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Q & A: Heating and cooling

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Most recent answer: 10/07/2014
Q:
A question that arose while refilling a coffee cup in the UIUC Physics Dept. Business Office: is it easier for something hot to reheat something cool or for something cool to chill down something hot?
- Cheryl (age 53)
Urbana, IL USA
A:

Interesting question, Cheryl. Think about it this way: in both cases, heat is being exchanged between two objects with different temperatures. If you use something hot to heat something cool, the hot object is also being cooled down as it loses heat—so it's really the same situation as using something cool to chill down something hot.

If you let the two objects stay in contact for a while, they will eventually end up at the same temperature. This final temperature can be predicted if we know the intial temperatures, masses, and heat capacities of the two objects. Heat capacity (technically, "specific" heat capacity) is a property of a material that measures how much heat must be added or removed to raise or lower its temperature. For example, liquid water has a higher heat capacity than most metals. So if you transfer the same amount of heat into equal masses of water and of metal, the temperature of the metal will increase more.

There is one thing about heat capacity that might affect whether it's "easier" to do heating or cooling. As long as the material remains in the same phase of matter (gas, liquid, or solid), heat capacity tends to decrease a little bit with temperature. So a cold object might have slightly less capacity to absorb heat compared to its capacity to give heat if it were hot. This effect would probably only be noticable at extreme temperatures.

Rebecca Holmes


(published on 10/07/2014)

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