3 posts • joined Tuesday 22nd May 2007 18:18 GMT
To Big Boomer
Beware of talking down the heating effects of different radio frequencies. Domestic microwave ovens operate at 2.45 GHz for two main reasons:
a) The size of the resonant cavity and the cost-efficiency of producing the magnetron for that frequency are both close to ideal at 2.45 GHz.
b) The actual penetration depth at that frequency is in the cm range, the actual value depending on the salt content of the water but, again, ideal for food heating. The first resonant peak for the water molecule itself is above 1 THz and the highest peak is in the infrared range. Microwave ovens cook quickly because the penetration (to whatever depth) is immediate rather than the gradual with conventional heating. In practice though, a combination is used (the instruction to "stand for x minutes before serving" provides extra insurance that the heating-through process is as complete as it can be).
also, the frequency band around 2.45 GHz was one of the first pieces of radio spectrum globally assigned to ISM - Industrial, Scientific and Medical usage - well before there were such things as microwave ovens.
Actually, anywhere in the 900 MHz to 5 GHz range is technically fine for microwave cooking and some professional ovens (big ones) operate at 915 MHz. Remember also, that the medical treatment known as Diathermy (tissue heating) operates way down at 27 MHz (did someone mention CB radio?) while frequency-hopping Bluetooth devices are another user of the same spectrum.
The word "microwave" means nothing special other than "tiny wave". Depending on which text book you read, the microwave range starts as low as 300 MHz (the VHF/UHF boundary) or 1000 MHz (a nice round number). They're just radio waves which are the lowest energy part of the electromagnetic spectrum which goes on to include infrared, visible light, ultraviolet, x-rays and gamma rays.
Ron Schmitt's "Electromagnetics Explained" (pub: Newnes) is a good primer for those wishing to understand the phenomenon a bit and be able to debunk the quacks with their patches and crystals.
One myth needs correcting
I go along with the sentiments of the majority of contributors but there is is one persistent myth in Guy's commentary:
Water molecules do not resonate at 2450 MHz and microwave ovens need not operate at that specific frequency. In fact, ANY radio frequency from 3 kHz to 300 GHz will heat in a similar fashion if enough power is applied. Indeed folk or a certain vintage will remember a medical deep tissue heating treatment called diathermy which operated at 27 MHz (CB territory!). This is why TV transmitters (470 - 860 MHz) operate on low power when some poor souls have to climb the mast to paint it or replace the bulbs in aircraft waring lights.
Microwave ovens for domestic use operate at 2.45 GHz because
a) A licence free slot is available there worldwide for ISM (Industrial, Scientific, Medical) use.
b) The RF circuitry eg the magnetron frequency generator are relatively cheap and easy to mass produce at that frequency.
c) The internal dimensions of the cavity in which the heating takes place and the waveguide feed are convenient for domestic equipment.
Some professional ovens however work at 915 MHz (GSM mobile phone territory!) where the functioning parts are pro-rata larger.
The basic mechanism of the heating is the same as for conventional infrared ie vibration due to absorbed energy except that in the case of microwave ovens this is accomplished due to the propensity of bipolar water atoms and some fat molecules to twist in the applied electromagnetic field leading to friction.
Microwave and 2.4 GHz
Just to add to the "oven" comments, another reason for the use of 2.45 GHz is the convenient size of the magnetron, waveguides and cavity at that frequency and the economies of scale of manufacture of these items.
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