yes sorry
so for the next few minutes i'm going to
talk about literally nothing i'm going
to talk about the vacuum void
nothingness and and how scientists
really really need a vacuum needs
something that's empty of molecules to
do a lot of experiments it's very very
important for the work we do where we
want to manipulate individual atoms and
molecules you want to keep it in a
vacuum and make sure that you get as few
molecules sort of contaminant molecules
interacting or bonding to the surface as
you can however you can never ever reach
a perfect vacuum and that's related
fundamentally again to quantum mechanics
uh the unit we use for pressure well we
use a range of units we actually one of
the the interesting ones is tor
which stems from toricelli and toricelli
was one of the first people to build it
was was the first person to build a
barometer and
760 torr is
the atmospheric pressure and that stems
from to a certain extent stems from the
height of uh of a column of mercury that
atmospheric pressure can actually um
sustain
the other unit we use is the millibar
which is perhaps more familiar but the
unit the the si unit the proper si unit
and this is in terms of newtons per
meter squared is the pascal and so
atmospheric pressure is ten to the five
a hundred thousand very roughly ten to
the five pascal but when i just troll
that number that 100 000 pascal that
that's just a number it doesn't really
make it mean a lot but if we think about
what that actually means in terms of
force and the force on an area it's a
huge force 100 000 newtons per meter
squared is equivalent to about a ton of
force on on an area which is about a
foot square a huge huge amount of force
just raining down on us and we forget
just i guess
we're just so used to to to living with
that we forget how much force that that
can actually impart
so the the vacuum in a vacuum cleaner
compared to the experiments we do is
just
terrible useless nonsense we couldn't do
anything with that type of vacuum turns
out that um the vacuum in a vacuum
cleaner if we start off with atmospheric
pressure
of about a hundred thousand pascal the
vacuum in a vacuum cleaner is roughly
something like 70 or 80 000 pascal so
not a very very big change the key thing
there is though what's happening is that
sometimes we get this idea that
when we use a vacuum cleaner that
something is sort of actively sucking
the more important thing is that what
you've got is air pressure um that is
it's the force of the of the air
streaming into a region where there is
nowhere or the amount of air has been
reduced that actually gives you that
force that allows you to clean things
dust isn't sucked into a vacuum cleaner
it's pushed in it's effectively pushed
in yeah it's it's entrained with the air
something called viscous flow so it gets
entrained with the air and it's it's
basically pushed in and it's driven
along with the air so this idea of
sucking is is actually not quite what's
happening i'll see a pretty vivid
demonstration of that later on we really
want to work at not a hundred thousand
pascal or eighty thousand pascal we
actually want to go down and work at
nanopascal pressures so 10 to the minus
9 over pascal and that the pressure we
work with in the lab in our routine
experiments is comparable to that you
actually get in the surface of the moon
so we want to remove all those molecules
and get down to those type of pressures
right so what we have here is a tube
which i'm about to evacuate we talked
earlier about just how much force air
and air pressure can impart hopefully
we're going to see quite a dramatic
illustration of that so what i'm gonna
do is gonna pump the the air out of this
pipe with a rotary pump a vacuum pump um
in here there's a ping pong ball and
i've got a very simple seal here and a
very simple seal here the seal on this
end is just the end of a
plastic cup we put that on there that's
going to be held in place basically by
the air pressure it'll be a vacuum and
it'll be air pressure holding this on
this side i've just taped it up with
some tape
i'm going to pump it out i'm going to
leave it for a few seconds and then i'm
going to put the stanley knife rupture
the seal at the back here the air will
rush in and the ping pong ball will go
flying at the end if we do this right
the ping pong ball should fly out at
about 400 miles an hour or something
like that and hopefully we'll pulverize
the coke can right let's try
yes sorry
good so let's see what it did
but certainly pulverize the coke can and
in fact with a longer length and a bit
more oomph you can actually go straight
through the coke can
you should take from that the the the
rather large forces that air alone can
actually impart the the as i said the
ball it's a ping pong ball where is it i
don't know
it didn't go in the bin did it
oh that's fantastic actually in this
case it hasn't been the the ping-pong
ball hasn't been damaged but that's a
standard ping-pong tennis ball tennis
table tennis ball 40 millimeters
diameter very very very lightweight and
it ended up in that bin and it ended up
in that bin and it pulverized the cocoa
so that's the the force that air
pressure alone can actually exert a
really nice way of looking at at vacuums
are getting getting a good insight into
what a vacuum is is to look at something
called the mean free path how far can a
particle get before on average before it
smacks into another particle and um in
at atmospheric pressure that turns out
to be a very very very small distance
it's about 100 nanometers so on average
an a molecule in the air can only travel
about 100 nanometers a hundred
billionths of a meter before it smacks
into another one
when we get to down to really really low
pressures like ten to the minus nine and
nanopascal which is what we work with in
the lab with pressure on the moon then
what we have is a mean free path that is
many thousands of kilometers so those
those molecules have a very very lonely
journey through through the void before
they actually bounce into another
molecule it's amazing from one
perspective and that we can get down to
those pressures but the amount of pain
sometimes that's involved in trying to
get get our experimental equipment to
get down to those pressures can be can
be quite tricky for example one of the
things we have to do with our stainless
steel vacuum chambers to get down to
those pressures regardless of sort of
the the quality of the pumping we use is
we have to bake them we literally have
to put them in an oven and cover them
all tin foil make them nice and toasty
and and bring the temperature up to
about 150 degrees and the reason we have
to do that is because if we don't do
that all the water that's in it that
covers all these surfaces in air there's
a thin water film just sits there and
over course of time just gradually those
water molecules come out and pollute our
vacuum basically we have to get rid of
all those molecules okay let's have
another go
better vacuum build up and now let's try
it
yes yes that was good
oh
it shattered a bit of it we're getting
close to her maybe if we keep improving
if we go to a second generation version
we might actually punch right through
you