Open the simulation by clicking on the image. Start the simulation (button with the triangle icon). In the simulation, there are two containers separated by a non-thermally conductive wall, in which there is a liquid at the same temperature at the beginning of the experiment. The container on the left is covered with a lid, which can also be moved with the mouse.

Convection is not shown in the simulation.

When the simulation starts, we see the cooling of the bodies. Both bodies have a temperature sensor that can be moved with the mouse. Propeller-like icons indicate the speed of gas movement; they too can be relocated in the simulation.

We have probably all experienced that if you cover a coffee or tea cup with a lid, it cools down more slowly. Slower cooling means that the liquid loses heat energy more slowly.

Why does an uncovered teacup cool faster than a covered teacup?

The water evaporates, and since the water also warms the air, convection occurs with the open cup, where warm air moves higher and cooler air moves above the tea cup. Because cooler air is constantly coming over the teacup due to convection, the tea also cools down faster.

A more interesting question is why the tea or coffee under the lid cools down more slowly. Liquid evaporation and convection also take place under the lid. The trick is that when the water evaporates under the lid, it creates an environment called saturated steam: the air simply cannot hold more water. And although water evaporates, just as much of it condenses. An energetic balance is created: what is lost in evaporation is regained in condensation.