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My Favourite LC Questions (Part 4): The Physics of...Waterfalls?

I do enjoy a LC question that ties in with a good story, and in 2016 q7 there was a question that connected Niagara Falls, heat pumps and fridge/freezers through the stories of James Joule and Lord Kelvin, with indirect associations to the brewing of beer and Irish history*. What more could you ask for?**

Reading on the background to the question taught me how Joule was a scientific outsider, and how, while working in the family brewery in Manchester, he thought it might be a good idea to replace some of their steam-driven apparatus with the newly developed high tech electric motors. That lead him into a series of experiments on efficiency, comparing the energy output of burning coal with that of a zinc battery. This led in time to a deepening of the concept of the conservation of energy - and the revelation that heat was just another form of that energy. Before this, it was believed that a fluid existed called caloric, and that the temperature of a body was believed to be controlled by the amount of a caloric it contained.

Joule's work was spectacular, and laid the foundation for much of our modern understanding of energy. But when he tried to share his findings with the broader scientific community, he was largely ignored: he was after all, just a country brewer and not one of London's scientific elite. It was only when the renowned scientist William Thomson befriended him that his work was recognised.

Apparently, the friendship between the two was cemented when Joule took time out from his honeymoon in Chamonix to join with Thomson in an attempt to measure the temperature difference between the top and bottom of the waterfall there, providing the theory with which this question begins....

For anybody interested, I've tried to compress some of the story, and the LC question with attendant marking scheme, into a PowerPoint here. I found it a useful enough class with which to tidy up the study of heat and temperature.


* The Irish history connection: Thomson was appointed to the House of Lords in 1892 and given the title Lord Kelvin. I've often seen it noted that he was one of the first scientists to be so honoured. But according to this, the honour was as much for his opposition to Home Rule as it was to his contributions to Thermodynamics.

** I spent a month in 1986 working in a motel in Niagara Falls, stranded there because I had missed a flight to New York. I enjoy telling that story to my captive audience of LC students too, explaining how it would all have been different if only mobile phones had existed back then. They don't care, of course, but it amuses me.

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