CHAPTER ONE MAGNETS AND MAGNETIC FIELDS Chapter Permanent magnetic material is an object made from a

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    CHAPTER ONE

    MAGNETS AND MAGNETIC FIELDS

    Chapter outlines

    1.1 Molecular theory

    1.1.1Domains and Domains walls

    1.1.2 Suitability criteria

    1.1.3 Coercivity mechanisms

    1.2 Magnetic materials

    1.2.1 Permanent magnets

    1.2.2 Electromagnets

    1.3 Magnetic storage

    1.4 Magnetism and super conductivity

    1.5 Magnetic shielding

    1.1 MOLECULAR THEORY

    The first accounts of magnetism date back to the ancient Greeks who also gave magnetism its

    name. It derives from Magnesia, a Greek town and province in Asia Minor, the etymological

    origin of the word “magnet” meaning “the stone from Magnesia.” This stone consisted of

    magnetite (Fe3O4) and it was known that a piece of iron would become magnetized when rubbed

    with it. More serious efforts to use the power hidden in magnetic materials were made only much

    later. For instance, in the 18th century smaller pieces of magnetic materials were combined into a

    larger magnet body that was found to have quite a substantial lifting power. Progress in

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    magnetism was made after Oersted discovered in 1820 that a magnetic field could be generated

    with an electric current. Sturgeon successfully used this knowledge to produce the first

    electromagnet in 1825. Although many famous scientists tackled the phenomenon of magnetism

    from the theoretical side (Gauss, Maxwell, and Faraday) it is mainly 20th century physicists who

    must take the credit for giving a proper description of magnetic materials and for laying the

    foundations of modem technology

    Theory

    A popular theory of magnetism considers the molecular alignment of the material. This is called

    webber‟s theory; this theory assumes that all magnetic substances are composed of tiny

    molecular magnets. Any unmagnetised material has the magnetic forces of its molecular magnets

    neutralized by adjacent molecular magnets, thereby eliminating any magnetic effect. A

    magnetized material will have most of its molecular magnets lined up so that the north pole of

    each molecular magnet points in one direction and the South Pole faces in the opposite direction,

    thus a material with its molecules aligned will then have one effective North Pole and one

    effective North Pole in the opposite direction

    Figure 1.1: webber‟s theory (magnetizing by stroking)

    Domain theory

    A more modern theory of magnetism is based on the electron spin principle. From the study of

    atomic structure it is known that all matter is composed of vast quantities of atoms, each atom

    containing one or more orbital electrons. The electrons are considered to orbit in various shells

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    and subshells depending upon their distance from the nucleus. The structure of the atom has

    previously been compared to the solar system, wherein the electrons orbiting the nucleus

    correspond to the planets orbiting the sun. Along with its orbital motion about the sun, each

    planet also revolves on its axis. It is believed that the electron also revolves on its axis as it orbits

    the nucleus of an atom. It has been experimentally proven that an electron has a magnetic field

    about it along with an electric field. The effectiveness of the magnetic field of an atom is

    determined by the number of electrons spinning in each direction. If an atom has equal numbers

    of electrons spinning in opposite directions, the magnetic fields surrounding the electrons cancel

    one another, and the atom is unmagnetised. However, if more electrons spin in one direction than

    another, the atom is magnetized. An atom with an atomic number of 26, such as iron, has 26

    protons in the nucleus and 26 revolving electrons orbiting its nucleus. If 13 electrons are

    spinning in a clockwise direction and 13 electrons are spinning in a counterclockwise direction,

    the opposing magnetic fields will be neutralized. When more than 13 electrons spin in either

    direction, the atom is magnetized. An example of a magnetized atom of iron is shown in figure

    below.

    Why poles are called north and south

    A magnet suspended so that it can rotate freely horizontally will eventually settle down with one

    pole facing north and the other south.

    One pole is therefore called the „north seeking pole‟, usually shortened to just „north pole‟, and

    the other pole is called the „South seeking pole‟, usually shortened to just „South pole‟. The

    magnet has been orientated by the Earth‟s magnetic field.

    Figure 1.2: Finding the North and South Pole of a magnet

    **A compass is an application of this effect.

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    Law of magnets: Like poles repel unlike poles attract

    A magnetic pole: is a region where the magnetic force is greatest

    A magnetic field: is a region where the magnetic force is exerted

    THE EARTH’S MAGNETIC FIELD

    The earth‟s magnetic field is similar in shape to that around a bar magnet. It is thought to be

    caused by electric currents flowing through the molten outer core of the Earth. At the present the

    field pattern is like that with a magnetic SOUTH pole situated somewhere below northern

    Greenland

    Figure 1.3: Earth‟s magnetic field and field lines of force

    1.1.1 DOMAINS AND DOMAIN WALLS

    A magnetic domain is a region within a magnetic material which has uniform magnetization.

    This means that the individual magnetic moments of the atoms are aligned with one another and

    they point in the same direction, when cooled below a temperature called the Curie temperature

    the magnetization of a ferromagnetic material divides into many small regions called magnetic

    domains.

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    Magnetic moment is thus the quantity that determines the torque it will experience in an external

    magnetic field. Magnetic moment is a vector having both magnitude and direction

    Curie temperature (Curie point) is the critical point where a material‟s intrinsic magnetic

    moments change direction. Magnetic moments are permanents dipole moments within the atom

    which originates from electron‟s angular momentum and spin.

    In magnetism, a domain wall is an interface separating magnetic domains. It is a transition

    between different magnetic moments and usually undergoes an angular displacement of 90 or

    180 degrees. The energy of a domain wall is simply the difference between the magnetic

    moments before and after domain wall was created, this value is usually expressed as energy per

    unit wall area.

    Figure 1.4: magnetised atom

    1.1.2 Suitability criteria

    To consider how suitable magnetic material is able to allow and sustain the formation of

    magnetic field within it we shall consider a magnetic property called magnetic permeability

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    Magnetic permeability (µ)

    In electromagnetism, permeability is the measure of the ability of a material to support the

    formation of a magnetic field within itself. In other words, it is the degree of magnetization that a

    material obtains in response to an applied magnetic field. Magnetic permeability is typically

    represented by the Greek letter μ. The term was coined in September 1885 by Oliver Heaviside.

    The reciprocal of magnetic permeability is magnetic reluctivity.

    In SI units, permeability is measured in henries per meter (H·m −1

    ), or newtons per

    ampere squared (N·A −2

    ). The permeability constant (μ0), also known as the magnetic constant or

    the permeability of free space, is a measure of the amount of resistance encountered when

    forming a magnetic field in a classical vacuum. The magnetic constant has the exact (defined)

    value µ0 = 4π×10 −7

    H·m −1

    ≈ 1.2566370614…×10 −6

    H·m −1

    or N·A −2

    ).

    A closely related property of materials is magnetic susceptibility, which is a measure of the

    magnetization of a material in addition to the magnetization of the space occupied by the

    material.

    Figure 1.5: Simplified comparison of permiabilities for ferromagnetic, paramagnetic, free space

    and diamagnetic

    1.1.3 Coercivity mechanisms

    In material science, Coercivity (coercive field or coercive force) is a measure of a ferromagnetic

    of ferroelectric material to withstand an external magnetic or electric field.

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    For ferromagnetic material the coercivity is the intensity of the applied magnetic field required to

    reduce the magnetisation of that material to zero after the magnetisation of the sample has been

    driven to saturation.

    Thus coercivity measures the resistance of a ferromagnetic material to becoming demagnetized;

    It is measured using a B-H Analyzer or magnetometer.

    Ferromagnetic materials with high coercivity are called magnetically hard materials and are used

    to make permanent magnets. Permanent magnets find application in electric motors, magnetic

    recording media (e.g. hard drives, floppy disks and magnetic tape) and magnetic separation.

    Material with low coercivity are said to be mag

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