively low spatial coherence. The line is mostly Doppler broadened (which depends on the ion temperature).
As the common laser transitions between electronic or vibrational states correspond to energies up to 10 eV, a different active medium is needed
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