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The vacuum reduce heat loss by conduction and convection.
A vacuum makes a good heat insulator.
Using vacuum as an insulator avoids heat loss by conduction. Heat transfer is minimised by reflective silver surfaces that are applied to the flask. This prevents thermal radiation from entering and escaping the flask.
The vacuum space between the two silvered surfaces make efficient heat insulation against heat loss.
This is due to insulation. Inside of the Thermos the liquid remains its placed temperature longer due to the insulating either stopping heat from reaching the fluid, or by stopping heat from leaving the fluid.
The main heat loss in a good thermos flask, is due to heat conducted through the material. (Glass or Stainless Steel). There should be little heat lost through the vacuum of the flask.
The vacuum reduce heat loss by conduction and convection.
To reduce heat loss by thermal radiation.
A vacuum makes a good heat insulator.
Using vacuum as an insulator avoids heat loss by conduction. Heat transfer is minimised by reflective silver surfaces that are applied to the flask. This prevents thermal radiation from entering and escaping the flask.
The vacuum space between the two silvered surfaces make efficient heat insulation against heat loss.
This is due to insulation. Inside of the Thermos the liquid remains its placed temperature longer due to the insulating either stopping heat from reaching the fluid, or by stopping heat from leaving the fluid.
Thermos Flask prevent loss or gain of heat by separating the internal layer and the external layer with a vacuum (absence of air molecules). Heat is usually transferred through different media (air molecules and other molecules). Without these media, heat can't pass from one place to another, except for radiation which doesn't need any media (it can travel through vacuum)
The flask makes use of heavy insulation, either by vaccum, air cushion or just filled with very poor heat conductors to slow the heat loss from a hot drink or warming from the enviroment.
Vaccum between thermos walls does NOT prevent radiation, however the other forms of heat transfer(convection and conduction) are more or less eliminated. Radiation effect of heat transfer may be reduced by applying a layer of reflective material inside the bootles - to reflect the emmision back inwards.
A vacuum between two glass sheets will greatly reduce heat loss through convection, and also through conduction. The fact that the glass is coated (to convert it to a mirror) also reduces the loss by radiation.
There is a dead air space between the liner of the thermos and the outside. This space acts as insulation preventing conduction. In a really good thermos, or Dewar flask, the space is actually a vacuum. Vacuum is an even better insulator than air is, since heat doesn't conduct across a vacuum at all.