Some metal containers such as the Nissan Thermos ones, even if 100 C water is filled inside, the container is still cold to the touch on the outside. It won't be even warm:

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At the same time, some that look similar are so hot on the outside that holding it for a couple of seconds won't be possible:

enter image description here

Also, some teapot at Chinese restaurant or Vietnamese Restaurant, it can be boiling hot water inside, but the handle is also cold to the touch:

enter image description here

The question is, for the Nissan one and the ones at Chinese restaurants, if the metal part is all connected, from where the liquid is, to the part that the hand can touch, why isn't the handle or outside too hot to hold, but can be not much different from room temperature?

(the Nissan one is vacuum inside, but the metal container part is all part of a single piece.)

  • 1
    $\begingroup$ The first one is a Dewar bottle. It has vacuum between inner and outer cyllinders. The second one probably doesn't have this insulating layer. And the third probably has a handle made from a material with low heat conductivity or with thermal bridging. $\endgroup$
    – liberias
    Nov 26 '12 at 0:35
  • $\begingroup$ I thought if I look at the inside the flask and follow to the outside of the flash, it is all one piece of metal? $\endgroup$ Nov 26 '12 at 0:48
  • $\begingroup$ Probably it is. $\endgroup$
    – liberias
    Nov 26 '12 at 2:17
  • $\begingroup$ even if the handle is highly conductive, and not bridged, it has a high surface area to volume ratio, so it will dissipate a lot of heat to the surrounding air, which means it will be much cooler than the bulk of the container a short distance from where it meets with it. $\endgroup$
    – Jaime
    Nov 26 '12 at 4:43

From the wikipedia article,

The vacuum flask consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near-vacuum which prevents heat transfer by conduction or convection. Heat transfer by thermal radiation may be minimized by silvering flask surfaces facing the gap, but can become problematic if the flask's contents or surroundings are very hot; hence vacuum flasks usually hold contents below water's boiling point. Most heat transfer occurs through the flask's neck and opening, where a vacuum is not present. Vacuum flasks are usually made of metal, glass, foam, or plastic, and have their opening stoppered with cork or plastic. Vacuum flasks are often used as insulated shipping containers.

design of dewar

From the design one can see that the metal inside container can transfer from conductivity heat to the outside surface through the small neck, from where also the radiative and convective heat losses can happen.

If you have ever cooked you will know that even if you stir the pot with a metal spoon the heat of the pot does not transfer to the handle since other mechanisms than conductivity keep the temperature low (air convective cooling for one). Thus the neck ring of metal through which any heat transfer must pass is too small to heat the outside larger metal mass and it is lost to air conduction, air convection and metal radiation at the neck , which are the reason such dewars will lose their internal heat after some time.


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