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Metal in microwave-why not?

  1. Oct 19, 2005 #1
    Like many I've heard the 'ol rule - DON'T put metal in a microwave oven.
    But the thing is, I've neve heard anyone explain WHY you shouldn't put metal in a microwave.
    I've seen it done, at least on TV, the metal sparks, but so what?

    What exactly is dangerous about putting metal in a microwave?
     
  2. jcsd
  3. Oct 19, 2005 #2

    Astronuc

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    Food absorbs microwave radiation by excitation of molecular bonds, which cause the atoms to vibrate. Typically, microwaves excite the OH bond in water.

    The metal will reflect microwaves, and (by itself, i.e. not in contact with a hydrogen-bearing organic substance (usually containing water)) it can cause electrical discharges if the power density is high enough. Any discharge can damage the microwave.

    Actually, if one puts a metal object (e.g. spoon) in a piece of food or in liquid containing water (e.g. coffee, tea, soup, stew) there is no problem.

    But put a metal object on a plate and one will likely see electrical discharges, like lightning.

    If one wraps (completely surrounds) food in Al foil, the food will not heat in the mircrowave, but one will get electrical discharges.

    Putting food on a metal plate is OK in some microwave models. The microwaves are simply reflected into the food.
     
  4. Oct 21, 2005 #3
    Hey. I don't get it. How does metal reflecting microwaves cause electrical discharges?
     
    Last edited: Oct 21, 2005
  5. Oct 21, 2005 #4

    Astronuc

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    A sufficient charge density builds up, usually on points, and IIRC, the atoms in the metal are ionized (electrons knock off) until a potential difference builds up, and then at a critical potential difference, a discharge (like lightning) occurs.

    The discharge (electrical surge) can damage sensitive microelectronics.

    The issue with reflecting microwave radiation is that microwave ovens are designed with shielding assuming the user (consumer) uses the microwave for the purpose for which it is intended, i.e. heating (cooking) food or drinks. Alternate uses can put the user at risk.
     
    Last edited: Oct 21, 2005
  6. Oct 21, 2005 #5
    Microwave Ovens normally use 2.4Ghz range and it disrupts Wi-Fi.

    http://www.wi-fiplanet.com/tutorials/article.php/3116531

    I have read, on a number of websites, that Microwaves cannot be focused, is that true?


    Disclaimer Notice:
    The issue with reflecting microwave radiation is the microwaves are designed with shielding assuming the user (consumer) uses the microwave for the purpose for which it is intended, i.e. heating (cooking) food or drinks. Alternate uses can put the user at risk. :))
     
  7. Oct 21, 2005 #6

    ShawnD

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    The sparking and discharging caused by putting metals in the microwave only happens when the metal is not a well-connected solid piece.

    Put a spoon in the microwave and run it for 10 seconds. Did it spark? Is the spoon hot?

    Put a piece of steel wool in the microwave and run that for 10 seconds. Did it spark? Is the steel wool hot?

    Actually test these yourself and report back what you find. It will be ok as long as you put the spoon and the steel wool on several thick glass/porcelain (dinner) plates. This is to prevent any electrical buildup on the spoon/wool from feeding back into the magnetron and damaging the microwave oven.
     
  8. Oct 21, 2005 #7

    Pengwuino

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    I'm pretty sure microwaves can be focused since that's what they use for satellite communications....
     
  9. Oct 21, 2005 #8

    Danger

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    A maser is a microwave frequency laser. I'd consider that focused.
     
  10. Oct 21, 2005 #9

    Astronuc

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    Yep, and one can do interesting things with a maser.
     
  11. Oct 21, 2005 #10

    Danger

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    Fishing comes to mind...:uhh:
     
  12. Oct 21, 2005 #11

    Mk

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  13. Oct 22, 2005 #12

    Danger

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    Oh great, Mk... now I'm going to have to rent the damned movie to see what you're talking about.:rolleyes:
     
  14. Jan 26, 2010 #13
    This pertains to focusing microwaves. I wouldn't consider a MASER a focused beam (coherent, not focused). However, you can focus microwaves, you just need something with a different index of refraction than the surrounding material (e.g., teflon) and shape the material into a lens. Many police RADAR units have a lens on the front to limit the beam to about a 6 degree divergence.
     
    Last edited: Jan 26, 2010
  15. Jan 28, 2010 #14

    Claude Bile

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    The microwaves (being EM waves) cause the free electrons in metal to slosh around, energising them. There is no readily available means for this energy to dissipate from the metal object, so it continues to build to the point where electrical discharging can occur.

    Claude.
     
  16. Jan 28, 2010 #15

    ZapperZ

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    In practically all objects, the "smooth" surface isn't that smooth. This is especially true for most household objects. When you put this object into EM field environment, these pointy parts of the object enhances the electric field at that point. When you have sharp, pointy geometry that can be as small as sub-microns in size, these can enhances the field quite a bit. Now what can happen here is that, depending on how sharp and how high the field is, you can initiate a field emission current - electrons that tunnel out of these tips via tunneling and the large bias electric field. This current can be quite high, or I should say, the current DENSITY can be quite high, since it is coming out of a very small area of the tip, and the current density is higher in metals than in an insulator because of the abundance of conduction electrons.

    Because of this current, a number of things can happen in this case (i.e. microwave oven scenario and not in a vacuum): (i) there can be heating effects that could melt the very tip, which in turn causes more adsorbed gas to be liberated and causing a discharge, or (ii) the emitted electrons themselves have enough energy to cause a discharge in the gasses already present in the microwave.

    Note that the magnetic field in the microwave can also induce surface currents on the metal, even at the tip, to also add to the heating effects. So the situation isn't that simple if one wants to really dig into this.

    Zz.
     
  17. Feb 2, 2010 #16
    The metal reflects the microwaves, and if they get reflected back into the magnetron, which generates them, it can be damaged or destroyed.
     
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