NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/physics/21596

Text File | 1992-12-22 | 3.5 KB | 72 lines

Newsgroups: sci.physics
Path: sparky!uunet!gatech!concert!uvaarpa!murdoch!kelvin.seas.Virginia.EDU!crb7q
From: [e-mail] (Cameron Randale Bass)
Subject: Re: can sound waves boil water?
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: University of Virginia
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Date: Tue, 22 Dec 1992 20:08:42 GMT
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In article <[e-mail]> [e-mail] (Daniel Seeman) writes:
>In article <[e-mail]> [e-mail] (CHEW JOO SIANG) writes:
>>I was thinking of this the other day - we all know that a microwave oven
>>works by resonating the water molecules at its natural frequency - this
>>causes a rise in its temperature. The question is, can we do the same
>>with sound waves - using it to resonate the water molecules. I know that
>>you need incredibly high frequecies to achieve it but is the concept
>>sound theoretically?
>
>Hi,
>
>The reason Microwaves vibrate the water to resonance is partially due to
>their wave length.  And that magical wave length is on the order of centimeters.
>A bit of simple calculation tells you the frequency for ---say, a 3cm Microwave
>would be:
>
>    f = c/l (l= wave length, c = speed of light, f = frequency)
>
>    f = ~3x10^8(m/sec)/.03m = 1x10^10 (cycles/sec)
>
>The above frequency is roughly resonance.  (The wave length I used is a bit too

     Banish this word 'resonance'.  The reason microwaves vibrate water
     is related to wavelength, but it is not at a 'resonance' in common usage
     of the term.  There are various 'microwaves' that dielectrically heat
     water.  Typical frequencies are 915 Mhz, 2.45 Ghz, etc., however
     these frequencies are not exclusive.  There is a range of frequencies
     in which heating can be effectively accomplished.  I suggest Arthur
     von Hipple's "Dielectrics and Waves" for an explanation of mechanisms
     from a classical approach to a quantum approach.

>small  ---take a look at the wave guide on you local micro wave receiver dish
>and see what it's dimensions are---but it made the calculation easy ;-).
>Now all you need to do is to find the speed of sound in water and find the
>wave length of sound that is needed to provide the same agitation that the
>micro wave made. Then, just design your sound amplifier/wave generator to these
>specifications.

     Sound is a different disturbance, whose properties are
     different from a electromagnetic wave.  Do not expect to
     have to go to Ghz ranges to effectively 'boil' water, especially
     considering DC current can boil water.

>Due to the wave's length and characteristics it would take longer for sound
>sources to heat water (otherwise you could hear your home's fast cooker
>working every night as it heats the evening's dinner...).  In this case, you
>are relying on pressure waves to eventually excite the molecules to resonance,
>rather than fluctuating electro-magnetic fields.

     Please strike this word 'resonance'.  A resonance for water in
     E&M waves is up in the tens of Ghz range.  Dielectric heating is
     not a resonance phenomenon in the noted range.

                               dale bass

--
C. R. Bass                                          [e-mail]
Department of Mechanical,
     Aerospace and Nuclear Engineering
University of Virginia                              [phone]