Re: Why don't shrinky dinks shrink in the microwave oven?

Hi Devin,

First of all, this is an excellent observation. To understand why an oven (toaster, conventional) works on shrinking the shrinky dink and a microwave doesn't, you need to understand that microwaves and ovens work differently to achieve the same goal--heating things. An oven works by conduction, that is heat is conducted by a heat source (usually gas or electicity). Convection is also invloved, but for simplicity lets just look at conduction.

Food placed in an oven warms from the outside in (perhaps you have had some food that was warm on the outside and frozen in the center). Almost anything (food, plastic, metal)placed near a source of heat will get hot (there are a few exceptions that depend on how well something "holds heat," but that is another story). Microwave ovens heat food by induction. The microwaves resonate, primarily with the water and fat molecules in the food, thus, transferring heat directly to the food. (Think of it like a radio, water and fat molecules can listen to microwaves, while other things have more trouble tuning in--this is what is meant by resonate).

Water and fat molecules have something in common, that is each has polar bonds in them (simply put, there is a distribution of charge between two elements. By charge I mean sharing of electrons, which is what makes up an element along with neutrons and protons. An example being water which is made up of oxygen and hydrogen, oxygen takes more than its share of charge from the electrons and creates an unequal distribution of charge.)

Items that have these types of unequal distributionof charge can be heated in a microwave (think dethawing something frozen in the microwave, you are actually heating the frozen water molecules in there, which in turn warms the food). If something does not have an unequal distribtuion of charge, then a microwave can not warm it. Polystyrene does not have an unequal distribution of charge (though it does not perfectly share the distribution of charge either), but because of this polystyrene will not shrink in a microwave, but will shrink if heated by conduction.

By the way, did you notice that the walls in you microwave are most likely made from a plastic? Also, if you feel the air when you open the door of a conventional oven, it is hot. When you open the microwave door, the air is not hot (air is nitrogen and oxygen mostly, neither have an unequal distribution of charge).

I hope this helps. The answer involves a lot of science and I hope it shows how interesting science is. Let me know if you need any clarification.

KIP