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Q & A: Plasma from a flame in the Microwave

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Q:
Hello physics Van! I am doing a study on plasma, and i need some help. Me and a friend discovered that if a fire is lit in the microwave, and a dome is put around the fire, (like a glass mixing bowl), and the bottom is rasied off the ground, then after approx. 10 seconds, a blue flash is created and then a plasmoid appears at the top of the bowl, humming at about 120hz. I was wondering, how does this happen?
- Michael Delayen (age 15)
Aden Bowman Collegiate, Saskatoon, Sk, Canada
A:

This sounds like a fun experiment, if you don’t mind making a mess of your microwave oven. Please get permission before set fire to anything inside your house (this whole experiment sounds dangerous and we don’t want anyone or anything to get hurt), and please be extra extra careful when experimenting with the microwave oven.

There is a safer version of this experiment that can be done with a grape sliced appropriately. Please see our answer here about grapes making plasma in the microwave.

When something burns with a flame, electrons are torn from their atoms as the atoms rearrange to form new molecules. Usually they get re-captured by the molecules, and this is one of the reasons why flames glow -- the electrons emit light as they lose energy spiraling in from their paths free through the air to being caught in orbits in the new molecules.

A microwave’s job is to set up a standing wave of electric and magnetic fields within a metal box. The electric fields alternately push and pull electrons left and right, or up and down. In a partially conducting material, the current that sloshes back and forth can heat up an object resistively. Even if the material does not conduct dc electricity at all, if it contains water molecules, their electric polarization directions flip back and forth with the field, making them jiggle and get hot.

If electrons are floating around freely, even for a very short amount of time, they can be shoved far away from their point of origin by the electric field. And then shoved back. And then forwards again. As they move back and forth, they crash into air molecules in the oven, and can knock electrons in them to higher-energy orbits. Then these electrons fall back, emitting light. That’s why you have a glowing blob of plasma over your flame. This plasma is hotter than the rest of the air, and so it tends to rise up to the top of your bowl.

I think they arrange the strength of the microwaves in ovens so that the back-and-forth motion of the electrons in a plasma that gets formed is not sufficient to knock other electrons free from the air molecules. If this were the case, even a small spark somewhere on a piece of food would eventually cause the whole oven to fill with plasma.

The reason the thing oscillates at 120 Hz has to do with how the microwaves are generated and shaped in the oven. Microwaves have a resonant cavity called a magnetron which resonates at a few billion Hz. Left to itself, the microwaves quickly dissipate (the energy goes into your food or gets dissipated in the resistance of the walls). The magnetron is constantly fed more energy from the electrical supply which plugs into the wall. Every cycle of power from the wall puts energy into the microwave cavity twice (a typical nonlinear circuit like a rectifier will make high-frequency noise twice per wall-power cycle -- the actual circuit of a microwave is probably more optimized to generate energy in the GHz range but to do it only on two places in the wall power cycle). Then the strength of the microwaves in the oven varies at 120 Hz.

The other reason it could oscillate at 120 Hz is that some microwaves have a metal "fan" on top which spins around on a shaft attached to a motor which runs off the wall current. Rather than cool anything off, this "fan" changes the shape of the metal side of the box by having irregularly shaped fan blades which constantly move. The microwaves make a standing wave pattern inside the oven, but the actual locations of the peaks and troughs of the standing wave depend on the shape of the box. By putting this "fan" in there, the peaks and troughs can be moved around -- so as not to burn spots of your food while leaving other spots frozen solid, a common problem with microwaves. If the fields change at about 120 Hz (not surprising given that the motor spins at a multiple of the line frequency), it can make your plasma oscillate like that.

Tom


(republished on 07/18/06)

Follow-Up #1: x-rays from microwave plasma?

Q:
Does this process emit x-rays when the excited electrons fall back into a normal orbit?
- rod (age 23)
ga
A:

I would expect very few x-rays from this process. The reason is that things just don't get hot enough to tear the most tightly bound electrons loose. There's a frequency-temperature relation: f is roughly comparable to kT/h where k is Boltzmann's constant, T is absolute temperature, and h is Planck's constant. The sorts of temperatures reached in the plasma would give infrared and visible light and maybe a bit of UV.

Mke W. 


(published on 10/02/14)

Follow-Up #2: microwave plasma

Q:
If microwaves are non-ionising EMR, then how are they able to produce plasma (ionised gas) from a flame
- David (age 21)
UK
A:

Nice question. When we say that radiation is non-ionizing, we mean that a single photon of that radiation doesn't have enough energy to knock an electron off some material. The effects of low levels of radiation are primarily from single-photon processes, so low levels of non-ionizing radiation cause very little ionization. 

In the microwave, there are very intense radiation levels. Rather than think of single-photon effects, it's more important to think of the simple classical effects of the very big electromagnetic fields. In a flame, where the heat already causes some ionization, the microwvae electric fields will accelerate electrons and ions, causing them to collide with neutral aprticles, creating some more ions. The resulting chain reaction gives enough ions to let us call the gas a plasma.

Mike W.


(published on 10/17/14)

Follow-up on this answer.