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Thermal Equilibrium - in a microwave?

  1. Apr 17, 2012 #1
    hi,

    i don't quite know how to pose this question but i'll try my best.

    if you have an object and heat one end of it, energy will move from an area of high energy, to an area of low energy. By this method, heat transfer will stop when all regions of the object are at the same temperature. (at least as far as i know, this is what will happen)

    so in a microwave, water molecules absorb energy in the form of 'microwaves'. if the energy of a photon is equal to planck's constant * it's frequency will the water molecule no longer absorb microwaves once it has the same amount of energy as the photons that hit it?

    i'm convinced i'm probably using 'classical' ideas etc to try and think about this, but i really don't know enough about photons and the like to make sensible assumptions. i was tempted to ask if the heating of the water has anything to do with electrons absorbing energy and changing 'energy level' - but i have a feeling that's not the case.

    so if anyone can explain to me whether my first idea is sensible or not, that would be great.

    P.S although my understanding of how a microwave works might be primitive, i'm really more interested in the actual absorption part of my question(s)

    thanks!
     
  2. jcsd
  3. Apr 17, 2012 #2

    sophiecentaur

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    In the case of a molecule, it's probably better to talk in terms of the whole system of charges rather than harking back to the old familiar Hydrogen Atom that is the first thing that comes into ones head. (Me too). Microwaves have very low photon energy c/w light and the energy levels involved are more rotational I understand.
    Also, the familiar, distinct levels that apply to the isolated hydrogen atom become spread into bands of energy levels (the Pauli exclusion applies vigorously in a condensed medium so that all atoms cannot have the same quantum numbers - the possible energy states are squeezed out into bands) So any one molecule could get several doses of microwave photons and this energy can be exchanged with adjacent molecules, resulting in thermal conduction. (More of a straight mechanical interaction) Water is a pretty poor thermal conductor but the microwaves do manage to penetrate some way into a large mass of watery food. But, afaiaa, it's normal thermal conduction that's needed to cook the insides of a large lump of meat.
     
  4. Apr 18, 2012 #3
    In typical usage, the microwave oven won't reach thermal equilibrium, so it's moot to consider. The oven puts out a certain amount of microwave power (on the order of 1kW) which is absorbed by the food molecules, and the food heats up according to the specific heat of the food.

    If you want to consider the idealized situation of thermal equilibrium, you can consider that the food also radiates some power which depends on the food's temperature (and roughly follows a blackbody distribution). Some of this power is lost through the walls of the microwave, so you don't reach thermal equilibrium, but rather some kind of steady state where the power loss from the food equals to power absorbed. You can perhaps estimate the temperature of the food using a blackbody. I think your food will be rather overcooked.
     
  5. Apr 18, 2012 #4

    sophiecentaur

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    I might point out that you seldom really get thermal equilibrium in any form of cooking. Long periods of boiling of a ham may bring the centre up 100C whilst the outside is also at 100. But in baking and roasting, most meat joints are 'done' when the inside has been at only 75 to 80C for a few minutes. The outside will be at 180C+ and nicely carbonising.
    Cake baking is a real problem as it is very easy to burn the outside and yet still have a soggy middle. Fan ovens have entirely different characteristics c/w/ conventional ovens as they produce much more heating effect at the surface for a given oven temperature.
    I wish someone would do some definitive work on appropriate conversion factors for different recipies.
    I'm showing you my feminine side here chaps - I love cooking and baking.
     
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