MAGNETISM & ELECTROMAGNETISM 3.1.1 MAGNETIC Magnetism & Electromagnetism htpp:// آµ = For vacuum Xm

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  • 3 Magnetism & Electromagnetism htpp://www.allonlinefree.com

    MAGNETISM & ELECTROMAGNETISM

    3.1.1 MAGNETIC PERMEABILITY Consider the relation

    β = µ (H + M) = µ0 (H + Xm H) = µ0 (1 + Xm) H = µ H

    Where µ = µ0 (1 + Xm), is called the magnetic permeability of the material.

    ∴ β = µ H

    Magnetic permeability (µ) of a medium is defined as the ratio of magnetic induction to the intensity of the magnetising field.

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    µ = For vacuum Xm = 0 and µ = µ0

    Hence magnetic induction in vacuum is β0 = µ0 H

    The ratio =

    is called the relative permeability µ0 Obviously µr = 1 + Xm

    We may also classify magnetic materials in terms of the relative permeability µr

    Diamagnetism : µr < 1 (For βi, µr = 1 - 0.00017) Paramagnetism : µr > 1 (For Al, µr = 1 + 0.00002) Ferromagnetsim : µr >> 1 (For pure Fe, µr = 1 = 200,000)

    Magnetic Flux φ The magnetic flux φ through a surface S is defined as

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    φ = ∫s = B.dS

    It represents the lines of induction crossing surface S. The SI unit of magnetic flux is a weber (Wb) If B is uniform and normal to area A, φ = BA If a = 1m2, φ = B i.e., magnetic induction is numerically equal to normal flux per unit area. Therefore, it is also called magnetic flux density.

    Magnetic Flux Unit of Magnetic Flux density is Tesla. Tesla id the density of 1 Wb of magnetic flux per metre square.

    3.1.2 What are the properties of the lines of force, also state the care and maintenance of the permanent magnet?

    Ans. Following are the properties of the magnetic lines of force:

    1 All the lines of force starts from the north pole and terminates to south pole outside the magnet and from south to north inside the magnet.

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    2. There is no perfect insulator for the lines of force. 3. All the lines of force try to follow the shortest path and exhibit the elastic nature. 4. The lines of force do not cross each other. 5. It produces the effect of magnetic shielding if a

    closed magnetic ring is placed between two poles the maximum lines of force will complete its path through the ring and a minimum through the air.

    6. The magnetic lines of force produces the induction effect. As the soft iron bar magnetises when the permanent magnet is brought near it.

    Care and maintenance: If the molecules of the permanent magnet are dc-arranged then the magnet is htpp://www.allonlinefree.com

    3.1.2

    Bar Magnet Solt Iron piece

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    demagnetised so the following precautions should be taken: (i) It should he used properly and should not be thrown. (ii) It should not be heated up. (iii) It should not be hammered (iv) If a pair of magnets is kept, the keepers should

    be used and magnets should be kept facing the opposite polarity.

    3.1.3 TYPE OF MAGNET

    Q What are the different types of magnets? Ans. These are the following types of the magnet. a) Natural Magnet b) Artificial magnet

    (a) Natural Magnet: A substance is found in the nature which exhibits the property of attracting the iron and its fillings, simultaneously the property of directing north and south. It is in the shape of a stone. In the ancient era it was used by the navigator for direction purposes. It is known as leading stone or Iode stone.

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    (b) Artificial magnet: The magnets which are prepared by the human beings by artificial means like touch method etc. are known as artificial magnets. These are prepared by iron or steel.

    The artificial magnets are of the following types: (i) Temporary magnet (ii) Permanent magnet. (i) Temporary magnets:

    The magnets which loses their magnetism as soon as the magnetising means or force is removed. All the electro magnets arc the temporary magnets. The metal used in this case is the soft iron. The temporary magnets are used in electric bell, buzzers, bell indicator, etc.

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    3.1.3

    Natural magnet

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    (ii) Permanent magnets. The permanent magnets are those which retain their magnetism for a long time. It is observed that if a piece of hard steel is magnetised, it acquire a substantial magnetism which it retains for an indefinite time. Such magnets are permanent magnets. The material used for permanent magnets are tungsten steel, high carbon steel and other hardened steel etc.

    Shapes:

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    3.1.4

    Temporary magnet

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    There are the different shapes like bar magnet, horse shoe magnet U-shape magnet and magnetic needle.

    3.1.4 MAGNETIC PROPERTIES OF MATERIAL

    Now in this section sonic discussions are necessary on magnetic properties of matter. Infact materials can be classified generally as (1) Diamagnetic, (2) Paramagnetic and (3) Ferromagnetic. However magnetic properties of most materials differ so little from the magnetic properties of the free space that they are of no

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    3.1.5

    Shape of permanent magnets

    Dar Magnet

    Horse Shoe Magnet

    U-Shape Magnet

    Magnetic Needle

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    practical use in the construction of magnetic circuits.

    (1) Diamagnetic Material. These materials hence permeabilities slightly less than the permeability of free space. Materials like silver, copper and hydrogen for example are Diamagnetic.

    (2) Paramagnetic Material. These materials have permeability is slightly greater than that of free- space Materials like Platinum, Aluminum and Oxygen are paramagnetic.

    (3) Ferromagnetic Materials. These materials have permeabilities much greater than space. Principal ferromagnetic material is IRON and various steel. But Cobalt and Nickel are also ferromagnetic and various alloys consisting of these elemental materials are used for special purposes as explained above. Theory of Diamagnetic, Paramagnetic Ferromagnetic Material. Now a brief discussion will be made to explain the theory of the three type of Magnetic Materials just stated above. Infact the magnetic properties of materials can be explained by

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    supposing that there exit within the atom themselves the equivalent of minute circuits in which currents flow. However in addition to the magnetic effects of the electrons moving in their orbits about the nucleus of the atom, which will be equivalent of currents, the electrons themselves are believed to spin about their own axes, and thereby also to produce magnetic effects then the magnetic behaviour of any atom of any particular element is determined by the combined effect, of al