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Thread: The Sparking Question: microwaves

  1. #1
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    The Sparking Question: microwaves

    Here's a brain buster. Why can I defrost frozen juice in the microwave without sparking, as the typical one has a metal top and bottom? I think, that forks etc spark because charges built up on pointed objects, and leave the fork in the microwave. However, even thou the can lid is metal, it is smooth <macroscopically>, which would make it harder to spark.. Is this the correct reason why?
    A point brought to me was that there is a teflon coating. This, I dont think has much to do with the non-sparking..
    No longer do I leave juice on the counter for 1/2 a day to defrost..

  2. #2
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    I think the microwaves short - or spark - when they have direct contact with the metal. The teflon would prevent that.
    You can actually put metal cans into a microwave; just wrap it in paper towel first. Wouldn't recommend it though; last thing I need is to get plutonium poisoning from a bowl of Chunky Beef.

  3. #3
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    hmmmmmmmmm but cant microwaves penetrate teflon? and dont baking pans have teflon on them, ive noticed they spark..???

  4. #4
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    Lol - got me; I'm just guessin' too.

  5. #5
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    haha :P me too. thats why I posted, to see if someone else had a clue!

  6. #6
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    not sure but I remember as a kid in the late 70's, early 80's, My mom had a set of bowls that had some gold leaf around the rim. we couldn't put the bowls in the microwave withough causing a lightshow.

  7. #7

    Re: The Sparking Question: microwaves

    Quote Originally Posted by kelly
    Here's a brain buster. Why can I defrost frozen juice in the microwave without sparking, as the typical one has a metal top and bottom? I think, that forks etc spark because charges built up on pointed objects, and leave the fork in the microwave. However, even thou the can lid is metal, it is smooth <macroscopically>, which would make it harder to spark.. Is this the correct reason why?
    I can't answer your question, per se, but you are correct that pointed objects (i.e. small radius) are much more likely to strip the electrons off of air molecules. Microwaves are also centimeter or so wavelengths, which is similar to the size of the prongs of the fork, so maybe they are pretty good antennas.

  8. #8
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    Kelly,

    You're right on the money.

    The microwaves induce a current on the metal and the current density is greatest at corner and points. Flat surfaces tend to have the smaller current densities.

    The power dissipated (i.e., converted to heat) at any given point is related to the square of the current density. So any place where the current density is high, you are likely to see sparks.

    This is also why after you rub your feet across the carpet, you can shock someone with your finger, but not your palm.

  9. #9
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    Put a cd in the microwave for a few seconds. It's like the 4th of July.


    And the surface of a cd is smooth, yet they spark.

  10. #10
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    You are quite correct; the charge buildup will arch more easily of pointed metal objects. But what you should consider is that the microwaves will induce current flow in the foil layer, causing it to heat up, and since this use might not have been tested while testing the container, it may release chemicals that is not so nice to have in your drink... Have you ever opened a container you have done this with and checked if it has any marks of melting, charring or arching? Another problem might be if you by accident let it stay to long in the oven in relation to the duty cycle you are using, so that you get steam buildup inside the container, and it explodes/ruptures or you get burned while opening it...

  11. #11
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    Quote Originally Posted by constible
    Put a cd in the microwave for a few seconds. It's like the 4th of July.


    And the surface of a cd is smooth, yet they spark.
    Ah, but the internal metalization is not.

    Anyway, this is general rule of thumb. Whether something sparks are not will also depend on
    • 1.)placement and orientation in the microwave,
      2.)power of the microwave
      3.)conductivity of the metal
      4.)thickness/skin depth of the metal
      5.)additional objects in the microwave.

  12. #12
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    Quote Originally Posted by constible
    Put a cd in the microwave for a few seconds. It's like the 4th of July.

    And the surface of a cd is smooth, yet they spark.
    A CD use metalized plastic, in essence, metal is deposited on a plastic substrate, there are a few different ways to deposit metal, sputtering or vapor deposition is likely the ones used for CDs. Its almost like a layer of extremely fine dust in a thin layer. It is to thin to conduct very well, and it has very little heat dissipation. When you add the possibility of potential gradients building up, you are in for a little firework...

    The CD might look smooth, but the small dust like particles are not necessarily that smooth. And I would think that their small size would make them nice emitters for arching...

  13. #13
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    Quote Originally Posted by TrAI
    You are quite correct; the charge buildup will arch more easily of pointed metal objects. But what you should consider is that the microwaves will induce current flow in the foil layer, causing it to heat up, and since this use might not have been tested while testing the container, it may release chemicals that is not so nice to have in your drink... Have you ever opened a container you have done this with and checked if it has any marks of melting, charring or arching? Another problem might be if you by accident let it stay to long in the oven in relation to the duty cycle you are using, so that you get steam buildup inside the container, and it explodes/ruptures or you get burned while opening it...
    For Kelly's specific case of frozen juice, if she doesn't see any sparks or other badness, I suspect she's okay. The juice will be lossy so the electric field should considerable stronger on the top of the top lid and on the bottom of the bottom lid (I'm assuming a nice glass carousel like in my microwave). Any sparks should be quite noticible.

    When cooking large things like a whole chicken, you can cover parts of the chicken that cook faster with aluminum foil to shield them and slow down the cooking.

    However your point about pressure build up is well-taken. This could easily occur; so becareful, Kelly.

  14. #14
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    Quote Originally Posted by Wiley
    For Kelly's specific case of frozen juice, if she doesn't see any sparks or other badness, I suspect she's okay. The juice will be lossy so the electric field should considerable stronger on the top of the top lid and on the bottom of the bottom lid (I'm assuming a nice glass carousel like in my microwave). Any sparks should be quite noticible.

    When cooking large things like a whole chicken, you can cover parts of the chicken that cook faster with aluminum foil to shield them and slow down the cooking.

    However your point about pressure build up is well-taken. This could easily occur; so becareful, Kelly.
    Hmmm.. I think I have misunderstood what kind of container it was Kelly had, I was thinking along the lines of paper/cardboard container with a plastic layer inside, that is the usual way that juice, milk and some other liquids are packaged where I live, some of them has a layer of aluminium foil under the plastic layer(to reflect light and heat, I expect). I see now that Kelly mentioned a can, it escaped me the first time i read the post. If the can body doesn not use metal foil or metalized paper/plastic this would not be a problem, a metal lid would likely be much thicker than foil, so it will not heat up much...

  15. #15
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    Point well taken! thankyou for the tips. To answer your question, I have inspected the container, and no melting, arching or any sort of badness going on. In fact I have noticed that even though the juice is fully thawed, the metal lids are hardly warm at all. Maybe the "metal" isn't really pure metal after all, maybe it is mixed with other goodies..

  16. #16
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    I had a mug that had gold foil trim around the lip. The trim sparked, so I didn't put it in the nuke box again.
    If you want a "fun thing to do at home" cut a carrot in half, separate it only a little bit, and place it in the microwave.

  17. #17
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    You can also use a microwave to burn out the face of Andrew Jackson on the new U.S. twenty dollar bill. The bill has an embedded metal strip. Depending on your degree of paranoia, the strip is there to thwart counterfeiters or to allow the government to track innocent citizens.

  18. #18
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    Quote Originally Posted by Bob
    You can also use a microwave to burn out the face of Andrew Jackson on the new U.S. twenty dollar bill. The bill has an embedded metal strip. Depending on your degree of paranoia, the strip is there to thwart counterfeiters or to allow the government to track innocent citizens.
    Hmmm... It's quite interesting the ways one has found to protect against counterfeiting. Here in Norway, silver thread/strips have been in use for some time. The latest version of paper money over here employ quite a lot of different safeguards; watermarks, serial number, micro writing, silver strip with text(can be read by holding the bill up to the light), alignment control prints(to check the alignment of the prints on the two sides if one holds them up to the light), small fibers of fluorescent(under UV/black light) material intermixed with the paper fibers, a small picture printed in fluorescent ink(that too is only visible under UV/Black light), a weak line of metallic printed numbers(for example the 500 kroners bill will have the number 500 printed several times down this), off course special inks and papers, the print is quite complex pictures that might make moiré patterns if scanned on low resolution, and lastly a strip of a silver like metallic color with holographic prints. Thats the ones i know about anyway... I guess that as the technology for high quality printing now is available to anyone, the anti-counterfeit techniques has to be quite advanced too...

  19. #19
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    TrAI: The juice containers I guess are different in Canada than in Norway. These ones I'm refering to have a cardboard cylindar with metal tops and bottoms, with a plastic pull tab around the perimeter of the top to open. I have a astronomy friend here who is from Norway and he explained to me the type you were refering to...
    PS those money stats are interesting.

  20. #20
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    Quote Originally Posted by kelly
    TrAI: The juice containers I guess are different in Canada than in Norway. These ones I'm refering to have a cardboard cylindar with metal tops and bottoms, with a plastic pull tab around the perimeter of the top to open. I have a astronomy friend here who is from Norway and he explained to me the type you were refering to...
    PS those money stats are interesting.
    Yes, I see what you mean.

    Well, It's easy to forget some times that some things are quite dependant on how the local industries have chosen to solve a specific problem. The type you are referring to will likely have thicker metal with more heat dissipation than a the types I was thinking of, and as long as it doesn't get hot or create arching, it is probably Ok. But be careful not to overheat the juice. :wink:

    By the way, "Norges Bank" have pictures of the notes and the security features for some of them on it's site here:
    http://www.norges-bank.no/english/no...ins/notes.html
    This version seem to have the features for all the bills, it's in norwegian though...
    http://www.norges-bank.no/sedler_og_mynt/sedler.html

  21. #21
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    Mythbusters had a failry entertaining show about microwave ovens. they showed what happens when all sorts of metallic stuff is put inside- the big ball of aluminum foil was like watching a really wicked thunderstorm from a distance. they even screwed around with the parts from 4 or 5 microwaves to try to make a super duper microawave oven, but it didn't work.
    they mentioned putting a dog inside one to see what happens, but the poodle got pardoned.

  22. #22
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    For microwave oven experiments of the more extreme sort, you might want to visit Sam Barros' site;

    http://www.powerlabs.org/uwavexp.htm

  23. #23
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    Have you tried placing the frozen juice can on the side instead of the bottom? It might roll around some, but when it is places on its side it will probably catch more rays. Some microwaves seem to have less power at the center of the carousel due to hotspots. of you ever warmed up a plate of leftoevers you'll notice the outside gets hotter. So maybe you'll get sparking whenthecan's on its side.

  24. #24
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    Quote Originally Posted by novaderrik
    Mythbusters had a failry entertaining show about microwave ovens. they showed what happens when all sorts of metallic stuff is put inside- the big ball of aluminum foil was like watching a really wicked thunderstorm from a distance. they even screwed around with the parts from 4 or 5 microwaves to try to make a super duper microawave oven, but it didn't work.
    they mentioned putting a dog inside one to see what happens, but the poodle got pardoned.
    I love Mythbusters.

  25. #25
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    Also loved that myth busters episodes LOTS OF SPARKS.

    Agree with sharppoinited objects geerated sparks, as I have used needles to produce coronal discarges, which occur where current densities are highest at the point.

    If you are interested see what the difference in your cars radio reception is when the antenna has a little ball on the end or has a pointed end.

  26. #26
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    Quote Originally Posted by Jpax2003
    Have you tried placing the frozen juice can on the side instead of the bottom? It might roll around some, but when it is places on its side it will probably catch more rays. Some microwaves seem to have less power at the center of the carousel due to hotspots. of you ever warmed up a plate of leftoevers you'll notice the outside gets hotter. So maybe you'll get sparking whenthecan's on its side.

    hmmmmmmmm maybe your on to something!!!!!I'll try it.. My poor microwave..haha

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