3/19/97 - How does a Thermos keep things hot?

How does a Thermos keep things hot?

(Lansing State Journal, March 19, 1997)

A Thermos keeps things hot by preventing the transfer of heat to the environment. To understand how this is done, we must understand heat. In a gas, liquid, or solid, atoms and molecules move in different directions, banging into each other.

When the fast moving molecules of your hot coffee touch the slower molecules of your cold cup, the cup gets heated and the coffee gets cooler. We call this conduction.

When air moves by the cup, the air gets heated, and the coffee and the cup cool down. This is convection.

The coffee also radiates heat. In this process, heat is give off by photons (light).

In 1885 Sir James Dewar invented a vacuum bottle, an effective way to stop the flow of heat from liquids he was working with. He sold his invention under the trade name Thermos. Dewar made the first Thermos from a glass bottle because it is a poor conductor of heat. He removed the air between the walls of the bottle so heat would not be lost through convection. Finally he covered the glass with a reflective coating to limit the heat lost by radiation.

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