From: dseeman@novell.com (Daniel Seeman)
Newsgroups: sci.physics
Subject: Re: can sound waves boil water?
Date: 22 Dec 92 21:37:53 GMT
Organization: Novell Inc., San Jose, Califonia

In article <1992Dec22.200842.9033@murdoch.acc.Virginia.EDU> crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:
>In article <1992Dec22.191142.7093@novell.com> dseeman@novell.com (Daniel Seeman) writes:
>>In article <1992Dec22.151439.29874@nuscc.nus.sg> eng10370@nusunix1.nus.sg (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                                          crb7q@virginia.edu        
>Department of Mechanical, 
>     Aerospace and Nuclear Engineering
>University of Virginia                              (804) 924-7926

Hi,

Dale, you are of course right.  I had some trouble with that word resonance too
when I was writing this response, but continued on my thought anyway.  I suppose
that points to my laziness, (read not much confidence in my intuition---"hey, I
knew that! Why didn't I just say it!")  Still, I hope (at least for discussion's
sake) that some of the information is ---well--- accurate...

Happy Holidays!

dks.
Newsgroups: sci.physics