Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Fwd: Microwave boiling water]



I'll do it, but I expect that the TM is accurate w/in its resolution (1/2 C
deg -- it's got a mirror next to the scale!). Since boiling ceased, while
"scratching, @ 100, I assumed calibrated.

I've done some more kitchen phys.: very very prelim. results. Three mills
of al. foil cuts transmission to ~ 10% about half cm of water v. ~
equivalent to one mill. al. ~~ 50% see ff. I used two expanded poly
styrene cups ~ 1/3 full (190g) one covered xcpt bot. with successive layers
of HD al (one mill.). After 1st or 2nd trial I placed the cup on four layers
of foil. Didn't notice much diff. in placement in the oven. (fan working
--varies the dimensions of the resonant chamber and though only 1/3 rd full
included a node wherever placed.) 2nd xpt.: small cup inside larger one
separation ~ 1 cm sides & @ bot. 4 cm. provided by a plastic jar top. iI
used a glass to weigh the inner cup (a mistake as I couldn't find its SH
literally, so did mixtures <0.65 J/g.-C°> any one have a better value? Next
time I'll use lead shot. I covered the top with four mills of al. foil
overlapping the outer water level by 2 cm..

I heated the apparatus (don't laugh) for six min. removed it and the foil top
and measured the inner and outer "calorimeters'" temp. for about an hour and a
half. The initial temps. were 46 and 63 ° C -- not very meaningful 'till
calc'd. their heat caps.


later ,

bc

non metallic TM? Alcohol has the same bond as water so no go;
semi-conductor ones also out? Use IR radiometer!!!!

Tim Folkerts wrote:

It's always nice to see real experiments to test a hypothesis.

But as an experimentalist, I do have one question. How well is your
thermometer calibrated? As a control, you might try boiling water the
conventional way and see what temps you get in a pan on the stove top.

Also, is there a good, non-metallic thermometer that could go directly in
the microwave? It seems that the time delay is still a problem.

Tim Folkerts

High!

Finally done the expt. I promised. Results:

Equip: Glass measuring cup nearly full of tap water (Most of Monterey
county's
water is well water -- aquifer filled by Salinas river, so very hard.),
dial
stem bi-metalic thermometer, and the oven.

First several heatings: on removal, boiling vigorously when I opened the
door.
By the time I could read the TM it was 100 C and not boiling.

After several "trials" on removal, boiling sl., but continuously, temp.
107
Probably sl. higher at equilibrium while being heated. In ~ 3 s. stopped
boiling @ ~105 C. Cools more slowly to 100 (and below). If I rub the
bottom
of the cup with the end of the TM, it boils and the temp. drops more
rapidly
to 100 C., at that point it them stops boiling

bc

"Bernard G. Cleyet & Nancy Ann Seese" <georgeann@REDSHIFT.COM>
@lists.nau.edu: Forum for Physics Educators" <PHYS-L on 02/06/2000 09:15:12
PM

Please respond to "phys-l@lists.nau.edu: Forum for Physics Educators"
<PHYS-L@lists.nau.edu>

Sent by: "phys-l@lists.nau.edu: Forum for Physics Educators" <PHYS-L

To: PHYS-L@lists.nau.edu
cc:

Subject: [Fwd: Microwave boiling water]

--Boundary_(ID_6RP4lgVu7XAtNnXH6lzquw)
Content-type: text/plain; charset=us-ascii; x-mac-type=54455854;
x-mac-creator=4D4F5353
High!

Finally done the expt. I promised. Results:

Equip: Glass measuring cup nearly full of tap water (Most of Monterey
county's
water is well water -- aquifer filled by Salinas river, so very hard.),
dial
stem bi-metalic thermometer, and the oven.

First several heatings: on removal, boiling vigorously when I opened the
door.
By the time I could read the TM it was 100 C and not boiling.

After several "trials" on removal, boiling sl., but continuously, temp.
107
Probably sl. higher at equilibrium while being heated. In ~ 3 s. stopped
boiling @ ~105 C. Cools more slowly to 100 (and below). If I rub the
bottom
of the cup with the end of the TM, it boils and the temp. drops more
rapidly
to 100 C., at that point it them stops boiling

bc

"E.C. Muehleisen" wrote:

If I understand correctly, the water that is boiling after microwave
heating
is above 100 C. The energy supplied by a microwave (klystron) tube is
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
layers
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?
______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com

--Boundary_(ID_6RP4lgVu7XAtNnXH6lzquw)
Content-type: text/html; charset=us-ascii
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
<html>
High!
<p>Finally done the expt. I promised.&nbsp; Results:
<p>Equip:&nbsp; Glass measuring cup nearly full of tap water (Most of
Monterey
county's water is well water -- aquifer filled by Salinas river, so <b>very
hard.),&nbsp; </b>dial stem bi-metalic thermometer, and the oven.
<p>First several heatings: on removal, boiling vigorously when I opened
the door.&nbsp; By the time I could read the TM it was 100 C and not
boiling.
<p>After several "trials"&nbsp; on removal, boiling sl., but continuously,
temp. 107&nbsp; Probably sl. higher at equilibrium while being heated.
&nbsp;
In ~ 3 s. stopped boiling&nbsp; @ ~105 C.&nbsp; Cools more slowly to 100
(and below).&nbsp;&nbsp; If I rub the bottom of the cup with the end of
the TM,&nbsp; it boils and the temp. drops more&nbsp; rapidly to 100 C.,
at that point it them stops boiling
<p>bc
<br>&nbsp;
<br>&nbsp;
<p>"E.C. Muehleisen" wrote:
<blockquote TYPE=CITE>If I understand correctly, the water that is boiling
after microwave heating
<br>is above 100 C.&nbsp; The energy supplied by a microwave (klystron)
tube is
<br>funneled into a rather narrow surface layer.&nbsp; That layer can
become
<br>superheated since the the power input exceeds the power output from
the
<br>layer into the remaining mass of water.&nbsp; The fact that water has
a rather
<br>high specific heat exacerbates the situation.&nbsp; Thus when the
surface
layers
<br>are superheated, the thermometric measurement probably weighs the
surface
<br>more than the&nbsp; bulk.&nbsp; Was the thermometer plunged into the
middle of the
<br>water or barely immersed?
<br>______________________________________________________
<br>Get Your Private, Free Email at <a href="http://www.hotmail.com
">http://www.hotmail.com</a></blockquote>
</html>

--Boundary_(ID_6RP4lgVu7XAtNnXH6lzquw)--