Field of Science

Grapes of Wrath



My youngest came home from a father-son event with a new interest in healthy foods. I put grapes on the table with dinner. "There are grapes for dinner," he exclaimed. Who are you and what have you done with son? ran through my mind.

At the end of dinner he puts two grapes on his plate and carefully cuts them nearly in half. Then he ducks into the kitchen. "Come on, Mom!" Warm grapes? He'd eaten all the chicken, there was nothing left on his plate to veronique.

He hits the start button and suddenly the grapes start arcing, and one actually bursts momentarily into flame. I'm stunned. No metal, but the arcing is clear. We try various experiments - do you have to leave the grapes connected (no), does it work with other things (carrots), can you char a grape (yes).

What's going on? Hang on, we were producing plasmas in the kitchen. Not the kind that circulates in your veins, but the kind that stars are made out of. Plasma is often called the 4th phase of matter - the iconic triad being solid, liquid and gas. (There are many other phases in which substances can exist, in fact - such as liquid crystals and supercritical fluids.)

Plasmas are gases in which a large number of electron are "free", rather than associated with a molecule or atom.

I'm still trying to come to grips with the idea that I can create a (very tiny) ball of plasma in my kitchen.

(Read more in the paper : "Microwave Mischief and Madness" by H. Hosack, N. Marler, D. MacIsaac of Northern Arizona University, The Physics Teacher 40, 14 (2002).

6 comments:

  1. It's a little sad that people don't have the connection to experimentation that would make clear how simple and easy things like the creation of plasmas are.

    Of course, kids today can't get their hand on "dangerous" chemicals like all the chem-freaks of my generation had trivial access to back in the day. We shall reap what we sow.

    But on a more hopeful note. Here's a relevant link about industrial uses of plasmas for etching which use fancy words to describe a process little more complicated than what goes on in the video.

    http://books.google.com/books?id=tSu--K8OUykC&pg=PA2202&lpg=PA2202&dq=plasma+etcher+design&source=web&ots=Z8ZMaW_PHb&sig=SapPOLWzymG9ARLFg-mJssUzCnA&hl=en&sa=X&oi=book_result&resnum=10&ct=result

    ReplyDelete
  2. I should know this on many levels, but is glass one of those liquid crystals. I know it has qulaties of liquids.

    ReplyDelete
  3. Cassandra,

    Thanks for the pointer to the more pragmatic use of plasmas!

    Stratoz - a glass is considered by some to be a separate phase, by others not (there are some thermodynamic criteria for deciding).

    Glasses (including the kind you most wondrously manipulate) are amorphous solids - no regular repeating patterns. Liquid crystals are yet another phase, with more low-level structure.

    ReplyDelete
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    ReplyDelete
  5. There are metals in the grapes. That is what's causing the arcing, the metal ions that grew in the grapes. These metal ions are what gives some of the distinctive flavors afterall.

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  6. solidlikearock -- metal ions are very different from metals in many properties, including arcing. Every food you put in the microwave contains metals ions, but most don't arc. And you can get water -sans any ions -- to produce a flaming ball of plasma.

    ReplyDelete


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