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Microwave ovens and sparks

Discussion in 'Electronic Design' started by Danny, Dec 21, 2003.

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  1. Danny

    Danny Guest


    What is it that causes the sparks when you put something metalic into a
    microwave oven?

  2. Ian Stirling

    Ian Stirling Guest

    The microwaves induce oscillating voltages in various bits of the oven
    and metallic objects inserted in them.
    In some cases, these voltages can increase over what the air can insulate.
    When this happens, you get sparks.
  3. Standing waves bouncing back and forth along conductors produce large
    voltages at the ends of the sections if the length is an odd multiple
    of a half wavelength and at all have wavelength points. Put this
    conductor in a nearly closed loop and those high voltages are opposite
    polarity and large enough to arc across the gap. Sheets of metal
    seldom arc, unless they are so thin that the RF currents burn them
    into small pieces (like the foil layer in a CD).

    You can experiment with this by putting a cup of water in the
    microwave near the loop, to prevent the fields from getting so high
    they reflect back into the magnetron. Lay a small loop of wire on a
    heat proof surface like pyrex, with the ends up off the glass, and
    close together. Trim the length of the wire in 1/16 inch (or
    millimeter) steps and you will find the length that produces maximum
  4. Mike

    Mike Guest

    Others have answered your query, I'll just add a bit of trivia: It doesn't
    have to be metallic: try diced raw carrots some time.

    -- Mike --
  5. I don't know for sure, but would hazzard a guess that it's due to the
    very short wavelength of the energy radiated by the oven. This would
    cause substantial potential differences to arise over the course of
    just a few inches with any reasonably conductive material, giving rise
    to arcing.

    Disclaimer: Of course, I may just be talking bollocks. I'm not an
  6. I often put my teacup in the mw to warm it up. I leave a teaspoon in
    it too and don't get any sparks. OTOH, if I use a teacup with a
    guilded edging, I get an indoor thunderstorm. Seems to back up your
  7. If you look at the burnt gilding, you will find that it is broken into
    similar length pieces of about 1/2 wavelength, each.
  8. Even if the turntable is rotating during the nuking?
  9. Yes.
  10. Funny. I coulda sworn that 2.xGHz had a wavelength in the vicinity of five
    inches. Most teacups are about half a wavelength *across*, but the gilding
    breaks up into pieces 1cm or less. Guess you don't need a half-wave to
    get heating, huh...
  11. That is 2.7 GHz, I think, with a free space wavelength of about 4.4
    inches, but a little less on the surface of ceramic, so a half
    wavelength is about 4 inches, and a half wavelength of about 2
    inches. HMO. You may be right. I seem to remember gilding burning
    into about 1 inch sections.
  12. I came up with a nice trick using a few string beans that started a
    wonderful fire on a paper plate every time. Place the string beans (for
    instance, five of them) in a rough circle with the ends nearly touching. Do
    this on a paper plate. Microwave.
    You may want to have that Mr. Coffee add-on handy- what was it called? Oh,
    yes, Mr. Fire Extinguisher.


    Chip Shults
    My robotics, space and CGI web page -
  13. Bob

    Bob Guest

    Gee, I wish I would have known about this trick when I was a kid. I thought
    that the only thing string beans were good for was shoving them up your
    nose, and thus causing your little sister to throw up.

  14. N. Thornton

    N. Thornton Guest

    Is this because carrots are metallic?

    Seriously, why do carrots do this?

    Regards, NT
  15. I read in that N. Thornton <>
    Yes; the colour is due to the high concentration of orangeonium.
    Water content.
  16. N. Thornton

    N. Thornton Guest

    Right, but other veges also have water content and arent prone to
    arcing the way carrots are. I think I know the answer here: guessing
    time: the carrot I expect has less water content than many other
    veges, and has a low enough content to encourage standing waves, but a
    high enough content to remain electrically conductive - thus creating
    the 2 requirements for arcing.

    Regards, NT
  17. I read in that N. Thornton <>
    Probably. I believe you can get courgettes to light up on US 120 V mains
    because they have a high enough cue. (;-)
  18. N. Thornton

    N. Thornton Guest

    hehe, have to try that one day.

    Regards, NT
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