Electric Coolers

You may have seen the new coolers that don't use ice, plugging into your car's cigarette lighter instead. These coolers rely on a process known as the Peltier effect, or thermoelectric effect, to produce cold temperatures electronically.

You can create the Peltier effect with a battery, two pieces of copper wire and a piece of bismuth or iron wire. Just connect the copper wires to the two poles of the battery, and then connect the bismuth or iron wire between the two pieces of copper wire. The bismuth/iron and copper must touch -- it is this junction that causes the Peltier effect.

The junction where current flows from copper to bismuth will get hot, and the junction where current flows from bismuth to copper the junction will get cold. The maximum temperature drop is about 40 F from the ambient temperature where the hot junction is located.

To create a Peltier cooler, the hot junction is placed outside the refrigerator, and the cold junction is placed inside. Normally, you create a module containing many junctions to amplify the effect. See the links at the end of this article for details on the Peltier effect.

Now let's take a look at what's going on inside a cold pack.

Cold Packs

Speaking of refrigeration and coldness, have you ever used one of those "instant cold packs" that looks like a plastic bag filled with liquid. You hit it, shake it up and it gets extremely cold. What's going on here?

The liquid inside the cold pack is water. In the water is another plastic bag or tube containing ammonium-nitrate fertilizer. When you hit the cold pack, it breaks the tube so that the water mixes with the fertilizer. This mixture creates an endothermic reaction -- it absorbs heat. The temperature of the solution falls to about 35 F for 10 to 15 minutes.