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Anna N

Making plasma in the microwave with grapes

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I am not about to try this.


 The video is not very good quality but it does illustrate what happens. 




Edited to add a quote from the article:


“It's a crowd-pleasing party trick: Cut a grape in half, pop it in the microwave, hit "start" — then sit back and be dazzled by the grape balls of fire.”


 Edited again to include the link 


Oops. Here’s the link. 

Edited by Anna N (log)
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Ahem. From the Department of Self-Aggrandizement: the following appeared in the Daily Gullet in March, 2003. A few of the references are dated, and I haven't reviewed it for recent historic revelation or scientific understanding (though I can vouch for the results of the experiments in a current-day microwave, and I do have a pretty good idea of why the grape thing works). The photos, unfortunately, were lost in a site upgrade.



A Man & His Microwave (Don't try this at home . . .)


David Scantland

Wednesday, March 19, 2003


YOU'LL ADMIT to having one, but not to using it: a microwave oven. Oh sure, it's alright for reheating leftovers and making popcorn. Maybe you melt chocolate or butter with it. But it's not like you have to have one.


Here's a secret: that inoffensive box sitting on your counter? It's heavy-duty science. I'm not saying it's like a death-ray or anything, but it's more than the glorified butane lighter you call a gas range, or the mutant offspring of a power drill, like a food processor. It's a war veteran -- a hero, in fact, and even though it's a senior citizen, it's the most high-tech thing you've got in your kitchen, with the innocuous exception of the digital timer. But you, serious cook that you are, have no use for it. 


Fine. As long as you're at it, I'll bet you've got a few other things around the house that aren't pulling their weight. Candles burnt down to their last inch. Seedless green grapes. Unsolicited CD-ROMs from AOL. Gather 'em up, and bring 'em into the kitchen. Let's see what your microwave oven can do besides heat water for your morning tea (which is not a great idea, by the way). It's showtime.


What's that? You want to know a little bit more about that handy appliance before we proceed to destroy it? Glad to oblige.


Like Teflon, our last topic, microwave ovens are thought by many to be a product of the US space program. Also like Teflon, they are not. 


The magnetron tube, which is the heart of a microwave oven, was invented in England in 1940, and was used in early radar systems. Legend has it that  Raytheon scientist Percy Spencer stood briefly in front of an operational tube in 1946. Shortly thereafter, he discovered that a chocolate bar in his pocket had melted. The inferences one could draw from this story ring false: in particular, the gullible speculation that it was six years before some heedless wartime engineer stood in front of one of these things -- or that it took that long for some reckless fool to point it at somebody. But the story is repeated so often that it has become indistinguishable from the truth. Regardless, subsequent experiments with popcorn and eggs convinced Raytheon management that the magnetron might have a post-war future in the kitchen.


Raytheon patented the concept and produced the first commercial microwave oven in 1953. It was the size of a refrigerator, weighed 750 pounds, and cost more than many houses: $3000 (US). Tappan developed a home-use unit in 1955, but it was still very expensive: $1,295. Finally, with the invention of a miniature magnetron tube, real countertop microwaves were introduced in 1967, priced at around $500. In the end, however, it took microelectronics and modern manufacturing techniques to create an affordable appliance. Starting in the early 1980s, microwave ovens began their manifest occupation of the middle-class kitchen.


Fun Facts About Microwave Ovens

Contrary to popular belief, magnetron tubes do not operate at the resonant frequency of water. In fact, not even close. What really makes a microwave oven work is friction. Microwaves pass through food molecules at a rate of 2450 megahertz. The molecules attempt to align themselves with the waves. Since each wave has a positive and a negative phase, molecules are constantly being jostled back and forth, almost five million times per second. If you've ever been in a meeting with an indecisive supervisor and a gaggle of brown-nosers, you know the sort of energy this creates. Unlike your dysfunctional office, however, a microwave oven puts this energy to work. The friction created by molecules rubbing together generates heat, and heat cooks food. Microwaves work well on other things, too, like fats. When these are excited, their temperature can far exceed that of water. Contact with scalding fat is what causes plastic containers to collect scars after repeated use.

Microwave ovens do not cook food from the inside out. Microwaves can't penetrate most food beyond about an inch. As long as the food has substantial water or fat content, however, that inch does get cooked with a fair amount of uniformity. The rest is cooked by conduction as heat moves to the interior of the food.


Your frying pan is more likely to give you cancer than your microwave. A few years back, a flurry of reports claimed that interaction between fats and plastic wrap could create dioxin. The claim has vague credibility. All of the necessary elements are present: chlorine compounds, hydrocarbons, and heat. But the subsequent public relations blizzard instead focused on new plasticizer formulations, consumer information alerts, and not much emphasis on scientific fact. No one has proven that dioxin can be produced in a domestic microwave oven, as long as it has been used for cooking food. However, dioxin and PCBs have been found in the detritus of triglycerides that develops when frying temperatures are applied to animal fats.


Happy now? Enough facts . . . on with the show.


Your Own Personal Lightning Storm and Mandlebrot Set (no degree required)


Go find a paper cup. Along the way, snatch up a CD that you don't mind sacrificing to the Science God (actually, a blank disc works best). Set the CD face up on the inverted cup in the center of your oven, and set the timer on high for ten seconds. Watch closely. You'll see energy blaze through the inside of the disk, and a number of very bright spots will appear. This is where the aluminum has been etched into fragments, some of which are just the right size to be microwave-tuned antennae. As soon as the CD starts to smoke, turn the oven off. You don't want to breathe much of this stuff. Congratulations! You've just vaporized aluminum. Let the CD cool and look at it. I don't know if those are really <a href="http://www.tssphoto.com/abstract/EABS0985.html" target="blank">fractal patterns</a>, but they're a close approximation. Bonus: you didn't have to learn non-linear equations.

Flaming Grapes: A Holiday Treat


Take a seedless green grape and cut it almost in half, leaving the halves connected by a bit of skin. Gently flatten the grape in the middle of a microwave-safe plate. Put the plate in your microwave. Set the power on high, the timer for 20 seconds, and turn the oven on. Soon you'll have a little Independence or Guy Fawkes Day celebration, right in the privacy of your own kitchen. It won't last long, because the grape halves will blow themselves apart, and the show will be over. Turn off the oven before they catch fire. Don't eat them.


The explanation for why this happens is left as an exercise for the student. Personally, I have no idea.


Movie Magic: A Cup Full of Plasma


Although regular citizens here on Earth don't see it much, plasma is actually the most common form of matter in the universe. It's the stuff of nebulae, cosmic clouds, and nuclear fusion. It's a lot like a gas, but it's also a superior electrical conductor subject to magnetic fields.


Technically, I don't know if this experiment produces a real, honest-to-God plasma, but it sure looks like one -- imagine the Aurora Borealis, or at least the flurry of alien ships that buzz Richard Dreyfuss in <i>Close Encounters</i>.


You'll need: 
- A one-quart heat-proof measuring cup;

- Three or four paper cups, to elevate the measuring cup;

- A small candle (in keeping with the "honest-to-God" theme, a votive candle works well -- just remove the metal collar first);

- Three or four burnt wooden matches; and

- A one-cup custard dish, to elevate the candle


Need I add that you should be prepared to sacrifice all of these items -- plus your microwave?


Invert the custard cup and set the candle on top of it. Arrange the matches on top of the candle so that they will be close to the flame. (Yes, you're going to light the candle.) Place the paper cups, also inverted, in a circular pattern around the custard cup. You're going to turn the measuring cup upside-down and set it on top of the paper cups, so check for spacing and stability.


Light the candle and position the measuring cup. Close the oven door and set the timer for two minutes on high. 


If you've ever seen St. Elmo's Fire (ball lighting, not the movie), you'll be somewhat familiar with what happens next. The carbon in the matches vaporizes in the bath of microwave energy. A sphere of energized gas forms around the carbon particle(s) and floats in the air. Depending partly on the size of the sphere, it may drift around the cup, or it might remain relatively still. Usually, they don't last very long, but occasionally one lingers. In any case, they have a strange ethereal beauty. They're also ephemeral -- once you turn the oven off, the balls collapse. But while it lasts, what a show: a group of brightly burning, short-lived gas balls. Hmmm. Come to think of it, it is like the movie.


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3 hours ago, Dave the Cook said:

Ahem. From the Department of Self-Aggrandizement: the following appeared in the Daily Gullet in March, 2003. A few of the references are dated, and I haven't reviewed it for recent historic revelation or scientific understanding (though I can vouch for the results of the experiments in a current-day microwave, and I do have a pretty good idea of why the grape thing works).


The link that Anna provides does give a pretty good explanation on why this grape thing works.  🙂 


Check the video at the end of the article or even better, go to to The Proceedings of the National Academy of Sciences where the Canadian researchers published their paper, Linking plasma formation in grapes to microwave resonances of aqueous dimers



Edited by FauxPas (log)

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Note (before trying this at home) that the researchers accumulated quite a "microwave graveyard" in the process.

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