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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 |
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| 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 spiralling 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) |
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