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: [Phys-L] molasses kablooey



Hey all, apparently several veggies have the same behavior ....

https://www.npr.org/sections/thesalt/2016/10/03/495975189/snap-crackle-kale-the-science-of-why-veggies-spark-in-the-microwave

and

http://www.radiolab.org/story/big-little-questions/

The Radiolab episode is fun, jump to about 34 minutes in this one for the
microwave story.

Happy holidays, all,

Roger

Roger Key
Instructional Support
559-278-2728
[web <http://www.fresnostate.edu/csm/physics/>][facebook
<http://www.facebook.com/FresnoStPhysics>][youtube
<http://www.youtube.com/FresnoStPhysics>][twitter
<http://www.twitter.com/fresnostphysics>]

------------------------------

On Tue, Dec 26, 2017 at 8:49 AM, John Denker via Phys-l <
phys-l@mail.phys-l.org> wrote:

On 12/26/2017 08:41 AM, Bill Norwood via Phys-l wrote:

- This effect persisted even with a half cup of water accompanying the
bottle in the mw oven.

Very smart.
-- It protects the magnetron.
-- It makes the experiment more reproducible.
-- It rules out some of the simpler hypotheses.

- I am suspecting that the iron in the iron-rich blackstrap molasses is
reacting to the microwaves,

There's also a metric boatload of potassium in all
molasses, sufficient to make a nontrivial contribution
to the conductivity and hence RF dissipation.
https://health.gov/dietaryguidelines/dga2005/
document/html/appendixB.htm

The iron ions contribute in principle, but the molarity
is much smaller. I doubt the magnetic properties of the
iron have much to do with it. The microwave E-field does
the work.

Also note that pure liquid water absorbs somewhat at 2.45
GHz, for reasons having nothing to do with conductivity,
but instead "somehow" related to oscillations for which
hydrogen bonding allegedly supplies the restoring force.
This contribution is /absent/ in ice at 2.45 GHz.
http://www1.lsbu.ac.uk/water/microwave_water.html

- There was no heating of the bottle.

That's as expected, but confirmation is nice.

I do not understand what is going on,

I don't know for sure, but there are some obvious hypotheses
that you can easily check.

Hypothesis: Molasses is a better absorber, per unit
mass, than the water you are using. For definiteness,
let's assume 100x better.

You can check this by putting 1 cup of water and one
cup of molasses in the oven at the same time, and
doing the calorimetry.

Note that the conductivity of tap water varies
wildly from place to place. You could make this
experiment more reproducible by starting with DI
water and adding a small measured amount of salt.

Now if you put 100 g of water and 1 g of molasses
in the oven at the same time, they should absorb
comparable amounts of power. However, that's
100x more power /per unit mass/ for the molasses,
so it heats up much more quickly.

Let's call this /power hogging/. Note that if
you used 100g of water and 100g of molasses, the
molasses would absorb only twice as much as before
(i.e. 99% of the available power, not just half)
but it would have 100x more heat capacity, so it
would heat up much more slowly.

The rest of the story is prosaic:

a) Due to microwave heating, eventually it gets
hot enough to ignite.
b) Now chemistry takes over, it it gets very much
hotter.
c) The combustion products stay hotter longer
due to microwave heating of the plasma. I'm
not sure how important this is, but it's a
hypothesis to consider.

Note there are multiple stages:
*) At the beginning, the molasses is a liquid
with ionic conductivity.
*) Much later, there is charred sugar which
conducts in the solid state.
*) Things are messy in the intermediate regime,
where it is hot enough to drive off most of
the water but not yet hot enough to char the
sugar. I hypothesize that the molasses when
purchased is already caramelized enough, so
this is not a problem. It's also possible
that there is some absorption in dry pure
sugar, due to some screwy vibrational mode.
You could check all this by seeing how well
microwaves are absorbed by
-- dry refined white sugar
-- dry brown sugar
-- molasses dried by long gentle heating

=============

Note that for maximum absorption you want the
conductivity to be high enough so that the
microwaves don't just pass through and get
absorbed somewhere else ... but not so high
that they just get reflected. The relevant
concept here is /impedance matching/. You
can look it up. You can teach it at the HS
level easily enough, using a voltage source
in series with a fixed source resistance Rs
and predict then measure the power dissipated
in a variable load resistance Rl. There's no
power delivered to the load when Rl « Rs or
when Rl » Rs. In fact Rl = Rs is optimal.
_______________________________________________
Forum for Physics Educators
Phys-l@mail.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l