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The Restless universe
Introduction to The restless Universe

1 The lawful Universe

2 The clockwork Universe

3 The irreversible Universe

3.1 Thermodynamics and entropy 1/3

3.1 Thermodynamics and entropy 2/3

3.1 Thermodynamics and entropy 3/3

3.2 Equilibrium and irreversibility 1/2

3.2 Equilibrium and irreversibility 2/2

» 3.3 Statistical mechanics 1/2

3.3 Statistical mechanics 2/2

4 The intangible Universe

5 The uncertain Universe

6 Closing items

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Other titles in the Physical World series

Describing motion

Predicting motion

Classical physics of matter

Static fields and potentials

Dynamic fields and waves

Quantum physics: an introduction

Quantum physics of matter

3 The irreversible Universe

3.3 Statistical mechanics

Part 1 of 2 | Part 2

For a printable version of '3 The irreversible Universe' click here

You saw earlier that very strong claims were made for Newtonian mechanics. Many regarded it as a basic framework that would underlie all scientific explanations. It is therefore natural to ask about the relationship between Newtonian mechanics and thermodynamics:

Do they contradict one another?

Are they separate aspects of the truth?

Can thermodynamics be derived from Newtonian mechanics?

These are not easy questions. Thermodynamics was specifically designed to deal with concepts like temperature, heat and entropy which had no clear Newtonian interpretation. The gulf between the two subjects can be illustrated by taking, say, a glass of water in a state of equilibrium. We now know that this contains an enormous number of molecules (roughly 10 to the power of 24 ), each feeling electrical forces due to other molecules and moving rapidly around, colliding with other molecules in the liquid and the glass. The Newtonian world-view would require us to keep track of each and every molecule, building up an immensely complicated and detailed description. Of course, this is utterly beyond our powers. Even if it were possible, the results would provide little or no insight. It would be like looking at a painting under a microscope when its true significance is only apparent from a distance of a few metres. Thermodynamics adopts a more practical viewpoint. Rather than tracking each water molecule in detail, it uses just a few well-chosen variables - including energy, volume, pressure, temperature and entropy - to characterize the state of the water as a whole. The amazing thing is that this works. The thermodynamic description is massively incomplete, yet it is sufficient to make useful predictions.
Continue on to 3.3 Statistical mechanics, part 2 of 2

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A note on powers of ten and significant figures

Some highlights of physics

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Acknowledgements

Index

S207 The Physical World
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