Thermal Insulation

When we put hot water inside thermos, water will still hot although it has been several hours. This also happens when we put cold water inside thermos. After hours, water will still cold. How come?

Some may think how thermos knows that inside is hot or cold thus thermos keep the water hot and cold. The answer of this question is Thermal Insulation. As we know that heat is transferred spontaneously from higher temperature to lower temperature. But if between hot and cold temperature is blocked by thermal insulation, heat transfer will be minimized. Actually it is impossible to makes heat transfer rate is zero. It means that although we put thermal insulation between two temperatures, heat transfer still occurs, but very small. By this explanation, we can say that hot water inside thermos also becomes cold but it needs long time.

When hot water is put in room temperature, water will transfer heat to the surrounding whether by conduction, convection or radiation. But when hot water is inside thermos, heat transfer is minimized. Thermos wall is made by thermal insulation material which has very small coefficient conductivity. This cause heat transfer from water to surrounding is blocked. So the temperature of water will stay hot until long time (longer than we put without thermos).

It also happens when cold water is inside thermos. Heat transfer should be from outside thermos to inside thermos because temperature water inside thermos is colder than surrounding. But this heat transfer is minimized by thermal insulation material. Therefore temperature water inside thermos will be kept cold for long time.