Artisan, on East Sheen Lane, is one café in a small chain of coffee shops in West London (four cafés at the time of writing). Although there was plenty of seating inside, most tables were already taken when I arrived shortly after lunch suggesting that this is a very popular local café. There are many details to notice in this friendly corner shop coffee house. Firstly, the counter, on the left as you enter, was decorated as if supported by a door fixed on its side, one of many quirky features. When it arrived, my black Americano came with a most fantastic crema on top which cracked to reveal the coffee beneath, appearing as if it were a meandering river. Adjacent to my table was a sliding door, presumably leading to the toilets, that had a counterweight hanging from its side, I’m sure that could have led to a series of thoughts on Greek science and Archimedes.
There was also plenty to notice on the counter itself, a sign for two tip jars suggested you either tipped in one or the other depending on whether you wanted to “see into the future” or to “change the past”. As with previous ‘honesty box’ type experiments, it would be fascinating to know which box gets more coins and whether this correlated with external events in the East Sheen area and around. Still, I digress. Also on the counter was a wheel, a bit like the wheel of the Wheel of Fortune TV show. In this café, the wheel offered different coffees or cakes rather than prizes. As the wheel is spun, it is slowed by friction acting against pins that stick out from the circumference of the wheel. When learning about angular momentum and wheels in physics we always assume the ideal of a frictionless wheel without losses. We assume that it spins forever. The wheel in Artisan was quite far from this ideal, the whole idea being that the friction eventually stops the wheel and the pin points to your ‘prize’. So how do we reconcile these two ideas of the wheel? How efficient can water wheels be? And how efficient can engines be?
This was a question that occupied Sadi Carnot (1796-1832) (named after the Persian poet Sa’di of Shiraz). Carnot was interested in how to optimise steam engines. Although steam engines were being engineered to be increasingly efficient, Carnot realised that people still did not understand what the maximum efficiency of a steam engine could be. Carnot worked on the principle that heat was a fluid (caloric) and so steam engines could be understood analogously to water wheels. Even though we no longer have this understanding of heat, Carnot’s ideal engine is still relevant for today. He discovered that, for an ideal engine (that is an engine that works without frictional losses etc.), the maximum amount of work that you could extract from the engine depended only on the temperature difference between the maximum working temperature and ambient temperature of the engine (not on the details of the engine such as whether it used steam as its working fluid). In practise this means that a steam turbine (which operates between approximately 543 °C = 816 Kelvin and 23 ºC = 296 Kelvin) has a maximum efficiency of 64%. Were you able to design a frictionless engine made from a cup of coffee (typical drinking temperature 60 °C = 333 K), it would have a maximum efficiency of around 10%
Of course, a real engine made from a cup of coffee would encounter frictional losses etc. which would reduce its efficiency. So while we may think that an efficiency of around 10% is not that bad (particularly if we’re making the coffee anyway), once we’ve allowed reality to enter into our calculations, the actual efficiency is much lower. This is probably best summarised as: The best use of coffee is in drinking it, and where better than Artisan coffee if you find yourself in East Sheen (or Putney, Stamford Brook or Ealing)?
Artisan Coffee is at 139 East Sheen Lane, SW14 8LR
