Physics for Geologists, Second edition

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Heat and heat flow 71 heat and the process is called conduction. If you watch a saucepan of water when it is nearly boiling, you see currents in it tending to bring the hot water from the bottom to the top. This is convected heat, and the process is called convection. Convection is essentially gravitational because warm water is less dense than cold water: it rises and displaces the cooler water above, which sinks and is duly heated, to rise again. (Note: once steam bubbles form, the process changes because friction around the rising steam bubbles generates a sympathetic flow.) Radiated heat consists of packets of electromagnetic energy that are converted to heat in the substance absorb- ing the radiation. Heat is conducted through materials at a molecular level. The kinetic theory asserts that molecules in solids, liquids and gases are in constant motion and behave as perfectly elastic particles with their mean kinetic energy proportional to their temperature. The greater energy of the hotter molecules is transferred to neighbouring molecules, and so spreads throughout the conductive material. It is a process of diffusion. Heat can be generated by various processes apart from the electrical. Fric- tion generates heat (as in the brakes of your car, or in your hands when sliding down a rope). Hammering a metal warms it. Compressing a gas heats it. Some chemical reactions give out heat (exothermic) others absorb heat (endothermic). A liquid can lose heat by evaporation. Some organic processes generate heat, particularly decomposition (as in a pile of grass cuttings). The laws of thermodynamics were formulated from practical observation (of steam engines especially) as well as experiment. The first law concerns the conservation of energy, such as the dissipation of energy by friction with the generation of heat. Joule found that the amount of heat produced in a mechanical device for churning water was proportional to the work done in churning it. This led in 1847 to the principle of conservation of energy, propounded by Joule in Britain and by Helmholtz in Germany. The second law concerns heat and temperature. If you put two bodies, at different temperatures, together, such as a cold egg in a saucepan of warm water, heat will flow from the hotter to the cooler until both are at the same temperature. The egg does not become colder and the water hotter. The hotter object is said to have greater entropy than the colder. Entropy is a measure of the disorder in a system. The Third Law says that entropy is zero at absolute zero temperature. The laws of thermodynamics have been formally stated in various ways. Feynman et al. (1963, vol. 1: 44-13, Table 44-1) summarized them as follows: Copyright 2002 by Richard E. Chapman First Law: Heat put into a system +Work done on a system = Increase in internal energy of the system:
72 Heat and heat flow Second Law: A process whose only net result is to take heat from a reservoir and convert it to work is impossible. No heat engine taking heat Q1 from TI and delivering heat Q2 at T2 can do more work than a reversible engine, for which The entropy of a system is defined this way: (a) If heat AQ is added reversibly to a system at temperature T, the increase in entropy of the system is AS = AQ/T. (b) At T = 0, S = 0 (Third Law). In a reversible change, the total entropy of all parts of the system (including reservoirs) does not change. In irreversible change, the total entropy of the system always increases. Heat flow The interior of the Earth is very hot: the surface is sufficiently cool for us to live on it comfortably. There is a continuous transfer of heat from the interior of the Earth to the surface, where it is dissipated into the atmosphere by conduction and convection in air and water. Some of this heat is due to radioactivity, some to the deep-seated primeval heat of the Earth arising from gravitational processes during the early stages of the Earth's formation. The rate at which heat is transferred by conduction near the surface (where convection cannot be important because of the rigidity of the crust) depends on the thermal conductivity of the material conducting the heat, and its specific heat capacity; and these give rise to a geothermal gradient. Heat flow is a process of diffusion, and a finite time is required for a stable thermal gradient to be achieved. The thermal conductivity of rocks is very small. Seasonal changes of temperature are only felt within metres of the surface of the Earth, which is why some people like to live in caves. Even relatively shallow boreholes produce water at a virtually constant temperature. It has been calculated that we would not feel heat from an original source below a depth of a few hundred kilometres, created with the world, because it would not yet have reached the surface. The volcanic island of Lanzarote in the Canary Islands has areas in the Timanfaya National Park where the surface is cool enough to walk on, but a small excavation will find heat enough for spontaneous combustion of dry twigs. Temperatures of 600°C occur at depths of 10 m, and there is even a restaurant that cooks using this heat. 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
Page 37 and 38: 20 Force Figure 2.3 Torque is the p
Page 39 and 40: 22 Force How does momentum differ f
Page 41 and 42: 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: Heat and heat flow The noun heat ha
Page 91 and 92: 8 Electricity and magnetism Electri
Page 93 and 94: Electricity and magnetism 77 Figure
Page 95 and 96: Electricity and magnetism 79 rivett
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
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Fluids and fluid flow 123 ardentes,
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Fluids and fluid flow 125 Figure 12
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- 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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