Potato peeling and statistical thermodynamics

I mentioned the ‘potato peeling rule’ in a Tweet this morning, prompted by the second year Physical Chemistry exam which is actually taking place as I write this. I should start by acknowledging the originator of the term, my colleague John Staniforth (you can find his Facebook profile here). Many of the equations in statistical thermodynamics are quite complex and long-winded, and have terms in them like Planck’s constant (h) to the power 3 (or even 4). The type of calculators that our students have these days enable them to input complete calculations in one go (at least in principle), but even these wonderful gadgets can’t usually cope with numbers larger or smaller than 10 to the power plus or minus 99. So they happily try to calculate an entropy using the Sackur-Tetrode equation (for example), and get an answer of zero. The solution is of course to break the equation down into separate sections, and (horror of horrors) to cancel some powers of 10 by hand. Going back to John, he was helping me in a problem class a few years ago, and to explain the principle, came out with this (now) immortal phrase: ‘The champion potato peeler peels one potato at a time’. So, ever since, I have called this procedure the ‘potato peeling rule’. It remains to be seen whether it has been applied in today’s exam!

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