Why does stirring a hot cup of coffee cool it down?

Question

Why does stirring a hot cup of coffee cool it down?

Answer 1

Rate of cooling depends on, among other things, the contact surface and the temperature difference between the objects. So the surface of a cup of coffee is colder then the lower part. However when you stir, hot coffee from the lower part comes up and quickly cools down as the difference between the surface of the coffee and the air in the room is greater.

Answer 2

Reesie is on the right track. By stirring the hot coffee to the surface - where it is cooled by the air- you cool the coffee. The surface coffee is then cooler than the rest of the coffee in the cup and is cooled slower because of it. Cooling is more efficient when the temperature difference is bigger. To more efficiently cool the coffee you would want the hottest coffee in the cup to be in contact with the air. It is accomplished by stirring.

Another effect of stirring is that if you use a metal spoon it will lead heat better than the coffee itself and therefore further cool the coffee.

To sum up: Stirring coffe with a metal spoon cools it more efficiently than not stirring or not using a metal spoon. If you are lazy but still want to cool your coffee faster than normal you just place a metal spoon in the cup!

Hope it helps.

Answer 3

Two reasons. First, stirring increases convection, which is one of the ways heat is transported. Coffee molecules move quicker within the liquid, which means more of them can be near the surface border with cooler air in a given time. Molecules near that border exchange their heat to the air.

Also, stirring increases the motion of the surface coffee with respect to the surrounding air. This cools it in kind of an inverse way as blowing on the coffee would. Instead of the air moving over the coffee, carrying away heat, the coffee moves under the air (the stirring action also moves the air unless you can find a way to "internally" stir the cup), and the relative motions helps the heat transfer more effectively from the liquid to the air.

Answer 4

Hot Cup Of Coffee

Answer 5

The teaspoon used to stir the coffee is a cold object. The heat from the coffee is partly transferred to the teaspoon while stirring the coffee, thus cooling it down. Heat moves from a hot object to a cold object, and the hot object loses heat (cools down) in the process.

Answer 6

it keeps thermal layers from forming in the coffee allowing heat to escape rather than get trapped. The additional energy generated by teh mostion of stiring is insignificant. Liquids that don't mix generally will form thermal layers. The oceans for example which is how submarines try to hide form sonar by going into a "thermocline" or themal layer (an area where the temperature changes suddenly) which causes the sonar waved to bend and distort.

Answer 7

my guess would be because the surface area of the coffee is then increased by moving it around compared to when it is standing still. When more of the surface area is exposed to cooler temperatures then it cools down faster.. I could be completely wrong, just trying to make a guess.