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Introduction |
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The first magnetic phenomenon observed were those associated with naturally occurring magnets, fragments of iron ore found near the ancient city of Magnesia. These attracted unmagnetised iron. The attraction was maximum at certain regions of the magnet called the poles. |
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The Bar Magnet |
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The compass needle always lies along the direction of the field. The figure below shows the lines or pattern of the field, when the compass needle is placed at several places. These lines do not really, tell us the effect that magnet has on the other. |
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Magnetism and Gauss's Law |
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Gauss's theorem in electrostatics indicates that isolated charges exist and that electric lines of force do not form closed loops. The situation is different in magnetism. |
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The Earth's Magnetism |
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A magnetic compass was used to help the sailors for navigational purpose. But recently it has been discovered that some migrant birds have magnetic sensors in their heads, which help to guide them using the Earth's magnetic field. |
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Magnetization and Magnetic Intensity |
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The ultimate source of magnetism is the magnetic dipole moment, associated with an atom due to orbital motion and intrinsic spin. This suggests that all substances possess magnetic property as energy material consists of atoms having electrons revolving around the nucleus. |
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Magnetic Properties of Material |
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Michael Faraday discovered that a specimen of bismuth was repelled by a strong magnet. Diamagnetism occurs in all materials. These materials are those in which individual atoms do not possess any net magnetic moment. |
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Permanent Magnets and Electromagnets |
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Materials for making permanent magnet should possess high residual magnetism i.e., when the magnetising field is reduced to zero, the intensity of magnetisation is high. |
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Summary |
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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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Numerical - 01 |
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A short bar magnet placed with its axis at 300 with a uniform external magnetic field of 0.16 T experiences a torque of magnitude 0.032 J. |
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Numerical - 02 |
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A bar magnet of magnetic moment 1.5 lies aligned with the direction of a uniform magnetic field of 0.22 tesla. |
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Numerical - 03 |
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The radius of the coil of a tangent galvanometer is 0.16 m. How many turns of the wire should be wound on it if a current of 40 mA is to produce a deflection of 45o? Given: horizontal component of Earth's field is 0.36 x 10-4T. |
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Numerical - 04 |
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A circular coil of 16 turns and radius 10 cm, carrying a current of 0.75 A rests with its plane normal to an external field of magnitude 5.0 x 10-2 T. |
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Numerical - 05 |
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A telephone cable at a place has four long straight horizontal wires carrying a current of 1 ampere in the same direction east to west. The Earth's magnetic field at the place is 0.39 gauss, and the angle of dip is 35o. The magnetic declination is nearly zero. |
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Numerical - 06 |
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The core of a toroid having 3000 turns has inner and outer radii of 11 cm and 12 cm respectively. The magnetic field in the core for a current of 0.70 A is 2.5 T. |
