Physics for Geologists, Second edition

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78 Electricity and magnetism Figure 8.2 The field between two electrodes in a homogeneous, isotropic medium. Lines and surfaces of equal energy, called equipotential lines/surfaces, are normal to the flow paths. (This is a map of part of a field, not of a block.) energy or potential by virtue of the elevation to which water would rise unassisted in the boreholes (the static water level). A contour map of the elevation of the water surface in boreholes drilled to the same aquifer is a map of the energy of the aquifer, as we saw in Figure 3.7. The contours are lines of equal energy (equipotential lines) and flow lines are normal to them. The surface contoured is a notional surface called the potentiometric surface. Magnetism Magnetism is one of the fundamental forces of the universe. The mineral magnetite, FeO . Fe203, commonly with titanium oxide, Ti02, has been known for more than 2500 years for its properties of attracting iron and orientating itself in the Earth's magnetic field. A bar magnet has a magnetic axis, the ends of which are called poles. The end that seeks the geographical North when freely suspended is called the North pole or North-seeking pole (or +); the other is the South or South-seeking pole (or -). We can magnetize bars of ferromagnetic metals (iron, nickel, cobalt and some of their alloys) by putting them in a magnetic field; by cooling them when aligned N-S; by stroking them with another magnet (the new pole at the beginning of the stroke is the same as the stroking pole); and by hammer- ing them when oriented in a magnetic field. This last is the reason why steel ships and boats become magnetized during their construction, particularly Copyright 2002 by Richard E. Chapman
Electricity and magnetism 79 rivetted ships.l All substances are magnetizable to some degree, but few are usefully magnetizable. The ferromagnetic and most other materials are paramagnetic; that is, they become magnetized with their direction of magne- tization in the same direction as the magnetizing field. A few are diamagnetic (bismuth, for example) and acquire a different magnetism so that when suspended in the terrestrial magnetic field become aligned roughly East-West rather than North-South. If you cut a bar magnet in half, you have two bar magnets, each with a N and a S pole. Cut these in half and you have four magnets, and so on. It is therefore inferred that magnetized metal is magnetized on a molecular scale. Permanent magnets, as the name implies, retain their magnetism for a considerable time, but all magnets lose some of their magnetism with time. The magnetism remaining after the removal of the exciting source is called remanent magnetism. Materials lose their permanent magnetic properties when heated above a temperature known as the Curie point. At atmospheric pressure, the Curie point of nickel is 330°C; iron, 770°C; and magnetite, 580°C. As pressure increases, the Curie point falls. Of the minerals, only magnetite and, to some extent, pyrrhotite are affected by a bar magnet, but many are affected by an electromagnet. The various magnetic properties of minerals allow their separation using an electromagnet. Place a piece of thin glass on a bar magnet, and scatter iron filings on the glass. The filings orientate themselves in a pattern, showing the field (Figures 8.3 and 8.4). Drop the magnet into iron filings, and most attach themselves to the poles of the magnet, few near the centre. Coulomb defined a unit magnetic pole as one that repels an identical pole placed 1 cm away in a vacuum with a force of one dyne (10 kN); and found that a pole of strength p repels a like pole of strength p' at a distance of r cm with a force proportional to pp'/r2: PP' Fo = ko-, r2 where Fo is in newtons, p in Am, r in m, and ko in N The proportionality constant ko = p0/4n = 100 x IO-~NA-~ where KO is the magnetic permeability of free space (also in units of NA-2). It will be noticed that gravity, magnetism and electricity all have similar laws, with the force of attraction proportional to ~1 k2/r2, where IJ, is a quantity of mass, magnetic 1 A property that was made use of first by the Germans in the Second World War, with a mine that was actuated by the ship's magnetic field. The reply was to remove the magnetism of the ship with electric cables round the ship with which the opposite field was generated - degaussing. The mines were swept by towing cables from which a strong magnetic field was generated. Copyright 2002 by Richard E. Chapman
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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
Page 43 and 44: 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: Electricity and magnetism 77 Figure
Page 97 and 98: Electricity and magnetism 8 1 defin
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
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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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