# Tea stored in a thermos is an isolated system or closed system?

My opinion is that since we can create some extra motion in tea by shaking it we will be providing some extra kinetic energy to tea and thus increase in internal energy and therefore it is only a closed system not an isolated system.

• Is there a difference between an "isolated" and a "closed" system? Commented Oct 28, 2022 at 20:33
• It's closed until you get thirsty ツ. Commented Oct 28, 2022 at 20:37

If you move your thermos, the system os not isolated, since you can do work on it. And the work you do is transferred to the fluid that moves until the motion is dissipated by viscosity.

Close for sure, as a first approximation, while you keep the thermos closed, since it can't exchange mass with the external environment.

Thermos designers would like it to be also isolated, and exchange no heat with the external environment, to keep your beverage as hot (or as cold) as you pour it in the thermos.

The assumption to treat thermos as an isolated system, depends on the ability of the designer and producer and on the time scale you're considering (the shortest the time scale, the smallest the heat exchanged through its boundaries).

Note. To be more precise, it's the internal volume of the thermos to be closed and isolated, since the external surface can easily exchange heat with the external environment.

Almost adiabatically almost closed: almost because there is no perfect heat insulator and almost closed because there is no perfect "diffusion" insulator. Otherwise it can accept work by shaking the thermos and thereby increase its internal energy via fluid friction.

An isolated system is one that exchanges neither mass nor energy with its surroundings in the form of heat or work.

If one considers the thermos insulation to be perfect, then there would theoretically be no energy transfer in the form of heat.

If one considers the thermos to be rigid, there can be no boundary work (expansion or compression of the contents).

However, shaking the thermos is the equivalent of doing stirrer work on its contents, which increases its internal energy due to internal viscous friction.

Hope this helps.