A: You know, everybody asked relatively mundane questions this week – even Jake, though asked in a goofy tone. Unless I were to make up patently ridiculous answers, it’s pretty difficult to make any response humorous. So I chose the question that I recklessly assumed my half-dozen readers know the least about. The Thermos.
The Thermos, of course, is a colloquialism of “Vacuum Flask” much in the same way the we Americans use Kleenex to describe a tissue. For simplicity, and the sake of efficiency in my otherwise bombastic typing style, I’ll refer to our subject as a Thermos.
Continuing down the road of reckless assumption, I’m going to postulate that we all know the main purpose of a Thermos: to keep hot things hot, and cold things cold. A rather rustic description, but adequate no less. So, to paraphrase the question posed by my brother Mugs: How does it work?
The theory behind a Thermos is to remove as much of the transfer of heat between an object and its surroundings as possible. This works because of a vacuum surrounding the flask (go figure). “Why does this work?” you ask. Why, I’m glad you inquired! Let’s talk about the riveting world of heat transfer!
There are three ways to transfer heat: conduction, convection, and radiation.
Conduction is the direct transfer of heat from one body of matter to another: the heating coil on your stove gets hot, and it’s in direct contact with the bottom of your sauce pan. It conducts heat from the coil to the pan, bringing your red sauce to a slow boil that spits flecks of staining marinara all over your stove top and surrounding counter. Next time put a lid on it, buddy.
Convection is transfer of heat through flowing matter, most typically gas or liquid. This is how a convection oven works – hot air blows over the food. Because the air is moving, the turkey in the oven that you forgot to thaw doesn’t get a chance to cool off the surrounding air. That surrounding air is constantly replaced. Convection is the biggest culprit in home heating and cooling problems, and that’s what 99% of home insulation is designed to block.
Radiation is transfer of heat through electromagnetic waves or particles, such as microwaves or light. There need not be any contact between matter for radiation to work because radiation heats by exciting the particles with which it comes in contact; in the same way you warm up by jumping around in place while waiting at a cold bus stop, the particles heat up when the radiation causes them to vibrate in place.
The vacuum in a Thermos prevents two of the three forms of heat transfer – conduction, and convection. The inner wall of a Thermos is surrounded by a vacuum which cannot transfer through conduction because there’s no matter inside the vacuum. The only places to conduct heat are where the inner wall is bonded to the outer wall near the cap, and the cap itself.
There is almost no way to transfer heat by convection when there’s an air-tight seal on the Thermos. No air or moisture flow can effect the contents. There are minimal effects of what are called, “convection currents,” but the impact is so negligible as to be nearly ignorable (unless you kept something in the Thermos for weeks).
Even though radiation can heat through a vacuum, we rarely have to worry about radiation affecting the contents of a Thermos. The outer wall of the Thermos blocks light, and you wouldn’t be microwaving a Thermos unless you intended to heat it. Certainly we don’t worry about radiation cooling the contents of the Thermos.
There you have it! My thermodynamics course in college found a use after all – the bloated pedantry of a blogger!
Start thinking of your questions for next week’s installment!