SPARKS reply

["Prof. John P. Ertel (wizard)" <jpe@nadn.navy.mil>]

In response to David Abineri's question about micro-wave ovens:

The M-W(Micro-Wave) oven is one of my favorites to use as a practical
example of field polarization in E&M theory sections (as well as the Intro
level E&M)!

Placing metal foil (or any thing else metallic) which is almost entirely
in the horizontal plain causes very little absorption of energy from the
M-W cavity --- in fact, it really just redefines the bottom of the cavity.
This is because E-fields in the cavity are almost totally polarized in the
vertical direction.  This also explains why the manufacturers can have
give you a "tray" or "grill" in the middle which is usually made of
chromed steel.  Since its thickness in the vertical direction is small, it
cannot absorb much energy either; and therefor, makes a good separator of
the cavity for cooking two sections of meals.

On the other hand, metal items that are vertical, like the side walls of a
pan, the corners & edges of foil, the "gold" or "silver" in the designs on
cups, etc., would absorb a substantial part of the local field energy
density.  The E-fields at these sharp edges must be huge which would also
explain the arcing & sparking that you see.

Some years ago, I used to do a demo with a M-W oven which I found broken
in a yard sale.  It really took little to repair the power supply but I
purposely did not replace the interior compartment light.  I would place
an old fork (also gotten from a yard sale) with its tines bent up and
splayed outward into the center of the cavity along with a small cup of
water*.  When the M-W was turned on, there would be a rather spectacular
lightening show in the otherwise dark cavity from the tips of the fork.
The tines of the fork would get too hot to handle almost immediately while
the body of the fork would become only slightly warm (hot near the tines
obviously).

Hope this is of some help.

*BTW: The water was to make sure that the cavity wasn't depositing too
much energy into a very localized part of the cavity (namely the fork).
The Tech Support folks at TAPPAN deserve credit for the idea of the water
in the cup.  I actually was calling them to learn how to "turn down the
power" delivered by their "magnetron tube" (which is never designed to be
adjusted).  This inability to design a cost-effective M-W source with
adjustable power is why ovens act the way they do at "lower" power
settings.  Haven't we all noticed that the manufacturers "adjust" power
delivered by building in periodic on/off cycles?!



On Sat, 22 Mar 1997, David Abineri wrote:

> Does anyone know why, when metal is placed in a microwave oven, sparks
> are generated? What is the process by which they occur?
>
> My students tell me that foil lying flat in the oven does not spark but
> when wrapped around a sandwich it does spark. Not being willing to risk
> damaging mine, I wonder if someone knows the details of the spark
> production.
>
> Thanks for any insights.
> -- 
> David Abineri   
> --

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