Here is a physics in the kitchen
experiment that can be rather fun, involving grapes and plasma. Later I will do one involving Pyrex, canola
oil, and invisibility.
The first experiment has received quite a
bit of interest lately, and it is easy for anyone to do, and safe (though you
should be cautious around microwaves).
Yes, it is the grapes in the microwave experiment. There are many YouTube videos of this
experiment, so I am not claiming anything special for my blog or my
demonstration.
By the way, you shouldn’t let kids do this
unsupervised, if only so they don’t get the idea that experimenting with
microwaves is fun. That could be
dangerous. Also, if you have a
pacemaker, be careful around microwaves.
Make sure the power is off before opening the door, because leaking
microwaves can interfere with pacemakers.
Here’s a video (about 10 seconds of the
main part of the demonstration).
Step 1 – Put two grapes in a microwavable
bowl – they should be touching.
Step 2 – Put Bowl in Microwave, ensure
grapes still touch.
Step 3 – Close Door and Start Microwave –
Note there is a Faraday Cage, so visibility is somewhat obscured.
Step 4 – Watch for Fireworks
Step 5 – Even more fireworks
My experiments usually lasted for about 30
seconds before the grapes gave up the ghost, generally with some smoke at the
end.
There’s actually a lot of physics going on
here, as you can tell from the abstract below, taken from a paper by some
researchers at McMaster University lately.
The sparking of
cut grape hemispheres in a household microwave oven has been a poorly explained
Internet parlor trick for over two decades. By expanding this phenomenon to
whole spherical dimers of various grape-sized fruit and hydrogel water beads,
we demonstrate that the formation of plasma is due to electromagnetic hotspots
arising from the cooperative interaction of Mie resonances in the individual
spheres. The large dielectric constant of water at the relevant gigahertz
frequencies can be used to form systems that mimic surface plasmon resonances
that are typically reserved for nanoscale metallic objects. The absorptive
properties of water furthermore act to homogenize higher-mode profiles and to
preferentially select evanescent field concentrations such as the axial
hotspot. Thus, beyond providing an explanation for a popular-science
phenomenon, we outline a method to experimentally model subwavelength field
patterns using thermal imaging in macroscopic dielectric systems.
That’s a nice sounding explanation, but
what does it mean? My best effort at explanation
is below (aided by the CBC story), but you can read the scientific paper for yourself,
to double-check.
Basically, the microwaves interact with the
grapes to form a plasma, which is a state of matter characterized by unbound
charged particles. A grape is about the
right size to create a microwave resonance – roughly one-hundredth the size of
the standing wave that a microwave creates to cook food. It also contains water, which has the right
refractive index to aid in the resonance effect. A microwave of about that size can get caught
in the grape, bouncing around inside, being reflected from the skin. This heats the grape.
The nearby grape has the same thing happening
inside it, and the trapped microwaves can go between the grapes, creating a
strong electromagnetic field, which energizes electrons and ions within the
grapes, resulting in a plasma state within the grapes, and that gives you the
light show.
Apparently grapes aren’t really necessary,
as any similarly sized objects that contain water can have the same
result. The researchers also warn
against making a habit of this, as it seems like it can cause damage to the
microwave over time, though they haven’t determined the exact cause of that.
It is worth noting that the state of matter known as plasma is actually very common throughout the universe, as that's basically the state that things like stars are in. And let's not forget the various plasma storms of Star Trek fame (or of Northern Lights fame).
An interesting thing about microwave ovens,
is that the thing that generates the waves (a magnetron) was a top secret
development for the allies in WW2, which resulted in some very high resolution
radar sets. These were instrumental in
such matters as finding enemy U-boats that had surfaced mid-ocean to re-charge
batteries or travel on the surface. Thus, after the technology was commercialized
post-war, they were often called radar ranges (which explains a visual joke in
the movie Airplane – “Gunderson, check the radar range!”).
Sources:
Linking plasma formation in grapes to microwave
resonances of aqueous dimers
Hamza K. Khattaka, Pablo Bianuccib, and Aaron D.
Slepkova
Procedings of the Natioal Academy of Sciences
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Now that you have read some real science
(physics), you should read some science fiction. Either of the Kati of Terra series or the
Witch’s Stones series would be excellent choices. Alternatively, you could try the short story
“The Magnetic Anomaly”, which has lots of physics, and plenty about electromagnetic
fields, which can affect brains as well as grapes. J
Kati of Terra
How
about trying Kati of Terra, the 3-novel story of a feisty young Earth woman,
making her way in that big, bad, beautiful universe out there.
The Witches’ Stones
Or, you might
prefer, the trilogy of the Witches’ Stones (they’re psychic aliens, not actual
witches), which follows the interactions of a future Earth confederation, an
opposing galactic power, and the Witches of Kordea. It features Sarah Mackenzie, another feisty young
Earth woman (they’re the most interesting type – the novelist who wrote the
books is pretty feisty, too).
The Magnetic Anomaly: A Science Fiction Story
“A geophysical crew went into the Canadian
north. There were some regrettable accidents among a few ex-military who had
become geophysical contractors after their service in the forces. A young man
and young woman went temporarily mad from the stress of seeing that. They
imagined things, terrible things. But both are known to have vivid
imaginations; we have childhood records to verify that. It was all very sad. That’s
the official story.”
