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Beginner Question on the Electromagnetic Spectrum

  1. Jul 17, 2009 #1
    Hi,
    I'm new to the subject and I've been trying to catch up quickly, but something that's been bothering me is why exactly do Microwaves heat waters, fats, etc., even though things with a similar frequency and wavelength(Radio waves, Infared) do not, or at least as quickly?
     
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  3. Jul 17, 2009 #2

    rock.freak667

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    It has to do with http://en.wikipedia.org/wiki/Resonance" [Broken].
     
    Last edited by a moderator: May 4, 2017
  4. Jul 17, 2009 #3

    negitron

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    Microwaves ARE radio waves. Specifically, they are radio waves with a frequency of 2.45 GHz (for consumer microwave ovens; industrial ovens typically used for things like curing adhesives often use 915 MHz). The 2.4 GHz band used for many things like cordless phones and other wireless devices is fundamentally identical; the major difference is power. A typical consumer oven produces a power output of around a kilowatt (1000 Watts), while wireless devices seldom emit more than a few watts. Ovens also contain the RF using a reverse Faraday cage so the energy bounces around in the oven cavity until most of it is absorbed by the food inside (the rest is absorbed by the cavity walls and other components inside like the turntable, etc. or else it escapes. Standing beside a radio transmitter, you only absorb what radiation strikes you directly; the bulk of it misses you and continues off into the distance.
     
  5. Jul 17, 2009 #4
    So the main difference is the fact that a Microwave uses more power than similar wave transmitters?
     
  6. Jul 17, 2009 #5

    negitron

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    Wrong.

    Microwaves heat by a process called dielectric heating in which polar molecules (such as water, fats and sugars) are rapidly rotated by the alternating electric field of the RF, generating heat by slamming into other molecules around them in the process. Resonance is not a factor because 1) liquid water doesn't have a discreet resonance; the molecules in close contact with one another smear out any resonances through various intermolecular forces and 2) the strongest natural resonance of water molecules doesn't occur until around 30 GHz or so, many times higher than the frequency at which ovens operate.
     
    Last edited by a moderator: May 4, 2017
  7. Jul 17, 2009 #6

    negitron

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    Exactly.
     
  8. Jul 17, 2009 #7
    Thanks a lot!
     
  9. Jul 17, 2009 #8
    Is dielectric heating always what occurs when electromagnetic waves of any frequency heat polar liquids?
     
  10. Jul 17, 2009 #9

    Born2bwire

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    Any conductive media will also generate currents in response to the electromagntic waves. The resistance of the medium will dissipate these currents as heat loss. A medium like water however is not very conductive so the primary means of heating is done through rotation of the polar molecules.
     
  11. Jul 18, 2009 #10
    What about a nonconducting solid like a microwaveable bowl? Why doesn't it heat up?
     
  12. Jul 18, 2009 #11

    Born2bwire

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    Well... it would appear that your microwaveable bowl does not heat up because it is non-conductive and non-polar and thus not affected by ohmic or dielectric heating. There may be other mechanisms for heating but none that I can think of offhand.
     
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