Chemistry Blog

«

»

Jul 09

Why do grapes and microwaves = plasma?

by Mark | Categories: fun | (41833 Views)

If you:

  1. Take a grape
  2. Cut it equatorially, but just leave a bit of skin connecting the two hemispheres.
  3. Dry the new surfaces with a paper towel.
  4. Stick the grape in a microwave oven.
  5. Turn it on.

You get flashes of plasma emerging from the grapes. Like this…

But why does this happen? There are plenty of attempts to explain it all over the net e.g. here, here and here. But none of the explanations quite satisfy me. And what causes this plume when you put a glass over the grapes?

I’ve tried a few variations on the method. For example, you don’t have to leave the skin bridge as long as the two hemispheres are touching. Cutting the grapes longitudinally doesn’t seem to work. And I’ve never managed to get the addition of the glass to make a difference.

Anybody got a good explanation or further refinements?

 

p.s. The grapes get very hot and don’t do this in a microwave oven that you are particularly attached too.

7 comments

Skip to comment form

  1. Brandon

    The plume makes sense. As the grapes heat up they’re going to dry out. With a glass on top there’s nowhere for the water vapor to go, so you get a region of high humidity.

    High humidity = higher electrical conductance = more plasma.

  2. CHM

    Have you actually tried it? Doesn’t work for me, in all of the four configurations proposed. Microwaved for one minute each, nothing happened. Oh, except that the grape’s juices went boiling.

    I’ve been hearing/seeing it for too long, and always thought it was bogus…

  3. Mitch

    I think it works by placing a small piece of metal inside the grapes off camera.

  4. sam

    I’m not sure what is happening. And I have never tried it myself. But the fact that you need the grape halves in contact makes me think that there is some current flowing. The explanation by the Naked Scientist site is very compelling.

    Here’s a nano scale version of the same effect:
    http://www.stanford.edu/group/cpn/highlights/Fluorescence.pdf

    But I wonder if that’s the kind of explanation, while theoretically sounds great, is just too complicated for the real world. Maybe the grapes are just in hotspots in the microwave oven and the sugar catches fire a little. There certainly are hotspots in the microwave, which is why they spin the food around.

    Anyway, I’m hoping it’s the grape-as-antenna explanation and hope it is correct.

  5. Mark

    Yes it works for me. The first video is mine. And I promise there is no hidden metal in the grapes.

    Try finding a hot spot and putting the grapes in it.

  6. mike brown

    Basically because of all of the pesticides and metal contaminants in our food.

  7. pd

    I’d guess the geometry is important. Microwave ovens operate at about 3cm wavelength, so cut grape forms a good old half wavelength dipole antenna and current generated at small joint is enough to get plasma.

    Suggestions for further research (I’ve no microwave so I can’t do that):
    -try smaller / larger fruits (plums?)
    -disable rotation of the dish ant try different orientations

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>

Powered by sweet Captcha