Magnetism
is a result of electrons spinning on their own axis around the nucleus
In magnetic materials, the atoms have certain areas called domains. These domains are aligned such that their electrons tend to spin in the same direction
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| Image.2 Magnetic Domains |
The alignment of these domains results in the formation of magnetic poles at each end of the magnet. These poles are called the north pole and the south pole. The law of magnetism states that like magnetic poles repel and unlike magnetic poles attract one another
Magnetic Flux
The group of magnetic field lines emitted outward from the north pole of a magnet is called magnetic flux. The symbol for magnetic flux is F (phi).
Magnetic Flux Density
Magnetic flux density is the amount of magnetic flux per unit area of a section, perpendicular to the direction of flux. Equation (1-11) is the mathematical representation of magnetic flux
density.
𝗕﹣⌽ ∕ 𝐀
where
B = magnetic flux density in teslas (T)
F = magnetic flux in webers (Wb)
A = area in square meters (m2)
The result is that the SI unit for flux density is webers per square meter ⟮ 𝐖𝐛∕𝑚𝒎² ⟯. One weber per square meter equals one tesla.
Magnetic materials
Those materials can be either attracted or repelled by a magnet and can be magnetized themselves. The most commonly used magnetic materials are iron and steel. A permanent magnet is made of a very hard magnetic material, such as cobalt steel, that retains its magnetism for long periods of time when the magnetizing field is removed. A temporary magnet is a material that will not retain its magnetism when the field is removed. Permeability (μ) refers to the ability of a material to concentrate magnetic lines of flux.
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| Image.3 Magnetic Attraction & Repulsion |
Those materials that can be easily magnetized are considered to have a high permeability. Relative permeability is the ratio of the permeability of a material to the permeability of a vacuum (μo). The symbol for relative permeability is μR (mu).
Magnetic materials are classified as either magnetic or nonmagnetic based on the highly magnetic properties of iron. Because even weakly magnetic materials may serve a useful purpose in some applications, the classification includes the three groups described below.
Ferromagnetic Materials
Some of the ferromagnetic materials used are iron, steel, nickel, cobalt, and commercial alloys, alnico, and per alloy. Ferrites are nonmagnetic but have the ferromagnetic properties of iron. Ferrites are made of ceramic material and have relative permeabilities that range from 50 to 200. They have commonly been used in the coils for RF (radiofrequency) transformers.
Paramagnetic Materials
These are materials such as aluminum, platinum, manganese, and chromium. These materials have a relative permeability of slightly more than one.
Diamagnetic Materials
These are materials such as bismuth, antimony, copper, zinc, mercury, gold, and silver. These materials have a relative permeability of less than one.
Electromagnetism
The relationship between magnetism and electrical current was discovered by a Danish scientist named Oersted in 1819. He found that if an electric current was caused to flow through a conductor, the conductor produced a magnetic field around that conductor.
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| Image.4 Electromagnetic |
The polarity of a Single Conductor
A convenient way to determine the relationship between the current flow through a conductor and the direction of the magnetic lines of force around the conductor is the left-hand rule for current-carrying conductors, as illustrated in the image.4 The student should verify that the lefthand rule holds true for the examples shown in image.4
Magnetic Field and Polarity of a Coil
Bending a straight conductor into a loop has two results:
(1) magnetic field lines become denser inside the loop.
(2) all lines inside the loop are aiding in the same direction.
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| Image.5 Coil |
When a conductor is shaped into several loops, it is considered to be a coil. To determine the polarity of a coil, use the left-hand rule for coils in image.5.
Adding an iron core inside of a coil will increase the flux density. The polarity of the iron core will be the same as that of the coil. The current flow is from the negative side of the voltage source, through the coil, and back to the positive side of the source as so in image.6
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| Image.6 Electromagnetic field by Carring Current |
Magneto Motive Force
Magnetomotive force (mmf) is the strength of a magnetic field in a coil of wire. This is dependent on how much current flows in the turns of the coil. the more current, the stronger the magnetic field; the more turns of wire, the more concentrated the lines of force. In the current times, the number of turns of the coil is expressed in units called "ampere-turns" (At), also known
as mmf. The equation is the mathematical representation for ampere-turns (At).
Fm = ampere-turns = NI
where
Fm = magnetomotive force (mmf)
N = number of turns
I = current.
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Summary
- Magnetic flux - a group of magnetic field lines that are emitted outward from the north pole of a magnet.
- Magnetic flux density - the amount of magnetic flux per unit area of a section, perpendicular to the direction of the flux.
- Weber - a measure of magnetic flux.
- Permeability - The ability of a material to concentrate magnetic lines of flux.
- Ferromagnetic materials - iron, steel, nickel, cobalt, and commercial alloys with relative permeability ranging from 50-200.
- Paramagnetic materials - aluminum, platinum, manganese, and chromium with a relative permeability of slightly more than one.
- Diamagnetic materials - bismuth, antimony, copper, zinc, mercury, gold, and silver with a relative permeability of less than one.
- Magnetomotive force (mmf) - the strength of a magnetic field in a coil of wire dependent on current flowing through a coil.
- Ampere turns - current flowing through coil times the number of turns in the coil.
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