Food and drink seems to have been a recurring theme in my undergraduate lectures recently which as we are approaching a festive season is perhaps not inappropriate. At the moment, I am teaching thermodynamics to three hundred first year undergraduate students. Zeroth and first laws of thermodynamics before the Christmas break and then the second and third laws in the New Year. Toast, pizza, barbecued steaks, hot coffee, bottled water, and cold milk shakes have all featured as Everyday Engineering Examples of thermodynamic systems in recent lectures. We can define a thermodynamic system as a quantity of matter capable of exchanging energy with its environment. And, most food preparation processes involve heating, chilling and, or doing work on the food by stirring, beating etc. which are all forms of energy exchange, so the opportunities for Everyday Engineering Examples are many and varied.
In one recent lecture, I asked the class to consider the quickest way to cool your coffee with milk. It was a multiple choice question to which students could respond in real-time using their phones and a website called polleverywhere.com. There was more than one correct answer depending on the assumptions you made about the quantity and temperature of the milk as well as the temperature of the coffee and environment. The core issue is that the rate of cooling is proportional to the temperature difference. While discussing the possible answers, I made a throw-away remark about stirring the coffee involving doing work on the coffee and thus increasing its internal energy and temperature, which would be a step in the wrong direction. I was delighted when one of my students picked me up on this and sent me this link about stirring tea.
It is great to know that at least one student is listening and sufficiently engaged to do a little research. Only 299 left to inspire!
The hot coffee will transfer heat to its cooler surroundings by natural convection and radiation at its free surface and by conduction through the ‘walls’ of the cup. Similarly, the cup will transfer heat to its surroundings by natural convection and radiation from its outer surfaces. This process will establish a temperature gradient in the coffee that will induce a very slow convection flow that would be accelerated by stirring, i.e. introducing forced convection. This is likely to increase heat transfer slightly by carrying hotter coffee to the surfaces. The additional heat transfer (loss) might be more or less than the work done to stir the coffee. Who would have thought something as simply as stirring coffee or tea could be so complicated!
‘The Thermodynamics of Pizza‘ by Harold J. Morowitz, Rutger University Press, 1992.