If heat is a consequence of particle motion, why does stirring liquids decrease their temperature?



If heat is a consequence of particle motion, why does stirring liquids decrease their temperature?

In: Physics


Stirring does two things, first stove tops don’t heat evenly, so stirring mixes the hot water to colder water areas, distributing the heat. Second is expose the liquid or what have you to the air which us cooler and more capable of accepting the heat.

Stirring doesn’t cool liquid, it heats it.

But the mixing bring warmer internal liquid to the surface where it loses heat to the air. And that far outweighs the heating from friction.

Heat is a value proportional to the kinetic energy of the molecules with respect to the degrees of freedom of the molecules. In other words, throwing a block of ice will not heat it up, because the kinetic energy of the molecules is all traveling in the plane of freedom, however when ice molecules move against the crystal lattice and begin to break free, that is heating up. Stirring a liquid increases the surface area which allows better heat transfer between the liquid and the other heat sinks in the system.

Good question, and some good answers, but I’ll take a swing at it.

Heat always flows from hot to cold. When your coffee sits, the liquid next to the cold air cools, but the liquid down inside the cup doesn’t cool as quickly because it’s not in direct contact with the cold air. Stirring the coffee brings the warmer liquid up into contact with the cold air so it can give off its heat more easily. Heat transfer depends on the temperature difference between the two substances. The little layer of cooler coffee at the top of the cup is actually insulating the hotter coffee down inside the cup. The heat has to flow into the cool coffee from the hot coffee and then into the cold air, which takes longer.

As others have pointed out, the act of stirring does in fact impart energy to the coffee and heats it up. For this same reason, a fan in a room is actually heating the room. It cools the people by forcing air over them and carrying heat away. So always turn off a fan when you leave a room: it is actually heating the room while you are not in it.