this is a working siphon unaided by any
machine liquid is moving through a hose
from one container to another
the force is driving this process are
complex and interesting let's
investigate
start by setting a cup full of water on
a block and an empty Cup on the table
position a hose so that it connects both
cups
I colored the water to make it easier to
follow this process
to start the siphon we need to fill the
hose with liquid
to do this insert a pipette into the
hose
squeezing the bulb forces air out of the
tube creating bubbles
slowly releasing the bulb we see liquid
moving into the tube
we might say that the pipette is sucking
liquid into the tube
actually the liquid is being pushed into
the tube by the external air pressure
when we release the bulb a partial
vacuum appeared external air pressure
pushes water toward this vacuum
to start the siphon remove the pipette
while holding the end of the tube below
the level of the liquid
it is important to keep this end of the
tube below the liquid level or the
liquid will run back into the upper cup
once started the liquid will continue to
flow
one of the surprising things about this
is that the liquid actually flows uphill
as it exits the upper cup
this process requires both air pressure
and gravity
if we look at the siphon in its start
position we see that the column of
liquid in the tube on the left
is much shorter than the column of
liquid in the tube on the right
the column of liquid on the right is
heavier than the shorter column
pulled by gravity it starts to fall
flowing towards the lower cup pulling
water from the upper cup with it
this flow continues until one of two
things happen either the liquid is all
removed from the upper cup or an
equilibrium is reached
this occurs when the liquid level in
each cup is the same
this siphon has stopped flowing
equilibrium has been reached the level
is the same in each cup
when the siphon starts the long column
of liquid weighs more than the short
column
when the two columns have the same
vertical height forces are balanced and
the flow stops
sitting both cups on the table top we
can see the siphon starting to flow
stopping when both cups are at the same
level
there is another question
why does the column of water stay
together why doesn't it break in the
middle and flow in two directions
air pressure is responsible for this if
the column pulled apart that would
create a vacuum and air pressure would
push the column back together
if you drilled a small hole in the
siphon tube at the top this would
introduce air and the liquid would flow
in two directions
if you're teaching about liquids there
are some interesting things you can do
with siphons
mixing two colors with a siphon creates
some dramatic effects
foreign
you can create a siphoned stair with a
number of blocks starting each section
of the siphon in sequence
foreign
science related activities and videos at
our website hilarroad.com follow the
projects link