MadSci Network: Physics

Re: why does a cup of coffee cool down faster with a metal spoon in it??

Date: Fri Jan 22 02:34:27 1999
Posted By: Michael Kay, Staff, Chem, Haz. Mat Mgmt, Health Physics, Nuclear Science, AMBRY, Inc
Area of science: Physics
ID: 916879585.Ph


Let's start with a cup of coffee. We will use thermodynamics, which is part 
of chemistry called physical chemistry, and also part of physics. One of 
the laws of thermodynamics says that heat flows from hot to cold 
spontaneously. That means heat, a form of energy, goes from a hot object to 
a touching cooler object until they are both the same temperature. That 
means that the coffee will cool down by heating up the surrounding air, and 
even the table or saucer the cup is on, and eventually, all of them will 
end up at the same temperature. For all practical purposes, this will be 
the room temperature.

The coffee in a cup touches the cup and the air. The outside of the cup 
touches the air. So, all the heat has to be absorbed by the air to cool the 
coffee. The surface area of the coffee and the surface area of the cup can 
be measured. That area is acting like a radiator and heating the air. The 
coffee is cooling at a rate determined by how fast the heat can flow 
through the cup and then into the air, and from the surface of the coffee.
You know heat is flowing through the cup because it gets hot (if it is a 
ceramic or glass cup) or warm if it is a styrofoam cup. As a note, since 
the styrofoam cup only gets warm, it is a better insulator, letting less 
heat flow, and the coffee stays hot longer since most cooling must happen 
at the surface of the coffee where it meets the air.

Now let's put a metal spoon in the coffee. Metals are good conductors of 
heat. The bowl of the spoon is in the coffee, and as it is heated, the heat 
also travels up the handle of the spoon. If you use a silver spoon, and the 
coffee is very hot, the handle of the spoon may become too hot to touch 
because silver is an excellent conductor of heat. The handle of the spoon 
in the air acts like a radiator fin. It increases the surface area exposed 
to the air and transfers heat to the air. It is this increased heat 
transfer to the air that causes the coffee to cool faster with a metal 
spoon in the cup. 

This does not happen with a plastic spoon in the cup because plastic is not 
a good conductor of heat.

If you are interested and want to learn more about heat, I would go to the 
school library and look up HEAT in general science books. Your science 
teacher may be able to direct you to the right books in the library.

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