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If infared energy makes things hotter, why do microwaves use radio waves?

  1. Aug 28, 2010 #1
    If infared radiation is responsible for somethings heat, why do microwaves use radio waves?
     
  2. jcsd
  3. Aug 28, 2010 #2

    cjl

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    Infrared does make things hotter, but it isn't the only way to make things hotter. Microwaves use microwave radiation, which is actually somewhat shorter wavelength than radio, primarily because they penetrate the food somewhat better than infrared do. They still heat from the outside in, but they penetrate farther and heat the middle more quickly than if you were to heat the food purely with infrared radiation.
     
  4. Aug 29, 2010 #3

    Redbelly98

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    Electromagnetic radiation of any wavelength can cause heating if it gets absorbed.
     
  5. Aug 31, 2010 #4
    Microwaves are used because their wavelength is proportional to the dimensions of the cavity of the appliance (18 - 24 inches or so). The waves bounce back and forth inside the cavity and are in phase. Electromagnetic waves of a longer or shorter wavelength would go out of phase and cancel each other out.

    As I understand it, the magnetic field portion of the microwave oscillates back and forth, causing the H2O atoms (which act as dipole magnets because of the orientation of the H atoms) to flip back and forth as the positive and negative waves transverse them. The kinetic energy from the flips is absorbed by nearby molecules (carbohydrates, protiens...) by collisions. This kinetic energy gradually builds throughout the food, and is manifested as heat, and occurs throughout the food, not just the inside or outside.

    As the previous poster said, any electomagnetic wave will heat the outside of an object if it is intense enough.
     
  6. Aug 31, 2010 #5
    Water has a resonance around 2.4GHz, which is where your home microwave operates. The molecules absorb the uW energy and wiggle more, which means they get hotter. Strangely enough your normal 802.11b wireless i'net also operates at that frequency and tends to get lost in the mist as well...
     
  7. Aug 31, 2010 #6
    No, microwaves are used because they are very readily absorbed by water molecules.

    No, any electromagnetic wave will heat an object, period. Less intensity merely heats the object less.
     
  8. Aug 31, 2010 #7
    I'm having some confusion over absorption and resonance. When I think of absorption, I think of an atom's electron moving to a higher orbital, and then emitting a less energetic photon as it moves back to a ground state. I'm not sure if that applies to whats going on with the water molecule in a microwave. Not sure about resonance, but I think we're saying about the same thing, the water molecules have kinetic energy because of the wavelength of the microwave.


    I like your explanation better.
     
  9. Aug 31, 2010 #8

    Pythagorean

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    Still, why is infrared associated with heat? What's the physical mechanism behind using infrared detection as a substitute for heat detection? Why not ultarviolet or visible spectrum, for instance?
     
  10. Aug 31, 2010 #9
    You CAN use visible spectrum, if the body's hot enough. Fire a piece of wood and it will radiate red. Infrared is emited at lower temperatures because it requires less energy (less heat to put into body) to radiate.
    Also it has probably something to do with emission spectrum of material, though I'd rather not talk about such things yet.
     
  11. Aug 31, 2010 #10

    Redbelly98

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    And to further answer Pythagorean's question ... we're usually interested in detecting heat from room-temperature objects, or slightly warmer or cooler. But at those temperatures no detectable visible or uv radiation is radiated, only infrared.

    Think of how hot a stove burner has to be before you see it visibly glowing red. And the sun is hotter still, hot enough to radiate uv which you can detect by getting a sunburn.
     
  12. Sep 1, 2010 #11

    Pythagorean

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    ok, so it's a matter of the "scale of common interest".
     
  13. Sep 1, 2010 #12
    I like it, a very advanced method of discovering if an object emits UV, haha :D
     
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