ding-dong Michael here today we're gonna
talk about this a thing that you can
make at home what does it do well take a
look I have here a glass I'm gonna fill
this glass with water that I've dyed
blue so it's easier to see all the way
up to the very top of the glass just
like that perfect now watch what happens
when I place this tube in the glass
pretty cool right the water flows out
and it keeps flowing out but how is this
happening I mean I just put a tube in
the glass the tube has no moving parts
no pump what's pushing the water out of
the glass well this isn't just a - no no
it is a self-starting siphon I first
learned about these from the Curiosity
show and I am incredibly grateful
because I mean they're a blast it's
almost like magic but it's not magic it
has everything to do with the shape of
the tube and physics now to see how it
works let's keep watching because it
won't run forever eventually the flow
will stop and that will happen when the
water level in the glass is that the
same height as the exit point of the
straw the flow will get weaker and
weaker as we approach that point
eventually it'll just turn into a few
drops and then it will stop and the
system will be in equilibrium perfect at
this point the forces acting on the
fluid cancel each other out and the
fluid stops moving but what are the
forces well for a siphon like this the
most relevant forces are weight forces
every single molecule of water and air
in this system is being squeezed by the
weight of everything above it the lower
down a molecule is the more stuff there
is above it and therefore the larger
weight weighs down on it so it's under
greater pressure now because water and
air can flow the result of this pressure
isn't just a tendency to move down when
possible oh no water and air molecules
can slip by each other and so when under
pressure they're ready to move in
whatever direction they can
think about an air molecule right here
by the exit of the straw it is being
squeezed by the weight of air above it
against all the molecules below it and
if that straw was suddenly evacuated of
water and air
you better believe that under pressure
those molecules of air outside would
rush inside all right so now that we're
thinking about pressure let's look at a
picture all right here is a diagram of
what we have been looking at a container
filled with water and a tube that is
also filled with water the exit of the
tube is at the same height as the liquid
in the container and nothing is flowing
why well let's take a look at the
pressures at work first of all it's
important to note that pressure depends
on depth and that's really it I mean it
also depends of course on acceleration
into gravity and the density of the
fluid that you're in but those don't
really change much over these small
distances it's the depth that matters
not the volume of water just the depth
so here at the entrance where is the
entrance
well the entrance is deep in the
container and how deep well it's this
distance deep and I'm gonna call this h1
perfect now before we get too far let's
talk about some other depth that exists
the entrance is also in much deeper
water in the tube let's divide the tube
in half and now I'm going to call this
half the outside half and this half the
inside half so there's a lot there's a
lot more water the water goes up a lot
higher here in the tube so what is this
depth well from the surface of the water
in the container down to the entrance
this is also h1 all right same depth but
the water goes up higher in the tube
there's also this extra depth I'll call
this h2 so h2 plus h1 equals the total
depth on the inside of the tube over
here on the outside how deep is the
water at the exit well it is let's say
h3 deep this is the distance from the
top of the tube to the exit now let's
talk about the pressures created here
we've got a pressure proportional to H
one that is coming from the water in the
container
and that's gonna cause water to go into
the straw if possible but we also have a
column of water h1 tall inside the straw
that's creating a pressure in the
opposite direction so they both cancel
out let's not forget about air when it
comes to canceling out by the way
there's also air pressure right air
pressure pushing on the water in the
container and if this straw was empty
that would that would cause the water to
go into the straw but there's also air
out here there's also air in contact
with the exit and that air pressure is
equal to this air pressure so they also
cancel out all we're left with are h2
and h3 the depth of water above the
water in the container on both the
inside and the outside of the tube if h2
is equal to a ch3 that means we have the
same pressure out of the straw into the
container as we do out of the straw into
the environment and so nothing will move
however if I pour more water into the
container and say raise the level of
water in it up to here so this is now
all water well now there is a greater
pressure out of the container because h2
has become much smaller h2 is now only
this large and if h2 is smaller than h3
if the depth of water on the inside
above the containers level is smaller
than the depth of water on the outside
there's more pressure here moving out
and so the water will flow out until the
water level in the container has dropped
back down to the same level as the exit
at which point we'll be back at
equilibrium there will be this situation
h2 and h3 equal depths on the inside and
the outside and the water will stop
moving now let's see this in real life
water will flow in the direction of the
side of the siphon that has the deepest
amount of water in it that isn't already
under water right now those depths are
equal so water isn't flowing but if I
add some more water to the glass the
water level in the glass will go up and
the depth of the water in the siphon
that isn't also under water will get
smaller relative to the
outside so the water will flow to the
outside watch this it's regulating
itself it's really beautiful this will
continue until once again the water
level in the glass is equal in height to
the exit of the straw now let's say you
don't already have a straw that's full
of liquid how do you get to this point
well there's a couple of ways to do that
one way is to I kind of feel like this
should finish it's just doing such a
good job yeah hmm thank you
all right so let's say your straw isn't
already filled with liquid how do you
get some siphon action going well let's
fill this up to the brim that makes our
job a lot easier because remember we
only need the water on the outside of
the siphon to go below the water level
in the glass so the higher that level in
the glass is the the less down we're
gonna need the water to have gone to get
the siphon to happen
now if I put this empty straw in the
glass not much will happen but I need
the water level on the outside to get
below the water level in the glass and I
can do that by just like sucking on the
straw like that perfect now it's working
but let's say you want to siphon a
liquid that you don't want in your mouth
well there are a number of different
options available to you all kinds of
devices to draw the liquid through the
siphon this is also a really fun thing
to do with a straw cover one end of the
straw and place it in the glass because
you're covering one end the air won't be
able to escape when you put it in so the
air inside the straw will get compressed
its pressure will be greater and it will
push down on the water in the straw more
so than atmospheric pressure does when
you then release your finger the
compressed air inside will escape and
the water in the straw will rise up to
the level of that it is in the glass
however it won't stop there because
while the water is being pushed up by
pressure but what is pressure right it's
force it's force per area so that force
accelerates the water by the time the
water reaches the level that it would
normally stay at in equilibrium it still
has velocity it's going to be slowing
down but if I do this just right it
won't
slow down or get reversed until it's too
late and the waters already passed over
the bend and fallen down the other side
below the water level in the glass let's
see if I can do this I'm gonna use my
thumb to cover the straw I'm gonna go in
and when I release my thumb there you go
the water is flowing out and nothing had
to get in my mouth now that's pretty
cool but what about this self working
siphon how does it work because I don't
even have to cover it over with my thumb
where is my I found it so yeah how does
a self-starting siphon work I don't need
to use my finger or suck on it to get it
started it just well true to its name is
self starting well to see how this works
let's go back to the overhead camera
here's a cardboard model of the shape of
our self starting siphon I'm gonna use
this because this is kind of wet alright
so what happens when I put the self
starting siphon into a full glass of
water well at first the siphon is full
of air as it goes down into the glass
the water in the glass enters the straw
and pushes out the air but if I put this
in really quickly through look what
happens well there's enough pressure
here for the water to get pushed all the
way up to the top but that can't happen
because of the bend and the straw so the
water reaches this bend and it's still
under pressure there's still a force
acting on it so it continues to be
accelerated but now it's going down oh
so it's also being accelerated by
gravity so it's velocity gets larger and
larger and larger it turns that bend and
it comes all the way up here once the
water in the straw has reached this
level it will not be accelerating up
anymore however after all of this it may
have reached we hope if I put the straw
in fast enough a velocity sufficient to
continue on around this third Bend
before being stopped and reversed until
it is low enough maybe like say here
that it's below the water level in the
container when that happens it will
continue flowing and we've got ourselves
a self started siphon to make your own
self starting siphon you will need a
full glass of water
three bendy straws and some scissors now
one of these straws will be our middle
Bend and we're going to use it to ensure
that one hump is higher than the other
I'm going to do that by cutting the
straw a bit shorter on one side of the
bed
that should be about enough perfect now
as you can see we're going to be able to
join the straws together to make an M
shape where one side is higher than the
other to join them I like to cut the
straw at a bit of a diagonal like that
so I get a nice sharp point in a gradual
widening that way I can push them
together
let the plastic deform and they come
together hopefully with a really nice
airtight and watertight seal yeah is
that good
all right well the best way to check is
to cover one end with your finger and
suck on the other end if the feel is
good then it should hold the vacuum I'm
happy with that
very good all right now let's join up
the other side again I'm going to cut at
a diagonal just like that and then I'm
gonna push them together if you're
having trouble with your seals you can
always put some tape around the joint
and that might be all you need to do but
it's extremely important that they be
air and water tight let's see how well
I've done cover one end suck yeah see
that that's good
got a nice glue feel alright we're not
quite done because I need to shorten one
end I want the lower hump to be
submerged in the water and right now the
straws is a bit too long so when the
higher hump is at the edge of the glass
I think I'll need it to be maybe I don't
know that short to be safe let's make
sure it's gonna fit in the glass and
let's make sure the seal is still good
okay all right I think we're ready I
think we're ready to test our homemade
self-starting siphon in three two one
Hey look at that how cool I love making
these things so please make some at home
and play around with pressure and what
pressure can do it is a blast for more
hands-on science check out our curiosity
box stay safe out there stay dry and as
always thanks for watching
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