Hey Reader, Happy Wednesday!

Last week, I visited Pittsburgh, Pennsylvania with my friends, and revisited my grad school after 5+ years.

Let’s look at how a thermos flask works this week. I aim to write my newsletter issues in a way one can follow them while traveling on a bus, having a coffee, waiting for food, etc. Let's jump in!

How it works: 15-second answer

A thermos flask slows down heat transfer to keep your coffee hot. Firstly, it uses a vacuum between the inner and outer glass layers of the flask, blocking heat transfer between the outside world and coffee through direct contact. Secondly, a tight seal prevents air from circulating. Finally, infrared radiation is minimized by silvering the flask inside, reflecting heat back into the flask. These combined features allow the thermos to maintain the temperature of its contents for a longer time.

Answer to the question I posed last week

Last week, I asked why if noise-cancelling headphones can cancel all noises, do we still hear high-frequency sounds like a car horn, etc.?

Even if the headphones can process the noise’s waveform and produce a sound to cancel it, aligning the high-frequency waveform exactly to cancel it out can be tricky and difficult. Even if there is a small misalignment, it can be a distortion noise. Also if we are walking on the streets, then we need headphones to be able to allow sounds like a car horn, crying of a baby, etc. so we can become aware and attend to it. Something that keeps us safe. Let’s get back to this week’s topic :)

Shoutout!

Thanks to Vasantha for asking this week’s question.

And a shoutout to my good friend and subscriber Caitlin Olson for answering last week’s question on Noise-canceling headphones correctly!

She runs a newsletter as well called 'Today You Should Know'. Do check it out and subscribe if you're interested.

💡 Today You Should Know Learn something new every Friday. Join 1,800+ nerds and satisfy your curiosity one question at a time with topics including science, technology, history, and more. Subscribe​

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How does a thermos flask work?

Now imagine you have a cup of hot coffee. Over time, the coffee will naturally cool down due to heat transfer.

Three types of heat transfer cause this to happen!

Let’s look at what they are and how thermos flask counteracts them:

Type #1: Conduction

If you leave the coffee on the table, heat will transfer from the coffee to the cup and the air through direct contact. If you take a metal bar and heat one end of it, the other end gets hot through conduction. This method of objects becoming hot and cool through direct contact is conduction.

How Thermos flask prevents Conduction: Introducing a vacuum layer between the inner and outer glass layers

A vacuum is a space of no air, atoms, or anything. It is nothing. Imagine you have a regular flask. Now you construct two layers of glass inside the bottle introducing a vacuum between the inner and outer layers of the glass by sucking air out during construction. This vacuum prevents heat transfer from coffee to the outside world through conduction because there are no molecules to pass the heat along, which slows down the cooling of your coffee.

Type #2: Convection

Convection is a property of liquids and gases. As the coffee’s warmth heats the air above it, that warm air rises due to low density and is replaced by cooler air from the surroundings. This speeds up the cooling of the coffee as the cooler air is now in contact with the coffee. This cycle repeats until the coffee and the surrounding air are at the same temperature. This form of heat transfer is called convection.

How Thermos flask prevents Convection: A tight Seal

In a thermos flask, the sealed lid stops air circulation, so there’s no convection happening. This keeps the coffee warm for longer.

Type #3: Infrared radiation

We feel the sun’s warmth from millions of miles away as it radiates heat in the form of infrared radiation. The coils or heating elements of an electric stove or toaster glow and emit infrared radiation, directly warming any object near them without the need for direct contact. Such forms of heat transfer are called infrared radiation.

How Thermos flask prevents Infrared radiation: The glass layers are silvered to make it reflective

Your coffee also emits heat in the form of radiation stored inside a flask. Infrared, like visible light, is reflected by mirrors and absorbed better by black objects. When infrared is absorbed, it results in a temperature rise. Hence, the glass layer is silvered like a mirror so that the surface becomes reflective. This causes the infrared radiation to bounce and reflect back into the flask, keeping the contents warm or cold.

These are the ways in which a thermos flask keeps the coffee hot and right!

Question of the week

Why does your coffee in a thermos flask eventually get cold even if they are sealed and left undisturbed for several hours? Reply with your thoughts to this email and we can have a discussion. I’ll answer this in the next week’s edition.

That’s all folks. Thank you for reading!

Have an amazing rest of the week, and take care!
Until next to next Wednesday,
Chendur

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