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High!county's
Finally done the expt. I promised. Results:
Equip: Glass measuring cup nearly full of tap water (Most of Monterey
water is well water -- aquifer filled by Salinas river, so very hard.),dial
stem bi-metalic thermometer, and the oven.door.
First several heatings: on removal, boiling vigorously when I opened the
By the time I could read the TM it was 100 C and not boiling.107
After several "trials" on removal, boiling sl., but continuously, temp.
Probably sl. higher at equilibrium while being heated. In ~ 3 s. stoppedbottom
boiling @ ~105 C. Cools more slowly to 100 (and below). If I rub the
of the cup with the end of the TM, it boils and the temp. drops morerapidly
to 100 C., at that point it them stops boiling
bc
If I understand correctly, the water that is boiling after microwaveheating
is above 100 C. The energy supplied by a microwave (klystron) tube islayers
funneled into a rather narrow surface layer. That layer can become
superheated since the the power input exceeds the power output from the
layer into the remaining mass of water. The fact that water has a rather
high specific heat exacerbates the situation. Thus when the surface
are superheated, the thermometric measurement probably weighs the surface
more than the bulk. Was the thermometer plunged into the middle of the
water or barely immersed?
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