Post Tagged with: "grape"

Why do grapes and microwaves = plasma?

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.

By July 9, 2012 9 comments fun

Magnetic grapes and NMR

When I was an undergrad I found NMR to be one of the trickiest techniques to get my head around. I think it was because the technique involves so many concepts that run counter to everything we’ve learnt before. After all in school we get told about ferromagnets and thats it. Then at uni suddenly someone is trying to tell us that actually there’s these other things called diamagnets and paramagnets, which means that even water is magnetic!

So now that I get to teach NMR I like to demonstrate diamagnetism right from the get go. Of course diamagnetism is really weak so you need a precision built, low friction setup. So I set about building one with ….

  • 2 grapes
  • A wooden skewer
  • A pin
  • An old film canister
  • A neodymium magnet. I get mine from emagnets. The stronger the better I use  one with 20 Kg pull.
The neodymium magnets are really powerful. So…
  1. Don’t let kids play with them.
  2. Don’t put them near your credit cards, phone, watch or any other electrical equipment.
  3. Don’t put 2 magnets anywhere near each other because they’ll fly towards one another, shatter and send chunks flying.
  4. If you have any medical implants don’t go anywhere near them.
  5. Read the safety instructions that come with the magnets.

The Build:

1. Push the 2 grapes onto either end of the skewer

2. Push the pin through the cap of the film canister, so that its pointing upward. Put the cap back on the canister.
3. This is the tricky but. You need to balance the skewer and grapes on the point of the pin.
4. Once you’ve managed that just put the edge of the magnet near one of the grapes and watch it spin.
There you go, the magnet repeals the water in the grapes and you have a nice down to earth demo of diagmanetism.
Then to cap it off show your audience the levitating frogs, and I promise they won’t forget diamagnetism after that.
Originally posted (in a slightly different form) at
By April 15, 2012 2 comments chemical education, fun, Uncategorized