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From: jp@lanl.ARPA
Newsgroups: net.physics
Subject: Re: Behavior of macaroni in a microwave oven?
Message-ID: <28264@lanl.ARPA>
Date: Sat, 13-Jul-85 14:45:15 EDT
Article-I.D.: lanl.28264
Posted: Sat Jul 13 14:45:15 1985
Date-Received: Mon, 15-Jul-85 21:00:50 EDT
References: <2970@nsc.UUCP>
Distribution: na
Organization: Los Alamos National Laboratory
Lines: 38

> 
>      Today, I had some macaroni and cheese for lunch(actually leftovers) which
>      I brought to work with me.  Before eating it, I microwaved it to warm it
>      up.  I noticed that some of the macaroni was really hot and a spot a couple
>      of inches away was still cold.
> 
>      This seems like really odd behavior.  Any ideas why it does this?

Yes.  The energy in your microwave oven is electromagnetic waves bouncing around
inside a closed metal box.  The reflections at the boundaries result in a
standing wave pattern with regions of high electric field (and, consequently,
low magnetic field) and vice versa.  Microwave cooking occurs from heating
by dielectric loss (mostly in the water).  The parts of the food that are
near electric field maxima heat more than those near electric field minima.
Most microwaves have a rotating metal blade that "stirs" the microwaves.  
That is, it disturbs the standing wave pattern in an attempt (usually not
very successful) to move the electric field maxima and minima around.  The
rotating table in some microwaves is an attempt to even out the cooking by
moving the food around in the standing wave pattern.  Unforunately, the 
middle of the rotating tray doesn't move very far.

Once, someone asked me how to measure the field distribution in a microwave
oven. (He was curing some kind of ceramic glue or something like that.)
Since my main job is measuring microwave field distributions in cavities
he thought I was a good person to ask.  He didn't seem fully satisfied with
my answer, however.  I suggested that he cover a pizza crust more or less
uniformly with cheese and place it in the microwave oven.  The electric field
maxima would be at the points where the cheese first began to melt.
Actually this is a tough problem because the frequency of the oven cavity and
the standing wave pattern depend on the distribution of food (dielectric
loading) in the cavity.

If all you want to do is cook hotdogs, just apply 120VAC from end to end
for 60 secs.  It works, and it's cheap.

Yours for scientific cooking,

Jim Potter  jp@lanl.arpa