MadSci Network: Physics |
Hi Arun,
That's a good follow-up question, and it brings up an important concept
about temperature. The temperature of something is basically a way for us
to measure the speed of the internal molecules. When we heat something in
the microwave, we are applying energy to the molecules that makes them
move faster and faster.
So, the velocity of the individual water molecules depends on the
temperature of the water. The equation breaks down like this:
3/2 k*T = 1/2 m*v^2
T is the temperature in Kelvin,
k is Boltzmann Constant = 1.38066 X 10^-23 J/K
m is the mass of a water molecule = 2.99 X 10^-26 kilograms
(18 grams/mole / 6.02X10^23 molecules/mole)
For water just above freezing, each molecules is moving around 615
meters/second. For water that's almost boiling, each molecules is moving
719 meters per second. These are average speeds.
Wait a second! Does that mean that the molecules in frozen ice are
travelling at 615 meters/second? Yup. All molecules of all objects that
are NOT at absolute zero are vibrating at some speed. 615 m/s is just
the point where the attraction between molecules is enough to keep them
together as a solid. 719 m/s is the point where molecular attraction is
enough to keep them as a liquid. If the molecules are travelling any
faster, they'll fly out of the liquid and become airborne. (also known as
boiling!)
The natural frequency of water is a bit more complicated, because it takes
into account the mass of water molecules, the attraction between
molecules, the distance between molecules, and some other stuff. Suffice
it to say that most microwave ovens put out a frequency of 2.5 gigahertz.
This means that these microwaves pulse 2,500,000,000 times per second.
This isn't the lowest (also known as "primary") resonant frequency for
water, but microwave manufacturers use 2.5 GHz because they want the
microwave to work at any and all water temperatures. There's lots more
techno-babble about resonance, matching, and the engineering of
microwaves, but that'll have to be a separate question.
I hope this helps!
Jeff Yap
Mad Scientist
References:
Kinetic Theory of Gasses
Simple
Harmonic Motion
How Stuff Works -
Microwaves
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