Microwave play

Kilowatts

A microwave (perhaps one you don't care about.)

Warning

We have done these activities and have not damaged or destroyed our microwave oven. We warn you however that it is theoretically possible to damage your microwave using these activities.

Be careful!
These activities make objects very hot.
Fire inside the microwave is very possible.

To Do and Notice/What's Going On?

Marshmallow (Peeps are best.)

Place a marshmallow inside the microwave.
Turn on the microwave in 10 second bursts.
Notice that it expands. Then "melts." Then turns brown.

The microwaves heat the marshmallow. Causing the air trapped inside to expand. The heating eventually melts the marshmallow releasing the air.
The temperatures rises to the point where the sugar in the marshmallow caramelizes and becomes brown.

Lightbulb

Place a lightbulb in the microwave.
Turn on the microwave in two second bursts.
Notice that the lightbulb glows brightly.

The microwave delivers a kilowatt of energy to the lightbulb.
The lightbulb is designed to handle only 100 Watts.
The filament acts as an antenna giving off sparks and heating to incandescence.
Eventually the high energy will melt the glass bulb.

CD

This will destroy the CD.

Place the compact disk inside the microwave.
Turn the microwave on for 2 seconds or so.
Notice the sparks across the disk.
Remove the disk from the microwave and hold it up against the light, notice the pattern of lightning like clear regions in the metal of the CD.

Notice the pattern on the CD. Notice the size of the pattern.

The microwaves build up electric charges on the metal of the CD. When the charges build a high enough voltage, an electric discharge takes place which vaporizes the metal on the CD leaving behind a visible lightning bolt track.
The pattern has a size about one-half of a microwave wavelength.
It often follows a radial and tangential pattern.

Steel Wool

Put a ball of steel wool onto a glass plate in the microwave.
Turn on the microwave for a 30 seconds
The steel wool will be laced by electric arcs and will burn.
The smoke from the burning steel wool will also absorb microwaves so you will create a glowing ball of incandescent gas, similar to reports of ball lightning

Scrape up the remains of the burned steel wool and use it as "iron filings" in magnetism experiments.

What's Going On?

An object covered with charge will be at a high voltage.

The electric field around such an object will be largest at points such as the ends of your fingers.

Charges will flow off the charged object at the points. These charges will strike air molecules and create a wind flowing away from the point. Thus when you point at the dry ice mist a wind strikes the mist and blows it away.

Going Further

Use other electrostatic generators and find their effects on the dry ice mist.

An Activity by Paul Doherty,

Thanks to Jan Sjokvist of The Xperiment Huset in Vaxjo for showing me some of these activities.