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 The science of magnetism is old. It has been known since ancient times that magnetic materials tend to point in the north-south direction: like magnetic poles repel and unlike ones attract; and cutting a bar magnet in two leads to two smaller magnets. Magnetic poles cannot be isolated. |
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When a bar magnet dipole moment m is placed in a uniform magnetic field B. |
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| (a) the force on it is zero, |
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| (b) the torque on it is m x B. |
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| (c) its potential energy is -m.B. where we choose the zero of energy at the orientation when m is perpendicular to B. |
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Consider a bar magnet of size l and magnetic moment m. At a distance r from its mid-point, where r >> l, the magnetic field B due to this bar is |
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Gauss's law for magnetism states that the net magnetic flux through any closed surface is zero. |
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The earth's magnetic field may be approximated by a dipole with moment 8.0 x 1022 A m-1. This dipole is aligned making a small angle with the rotation axis of the earth. Its magnetic north pole Nm is near the geographic south pole Sg and its magnetic south pole Sm is near the geographic north pole Ng. The magnitude of the field on the earth's |
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Three quantities are needed to specify the magnetic field of the earth on its surface - the horizontal component, the magnetic declination, and the magnetic dip. Please refer to the text for precise definitions of declination and dip. |
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Consider a material placed in an external magnetic field B0. The magnetic intensity is defined as, |
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| The magnetisation M of the material is its dipole moment per unit volume. The magnetic field B in the material is |
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| B = m0 (H+M) |
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For a linear material M = cH. So that B = mH and c is called the magnetic susceptibility of the material. The three quantities, c, the relative magnetic permeability mr, and the magnetic permeability are related as follows: |
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| m= m0 mr |
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| m r = 1+x |
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Magnetic materials are broadly classified as: diamagnetic, paramagnetic, and ferromagnetic. For diamagnetic materials c is negative and generally very small, for paramagnetic c is positive and small. Ferromagnetic materials have large c and are similarly characterised by non - linear relation between B and H. They show the property, of hysteresis as explained in the text. |
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