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B Why can't I kill the ice cubes >:o (in a Microwave Oven)

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  1. May 6, 2016 #1
    I recently talked to a friend of mine about microwaves.
    Specifically that (to my knowlege) the microwave radiation is primarily absorbed by a rotational
    degree of freedom of water molecules.
    Which got me thinking.
    1.First of all what would happen if you put a extremely cold ice cube ( with no liquid water on surface)into the microwave ?
    Wouldn't the water in the ice be unable to absorb the radiation because it is bound in the crystal lattice ?

    2.Second what happens when you don't put anything into the microwave ?
    Resonance catastrophe ???

    Is the radiation just absorbed by the walls as the Intensity in the box builds up more and more?
    Maybe gas discarge ... that would be cool, but i doubt it would happen.
    Is doing it actually dangerous, since a observer might get hit by high levels of microwave radiation "leakage"?

    The third question is the one from the title:
    Why can't i kill the ice cubes :DD?
    When you freeze(glass) water bottles they will get destroyed by the expansion of water to ice.
    But when i put a half frozen ice cube into the microwave and the ice melts form the inside out,
    the ice cube doesn't collapse in on itself(implode) because of the contraction.

    That was a huge disappointment.:sorry:

    I heard some quiet crackling but no big structural difference can be observed.

    3.How are these stresses released ?
    Does air somehow enter through cracks ?

    BTW If you replicate this make sure you let the ice cubes lay around for a while before putting them
    in the microwave, since a surface layer of water might warm the initially cold surface wich would make it crack due to thermal shock.


    PS I realized something similar must also happen when freezing ice cubes from the inside out...
    When the outer shell freezes it takes a shape that would accomodate some volume of water.
    However this water expands when freezing and pushes on the outside shell.
    I have actually observed the surface bulging out on some ice cubes.
    I am confused how it can change shape so much without completely breaking apart.
    Could someone explain it ?
     
    Last edited: May 6, 2016
  2. jcsd
  3. May 6, 2016 #2
    Ice does not absorb microwaves as well as liquid water, but I think some heating does occur. Also remember whatever the ice is touching while in the microwave.

    Plastic water bottles will stretch to accommodate freezing. The manufacturers would be pretty dim not to anticipate this situation. However when you thaw them, the water IMHO tastes nasty, and I'd bet has chemicals from the plastic that you wouldn't want.
     
  4. May 6, 2016 #3
    If you leave a microwave on for too long it will turn into a time machine. Don't believe me? Next time you're hungry, put a frozen burrito in the microwave and just experience the time distortion that takes place until the burrito is ready. Yeah!
     
  5. May 7, 2016 #4
    While i appretiate your comment my question remains unanswered.:frown:

    At least question 2 and 3 ... Alqr did a nice job in answering the first one.
    Ps: I don't think the elasticity of the water bottles is intended to protect them from cracking due to the expansion.
    That is probably just a pleasent side effect.If you make the bottles walls very thin they are always somewhat deformable.
    (Guess why it is called "plastic"o0))
     
  6. May 7, 2016 #5

    russ_watters

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    Ice melts fine in a microwave.
     
  7. May 7, 2016 #6

    anorlunda

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    That is not supposed to happen, but it can.

    I owned one of the original Amana Radar Range microwaves. Three times it got mistakenly turned on with nothing inside. Three times it melted a hole in the glass tray leaving a red hot glass marble about 3 cm in diameter. When I went to buy new trays, the repair people at Amana refused to believe me when I told them what happened.

    I can't explain those events. The glass just doesn't absorb the microwaves. Nevertheless, I am an eye witness. It did happen.
     
  8. May 7, 2016 #7
    Wow that sounds crazy :))
    Some uneducated speculation on my part: SPOILER most of it is highly unlikely!
    If your microwave is really "impermeable" or in other words doesn't leak any radiation,
    then i can imagine the radiation building up to levels where you really might get gas discharge ...
    Maybe the electric field at a "antinode" of the resonating microwave was high enough to cause an electron avalanche after some free electron has been randomly produced by radioctive decay or something.
    (There are some videos of almost in half-cut-grapes/flames put in a microwave producing a "stable" plasma(very cool))
    The glass gets heated by some gas discharge ... and maybe it is like a positive feedback loop.
    As the glass gets hotter and gains "mobility" it becomes better at absorbing microwave radiation.
    So therefore the glass doesn't heat up equally but just on some parts wich already were hot.
    In some sense the electronic stucture of SiO2 is similar to H2O (it is simply far more massive), so it makes sense that it would also absorb microwave radiation.

    Actually ... i don't think gas discharge would happen :rolleyes:.That is just wishful thinking on my end.
    Glass probably absorbs some microwave radiation at all times and that starts the "chain reaction".
    However the positive feedback loop seems plausible to me.

    Still that story seems too good to be true ...
    When red hot the glass would radiate quite a lot of power away but since the metal surfaces inside are pretty reflective the energy loss might not be that big.
    Additionally microwaves have a lot of power: somewhere around 0.4 to 2 kW

    while i wouldn't want any damage to my microwave it would certainly be a sight to behold...
    maybe i should try to recreate that o0)
    (almost) molten glass... in a damn microwave:wideeyed:

    PS Is one of you feeling like explaining question 3 ?
    3.How are these stresses released ?(In ice cube freezing from the outside and ice cube melting from the inside)
    I am confused how it can change shape so much without completely breaking apart.
     
  9. May 7, 2016 #8

    FactChecker

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    If you say that an ice cube melts from the inside out, doesn't that contradict your theory that the water absorbs more radiation?
     
  10. May 9, 2016 #9
    It sure would, but we have a little misunderstanding here.
    It you put a regular ice cube into a microwave oven it will (primarily) melt from the surface.
    However, i wanted to see wether ice cubes who melted from the inside would "implode".
    Specifically for that purpose i took half-frozen ice cube, so the water inside it would absorb most of the radiation, making it melt from the inside.
    Sadly my the ice cube does not shatter.
    That is what inspired my earlier question:
    3.How are these stresses released ?(In ice cube freezing from the outside and ice cube melting from the inside)
    I am confused how it can change shape so much without completely breaking apart.
     
  11. May 9, 2016 #10

    FactChecker

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    Ok. I'll buy that.
     
  12. May 11, 2016 #11
    AFAIK the heating from the inside is misleading: It's NOT heating from the outside. MW radiation is absorbed in an inverse exponential from the surface. If the object has low absorption, it will heat more or less uniformly.

    You can get differential heating due to part of the material thawing, which greatly increases it's absorption.

    If there is nothing in it, then the MW antenna in a recent appliance doesn't couple well with the load, so less power is used. Eddy currents in the shield will cause the cabinet to get warm.

    Ice: Ice is plastic under pressure: It starts to flow around 50-100 psi (100-200 feet depth in a glacier) Look at your ice cube tray: None of the cubes have flat tops.

    If the cube melted from the inside (video, please...) then the maximum pressure on the outside is only 15 psi. I would expect it to sit there until a crack formed, then possibly jiggle as the air rushed into the crack. If you melt half the interior of the cube, and the shrinkage is 14% (It's around that.) then the shrink volume is about 7% of the volume of the cube. Air is about 1/800 the density of water. So the air that rushes in will be about 1/5600 of the mass of the cube. I'll stick with jiggle.
     
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