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Old 10-06-2009, 09:40 AM
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robby robby is offline
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Join Date: Dec 2000
Location: Connecticut, USA
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Originally Posted by treis View Post
I think its time for a thermo booster shot. You neglected the energy required for the phase change from liquid water to solid ice. Furthermore, what you calculated is the amount of energy removed from the water. This is not the same as the energy required to run the refridgerator.

A refridgerator is different from, say, a stove. A stove provides all of the energy required to heat something up. A refridgerator, on the other hand, is a heat pump. It works by coaxing heat to move from the inside of the refridgerator to the outside, i.e. your kitchen. Your refridgerator is running a vapor compression cycle which, IIRC, has a coefficient of performance in the high 3s. In other words, for every one Joule your refridgerator uses it transfers 3 joules of energy from the inside of the refridgerator to your kitchen.
Exactly. Also, for all of you posters before treis, you are being sloppy in your terminology.

You have to remove heat to cool and freeze liquid water. Referring to the "number of joules needed" or stating that cooling "takes 25 calories" and that freezing "takes 80 calories" does not make much sense.

To effect the cooling and freezing, the refrigerator's heat pump does indeed require energy, but realize too that no heat pump is 100% efficient.

P.S. I also took two semesters of physical chemistry and two semesters of thermodynamics and learned to hate the whole subject with a passion. The worst part for me were the thermodynamic equations involving partial derivatives.