fun with microwaves - a physics case study

[binary hero]

OK, the majority of this is about metal in microwaves, and cool, but not amazing stuff. But then, at the bottom, it has about creating ball lightning in a microwave...i can't wait until i get home

Grapes in the microwave oven.

This is a lesson that it is not necessarily metal which is most dangerous to put in the microwave.

Take a relatively think skinned grape and cut it in half, but not all the way. Leave a small bridge of skin connecting the two. Dab the excess juice from the grape and place the whole thing like an open book in the microwave oven. After 10 seconds of cooking, a large spark blows the two halves apart!

I first found this demo at the web site of Patrick R. Michaud.

Our suspicion is that the grape halves act as a small dipole antenna for the microwaves. Optimally such an antenna is 1/4 the size of the wavelength, which is pretty close for our grape. The skin flap is a conductor and currents run back and forth across it. As it heats up, it dries out, increasing it's resistance further, causing more heat, etc. Finally it ignites. The smell of burnt sugar fills the oven after it is turned off. The spark will not occur if the grape is too wet. Juice boils up and covers the skin bridge and there is no spark.

On rare occasion we noticed rising clouds of luminous plasma (often referred to as 'ball lightning' in other microwave oven web pages). While testing for repeatability (so we could demonstrate it in front of a large group of high school students) we ran out of grapes. Carl Acampado, one of our undergraduate assistants, started to cut used grape halves in half again, as shown in the next figure:

[shyam]

I feel like trying that.. but i dont want to fuckup the microwave. Is it safe?

[MetallichicA]

THAT'S AWESOME. Way better than the old stick-an-AOL-cd-in-your-microwave-and-watch-the-fireworks.

Now, I just need to find an old microwave....

[simon snowflake]

I must resist
I must resist
I must resist
I must re..
I must re..


I must get a old microwave

[Orgone_Man]

I've done it a few times, it won't screw your microwave up that I can tell. Half the time it won't work, but if it does, its pretty cool. Looks just like a strobe light (intermitten flashes). I have not seen ball lightning though.... i guess that is a rare occurance. You can stick a cd in the microwave to also get neat effects.....but don't expect to use the cd again.

[MetallichicA]

Which is why you....

Originally posted by MetallichicA
stick-an-AOL-cd-in-your-microwave-and-watch-the-fireworks.

[roby0215]

I can see the used microwaves being snatched up at a yard sale near you.

[liquidmotion]

Originally posted by simon snowflake
I must resist
I must resist
I must resist
I must re..
I must re..


I must get a old microwave

simon, your avatars never cease to amaze me.