|MadSci Network: Engineering|
Your question: If microwaves work by changing the polarity of water molecules and creating friction...heat, then, it would seem that a substance, free of water would never get hot...or is it the hydrogen atoms. Finally, would it only be theoretically possible to test that? If not, how would you do it? Dear WONDERING, This is a very good question because I have met many people with advanced degrees in science who have asked this very question. I think I have a brief answer that you will understand and may challenge you to study further. I am assuming that you are 13 or 14 years of age. It is true that microwaves interact with water due to the polarity of the water molecule. Interaction of energy with a molecule's natural polarity is called a dipole interaction. The reason why microwaves have a dipole interaction is they don't have enough quantum energy to interact with the other parts of molecules (inner atomic electrons, outer atomic electrons and bonding electrons) as do higher or more energetic forms of electromagnetic energy. Gamma rays, x-rays, ultraviolet light and visible light are all more energetic than visible light. Microwaves can't change the electronic structure around atoms or between atoms, but they can interact with electronic differences between atoms. Why do they only interact with water? Simply, they do not only interact with water. They act on the polarity differences between many materials! An important point to make here is, if the molecular dipoles are restricted because they are frozen into place, the microwave dipole interaction is much smaller than would be predicted. What are some other polar molecules that interact with microwave energy and will get hot in a microwave? You have many around the house. Be sure to get your parents approval before experimenting with any of these materials. A relatively safe one is...butter! Butter is made of fat. Fat is an organic molecule that has dipoles between carbon atoms and oxygen-hydrogen groups known as hydroxyls and also between carbon and oxygen atoms that are doubly bonded, known as carbonyls. These dipoles interact strongly with microwave energy. The interaction is easy to see. A few pads of cold butter placed on a saucer and exposed to microwave energy in a microwave overn for several seconds will melt. Melted butter will continue getting hotter. Cooking oils can be heated to very high temperatures in a microwave oven, so if you try one of these, watch the temperature carefully. NEVER DO THESE EXPERIMENTS WITH PLASTIC CONTAINERS. THE OIL OR BUTTER CAN EASILY GET HOT ENOUGH TO MELT THE CONTAINER. USE PYREX BOWLS OR MEASURING CUPS. Other substances that microwaves heat include: dried meat and fruits, wood, most decorative stones, tire rubber. BE CAREFUL, THESE SUBSTANCES CAN GET SO HOT THEY WILL BURN IN JUST SECONDS! SOMETIMES THE SMELL CAN BE TERRIBLE! Do not try substances such as shellac, laquer paints or anything else that can easily cause a fire. If anything else starts to burn or smoke, immediately turn the microwave oven off and keep the microwave door closed. Microwaves also interact with poorly bound or nearly free electrons (as I call them) in semiconductors such as graphite. Pencil lead is made from graphite. If you remove the wood from a pencil lead ( or just get a few mechanical pencil leads) and put it (them) in a pyrex measuring cup and turn the microwave oven on for just a few seconds, the leads will get very hot. BE CAREFUL WITH PENCIL LEADS! THEY CAN REACH TEMPERATURES IN EXCESS OF 500 CENTIGRADE IN LESS THAN A MINUTE! If you carefully try these experiments with your parent's approval and supervision, or if you can do these at school with teacher supervison, you will see that materials that don't contain water can be heated with microwave energy. However, materials that are not polar and not semiconductors that do not heat include: polyethylene (empty milk container), pure candle wax, quartz (very clean, dry sand), polystyrene (foam cup), liquid nitrogen, and dry ice (solid carbon dioxide). Just for fun, try the following experiment. What do you think will happen? The next time a regular incandesent light bulb burns out around the house, don't just throw it away. Get a clear glass that is big enough so that the light bulb (at least a 60 watt bulb) will fit inside the glass completely with the metal stem pointing toward the closed bottom of the glass. Now invert the glass with the light bulb inside it (so that the bottom of the glass and the light bulb stem point up) and put this in a microwave oven on top of a ceramic or glass microwave safe surface. Darken the room. Set the oven timer for 4 to 6 seconds. NO MORE! The bulb will get hot and will not burst (generally) if you follow the 6 second maximum rule. What else do you observe? Remember to watch very closely! REMEMBER, ALWAYS GET PERMISSION FROM AN ADULT TO DO THESE EXPERIMENTS. ALL OF THESE EXPERIMENTS CAN LEAD TO VERY HOT TEMPERATURES, FIRE, NOXIOUS FUMES AND SMOKE. ALWAYS HAVE A FIRE EXTINGUISHER HANDY. IF A FIRE STARTS, KEEP THE MICROWAVE OVEN DOOR CLOSED. IF POSSIBLE, DO THESE EXPERIMENTS IN A SCIENCE LAB IN A PROPERLY VENTILATED HOOD WITH PROPER SUPERVISION FOR MAXIMUM SAFETY. I hope I have answered your question and that you get to try a few of these experiments. Some of this could be science fair material. Dr. Peterson email@example.com
Try the links in the MadSci Library for more information on Engineering.