How Does A Thermos Know To Keep Hot Things Hot And Cold Things Cold?

Have you ever heard the old joke about the thermos that can keep hot things hot and cold things cold? The punchline is, “But how does it know?” It’s a funny question because, of course, thermoses don’t actually “know” anything. But the real question is, how do thermoses actually keep things at the right temperature?

Well, let’s go back to the late 19th century when Scottish chemist and physicist James Dewar was trying to study liquid oxygen, which boils at a bone-chilling -183°C (-297°F). He needed a way to keep it cool long enough for his experiments. This is where the laws of thermodynamics come into play.

The second law of thermodynamics tells us that heat always flows from hotter to colder areas. So, to keep things hot or cold, you need to slow down the heat transfer process. This is where Dewar’s genius invention comes in.

Dewar created a simple yet ingenious solution – he placed one flask inside another, with a near-vacuum layer in between. This vacuum layer prevents heat transfer through conduction and convection, while a reflective material inside the flask slows down radiation. This design effectively keeps the contents of the flask at the desired temperature by minimizing heat exchange with the surrounding environment.

So, the next time you enjoy a hot cup of coffee or a warm meal from your thermos, remember the science behind it. It’s not magic that keeps things hot or cold, but rather the clever application of thermodynamics by inventors like James Dewar.