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CYCLEOF MAGNETISATION & HYSTERESIS LOSS

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TOPICS TO BE DISCUSSED MAGNETIC INDUCTION(B) MAGNETISING FIELD(H) INTENSITY OF MAGNETISATION(I) CYCLE OF MAGNETISATION ENERGY DISSIPATION DUE TO HYSTERESIS CALCULATION OF HYSTERESIS LOSS IMPORTANCE OF HYSTERESIS CURVES

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INTRODUCTION: The proper selection of a magnetic material for various purposes involves the knowledge of their magnetic properties. Magnetic permeability and magnetic susceptibility play important role in determining the properties of the substance. Both of these are determined from graphical curves obtained between B and H and between I and H resp. These graphs called magnetization curves are obtained experimentally.

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Magnetic induction or Magnetic flux density(B) The number of lines of induction crossing per unit area normally through the magnetic material is measure of magnetic induction. Its SI unit is tesla(T) or weber/meter 2 (wbm -2 )

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MAGNETISING FIELD(H): It is that magnetic field which produces magnetism in a magnetic substance placed inside it. Its strength is known as magnetic intensity. It is defined as the force experienced by a unit north pole placed at a point. Its units are ampere/meter (A/m) or J/Tm 3 or N/Tm 2

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Intensity of magnetization(I) It is measure of degree of magnetization of a specimen. Quantitatively, it is defined as the magnetic moment developed per unit volume of the specimen when placed in a magnetising field. Its SI units are A/m or N/Wb.

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CYCLE OF MAGNETISATION 1 ) Retentivity: The magnetisation OF left in the specimen when the magnetising field H is reduced to zero. 2) Coercivity: The magnetic intensity H(OG) required to reduce the magnetic flux density B to zero. 3) Hysteresis: The lagging of magnetic flux density B (or intensity of magnetization I ) behind the magnetizing field (magnetic intensity)H. 4)Hysteresis loop: It is a closed B-H curve (EFGIJKE)or I-H curve.

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Energy dissipation due to hysteresis When the magnetizing field is applied on an unmagnetised material, the molecular magnets align themselves parallel to the field. Work is done by the magnetizing field against the mutual attractive forces. When the field is switched off, the material retains some magnetization. So the energy supplied to the material during magnetization is not fully recovered on removing the magnetic intensity. Hence negative energy has to be applied to bring I to zero. The balance energy retained in the material is converted into heat and is finally lost to the surroundings. It is known as hysteresis loss. It is equal to the area of B-H loop.

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CALCULATION OF HYSTERESIS LOSS To determine the hysteresis loss, consider an iron bar over which a wire is wound having n turns. When the current in the wire changes, the magnetic flux linked with it also changes and an induced e.m.f. is produced in the wire which according to Lenz’s law opposes the change in current. The induced e.m.f. is given by e=-nd φ/dt [-ve sign shows that =-nd(Ba)/dt direction of induced e.m.f. =-nadB/dt opposes the cause which produces it. The power consumed is given by P=ei =na(dB/dt)I

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But i=Hl/n Therefore ei = al H dB /dt Energy spent in time dt is given by dW= alHdB Therefore,net work done for a complete cycle of magnetisation is then given by W = al ∫HdB But HdB = area of B-H loop Hence work done per cycle of magnetisation =al*area of B-H loop Work done per unit volume per cycle of magnetisation=area of B-H loop. This loss of energy is dissipated in the form of heat and is known as hysteresis loss.

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The study of hysteresis curves gives the information about following properties of a magnetic substance. ( i)Susceptibility( χ ): It is measure of the ease with which a substance can be magnetised. χ =I/H Its value depends only on nature of material. It has no units. It is greater for soft materials than for hard materials.

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(iii) Retentivity : It is greater for soft materials than for hard materials. (iv) Coercivity: It is less for soft materials than for hard materials. (v) hysteresis loss: It is less for soft materials than for hard materials. (ii)Permeability( µ ) It is the degree to which the magnetic lines of force can penetrate a material. µ=B/H Its SI units are Tm/A or NA It is greater for soft materials than for hard materials.

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Importance of hysteresis curves : Knowing susceptibility, permeability and hysteresis loss for different magnetic materials their suitable selection can be made for practical and industrial applications e.g. for making electromagnets for making permanent magnets for making transformer cores

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