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Microwave ovens and sparks

D

Danny

Jan 1, 1970
0
Hi

What is it that causes the sparks when you put something metalic into a
microwave oven?

Danny
 
I

Ian Stirling

Jan 1, 1970
0
Danny said:
Hi

What is it that causes the sparks when you put something metalic into a
microwave oven?

The microwaves induce oscillating voltages in various bits of the oven
and metallic objects inserted in them.
In some cases, these voltages can increase over what the air can insulate.
When this happens, you get sparks.
 
J

John Popelish

Jan 1, 1970
0
Danny said:
Hi

What is it that causes the sparks when you put something metalic into a
microwave oven?

Danny

Standing waves bouncing back and forth along conductors produce large
voltages at the ends of the sections if the length is an odd multiple
of a half wavelength and at all have wavelength points. Put this
conductor in a nearly closed loop and those high voltages are opposite
polarity and large enough to arc across the gap. Sheets of metal
seldom arc, unless they are so thin that the RF currents burn them
into small pieces (like the foil layer in a CD).

You can experiment with this by putting a cup of water in the
microwave near the loop, to prevent the fields from getting so high
they reflect back into the magnetron. Lay a small loop of wire on a
heat proof surface like pyrex, with the ends up off the glass, and
close together. Trim the length of the wire in 1/16 inch (or
millimeter) steps and you will find the length that produces maximum
fire.
 
M

Mike

Jan 1, 1970
0
Hi

What is it that causes the sparks when you put something metalic into a
microwave oven?

Danny

Others have answered your query, I'll just add a bit of trivia: It doesn't
have to be metallic: try diced raw carrots some time.

-- Mike --
 
P

Paul Burridge

Jan 1, 1970
0
Hi

What is it that causes the sparks when you put something metalic into a
microwave oven?

I don't know for sure, but would hazzard a guess that it's due to the
very short wavelength of the energy radiated by the oven. This would
cause substantial potential differences to arise over the course of
just a few inches with any reasonably conductive material, giving rise
to arcing.

Disclaimer: Of course, I may just be talking bollocks. I'm not an
engineer.
 
P

Paul Burridge

Jan 1, 1970
0
Standing waves bouncing back and forth along conductors produce large
voltages at the ends of the sections if the length is an odd multiple
of a half wavelength and at all have wavelength points. Put this
conductor in a nearly closed loop and those high voltages are opposite
polarity and large enough to arc across the gap. Sheets of metal
seldom arc, unless they are so thin that the RF currents burn them
into small pieces (like the foil layer in a CD).

I often put my teacup in the mw to warm it up. I leave a teaspoon in
it too and don't get any sparks. OTOH, if I use a teacup with a
guilded edging, I get an indoor thunderstorm. Seems to back up your
findings.
 
J

John Popelish

Jan 1, 1970
0
Paul said:
I often put my teacup in the mw to warm it up. I leave a teaspoon in
it too and don't get any sparks. OTOH, if I use a teacup with a
guilded edging, I get an indoor thunderstorm. Seems to back up your
findings.

If you look at the burnt gilding, you will find that it is broken into
similar length pieces of about 1/2 wavelength, each.
 
P

Paul Burridge

Jan 1, 1970
0
If you look at the burnt gilding, you will find that it is broken into
similar length pieces of about 1/2 wavelength, each.

Even if the turntable is rotating during the nuking?
 
C

Clifford Heath

Jan 1, 1970
0
John said:
If you look at the burnt gilding, you will find that it is broken into
similar length pieces of about 1/2 wavelength, each.

Funny. I coulda sworn that 2.xGHz had a wavelength in the vicinity of five
inches. Most teacups are about half a wavelength *across*, but the gilding
breaks up into pieces 1cm or less. Guess you don't need a half-wave to
get heating, huh...
 
J

John Popelish

Jan 1, 1970
0
Clifford said:
Funny. I coulda sworn that 2.xGHz had a wavelength in the vicinity of five
inches. Most teacups are about half a wavelength *across*, but the gilding
breaks up into pieces 1cm or less. Guess you don't need a half-wave to
get heating, huh...

That is 2.7 GHz, I think, with a free space wavelength of about 4.4
inches, but a little less on the surface of ceramic, so a half
wavelength is about 4 inches, and a half wavelength of about 2
inches. HMO. You may be right. I seem to remember gilding burning
into about 1 inch sections.
 
S

Sir Charles W. Shults III

Jan 1, 1970
0
I came up with a nice trick using a few string beans that started a
wonderful fire on a paper plate every time. Place the string beans (for
instance, five of them) in a rough circle with the ends nearly touching. Do
this on a paper plate. Microwave.
You may want to have that Mr. Coffee add-on handy- what was it called? Oh,
yes, Mr. Fire Extinguisher.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip
 
B

Bob

Jan 1, 1970
0
Gee, I wish I would have known about this trick when I was a kid. I thought
that the only thing string beans were good for was shoving them up your
nose, and thus causing your little sister to throw up.

Bob
 
N

N. Thornton

Jan 1, 1970
0
Others have answered your query, I'll just add a bit of trivia: It doesn't
have to be metallic: try diced raw carrots some time.


Is this because carrots are metallic?

Seriously, why do carrots do this?


Regards, NT
 
N

N. Thornton

Jan 1, 1970
0
John Woodgate said:
I read in sci.electronics.design that N. Thornton wrote:
Yes; the colour is due to the high concentration of orangeonium.
Water content.

Right, but other veges also have water content and arent prone to
arcing the way carrots are. I think I know the answer here: guessing
time: the carrot I expect has less water content than many other
veges, and has a low enough content to encourage standing waves, but a
high enough content to remain electrically conductive - thus creating
the 2 requirements for arcing.


Regards, NT
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that N. Thornton <[email protected]>
wrote (in said:
Right, but other veges also have water content and arent prone to
arcing the way carrots are. I think I know the answer here: guessing
time: the carrot I expect has less water content than many other
veges, and has a low enough content to encourage standing waves, but a
high enough content to remain electrically conductive - thus creating
the 2 requirements for arcing.
Probably. I believe you can get courgettes to light up on US 120 V mains
because they have a high enough cue. (;-)
 
N

N. Thornton

Jan 1, 1970
0
John Woodgate said:
Probably. I believe you can get courgettes to light up on US 120 V mains
because they have a high enough cue. (;-)

hehe, have to try that one day.

Regards, NT
 
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