Aluminium foil in microwave oven

Dorfl

Muse
D
Joined
Jun 19, 2005
Messages
523
According to wiki: http://en.wikipedia.org/wiki/Microwave_oven#Hazards, putting a pointed metal object—like a fork—in a microwave oven is likely to produce arcing, but smooth metal should not have the same effect. However, I remember once microwaving a plastic container, which turned out to be coated with aluminium foil on the inside. It did spark all over its surface, leaving a lightning pattern of molten plastic. Does anyone have any idea how that could have happened?
 
It basically comes down to the definition of "smooth". While the surface of the foil may have looked smooth to you, it may well have contained many imperfections that were big enough to cause sparking.
 
Cuddles said:
It basically comes down to the definition of "smooth". While the surface of the foil may have looked smooth to you, it may well have contained many imperfections that were big enough to cause sparking.
Click to expand...

I guess that could be it. But it seems a bit misleading to use forks and metal decoration as examples of non-smooth things, if imperfections to small to see are actually enough to cause arcing. I mean, it was foil attached to soft plastic, so it shouldn't have been possible for it to be very non-smooth.
 
I 'boil' eggs in the microwave by encasing them in foil in a cup of water for 5 minutes. Perfect eggs, no pyrotechnics.
 
kerikiwi said:
I 'boil' eggs in the microwave by encasing them in foil in a cup of water for 5 minutes. Perfect eggs, no pyrotechnics.
Click to expand...

Well, as you probably figured out yourself too, the key ingredient there is the water around it. The frequency of a microwave oven is tuned to heat water. The foil keeps the microwaves from, well, directly heating the inside of the egg, and the water around is the energy sink. That's why you don't get pyrotechnics. Trying the same without water, now that could get more interesting to watch.
 
My last experince with a microwave oven was the one in the galey, it had stopped working, and I was asked to look at it*.
There was a overheat swicht on the transmitter that needed resetting.

I assume that if you start it with nothing in the microwaves will zip around the oven and reflect back into the transmitter and overheat it.

*Yea, at chance at doing anything, it is not as if I have any spares for it or much understanding of it.
There are a lot of safetyswichtes on the door, a controll transformer and print, and a high voltage transformer supplying a transmitter.
 
Not that I'd encourage this or anything, but immersing the base of an old-fashioned incandescent light bulb in a cup of water in a microwave will cause the bulb to light.

Of course, you have to turn the microwave on, silly.
 
HansMustermann said:
Well, as you probably figured out yourself too, the key ingredient there is the water around it. The frequency of a microwave oven is tuned to heat water. The foil keeps the microwaves from, well, directly heating the inside of the egg, and the water around is the energy sink. That's why you don't get pyrotechnics. Trying the same without water, now that could get more interesting to watch.
Click to expand...


This is amazingly awesome.

Dorfl I'm glad to hear that there was a happy ending.
 
HansMustermann said:
Well, as you probably figured out yourself too, the key ingredient there is the water around it. The frequency of a microwave oven is tuned to heat water.
Click to expand...

No, I'm afraid it's not. It's just high-energy RF. The chamber is shaped to set up a standing wave inside of it. Because of this, there are "nodes" of RF energy inside the chamber. There have been some efforts to visualize these with neon bulbs, but it proves problematic. This is why one part of the food will heat, and the other won't - and why newer microwaves have a rotating table.

With foil, it's not so much surface imperfections as the fact that it is very thin. The foil becomes charged(as does anything metal), but the fact that is so thin causes the RF to discharge. It simply can't hold as much charge without breaking down as a spoon can.

As well, microwaves do *not* heat from within. They heat from the outside in.
 
HansMustermann said:
Well, as you probably figured out yourself too, the key ingredient there is the water around it. The frequency of a microwave oven is tuned to heat water. The foil keeps the microwaves from, well, directly heating the inside of the egg, and the water around is the energy sink. That's why you don't get pyrotechnics. Trying the same without water, now that could get more interesting to watch.
Click to expand...

It's also a lot harder to create sparks underwater, or in solids.

In regards to the coating, I think it's due to the fact that thin films can heat up very quickly. After a small bit of film melts, or causes bubbling on the plastic, the surface will no longer be smooth, leading to sparking.
 
A thick conductive aluminum surface will not get hot at all but a thin one will. This is due to the higher resistivity of the thinner surface. Take a 2" dia disk and alter the thickness between .00001" and .01." The current flowing though the disk is pretty much the same but as the disk gets thinner the resistance increases and that current deposits more heat. At some point it will simply heat up and oxidize irregularly then sparks will fly. A CD with its thin metallic layer produces spectacular fireworks.
 
Sharp points on a metal object will more easily arc as the voltage gradient is quite high at the points and will ionize air which conducts producing sparks. High power microwave guides don't have pointy surfaces for that reason.
 
You must log in or register to reply here.

Back
Top Bottom