The first law of thermodynamics

I’ve used the word ‘energy’ a lot in recent posts (like yesterday’s), but haven’t really said what it is. We all have some idea of what energy means – it’s a resource you need to do something useful, such as propel a car, or play a video game, heat your house or power refigerator. (This last one is interesting – it can take energy cool something down as well as heat something up – but more on that at a later time). But pinning down what exactly it is is rather harder.

It was well into the 19th century before the concept of energy was properly formulated. Interestingly, this was not done by the ‘established’ scientific community, but rather by an English businessman more concerned with making money – James Joule. Joule owned a brewery – and what he wanted to know was whether the new-fangled electrical technology provided a cheaper form of energy that the established steam engines. That prompted his interest in what energy actually was, and he undertook a series of simple experiments on the heating of water that formed the basis for what is now known as the first law of thermodynamics, namely that ‘heat’ and ‘mechanical work’ are equivalent forms of energy. That is to say, mechanical work produces heat. Frustratingly for Joule, his brewing rather than pure science background rather predjudiced established scientific opinion against his ideas, and it took several years before Joule’s idea to become accepted. But his name rightly lives on as a unit of energy – the joule. (The ‘kJ’ on the back of your chocolate bar stands for ‘kilojoules’ – or a thousand joules.)

OK, so I still haven’t told you what energy actually is, and I’m not sure I can. My physics textbooks are a fat lot of use here. The best I can come up with it is an attribute that allows something useful to be done. Like making beer

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