Aluminium foil in microwave oven

[SpuriusMaximus]

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.

Well, that has some influence too, but think of it as a matter of a circuit. Without the water, the only sink for that energy is induction in the foil. With the water, the water itself is the perfect sink for those photons. Water is semi-opaque to the frequencies used in a microwave oven.

Sorta like a weak brewed coffee is to normal light. So basically think of pointing a laser at that foil, and inside a mirror cavity at that. You can probably see how it would make a difference if it's in a cup of coffee.

Or think pretty much shunting a tesla coil with a resistor. Without the resistor it will make pretty sparks, with it, you'll just get a hot resistor.

Well, neither is a perfect analogy, but I think they're good enough to illustrate a point.

 

[Dorfl]

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.

That could explain why the foil sparked so much. Since it was supported by plastic, I suppose it could have been very thin. Thanks.

 

[kerikiwi]

With the eggs, they are not fully immersed in water. Half a cup to start with and much less when finished.

 

[pgwenthold]

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.

Wow, this is about as wrong as can be.

The only thing correct is that there are nodal regions in the microwave. That's true and is easily demonstrated, not with neon bulbs, but with marshmallows.

Other than that, your description is wrong.

1) It's not "high energy RF." It is actually very low energy EM radiation, in the microwave range (duh) not radiofrequency. It is high POWER, but not high energy.

2) It most certainly works by exciting water (although fat is also affected). The frequency of the radiation (2450 MHz) is in fact right at the rotational frequency of water. The way it works is that the water (and fat) in the sample being irradiated gains rotational energy, and that rotational energy is converted to physical heating by collisional energy transfer with something else.

3) Because of this, the sample is absolutely heated internally, as opposed to externally. This is easy to establish. Just turn on the microwave with nothing in it, and you will notice that in fact the interior doesn't heat up (not recommended, btw). Compare this to a conventional oven, which gets hot inside.

4) Metal is a problem not because "it is thin" but because it reflects the microwaves, which messes up the standing wave pattern, creating a microwave discharge.

 

[wackyvorlon]

Microwave is very much RF, and there are a number of devices operating in the 2.4 GHz ISM band. I am aware of experiments performed by ham radio operators using microwave ovens and power cycling the magnetrons to switch the signal on and off.

 

[EHocking]

Thank you But it could as easily have ended in tragedy. If the sparks had not stayed on the outside of the foil, all my tortillas could have got covered with molten plastic

I think it also depends on the material.

For over 20 years I have "masked" food cooked in microwaves with aluminium cooking foil (i.e. thin ends of a fillet, or the bones of a drumstick) without any sparking.

To reassure myself that this was not poor memory, I just put two pieces, about 1" square, one crumpled, one flat an "cooked" them both for 2mins at 1000W. No sparks. Since it's not recommended to "cook" an empty microwave, I then repeated the same with a plastic cup half full of water and cooked for 5mins at 1000W. No sparks.

 

[Buckaroo]

4) Metal is a problem not because "it is thin" but because it reflects the microwaves, which messes up the standing wave pattern, creating a microwave discharge.

I also seem to recall from my physics class that the microwaves are at just the right energy to toss electrons up into the conduction band of many metals, thus creating electric nuttiness. Too lazy to look it up at the moment, though.

 

[kerikiwi]

I discovered that the dinner service I inherited from my great aunt really was gold leaf by using it in the microwave.

 

[pgwenthold]

Microwave is very much RF, and there are a number of devices operating in the 2.4 GHz ISM band. I am aware of experiments performed by ham radio operators using microwave ovens and power cycling the magnetrons to switch the signal on and off.

What do you mean by RF? 2.4 GHz is not radiofrequecy, it is microwave.

 

[wackyvorlon]

Microwave is considered RF.

 

[arthwollipot]

Microwave is considered RF.

Actually, no. Allow me to provide an illustration:

Microwave is defined as the electromagnetic spectrum between 0.3GHz and 300GHz. The Radio part runs from 300GHz to 3Hz. There's a line at 300GHz - on the one side is microwave, on the other is RF.

 

[Dorfl]

Actually, no. Allow me to provide an illustration:

[qimg]http://imgs.xkcd.com/comics/electromagnetic_spectrum_small.png[/qimg]

Ooh... Slinky waves!

In high school we had a four metres long slinky-spring thingy, which we could reach five overtones on. Good times, good times

 

[Speedskater]

My 40 year old Electronics dictionary reports:
microwave
A term applied to radio waves in the frequency range of 1,000 megahertz and upward.

 

[Cuddles]

Actually, no. Allow me to provide an illustration:

[qimg]http://imgs.xkcd.com/comics/electromagnetic_spectrum_small.png[/qimg]

Microwave is defined as the electromagnetic spectrum between 0.3GHz and 300GHz. The Radio part runs from 300GHz to 3Hz. There's a line at 300GHz - on the one side is microwave, on the other is RF.

Well, I wouldn't take xkcd as any particular authority. To start with, there is no part of the EM spectrum called "power & telephone" or "toasters", and cosmic rays are not necessarily electromagnetic at all. Also, the numbers you've given there don't work at all. You've defined microwave to entirely be a subset of radio, while also claiming that they're completely separate.

In any case, there is no standard definition of exactly what frequencies belong to which category. RF and microwave are generally separate, but there is plenty of overlap at the boundary where you could call it either. Microwave is generally considered above 1GHz, and certainly microwaves would usually be considered to use microwaves rather than RF. In the end what you call them is irrelevant, all that matters is the actual frequency.

 

[pgwenthold]

My 40 year old Electronics dictionary reports:
microwave
A term applied to radio waves in the frequency range of 1,000 megahertz and upward.

Yeah, but how do you get RF out of radio waves?

As mentioned, the dividing line between microwave and radiofrequencies is generally considered to be the 1 m wavelength. Longer is RF, shorter is microwave. Standard microwaves run at 2.45GHz, which is a wavelength of above 11 cm.

Moreover, the wavelength is chosen because it is efficiently absorbed by water, with the energy going into rotation (fats also have rotational constants near that region).

Microwave ovens are just a practical application of microwave spectroscopy.

 

[pgwenthold]

In any case, there is no standard definition of exactly what frequencies belong to which category. RF and microwave are generally separate, but there is plenty of overlap at the boundary where you could call it either. Microwave is generally considered above 1GHz, and certainly microwaves would usually be considered to use microwaves rather than RF. In the end what you call them is irrelevant, all that matters is the actual frequency.

I don't disagree, for the most part, and would just add that you also need to get the physics right.

 

[JPK]

Good morning EHocking.

I think it also depends on the material.

For over 20 years I have "masked" food cooked in microwaves with aluminium cooking foil (i.e. thin ends of a fillet, or the bones of a drumstick) without any sparking.

To reassure myself that this was not poor memory, I just put two pieces, about 1" square, one crumpled, one flat an "cooked" them both for 2mins at 1000W. No sparks. Since it's not recommended to "cook" an empty microwave, I then repeated the same with a plastic cup half full of water and cooked for 5mins at 1000W. No sparks.

You are correct. In fact foil is needed for masking in order to cook certain things, like the foods you mentioned above.
If you ignore the warning from The Tubes, (Don't watch the food cook.) You will notice the sparks occur between gaps in the crumpled foil. If the foil is smooth you will not see arcs.

 

[Speedskater]

Again from that 40 year old Electronics dictionary:

"radio frequency"
Abbreviated " r-f ". Any frequency at which coherent electromagnetic radiation of energy is possible.

"radio wave"
Also called a Hertzian wave, or a wave. An electromagnetic wave produced by rapid reversals of current in a conductor. Such a wave travels through space at approximately the speed of light.

 

[EHocking]

Good morning EHocking.


You are correct. In fact foil is needed for masking in order to cook certain things, like the foods you mentioned above.
If you ignore the warning from The Tubes, (Don't watch the food cook.) You will notice the sparks occur between gaps in the crumpled foil. If the foil is smooth you will need see arcs.

I can only disagree - because I don't see sparks or arcs.

The point of my post.

 

[marting]

"Microwave" has typically been used to describe EM frequencies requiring "plumbing" more than discrete, lumped component design. The latter has applied to higher and higher frequencies as semiconductor technology has miniaturized. Much design work at 1GHz is now similar to work done 40 years ago at 100MHz. That is, discrete lumped parts combined with "transmission lines," albeit much smaller ones.

The chart's cute. I suppose "toaster" waves are supposed to be those far IR emitted by toasters. It seems closer to the room temp black body radiation region.