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Homework Help: Microwave Oven!

  1. Sep 20, 2009 #1
    1. The problem statement, all variables and given/known data
    Microwave oven I. The glass window isn't important to the microwave oven's operation, but the metal grid associated with that window certainly is. The grid forms the sixth side of the metal box that traps the microwaves so they cook food effectively. What is the approximate dimension of the holes of the grid? Explain why the metal grid is critical in preventing microwaves from escaping outside to cook your brain.

    Microwave oven II. Ceramic plates, glass cups, and plastic containers are water-free and usually remain cool while the food is being cooked in a microwave oven. Even ice has trouble absorbing microwave power because of its crystal structure restrict the water molecules’ motion. Explain why only foods or objects containing water or other polar molecules cook well in a microwave oven. (Hint: water or polar molecules are electrically polarized and will tend to rotate into alignment with the field.)

    2. Relevant equations

    3. The attempt at a solution
    I'm not sure how to start the problem of finding dimensions of the holes in the grid. I know the reason that the grid is critical probably has to do with the electromagnetic properties of the microwaves and the way they interact with the grid. My assumption would be that the magnetic properties of the of the wave cause a charge to be induced in the grid of the same charge as the electric properties of the wave, then the wave is "reflected" due to the repulsion of the same charge. But I feel like that's not quite the right answer and was wondering what assumptions are wrong.

    For the second part, I'm thinking that the polarity of the water molecules causes them to line up with the electric field. If this is the case, then there's rotational motion of the molecules, along with vibration from the molecules being penetrated by the microwaves. The result of the food being warm is caused from atomic friction caused by the motion of the molecules.

    Please help if you can!!! I'd really appreciate some input.
  2. jcsd
  3. Sep 20, 2009 #2
    Consider the wavelength of microwaves. What aperture would be needed to significantly reduce the transmission of the microwaves through the grid? Try likening it to light passing through a single slit.
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