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Microwave vs metallic objects

  1. Nov 25, 2009 #1
    Does a microwave oven burns metallic objects?

    I never put spoons, forks, or aluminum foil inside a microwave because I'm afraid that the microwave oven will burn them.

    If I remember correctly, my brother put an aluminum foil inside and it burnt.
    Nonetheless, the interior of the microwave oven was metallic, and of course, it didn't burn.
    And also, I know that microwave is even less powerful than infrared, but then how did it burn the foil?

    please help
     
  2. jcsd
  3. Nov 26, 2009 #2
  4. Nov 26, 2009 #3

    diazona

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    I wrote a blog post about this a while ago, although unless you're familiar with the math, it probably won't tell you much more than pzona already did. Basically, the idea is that since the conduction electrons in a metal can move around more or less freely within the metal, they're able to absorb a lot of energy from the microwaves. This causes trouble in two ways: (1) all those moving electrons heat up the metal and can possibly cause sparks, and (2) the electrons also re-emit (a.k.a. reflect) microwaves back to the magnetron of the microwave oven, which can damage it if it's not built with the proper safeguards.

    That blog post I linked to, by the way, was motivated by a recent episode of Mythbusters where they tried to explode C-4 in a microwave oven... if I remember correctly, in an earlier episode, they actually put aluminum foil in a microwave oven to test precisely this effect (i.e. it catching on fire). If you ever get a chance to watch that episode, I'm sure you'd find it interesting.
     
  5. Dec 7, 2009 #4
    Microwaving foods with spoons in them, such as coffee or tea, is not dangerous at all - I do it all the time. The water is such a good microwave load that it reduces the microwave field so much that arcing can't occur. As to other metal - trays, foil, etc. In general these are also not dangerous as long as there is a reasonable food load in the oven. By themselves they can exhibit arcing (sparking) around rough edges. I did the original, and to this day, still the definitive research on this starting in 1977 and continuing into the early 80's.
     
  6. Dec 8, 2009 #5
    I recall putting a frozen candy bar in the microwave when I was young, not knowing the wrapper had an aluminum coating on the inside. There was a noticeable arc and some of the package burned away. I'm not sure why anyone would purposely put a metal spoon in the microwave. I don't see any real purpose to warming your spoon. Anway, in a metal the electrons are free to roam so it makes sense that microwaves would dislodge them and send current flying about.
     
  7. Dec 8, 2009 #6
    My guess is the photons do NOT have enough energy to knock electrons loose. Current(sparks) is induced by the magnetic field, "dipole moment". Here is what your link says:

    "When electric waves hit the surface of metals, electrons near the surface of these metals are given the order to move".

    I think Photons knocking electrons loose is "ionizing radiation", I dont think this is what is going on here, just my best guess.
     
  8. Dec 8, 2009 #7

    Born2bwire

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    Yeah I agree, he means the excitation of currents and charge densities. The electromagnetic waves are not knocking off electrons from the metals. Anytime a wave is incident on a conductor, the wave induces currents that create secondary waves to cancel out the incident wave inside the conductor. The result outside of the conductor is a reflected wave. But the currents are excited by both the electric and magnetic fields, though we typically treat the electric field's force as first order and magnetic field's force as a second order effect (but separating them as a first/second order is not really useful for electromagnetic waves). The actual reflection of the wave is not much of a concern, the entire microwave is a resonant cavity though we generally like to have some kind of load inside the microwave. The main problem is that the electric fields between the walls and the metal can become very high resulting in the air breaking down, creating the sparks. This is from the fields that build up due to the charge distributions on the metal. This is especially compounded when you have any sharp edges to the metal. A sharp point on a conductor will give rise to an effect known as field enhancement. This is where the electric fied in the volume immediately surrounding the point will be much higher than if we had a rounded geometry. This can greatly facilitate dielectric breakdown of the air. In addition, the currents in the metal give up energy in the form of heat due to ohmic losses in the metal. This can give rise to hot spots and once again if we have areas that have reasonably high current distributions we can have a large amount of heat generated (think of how hot a copper wire gets when you short out a battery (not an advisable recreational activity)).

    The "dipole moment" currents that you mention though are not excited in metals. These are displacement currents, currents that arise from the polarization of the atoms and molecules in a medium. The atoms are polarized by an applied electric field, resulting in a dipole moment from the displacement of the charge distribution within the atom. While the displacment current does not physically migrate, the changing electric field causes these dipole moments to also change in time which gives rise to a current, not unlike how an AC voltage in a wire oscillates the electrons back and forth without any net displacement. This is the main means of heating of food in a microwave, the food undergoes dielectric heating where the polarized molecules will rotate to stay aligned with the changing electric fields. This is the same thing as displacement current. Metals, however, primarily excite conduction currents. These are currents that flow around the surface of the metal due to the fact that the conduction electrons in a metal are free to move about the bulk. The main mechanism for energy to be dispersed by conduction currents is by the collision of the electrons with the bulk lattice, explained in the Drude model.
     
    Last edited: Dec 8, 2009
  9. Dec 8, 2009 #8

    alxm

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    I don't think aluminum foil would be a good idea; it'd easily get to temperatures (locally) which could cause it to melt (or more likely, oxidize).

    Unless you want your food filled with flakes of aluminum oxide. (Which I guess is safe in the sense that I believe it's too inert to do anything in your body.. Non-toxic in other wordS)
     
  10. Dec 8, 2009 #9

    Born2bwire

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    Mmmm... that tangy zest that can only come from alumina.
     
  11. Dec 9, 2009 #10
    Thermodave - the spoon is in the coffee for 2 reasons - to prevent superheating - very important!; also, I add sugar. Also, the spoon won't warm from the microwaves - it is too good an electrical conductor, so there is no power for heating generated in the spoon (P=I2 x R) - Your experience with the wrapper is that it is not aluminum foil, but an imperfect coating (on the scale seen in electron microscopes) - so now there is a resistance to the current flow and it heats - this is the principal of microwave susceptors - those gray trays used to crisp microwave pizza.

    AXLM: the aluminum foil won't heat for the reason given above. However, it i possible that some arcing may occur at edges, but only if there is a poor food load - very little or no food; frozen food (where the "no metal in the microwave myth originated); or dry crackers, etc.
     
  12. Dec 9, 2009 #11
    In simple terms, non-grounded metal in the microwave acts as an antenna. If you want to see real sparks cut something into 1/4, 1/2 etc wavelength of the microwave (doesn't even have to be a perfect conductor, like pencil lead), and it will cause great damage around it because of over-excitation. Things which are not "designed" like this will still receive a good deal of energy, though.. Besides burning the bottom of your microwave, it will re-transmit energy back into the magnetron possibly causing permanent damage to the microwave.
     
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