Classical thermodynamics is a beautiful branch of 19th-century physics that handles the concepts of temperature, heat and work in large systems in equilibrium. Using the tools of natural philosophy rather than applied mathematics, it analyses what can be deduced about the macroscopic properties of a system regardless of atomic or quantum or statistical explanations. The deductive process requires the four laws that are the subject of this book.

First-year undergraduates reading physics should all be exposed to a short course in thermodynamics because the subject is complete in and of itself: there are no intellectual puzzles still awaiting resolution. Furthermore, the mathematical structure is elegant. It satisfies Einstein's criterion that a good theory should possess beauty, as is the case with special relativity.

The four laws have the following to say. The zeroth law (it was an afterthought) says that if two thermodynamic systems are each in equilibrium with a third, then they are also in equilibrium with each other. From this law flows the quantitative concept of temperature.

The first law is the easiest to grasp, despite being expressed in several variants. It states that the increase in the internal energy of a system is equal to the heat energy added minus the work expended on the surroundings.

The second law can also be expressed in several ways, all of which lead to the concept that natural processes are irreversible despite the fact that the fundamental laws of physics are time-reversible. An engineer's definition of the second law states that it is impossible to extract heat energy from a high-temperature source and convert all the energy into useful work. Some of the heat energy must be dumped into a low-energy sink.

According to the third law, it is impossible to reach absolute zero in a finite number of steps. Physicists can now access nano-temperatures, but they will never touch the bottom of the temperature scale.

Peter Atkins's account of the core concepts of thermodynamics is beautifully crafted. We see how laws that were derived by considering the action of ideal steam engines do in fact drive the universe. Natural processes increase the amount of disorder or entropy in the universe. In the 1850s, that consideration led William Thomson to propose heat death as a possible final state of the universe, in which everything has run down and there is no free energy left.

In style, the book revives the rational deductive approach of the natural philosophers, and for me that is one of its delights. There is just the right number of clear diagrams and a refreshing absence of complicated graphs.

When I was a science publisher, I advised Stephen Hawking that "every equation halves the market". Hawking permitted himself * E=mc2 * ; Atkins has chosen * S=k logW * , on the grounds that it is inscribed on the tombstone of Ludwig Boltzmann, the pioneer in kinetic theory.

Simon Mitton is a fellow of St Edmund's College, Cambridge.

## Four Laws that Drive the Universe

Author - Peter Atkins

Publisher - Oxford University Press

Pages - 128

Price - £9.99

ISBN - 9780199232369

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