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almost eight years ago when this channel was fresh and before I had gray hairs in
my beard in fact before I had a beard I made a video showing that if you take a
grape and cut it almost completely in half and put it in the microwave you can
make some plasma
but the explanation we had in that video was lacking this was
me and my friend Dr. Stephen Bosi a fellow physicist so you should look
forward to that on another episode of veritasium to be honest nobody really
knew how this effect worked that is until now as of the publication of this
video three scientists have published an explanation in the Proceedings of the
National Academy of Sciences they studied this effect with high-speed
cameras thermal cameras electromagnetic modeling and they came up with an
explanation that I think is pretty satisfying and it extends way beyond
just microwave great plasma so naturally I called up these scientists yeah
actually saw your video before that your videos before joining the grape project
oh really that's why I was like already kind of
like interested before joining the project the scientists showed you don't
even need to use a grape you can get the same effect with hydrogel water beads
these are those tiny polymer beads which when you soak in water they increase in
volume like 100 times
to understand how microwaving a grape gives you plasma we first need to know a
few things like a typical household microwave uses a frequency of
electromagnetic radiation at about two point four five gigahertz that means
that the wavelength of microwaves inside the oven is about twelve centimeters
long. now if you've ever studied electromagnetic radiation you might have
the intuition that interesting things start to happen when the object or
obstacle is around the same size as the wavelength which a grape certainly is not
it's not twelve centimeters but the important quantity is how big is the
wavelength inside the object inside the grape twelve centimeters in air but the
like index of refraction of grape material must be close to like one point
three three or something right like at visible wavelengths it is a lot lower
but in the microwave regime the index of refraction is a almost 10 so you have a
much higher index of refraction so microwaves are traveling you know like
ten times slower through the grape than they are through yet air yes and that
means that the wavelength is a tenth its size through air so instead of twelve
centimeters it's about 1.2 centimeters right about the size of a grape now if
you take a single grape and place it inside the microwave don't cut it in
half what you find is that the microwaves actually become trapped
inside the grape and that's because of its high refractive index and its size
because turns out that when you have like a ball where approximately the
diameter is roughly the same as the wavelength of the microwaves in the material. It
actually turns out that it can be trapped in there so that it interacts its kind...
it's kind of it bounces at the borders of the of the ball and it can't get out
is it like total internal reflection? yes pretty much wow that's interesting I
hadn't thought of it that way so the microwaves become trapped inside
the grape and they actually form these resonant modes you can think of them as
standing waves just ways in which the electromagnetic fields like to
oscillate inside a grape such that the maximum electromagnetic field is
actually in the center of the grape when you put in the microwave you see that it
heats up not from the outside in like you would expect if it was just
absorbing the microwaves but rather from the inside out. if you add a second grape
to the microwave you see that the same thing happens the microwaves get trapped
in the second grape too- and the amplitude is highest in the very center of the
grape and so the heating occurs most of all of there. but if you move these two
grapes close together so they're closer than say a wavelength well then you can
start to get interactions between the electromagnetic fields in the one grape
and in the other and in fact if you get the grapes touching then the greatest
electromagnetic field actually occurs at that contact point between the two
grapes and so that is where you're gonna get the greatest oscillating
electromagnetic fields and that is where those grapes are going to get hottest
it's also interesting that you don't need to cut the grapes in half. two
grapes placed side-by-side will make this effect work as long as they stay in
contact. but that's why in a lot of the videos you see there's a watch glass so
the reason the watch glass is there is to keep the grapes together now
with the very strong electromagnetic fields at the intersection of the two
grapes what you can get is some sparks some breakdown of air that is the
electric fields are strong enough that they ionize the air creating those
Sparks and that is what leads to the plasma it creates these ions which can
then receive more energy from the microwaves. you can see it here pulsing
at 120 Hertz that's twice the 60 Hertz frequency of mains power which reflects
the fact that the amplitude of the microwaves is pulsing twice every cycle
and what is that plasma made of? well the scientists looked at the spectrum of the
plasma from a grape and they found that there's a strong potassium emission line
and also a sodium emission line so it seems like those ions must be pretty
common in the grape and when the plasma gets formed as those ions which are
sprayed up into the air and that is what we're seeing
who needs drugs?! now the size of the grapes is important which is something I
of course suspected because you don't see this happening with too many other
squishy fruits but you don't have to get them the exact right size and this is
because water absorbs microwaves so the scientists did some modeling of this and
if you had a material inside the grape that did not absorb microwaves then what
you would see is at certain particular sizes you get extreme amplification of
the electromagnetic fields it's basically one size that you'd have to
hit to get the amplification but made of an absorbent material like water this
broadens out the peaks so you get less amplification of the field but over a
broader range of sizes this is why a lot of grapes will work even if the sizes
vary. so in terms of like applications of this research what do you think I mean
obviously people like me are excited and because this is a fun trick but but
beyond that what do you think in terms of like applications of this? the main
limit in fabricating microchips right now and going smaller and smaller is the
lithography so how to make these very small features. this phenomenon shows
that two spheres of the right size and refractive index can focus
electromagnetic energy down to a tiny spot in between them in this case down
to around a millimeter from radiation with a 12 centimeter wave length if
similar focusing could be achieved with light it would provide a significant
improvement over current lithography techniques. So if we could somehow harness
this to do this lithography to be able to maybe write things very small you
could actually use light to to make really small features you could do like
every two nanometers marks spot with about 2 nanometers resolution - it would help
help continue this like Moore's Law yes it would help continue Moore's Law like
putting much more small things in a single chip so I'm glad I was finally able to
explain how this effect works did you enjoy it do you feel like you get it any
questions put them in the comments below also I
really like this video that I made this is my previous video and so if you
haven't seen it go check it out I'll also put a link to the original
grape video which is maybe not that good cuz it was eight years ago but anyway
thanks for watching
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How Microwaving Grapes Makes Plasma

14 分類 收藏
林宜悉 發佈於 2020 年 3 月 29 日
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