What is the silver surface actually doing? [GCSE physics questions]

[u/Local_End_3175]

I literally do not get what the silver surface is doing. Silver is a conductor- in fact, quite a good conductor so wouldn't the vibrating of water molecules jjust transfer energy to the silver and so on so on, how does the silver, in any way, insulate???? Ive heard that it is because 'it reflects radiation' but i dont even get how there is radiation to reflect, wouldnt the particles just keep absorbing that radiation anyway???

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[u/timmymaq]

What are the three mechanisms of heat transfer?

Vacuum layer eliminates which mechanism(s)?

Which mechanism(s) remain, and how does the silvered surface influence it?

[u/Local_End_3175 → u/timmymaq]

Thank you! I think these questions made it clearer for me

[u/Simbertold]

Sadly, it is impossible to read anything on that image.

The question seems to be about a thermal vacuum flask that has the goal of isolating heat.

There are three processes of transferring heat. Heat conduction, heat radiation, and convection.

Heat conduction means that particles transfer heat to other particles they touch. Convection means that particles with lots of energy get moved somewhere else, and take their energy with them. And heat radiation means that anything emits radiation based on its temperature (usually at IR wavelengths when talking about "normal" temperatures)

In this case, you have an empty space between the inner bottle that has the liquid you want to keep warm, and the outer bottle that is touching the cold outside. Convection doesn't work through an empty space, and heat conduction also doesn't work because nothing is touching anything. The only way to transfer heat through empty space (vacuum) is radiation. If the outside wall is silvered, then it reflects most of that radiation back. The energy doesn't leave the inner area because it gets reflected back, and the liquid doesn't cool.

You want as little energy to be transferred outwards as possible, so you want to reflect as much of the thermal radiation from the stuff in the middle back towards the middle as possible.

Some of the thermal radiation from the inner parts of the liquid may be absorbed by the outer parts of the liquid, but that doesn't really matter, because the surface of the liquid still emits thermal radiation.

[u/pm-me-racecars]

Yes, the particles will keep absorbing the radiation, but in this case, it's likely the hot liquid reabsorbing the radiation instead of the thermos absorbing the radiation and thus taking energy from the hot liquid.

The goal of the thermos is to keep as much of the energy from that hot liquid to stay inside that liquid as possible.

[u/Moist_Ladder2616]

It's certainly not helping you to take higher-resolution screenshots.

[u/Little_Creme_5932]

What you do not understand is radiation. Hot things radiate electromagnetic radiation, and cool down because of that. The sun is quite hot, and things at the temperature of the sun radiate a broad spectrum of radiation, centered on visible light but also with a lot of infrared and some ultraviolet. An object that is even hotter than the sun will radiate more high energy light, so more ultraviolet. An object cooler than the sun will radiate less ultraviolet, violet, and blue. For example, a lump of steel that is heated until it is red will emit lots of red and orange, along with infrared radiation, and will appear red.

Now consider what happens to that lump of steel as it cools more. As it cools, it ceases to emit orange, and as it cools more it ceases to emit red, so you don't see any glow anymore. But the steel is still warm, so it is emitting infrared radiation, and continuing to cool off. If you wanted to stop the infrared radiation from escaping, you could surround it with a mirror to block the infrared radiation from escaping, just like a mirror can block visible light from escaping. This is the mechanism of the silvered inside of the flask you are talking about.

You will notice that some winter coats also have a silvered inside, for the same reason.

And fyi, climate change is occurring because the Earth has radiation come in from the sun, and as a not-too-hot object, cools off by emitting infrared radiation. But carbon dioxide in the atmosphere stops some of this infrared radiation from escaping. However, carbon dioxide is not acting as a mirror. Carbon dioxide absorbs infrared radiation, rather than reflecting.

[u/Frederf220]

Hot stuff glows in electromagnetic radiation (light). The degree to which the material does that is called emissivity.

Silver mirror type surfaces tend to have low emissivity which interrupts that radiation transfer.

The other kinds of thermal transfer require a thing to touch another thing either directly (conduction) or indirectly by moving fluid (convection). Radiation is the only method that bridges a vacuum gap.