Need some E&M help!

I have an incredibly cool experiment for you to try. Take a grape, slice it almost all the way in half and open it up so you have two hemispheres of grape connected by a thin strip of skin. Now, put it on a plate in the microwave and zap it for 10 seconds or so.

(Seriously, try it right now.)

The microwaves have a wavelength of about 12.5 cm so the grape sections are about a quarter wavelength long. My explanation for the grape catching on fire (really, go try it) is that each of the grape halves is sampling a different part of the electric field so you set up a large potential between the halves until the central skin finally burns through.

Does this make sense?


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8 thoughts on “Need some E&M help!”

  1. That's exactly how I would explain it. Grape antennas tend to burn up much faster than metallic ones.

  2. Hmmm... what happens if you slice the grape completely in half but place them in the same position, just barely disconnected?

  3. Ok, it's good to know that I at least partially understand EM waves.

    My GPS (grape positioning system) wasn't very accurate in the microwave, so I couldn't test extremely close positions of the completely sliced grape. If the two halves were actually in contact, you get an arc which fails as the halves jiggle away from each other. If they're separated by ~1 mm you just get two hot half grapes.

  4. Well good to confirm that well-separated grape halves exhibit significantly different heating (hot vs burning).

    Regarding the positioning, that's why dielectrics were invented - a thin film to maintain electric separation between to pieces... so what about using a piece of lettuce/paper/etc in between the two halves? I guess that wouldn't help the "jiggling away from eachother" part, hmmm...

    Anyway, that's pretty darn cool. Now I want my own grapes.

  5. It's really an amazing thing, everyone really should try it.

    I need to get some sort of known dielectric material so I can see what kind of potential the grapes are actually generating. Dielectric breakdown in air is what 1000 V/inch? The grapes definitely wouldn't throw a 1 inch spark. I wonder about waxed paper though, what voltage do you need to arc through that?

  6. I think I remember that K_paper is roughly three times that of air. Have no idea if that's right, but can look it up later on, I'm sure there have to be references out there.

  7. Wax paper should solve the wetting problem. Likely a different dielectric strength than regular paper, though.

    Google says: Yep, definitely more. Something like 40 MV/m, with a kappa around 3.7.

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