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Some of you may know this experience (Grape + Microwave oven = Plasma video link):

  • take a grape that you almost split in two parts, letting just a tiny piece of skin making a link between each half-part.
  • put that in a microwave oven, and few seconds later, a light ball which seems to be a plasma appears upon that grape

Looking through google to understand why this phenomena happens, I have found either laconic or partial answers to that question.

In the big lines, this what I understand :

  1. Micro waves seems to create an electric current in the grape because of ions.

    • Why do grapes contains ions ?
  2. Suddenly the tiny link between the two half-parts is broken which creates an electric arc

    • How is that link broken ?
  3. In parallel, the grape is warmed up and a gas is raised upon the grape

    • What is this gas made of ? water ? sugar ?
  4. The combination of the electric arc in that gas creates a plasma

    • What is the necessary condition for a gas crossed by an electric arc to create a plasma ?

Is that correct ?

Are there any relevant parameters (microwave frequency, grape size, grape orientation) that make it works ?

Any idea of order of magnitude for the intensity involved, the voltage of such an arc, the temperature reached (I've read 3000 degrees !) ?

Has someone a complete explanation to provide (reference to physical principle would be appreciated !) ?

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since this came up again I watched the video(s). Is it possible that alcohol is formed by the radiation of the microwave? The "plasma" looks to me a lot like burning alcohol fumes ( I use alcohol to start the fire in my woodstove at my vacation cottage) –  anna v 13 hours ago
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3 Answers

There does seem to be a lot of mythology around about the "grape in a microwave" experiment. I have never see any publications on the subject in a respectable journal, however from chatting to other scientists there seems to be a consensus about what happens.

It's all rather boring really. The grape is the right size (about a quarter wavelength) and shape to act as an antenna that focusses the power in the middle. The skin joining the grape halves heats up, vapourises and bursts into flame.

If anyone feels in an experimental mood some obvious tests of this would be to change the grape size and shape, and see if that affects the flame. Less easy to do at home would be to try the experiment with a nitrogen atmosphere as that should prevent combustion.

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Do you mean this is not a plasma ? this is not about an electric arc crossing a gas ? –  JBE Sep 20 '12 at 14:36
    
Plasma means a large population of free electrons and ions, as opposed to a flame which just has a large population of excited-state electrons. They can certainly look similar though, as both give off light. I don't believe a microwave oven has enough power to create a plasma at atmospheric pressure, so what you see is likely vaporized particles heated to incandescence (a flame). –  user2963 Sep 20 '12 at 15:10
    
However, if you place a container evacuated to a low pressure in a microwave, it is possible to excite a plasma discharge. –  user2963 Sep 20 '12 at 15:11
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@JBE the word "plasma" tends to be used rather loosely. As I understand it the grape experiment produces a flame, though I'm sure there would be ions within it so maybe you'd describe it as a plasma. Doing the experiment in nitrogen would show which it is. I did Google for this but I can find no report of the experiment having been done in nitrogen or any other inert gas. I doubt the flame is an electric arc as I can't see how you'd generate the potential difference required. –  John Rennie Sep 20 '12 at 15:34
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@Georg - I don't know where that quote came from, but I'd guess that "large population of excited-state electrons" means a large population of atoms whose electrons have been raised to a higher energy state but remain bound to the atom. Actually in a flame I think it's only a small minority of atoms that are excited enough to emit light, so I wouldn't use the description "large population". –  John Rennie Sep 20 '12 at 16:13
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Youtube video: http://www.youtube.com/watch?v=vCNNqgKqnaQ

There are quite a few questions here: 1a) Because any living organism cantain ions. (Scientist can not answer the question "why" they answer the quation "how").

1b) From the video, it seems to be working for any size of grape (you cantry at home). The microwave frequency used in oven is the water molecule rotation frequency.

2a) Water molecules absorb the microwaves and get heated. "A microwave oven works by passing non-ionizing microwave radiation, usually at a frequency of 2.45 gigahertz (GHz)—a wavelength of 122 millimetres (4.80 in)—through the food. Microwave radiation is between common radio and infrared frequencies. Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy, when dispersed as molecular vibration in solids and liquids (i.e., as both potential energy and kinetic energy of atoms), is heat." http://en.wikipedia.org/wiki/Microwave_oven

That must weaken the link.

The arc can be up to a few thousands volt.

3a) The gas is probably mainly water plus some biological molecules.

3b) An electric arc through a gas is a plasma. Plasma temperature varies from a thousand degrees to several millions. Joule effect +microwave heating is responsible for this temperature.

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""The microwave frequency used in oven is the water molecule rotation frequency. "" There is no rotation of molecules in condensed liquids! –  Georg Sep 20 '12 at 15:56
    
@Georg: You would be inspired to check your physics. Rotaional spectroscopy is a field in itself. lsbu.ac.uk/water/vibrat.html –  Shaktyai Sep 20 '12 at 16:22
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Plasma is a state of matter which is composed entirely of charged particles or ions. By this definition, a flame is a plasma in itself because it consists of charged particles which can be demonstrated by placing a flame in a uniform electric field. The flame bends to a side in presence of a field. It may also be due to the fact that an electric field ionizes some constituents in the flame which causes it to be affected by the electric field itself. In either cases, it is not wrong to call a flame some sort of a "cold plasma". If a grape bursts into flame when put in the microwave, I do not think that people are wrong to call it plasma by any standards. As far as your question is concerned, you have I think already got your answer.

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