Why no Metal in Microwave Ovens?
Can Metallic Objects Be Inside; What About the Interior Walls?
Nov 11, 2007
Metal in Microwave Ovens
Most people have heard that they should not put metal in microwave ovens. Why not? What happens to metal in a microwave oven?
There are two things that happen when microwaves encounter a metallic object. One is that the metal reflects the microwaves. The other is that the microwaves set up electric currents in the metal.
Reflection
Metal reflects microwaves just as a mirror reflects visible light or the walls of a pool table reflect pool balls.
The interior walls of microwave ovens are made of metal. When microwaves hit one of the walls, they reflect back into the oven cooking chamber. If the walls were not metallic, the microwaves would escape the cooking chamber into the kitchen. The loose microwaves could strike the hungry kitchen occupants and heat the water molecules in their bodies. They could be severely burnt.
By design the metal walls of microwave ovens confine the microwaves to the cooking chamber. For microwave safety, the oven will not operate unless the door is closed. The small holes in the metal covering the window are designed to block microwaves yet allow light to pass through. Visible light has a wavelength of about 0.00005 cm while the microwaves used for cooking have a wavelength of 12.2 cm. Light can pass through these small holes, but microwaves cannot. Reflection keeps microwaves inside the oven.
If the food in the microwave oven is inside a metal container or wrapped in aluminum foil, the metal reflects the microwaves. No microwaves penetrate the food, and it stays cool.
If there is no food inside the oven or the food is enclosed in metal and cannot absorb the microwaves, the microwaves bounce around inside the oven. Eventually they reflect back to the magnetron, which is the part that produces the microwaves. The magnetron can overheat and be damaged.
Electric Currents
Microwaves are a form of electromagnetic waves, which are oscillating electric and magnetic fields. The oscillating electric fields cause electric currents in metals and other conductors. These currents do no harm in the electrical walls of the cooking chamber, but they can sometimes have some bad effects if thin metal objects are inside the oven.
Thin wires have more electrical resistance than thick wires, so a thin wire will heat up more than a thick one, if it carries an electric current. Thin metal objects inside a microwave oven can therefore get very hot from these electric currents.
Some packaged foods come with a sleeve containing a thin metallic foil layer. This sleeve is designed to get just hot enough to brown the food inside.
Thin metallic objects can also have electric charges build up on their sharp points. If enough electric charge builds up on a sharp, the insulating properties of air break down. The air can briefly conduct electricity. An electric spark arcs to the nearest other electrical conductor. This arcing can often happen with dishes that are not microwave safe because they contain small decorative strips of metal. If you see this arcing, turn off the microwave, be careful about touching the possibly hot dish, and cook your food in something else.
Metal inside a microwave oven will not harm the food or make it unsafe to eat. It can however prevent the food from cooking, create electrical sparks, or damage the oven.
Further Reading
Bloomfield, Louis A., How Things Work The Physics of Everyday Life, Wiley, 1997.
Microwave Ovens Heat Food Unevenly
