If heat is a consequence of particle motion, why does stirring liquids decrease their temperature?
In: Physics
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.
Latest Answers