I’m flagging

Energy, the subtle concept by Jennifer Coopersmith

Energy is a central concept in modern physics, and I hadn’t fully realised the long, long path before it was recognised as a central theme. People started out being fascinated by perpetual motion machines, and after many failing designs it became clear that such devices were impossible. But why?

Experimental work by Huygens on collisions had shown experimentally that mv^2 was being preserved, and Newton’s laws had shown that collisions preserved mv but this was just the beginning of the story. Galileo’s principle of relativity made people wonder about the conservation of such quantities, but it was really the need to explain heat and cold (thermodynamics) that made it necessary to understand energy.

Hence, things really look off with the invention of the stream engine. It was very unclear how a temperature gradient was being converted by a machine into useful work. Carnot developed his famous cycle, which used the gas laws to explain the work carried out during a single cycle of a heat machine. These laws had been found by various experimentalists over the years. The whole time, the calorific theory of heat was holding the theory back and it wasn’t until the acceptance of the atomic theory that things really moved forwards. Statistical mechanics and the Hamiltonian principle of least action helped with understanding, and eventually Boltzmann generalised the laws of thermodynamics with the notion of entropy.

This book develops the story well, with brief biographical information about the various players. There is very little mathematics, but many of the experiments are described. It is really interesting to see how the many experimental results were explained away, often with incorrect explanations that just happened to work, and good to see the years where experiment led the way, with theory often taking a long time to explain observed behaviour. All while the engineers were actively using the observed behaviour.

A great read!

Over the holiday I also read The big questions: Physics by Michael Brooks. This gives answers to the twenty most frequently asked questions about physics and was also a good read (though it is hard to know how they picked the particular questions).

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