Re: SPARKS

[Greg Marshall <gregm@magi.com>]

> > 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
> > --
>
> I am not sure but I think that it has something to do with whether or not
> there are "points" on the metal which will allow a strong enough field at
> the point to show that arcing. I am told that rounded edges are used to
> prevent this arcing behavior. I cannot find the address now, but there is a
> website that will tell more than you ever ned to know about microwave
> ovens.

I suspect it has more to do with the EM field strength in the location 
where the foil is. I would imagine that at the metal wall of the oven 
cavity the field drops to zero, and when the foil is nearly flat and 
lying against the wall the field is vastly lower than it is nearer the 
middle of the cavity. I seem to remember (maybe in TPT) a demonstration 
showing a block of frozen butter being melted in a microwave, with holes 
in it where the field was highest.
Certainly if the foil is crumpled and there are corners on it you would 
expect to see arcing preferentially from the corners.
Maybe someone who actually knows could respond....

Greg