Your question is a good
one! I must confess I have never had reason to understand the
details of a magnetron, but I will attempt to give you an explanation
that might help.
Question: what does
a magnetron have in common with a pokemon?
They are both examples of portmanteau,
where two words are combined into one new word.
Engineers use portmanteaux
all the time, although most of them don't even know what
the word means!
Weedle, #13 Pokemon
First of all, it is hard
to believe that people figured all this stuff out a long time
ago. Some very clever thought went into the magnetron designs
in the late 1930s, by mathematicians, then scientists, then engineers.
Raytheon was involved in productizing the device, the British
invented it but the way they manufactured it was labor intensive.
Percy Spencer came up with a way to replace expensive machining
with a stack of stampings which was much, much cheaper. Today
a secret invention that helped win WWII (enabling airborne radar)
is manufactured in China to heat your lunch! But I digress...
Now then, vacuum electronics
used to be the king of all electrical devices like radios and
televisions, before the "transistor age" which started
in the 1950s. Tubes, like transistors, can perform many functions,
like amplifiers, switches, TV screens, even computers (like the
ENIAC which consumed enough electricity to light up Eugene Oregon).
Electronics was much cruder way back in the day!
The way a vacuum tube works
is with sufficient temperature and very high voltage (electric
field), electrons can be boiled off of one metal and travel to
another one, through the vacuum, as opposed to through a wire.
The reason that TVs and radios used to have to warm up, was because
the heaters in the tubes had to get hot enough to boil the electrons.
This need for heat is the opposite of transistors, where heat
is considered the biggest enemy of reliability.
Electricity and magnetism
are very interrelated. The easiest examples to think about are
motors and generators. Although they don't all use permanent magnets,
they all exploit the interaction of electrons to a magnetic field.
A funny thing about that
interaction... when an electron is moving in one direction (say
East) if it encounters a magnetic field that crosses its path
(North-South) it is deflected upwards, not sideways! This is kind
of like (but unrelated) to a gyroscope, when you try to rotate
it in one direction it fights back at a 90 degree angle to the
force you are applying.
So now we get to the magnetron...
|In a "maggie"
the conductor at the center is heated. Then a huge DC voltage
(equivalent to a few thousand AA batteries connected in series!)
is put between the center conductor and the outer conductor.
The voltage is enough to really hurt you or kill you, so don't
mess around with an oven that is partially disassembled! The
voltage is stepped up from 120 volts that the power company
provides to your wall outlets, then converted from alternating
current (AC) to direct current (DC). Ask your teacher to explain
AC and DC... the result is that many, many electrons flow
through the vacuum from the center to the outer conductor,
concentrically. At this point, we don't have any conversion
of "house current" to microwave current, microwaves
are a form of alternating current but at 40,000,000 times
the frequency that the power company sent to your house!
This little Maggie is a fourth grader and hasn't learned to
sharpen her giant pencil. She's a pretty good swimmer though!
The giant magnet in a microwave
that you referred to is positioned to put an extreme magnetic
field up and down through the magnetron (north-south in magnet
lingo), while the electrons are moving from center to outside
(concentrically). The effect is that the magnet defects the electrons
sideways. By careful design, the magnet can spin the electrons
around the gap in the magnetron, when the force the magnet exerts
is equal to the centrifugal force of the spinning electrons. So
what you have is a "gas" of electrons, spinning like
a tornado around and around inside the magnetron! A beautiful
thing, but not yet a source of microwave energy.
from Wikipedia, electron path in red
Now picture when you are
driving in a car on the highway, and someone opens one of the
rear windows... and all you can hear is that wump wump wump noise
and it drives you crazy! This is because the car has a resonance
at a very low frequency. A flute also converts wind to sound,
but at a much higher pitch because the flute's resonant cavity
is so much smaller than the inside of a car. Both examples convert
one form of energy (wind) to another (sound). This is exactly
what happens in a magnetron! Those little chambers in the structure
resonate at a certain frequency when the electron cloud blows
past them. Thus one form of energy (electricity from the wall
outlet, which is stepped up to very high voltage inside the microwave
oven) is converted to another (microwaves). The energy is simply
picked off by sticking a wire or antenna (shown in brown)
into one of the magnetron's cavities, and the energy travels along
the wire and through a waveguide to a second antenna that sends
the energy to your food. A waveguide is merely a hollow metal
pipe that the wave energy can travel through with low loss, like
when you speak though a pipe and a friend listens on the other
end. Indeed, there are many analogies between microwaves and sound
waves, they are actually of quite similar dimension (wavelength),
the real difference is that microwaves travel at 1,000,000,000
feet per second while sound travels at "only" 1000 feet
Maybe I've given you too
much to think about all at once, let's just simplify it. The magnet
is used to spin the electrons in a circle, and the cavities are
there to steal energy from the spinning cloud and generate 2,400,000,000
cycles per second of radio waves, at a power level high enough
to cook your lunch. Note that the magnet doesn't supply a lick
of energy to the system (the power company and Mom's checkbook
get credit for that), the magnet just happens to guide the electrons
and trick them into converting their energy to something we can
use (a warm and tasty snack, just be sure to "run it off"