Physics for Geologists, Second edition

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80 Electricity and magnetism Figure 8.3 The lines of force in a bar magnet's field. Figure 8.4 The Earth's magnetic field closely resembles that of an axial bar magnet. NP and SP are the geographic poles. The dipole axis does not coincide with the geographic poles. Reproduced from Holmes 1965 Principles ofphysicalgeology, 2nd edn, London: Nelson. Figure 726, p. 988. pole strength, or electric charge and r is the distance between them. The strength or intensity of a magnetic field (H) in any position is defined as the force exerted on a unit north pole at that point. The unit of magnetic field strength used to be the oersted (oe), after the Danish physicist Hans Mrsted, Copyright 2002 by Richard E. Chapman
Electricity and magnetism 8 1 defined as the strength of the field at a point where a unit pole is repelled with a force of 1 dyne. The SI unit is the amperelmetre (A m-I), which equals 12.566 x lop3 oersteds. The origin of magnetism lies in electric currents, that is, moving electric charges, as first shown by Ampkre. In magnetic materials, the currents are due to electrons and protons bound to the atoms. The Earth's magnetism is more complicated. For treatment at greater depth, see Bleany (1984) and Feynman et al. (1963). Terrestrial magnetism The Earth has a magnetic field that is similar to that which would be gener- ated by an axial bar magnet near its centre. This has been known for about 400 years. We can measure the Earth's magnetic field with great precision. If we suspend a magnetized needle so that it can freely orientate itself in the magnetic field, we find that it is, in general, not horizontal but has a dip or inclination below the horizontal towards the nearer pole and a declination or variation from geographic north (Figure 8.5). The force can be resolved into three components: x, horizontal in the direction of geographic North; y, horizontal towards geographic East; and z, vertical. The force is vertical near the poles, horizontal near the Equator, but we find that the magnetic poles are not at the geographic poles. The South Magnetic Pole in 2000 was in Latitude 64.63"S, Longi- tude 138.36"E: the North Magnetic Pole, in Latitude 80.68"N, Lon- gitude 109.12"W - so they were not even diametrically opposite each other, but separated from the projected pole by 1400 nautical miles, or 2600 km. However, the geomagnetic poles are the poles of what is called the centred geomagnetic dipole, which is the dipole that gives the observed field most closely. In 2000, the geomagnetic poles were in Copyright 2002 by Richard E. Chapman Figure 8.5 Magnetic declination or variation (d) and inclination or dip (i).
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Physics for Geologists Second editi
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Copyright 2002 by Richard E. Chapma
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... vlii Contents Polarization 47 L
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Figures Copyright 2002 by Richard E
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Tables Copyright 2002 by Richard E.
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xiv Preface waves diminishes with d
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xvi Preface provides. This book is
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xviii Acknowledgements to K. Whaler
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xx Symbols mass refractive index bu
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2 Basic concepts Table 1.1 Dimensio
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4 Basic concepts Table 1.2 Basic SI
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6 Basic concepts pascal (Pa) and si
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8 Basic concepts all the relevant d
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10 Basic concepts relating them by
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2 Force The dimensions of force are
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14 Force The ratio of the Moon's ac
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16 Force Figure 2.1 The sum of the
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18 Force When a motorcycle takes a
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20 Force Figure 2.3 Torque is the p
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22 Force How does momentum differ f
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24 Force the weights - that is, the
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26 Force Figure 2.6 Wall-sticking i
Page 45 and 46: 3 Gravity Universal gravity When di
Page 47 and 48: 30 Gravity are significant to geolo
Page 49 and 50: 32 Gravity Figure 3.2 The tractive,
Page 51 and 52: 34 Gravity Mean sea level It might
Page 53 and 54: 36 Gravity The benefits of space fl
Page 55 and 56: 38 Gravity Figure 3.6 Profile of th
Page 57 and 58: 40 Gravity Figure 3.8 The siphon at
Page 59 and 60: 4 Optics Light is an electromagneti
Page 61 and 62: 44 Optics Figure 4.2 Refraction. (a
Page 63 and 64: 46 Optics The coeficient of reflect
Page 65 and 66: 48 Optics only the light oscillatin
Page 67 and 68: 50 Optics called Iceland Spar. Two
Page 69 and 70: 52 Optics Diffraction If you shine
Page 71 and 72: 54 Optics Figure 4.7 Geometrical op
Page 73 and 74: 56 Optics Figure 4.9 The mirrors in
Page 75 and 76: 5 Atomic structure and age-dating P
Page 77 and 78: 60 Atomic structure and age-dating
Page 83 and 84: 66 Electromagnetic radiation radiat
Page 85 and 86: 68 Electromagnetic radiation Source
Page 87 and 88: Heat and heat flow The noun heat ha
Page 89 and 90: 72 Heat and heat flow Second Law: A
Page 91 and 92: 8 Electricity and magnetism Electri
Page 93 and 94: Electricity and magnetism 77 Figure
Page 95: Electricity and magnetism 79 rivett
Page 99 and 100: Electricity and magnetism 83 is kno
Page 101 and 102: 9 Stress and strain Pressure, p, is
Page 103 and 104: Stress and strain 87 than some natu
Page 105 and 106: These three elastic constants are r
Page 107 and 108: Stress and strain 91 Figure 9.1 New
Page 109 and 110: Stress and strain 93 Figure 9.2 Com
Page 111 and 112: Stress and strain 95 Figure 9.3 Ide
Page 113 and 114: Fracture Stress and strain 97 We fo
Page 115 and 116: Stress and strain 99 from which we
Page 117 and 118: 10 Sea waves The nature of water wa
Page 119 and 120: Sea waves 103 Figure 10.1 Wave moti
Page 121 and 122: Sea waves 105 refraction by islands
Page 123 and 124: Acoustics: sound and other waves 10
Page 125 and 126: Acoustics: sound and other waves 10
Page 127 and 128: 12 Fluids and fluid flow Introducti
Page 129 and 130: Fluids and fluid flow 1 13 Figure 1
Page 131 and 132: Fluids and fluid flow 115 Figure 22
Page 133 and 134: Fluids and fluid flow 117 and the f
Page 135 and 136: water above a vertical thickness h
Page 137 and 138: Fluids and fluid flow 121 where V i
Page 139 and 140: Fluids and fluid flow 123 ardentes,
Page 141 and 142: Fluids and fluid flow 125 Figure 12
Page 143 and 144: - 14 $ 12 E 10 K .- 3 8 r cll + 6 0
Page 145 and 146: Fluids and fluid flow 129 the liqui
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Some dangers of mathematical statis
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Appendix 139 What would happen if y
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Notes 141 and since x is very large
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References Allum, J. A. E. (1966) P
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References 145 -(1950) 'Mechanism o
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Further reading 147 Trigg, G. L. an
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Physics for Geologists, Second edition

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