posted by sherri .
A friend asks for money to market a new product he invented. He claims to have invented a "cold plate" that works like a hot plate and will cool down foods and liquids that are placed on it. Should you invest??
Should I be thinking about the second law of thermodynamics here?
When you put electricity into a hot plate the electrical energy is converted directly to heat energy by the resistance of the wire. That makes the plate hot and the heat transfers to the food by conduction and radiation
Now if we could come up with a way to put electricity into the plate and make it cold, the heat in the food would be transferred to the cold plate by radiation and conduction thereby achieving the desired result.
However there is a problem. We do not have a way for the electricity to carry heat away from the plate. It puts power in. It does not take it out. It would have to run a machine that transfers heat from the cold plate to some warmer sink. That is called a refrigerator. In fact the surface of the freezing compartment in your refrigerator would work just fine for this purpose with the cold pipes full of expanded gas running through it. The heat generated by the electricity running the compressor is dumped to the air outside the refrigerator by coils out in the air behind the unit.
This would work fine, but is not anything like the simple concept of the hot plate because we need that entire machine between the plug and the wall to pull heat out of a cold thing and dump it to a hot place.