Cool calculations

It’s a hot day and you want a cold drink: how long will it take for your chosen beverage to be chilled to its ideal temperature? Fortunately, there’s a neat little online calculator that will give you the answer and save you the effort of constantly checking to see whether the drink is ready to deliver its perfect refreshment. The calculator is highlighted in the latest edition of Physics World (August 2021) and you can find it at https://www.omnicalculator.com/food/chilled-drink.

For example, a half-litre bottle of water, which has an optimum refreshment temperature of just six degrees Celsius, would take more than four hours to cool down from a fairly average summer day’s temperature if chilled in a fridge but only about one hour if the bottle is put in a freezer.

The “chilled drinks” webpage also gives some great tips for cooling drinks without a machine (using evaporation) and provides cooling graphs – which very obviously aren’t linear. This brings us the the physics part involving Newton’s law of cooling. Although these calculations aren’t in the GCSE Physics course, it’s worth noting that the non-linear curve is due to the fact that cooling is actually an exponential decay (just like radioactive half-lives, which are in the course).

It’s always useful in physics to try to link things together and to think about where things might lead. In the case of cooling, we might consider the Mpemba effect. This is the controversial idea, named after a Tanzanian student, that hot liquids can sometimes freeze faster than the same liquid starting at a lower temperature! There’s a nice summary of this unsolved mystery, which some people suggest may not actually exist, on the Yale Scientific website and another on the Chemistry World website.

Why not find out more about the Mpemba effect while waiting for your drink to cool? Or head over to the physicsworld.com website to listen to the latest podcasts about topics ranging from the physics of sport (inevitably) the effect of cosmic waves on self-driving cars (worryingly).