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Microwave oven

  1. May 17, 2004 #1
    Greetings all,

    I've recently been wondering why I see sparks of light when I stick certain metal objects into a microwave. I figured that it has something to do with photons supplying energy to the electrons. When the electrons gain the energy they move to higher energy levels, but when they return they emit light. Is this a viable explanation?
     
  2. jcsd
  3. May 17, 2004 #2

    HallsofIvy

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    No. What you are seeing are not "sparks of light". They are, in fact, actual sparks- that is electrical sparks because the microwaves are stimulating the electrons in the "metal objects" you foolishly put in the microwave. They can burn out the microwave and even conceivably start a fire! The manual that came with your microwave warns against putting metal objects in a microwave. Stop doing it!
     
  4. May 18, 2004 #3
    Thanks for the warning. These sparks must be emitting light otherwise I would not be able to see it? What exactly is the reason for these sparks to occur?
     
  5. May 19, 2004 #4

    dcl

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    On the topic of microwave ovens, is anyone able to explain why Ice doesn't melt inside when put a microwave?
     
  6. May 19, 2004 #5

    chroot

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    You can only damage the oven if the sparks actually complete a short circuit through the magnetron itself. It would take a very large piece of metal to do this, and probably won't ever happen. The only other danger is sending pieces of molten metal into the walls of the oven, damaging it cosmetically (though probably not functionally).

    The reason the sparking happens is because the oven cavity is a resonator -- it's filled with oscillating electric and magnetic fields. The electrons in the metal have to constantly move back and forth to keep the net field inside the conductor zero (conductors always have zero charge inside them, as the electrons are free to slide around to neutralize any field). Sharp point of metal can thus generate very very high voltages on them when the electrons are all sloshed in one direction. The sharper the point of metal, the higher the voltage. (Hence, a fork is more dangerous in a microwave than is a spoon.) When the voltage is large enough to exceed the breakdown voltage of the air, the air becomes a plasma and electrons jump across the air in a spark. Their collision with the atoms in the air produces the light that you see.

    - Warren
     
  7. May 19, 2004 #6
    The microwaves in the oven are at a frequency readily absorbed by liquid water. Ice doesn't absorb at that frequency as well and thus doesn't absorb the energy as efficiently.
     
  8. May 31, 2009 #7
    It is because the dielectric loss factor of ice is about 1/3000th that of water. That means that ice is about 1/3000th as able to absorb microwave energy to cause heating. The reason is that water molecules in liquid water are randomly located and essentially are free to rotate, and it is this rotation, as influenced by the microwave field that causes the heating (it's actually much more complicated but that should do for now). In ice, however, the water molecule is caught in a latice of other water molecules and is therefore not free to rotate so it can't generate heat. What is really interesting about this is that there is always some free water unless you are at at temperature of about minus 125 F (I'm not exactly sure of that number but it is close to that). So, what can happen, is that water starts to heat, the microwave field distorts so that area heats favorably, and you can end up with the crazy situation of boiling water inside or on the surface of an ice block.
     
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