[SI-LIST] Re: Honey, your conductors are ready!

  • From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
  • To: "si-list (si-list@xxxxxxxxxxxxx)" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 14 Nov 2014 23:03:00 +0000

Interesting responses.  I'm surprised that no one on this list braved the 
experiment (though I still can't recommend it, since I don't know all the 
things that can go wrong, as my wife will attest).  I'm a huge fan of Feynman, 
and would wager that he'd risk his microwave to try it.
Placing conductors in a microwave is not necessarily a bad thing.  If you buy a 
chicken pot pie that promises "even browning" you'll probably see it comes in a 
rather shiny bowl.  Put an ohmmeter to that bowl and you'll find that it's a 
conductor!  (or at least parts of it are)  The folks who make microwaveable 
food understand what conductors do when exposed to microwave energy and take 
advantage of it, we (signal integrity engineers who transmit 
microwave-frequency data) should also, I think.  And, we should understand the 
difference between the fundamental microwave mechanisms at play in a dielectric 
(chocolate bar) and a conductor.
I also don't think a simulation accomplishes the goal - I devised this 
experiment for a class I'm putting together, and I'm trying to remove a layer 
of abstraction for the audience.  Showing them the results of a simulation 
won't be nearly as effective as demonstrating that one piece of foil is hotter 
than heck, while the other is relatively cool, all with items everyone has 
access to.  I think a physical demonstration will allow people to "grok" why a 
rough surface (with more surface area) is a bad thing for a PCB trace.  And, as 
someone pointed out off-line, how would you prove correlation between your 
simulation and test results? :-)
I wonder what percentage of EM PHd's would predict correctly the results of 
this test? (I'm guessing Dr. Huray's students would fare well)
I guess we'll have to wait a bit for the results of the test, when I get back 
from China (unless my brother pulls through for me, he has an old microwave he 
was tossing and might be willing to try it).
Jeff Loyer
From: Loyer, Jeff
Sent: Friday, November 14, 2014 5:21 AM
To: si-list (si-list@xxxxxxxxxxxxx)
Subject: Honey, your conductors are ready!

I thought you all might enjoy hearing the results from an experiment I recently 
concocted (but haven't yet completed).  Here's how it was crafted:
Cut 2 identical squares of aluminum foil.  Shape one of those into a ball, 
really tight and compressed (sphere = minimum surface area).  Leave the other 
one flat (maximum surface area).  Put them both in the microwave and heat them 
for 15 seconds (not sure of exact time).  See if there's a significant 
difference in the temperature between them.  The flat sheet should be much 
hotter - more surface area for the microwaves to impinge upon.

Here's what I've learned from this exercise so far:

1)      Don't do this unless you're prepared to buy a new microwave, or can 
live with lots of singe marks.

2)      If you blow the microwave circuit breaker, reset it quickly if you have 
a freezer on the same circuit.

3)      Don't have your spouse try this while you're in business in China - 
wait until you can do it properly.

4)      The foil has to be small enough not to contact the walls.

As you can guess, I asked my spouse to try it.  I'm in China but was anxious to 
hear the results.  I should have waited! :-)   Here's the text from her 
synopsis:

"Well.  Our microwave looks like he[ck] - burned the door seal, and put black 
streaks on the back wall.  My flat piece of foil was not a square - it was a 
rectangle, so the edges grazed the sides of the box insides as the tray 
rotated.  There was lots of arcing, sparking, and color.  But as far as Kath 
and I could tell, neither piece got warm!!!!   Go figure!
I could try again.  [note from Jeff: how did I find this woman?!?!]
I began with a rectangular piece approximately 4 inches by 6 inches.    Should 
have tried a square.
Had difficulty making a ball out of the one piece, after a certain point of 
compressing it.  It was a little more of a prism than circular.  But close.
We weren't clear if you wanted both pieces in the microwave at the same time, 
or if the test should be run separately for 15 seconds.  We (Kath and I) 
decided to put them both in, beside each other, but not touching.  All the 
arcing activity caused the ball to roll under the rectangle, though.  At least 
so it seemed.  The pieces were lightly stuck together when we were done."

Anyway, I still think it's a cool demonstration of the fundamental principle 
that a layman can try (if they're brave or dumb).  There's a nice limit that, 
if the sphere is small enough to equal the skin depth, the two pieces will heat 
evenly (I think).  I'm anxious to try it when I get home.

If you choose to try this in the next week at the risk of your microwave and 
food in your freezer, please let me know what you find.

Cheers,
Jeff Loyer

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