Denise Shirley of Mosgiel asks :-

I saw a demonstration in which a lightbulb placed in a microwave oven glowed. Why did it?

John Campbell, who teaches physics at the University of Canterbury, responded.

Before anyone tries this, keep in mind that a microwave oven uses up to 600 watts of power whereas a standard lightbulb takes only 100 watts to make it incandescent, so the bulb can be destroyed doing this. I use a bulb with a broken filament, and also immerse the electrical contact end in a bowl of cooling water.

Metals contain some electrons which are free to move whenever an electrical force is applied to them. When an electrical current is forced through a metal wire the travelling electrons jostle the atoms of the metal, so heating up the wire.

The microwave oven works by generating very high frequency microwaves which oscillate at nearly ten thousand million times every second.

To prevent ourselves being cooked too, the microwaves are sprayed into a metal box. There are no electrical forces outside the box. There are also none at the inside surface of the metal box because the electrons move to cancel the force. (I show this in classes by putting grated cheese on toast against the metal wall. The cheese next to the wall doesn't melt.)

The metal walls reflect electric waves so there are places inside the oven where the two waves reinforce, giving a maximum of electrical force, and other places where the two waves cancel, giving a minimum of electrical force. These"hot" and "cool" spots explain why food must be rotated inside a microwave oven.

Hence the electric force along the filament isn't uniform so currents flow in the filament to try to cancel out the forces. Your light bulb will wax and wane as it rotates.

This heating of metals explains the advice to not use dishes with gold designs in microwave ovens. The thin gold foil can be heated to evaporation. A gold ring will also cause a ring of heating, and the stress can crack the dish.