Magnetism

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Magnetism is the property of a material that enables it to attract pieces of iron.

THE NATURE OF MAGNETISM

Magnetic Definitions

Permeability refers to the ease with which a material can pass magnetic lines of force.

Reluctance is the opposition that a material offers to the magnetic lines of force.

Temporary magnets easily lose most of their magnetic strength after the magnetizing force is removed.

Permanent magnets are made from substances such as hardened steel and certain alloys that retain a great deal of their magnetism.

Weber’s Theory

A magnetic substances are composed of tiny molecular magnets, each with a north and south pole. The two poles form a dipole. A magnetized material has most of its molecular magnets lined up.

If a magnet is split in have it becomes two magnets each with a north and south pole. The poles are in the same orientation as the original magnet.

Domain Theory

Electrons orbit the nucleus of an atom. They also spin, revolving around an axis, as they orbit.

The polarity of magnetic field is determined by the direction of electron spin. An atom is magnetized when it has more electrons spinning in one direction than in the other.

Atoms do not act independently but are bound in groups called domains.

When materials are heated they lose there magnetic characteristics. The Curie point is the temperature that a material becomes nonmagnetic.

How magnets are produced

An unmagnetized material can become magnetized by insertion into the coils of a temporary magnet known as an electromagnet.

Another way to produce magnetism is to stroke an unmagnetized material with another material which is already magnetized.

MAGNETIC FIELDS

The space surrounding a magnet where magnetic forces act is known as a magnetic field. The pattern can be seen by using iron filings.

Lines of Force

Lines of force are imaginary lines used to illustrate the pattern of the magnetic field. They are also called flux lines.

The lines emanate from the north pole and enter the south pole of a magnet

The Maxwell

One Maxwell is equal to one magnetic flux line

The Weber

The weber represents 1 times 10 raised to the 8^th Maxwells.

A microweber is 100 Maxwell

1. Magnetic lies of force are continuous and always form a closed loop.
2. Magnetic lines of force never cross each other.
3. Parallel magnetic lines of force traveling in the same direction repel one another. If traveling in the opposite directions they tend to unite with each other and form into single lines traveling in a direction determined by the magnetic poles creating the lines of force.
4. Magnetic lines of force tend to shorten themselves.
5. Magnetic lines of force pass through all materials
6. Magnetic lines of force always enter or leave magnetic material at right angles to the surface.

 Magnetic Attraction and Repulsion

1. Like magnetic poles repel each other
2. Unlike magnetic poles attract each other
3. The attraction or repulsion between magnets varies directly with the product of their strength and inversely with the square of the distance between them.

CLASSIFICATION OF MAGNETIC MATERIALS

Ferrognetism

Iron, cobalt, nickel (and various alloys) are called ferromagnetic materials. Ferromagnetic materials show a permanent, spontaneous magnetizing effect in the absence of a magnetizing field.

This effect is greatest at absolute zero. It is reduced by temperature.

Ferromagnetic materials with high retentivities, are called hard, and are used for permanent magnets.

Paramagnetism

Paramagnetic materials include aluminum, platinum, manganese, and chromium. They have mild attraction to a magnetic field.

Diamagnetism

Diamagnetic materials include bismuth, antimony, copper, zinc, mercury, gold, and silver. They can become weakly magnetized, buth their magnetic field is in the opposite direction from the magnetizing field.

TYPES OF MAGNETS

Permanent Magnets

A permanent magnet retains its magnetism (high retentivity) even when the magnetizing force is removed.

Loss of magnetism may occur through high temperatures, physical shock, or a strong demagnetizing force.

Application for Permanent Magnets

Loudspeakers are a common use for permanent magnets.

Temporary Magnets

A temporary magnet is a piece of soft iron that is magnetized while in the presence of a magnetizing field but demagnetizes the moment the iron is taken away from the magnetizing force. Soft iron possesses low retentivity.

Electromagnets

A coil produces an electromagnet field around a core. Soft iron is often used for the core to concentrate the flux lines and create a stronger magnetic field.

Applications for Electromagnets

Electromagnets are used for lifting objects, buzzers, bells, relays.

Electromagnets are used for tape recording. Flux in an air gap creates a fringing field that extends from the core. This magnetizes the tape passing by the magnet.

This is analog.

Disk drives are similar but use binary, ones and zeros to represent data.

Ferrites

Ferrites are nonmetallic materials that exhibit the ferromagnetic properties of iron.

Applications for Ferrites

Ferrites are used as magnetic cores. Eddy currents cannot form because the core is nonmetallic.

Ferrites also come in the form of small toroid (doughnut shaped beads) that block high frequency currents from passing through. This is known as filtering.

MAGNETIC SHIELDING

There is no known insulator for magnetic flux. Flux penetrates nonmagnetic materials.

If a magnetic material is placed in a magnetic field, the flux is redirected. 

MAGNETIC QUANTITIES

Magnetomotive Force (mmf)

Magnetomotive force is the magnetic pressure that produces flux. This is analogous to voltage in an electric circuit.

Reluctance

Reluctance is the opposition that a material offers to the magnetic lines of force. This is equivalent to resistance in an electric circuit.

Flux Density

Flux density is equal to the number of magnetic lines of flux per square meter.

Field Intensity

Field intensity is a measurement of the mmf needed to establish a specified flux density in a unit length of the coil.

Permeability

Permeability is the ease with which a material passes flux lines. It is the electrical equivalent of conductance.

Air and other nonmagnetic materials are assigned a permeability of 1. Magnetic materials have a value much greater than 1.

THE B-H HYSTERESIS CURVE

The two most important quantities are flux density (B) and the magnetizing force (H). Their relationship can be graphed as the B-H hysteresis curve.

With iron B and H are directly proportional. The induced magnetization eventually saturates when further magnetizing force does not produce an increase in flux density. This is when all magnetic dipoles have become aligned.

THE HALL EFFECT

The Hall effect was discovered by E. H. Hall in 1879.  When a magnetic field is brought close to a gold strip carrying current a voltage is produced. The Hall effect sensor is used to measure flux density (B). The gaussmeter uses the Hall effect.

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