today on Applied Science I'd like
to
show you an unusual property of rubber
bands and also demonstrate a mechanism
that I built that makes use of this
property so here's what you can do you
can try this at home just go to your
office drawer and pick out a couple of
regular rubber bands the bigger the
better this one is a size 64 and what
you want to do is stretch it really hard
and then hold it up to your lips so
let's switch to thermal vision so you
can see what I feel and so here's the
rubber band and it's kind of at room
temperature the scale here is about 27 C
to 34 C now if I stretch it really hard
you can see that the rubber band
actually warmed up and if I hold it to
my lips like this it's warm I can feel
it it's it's quite noticeable a change
so you might be thinking well that's not
that exciting I mean it's probably just
friction in the rubber you know car
tires heat up and all this however if
you keep the rubber band stretched like
this and either blow on it or just hold
it out in the air for a while and then
let it contract watch what happens it
actually gets colder than room
temperature and if I put it back up to
my lips it's it's noticeably cold so
when you try this stretch it really hard
so you get a good difference and like I
say blow on it a little bit so that it
comes back down to room temperature and
when you let it contract it's definitely
cold what a weird property right so I'll
let you think about why it does this at
first it might seem counterintuitive
because if you know you know gases get
cold when they expand and get hot when
you compress them it almost seems like
the rubber band is doing it in Reverse
where when it expands out it's getting
hot and when I let it contract again it
gets cold so we'll talk about this at
the end of the video but right now I
want to show you a mechanism that I
built it makes use of this property if
you search around on the internet you'll
see an unusual device that looks like a
bicycle wheel that's made out of rubber
bands basically the rubber bands take
the place of the spokes and if you shine
a light like a tungsten light on one
side of the wheel the rubber bands will
change shape and cause the wheel to
become off-center and it will roll
around exposing fresh rubber bands to
the light bulb so you might
getting the idea here that rubber bands
can act as kind of a part of a heat
engine just like the refrigerant does in
your refrigerator so I searched around
to see if anyone's tried building an
actual refrigerator from rubber bands
and I couldn't find anything so I
decided that would be a great thing to
give a shot I used the same idea of
having a wheel with an offset hub so
that half the rubber bands would be
stretched and the other half would be in
their natural state and then we know
that the stretched half is going to be
hot so we want to have that outside the
box and then the contracted state will
be inside the box and with some fans to
kind of blow the hot air off of the hot
ones and another fan to blow the cold
air off the cold ones and if you put the
wheel kind of right on the edge of the
boundary of the box you know this whole
system can work I know the ceiling is
not going to be that great of course but
really it's just a demonstrator just to
see if this thing can work at all I did
most of the modeling in SolidWorks and
exported all of the flat faces as SVG
files and then used my handheld CNC
router made by Schaefer to cut all these
pieces out so I've actually been looking
for you know to do a lot of projects out
of wood like this that would require a
wood router and I don't have one but
this this CNC handheld router is really
the best way to do it and if you're
watching this video in August 2016
you're in luck these are actually
available for pre-order now and I think
the price is going to be under 1,500 if
you're a quick so go to shaper tools and
I'll put a link in the description if
you're interested in this unlike a laser
cutter one thing that's nice about
having a router is that you don't have
to cut all the way through the material
like you can make a pocket very easily
so if you just want to pour away some
material it can do that and also this is
kind of thick for a laser cutter so I
don't really know if you'd want to do
half-inch thick plywood it leaves the
edge unburned which is another nice
thing and so for all these reasons this
really is the best tool to make wood box
type things with the joinery and
everything I created the gears using a
free SolidWorks gear creation tool and
the fit is actually even better than I
would have guessed the gears actually
work really
okay so let's see this thing in action
of course I'm using the thermal camera
again so that you can see the parts
heating up and getting cold as they're
moving and as you might guess it's not
like it's gonna cool your beer down or
anything like that I mean it's you know
barely a temperature change but you
can't actually see it pretty clearly on
the thermal camera the stretch of the
rubber bands is a little too much so
when I was prototyping this I had a
little jig on the table and I was
checking to see where I could reliably
stress the rubber band and how much
temperature change it actually created
it's about you know two degrees C for
the stretch that I was putting into it
and it's getting near the limits of what
those rubber bands can take I actually
thought I had a bit more margin than I
do but as you can see a lot of rubber
bands have snapped and partially that's
because of the wood breaking and
partially that's because the rubber
bands snapping but anyway this this
thing could use a redesign and so next
next time I'll let you know if it's in
better shape I ran it with the door
closed for just a little while you know
literally just five minutes or something
like that and you can just barely
perceive that it's slightly cooler
inside than it is outside although I
admit this might just be because I'm
standing near the thing and just my body
heat might be warming up the outside so
it might look like it's just looked like
it's getting cooler inside even though
it's the outside that's warming up so
coming back to these rubber bands the
actual reason that they get cool when
you let them contract demonstrates an
interesting law of thermodynamics so
think about this if you have like an
aerosol paint can and you spray some
paint out the can gets cold because the
gas expanded and everyone you know
intuitively knows that that's fine but
the rubber bands appear to work in
Reverse where you when they contract
they get cold so it almost seems like
it's sort of breaking the law but it's
actually the same thing and here's how
to think about it when you spray some
paint out of the can think about why the
gas even wants to come out of there
right like you've opened the nozzle
there's no physical reason that air
couldn't rush inside the can and make
the pressure even higher but it's
favorable for the universe to sort of
equalize disorder and so when you press
the nozzle because the pressure is
fire inside it wants to equalize with
the pressure outside and that's what
drives this whole process so what's
strange about the rubber band is that
when it's in the contracted state all
the molecules inside there are actually
very tangled up and when you stretch it
you're actually making it more ordered
you're actually putting the molecules
more straightened out so the stretched
version of the rubber band is much like
the compressed gas in the can because
you've done work to sort of segregate
those molecules and putting them inside
the can so if you sort of take your
hands off the system you're not applying
any work you're not controlling it the
universe tends toward disorder entropy
basically and that's what causes this
thermodynamic thing to happen it's the
reason that the rubber band wants to
return to its shape and it's also the
reason that as it contracts it's doing
work and it's getting cold so this is
the entropy different process which is
actually the same as a gas it just has
sort of an opposite appearing effect
okay I hope you enjoyed that and I will
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