00:00:09
okay so this brings us to the start of
00:00:11
chapter two does anybody have an idea
00:00:14
what this is
00:00:19
bubbles they're bubbles oxygen
00:00:25
they are they do contain atmosphere
00:00:28
atmospheric oxygen nitrogen
00:00:34
you could see this if you paired into a
00:00:36
relatively clear cocktail
00:00:39
ice it is ice right
00:00:42
and so what are those spherical things
00:00:46
in there
00:00:49
is it trapped air like an ice core
00:00:51
exactly so when water freezes it forces
00:00:54
the air out
00:00:55
right and it can't be dissolved in there
00:00:57
anymore
00:00:58
the oxygen nitrogen can't be dissolved
00:01:00
anymore so
00:01:01
um yeah it's trapped and then the last
00:01:04
question for you
00:01:05
is why is it spherical
00:01:14
water tension go lane yes exactly
00:01:17
uniform pressure
00:01:18
both those things um the the water the
00:01:21
water tension
00:01:22
basically holds the edges of bubbles
00:01:25
together
00:01:26
and it also it's the shape that has
00:01:29
minimal it takes the minimal amount of
00:01:32
surface area minimal energy
00:01:34
for per volume so we're going to go into
00:01:38
the properties of water now
00:01:40
we'll start with chemical and physical
00:01:41
properties talk about
00:01:43
water viscosity inertia and physical
00:01:45
parameters movement of water
00:01:47
and then forces that move water
00:01:51
this class is generally organized from
00:01:53
the basic physical stuff
00:01:55
up to the more complex
00:01:59
factors associated with the organisms
00:02:02
we'll hit the organisms
00:02:05
taxonomy sort of a little bit earlier on
00:02:08
just so we know what the players are
00:02:10
in the game
00:02:14
so there's really important chemical and
00:02:16
physical properties of water water is a
00:02:18
fairly unusual
00:02:19
compound because of hydrogen bonding
00:02:23
it has high density it has high surface
00:02:27
tension
00:02:28
high heat vaporization it takes a lot of
00:02:30
energy to heat it up
00:02:31
high heat capacity it's liquid at the
00:02:34
earth's surface
00:02:36
it's an excellent solvent it's really
00:02:39
important for weathering it
00:02:40
drives global biogeochemical over
00:02:42
geological time
00:02:44
so we can talk about a few of these
00:02:46
things real quickly
00:02:48
density does anybody know what uh how we
00:02:51
define a gram
00:02:54
what a gram is isn't it one cubic centim
00:02:58
centimeter of water exactly right so one
00:03:01
of our basic units of measurements a
00:03:03
gram
00:03:04
is based on one cubic centimeter of
00:03:06
water
00:03:07
at its maximum density and we'll find
00:03:09
out what temperature that is at
00:03:10
in just a little bit
00:03:14
all of these things surface tension heat
00:03:16
of vaporization
00:03:18
all are related to the fact that that
00:03:21
there's
00:03:21
hydrogen bonding that's keeping what
00:03:23
holding water together
00:03:25
can anybody think of other compounds
00:03:28
that are liquid
00:03:29
at the surface of the earth under normal
00:03:32
uh
00:03:32
normal temperatures i mean there's a
00:03:36
lava
00:03:36
but that's not normal i mean that's
00:03:39
normal for
00:03:40
a volcano but for most places it's not
00:03:42
so it's standard atmospheric temperature
00:03:44
and pressure are there other
00:03:46
other compounds that are liquid yeah
00:03:49
mercury mercury we've got mercury yeah
00:03:52
there's another there's another class of
00:03:54
compounds that is is liquid
00:03:58
oil is good oils and alcohols right
00:04:01
so or there's organic compounds other
00:04:03
than that
00:04:04
most of them are not liquid and and
00:04:07
they're not very abundant
00:04:08
so it's really important um
00:04:11
in addition to those properties ions
00:04:14
tend to be more soluble in warmer water
00:04:16
for the most part there's a few that
00:04:17
aren't
00:04:18
and gases are less soluble so when we
00:04:20
were at the lake yesterday
00:04:22
talked about oxygen solubility in water
00:04:26
and the warmer water gets the
00:04:29
less oxygen there can be in the water so
00:04:31
that's why the amount of oxygen in water
00:04:33
is a function of the temperature
00:04:36
there's an unusual relationship between
00:04:37
temperature and density which we'll get
00:04:40
to
00:04:42
right now so here's a graph of
00:04:45
temperature
00:04:47
in degrees centigrade versus density
00:04:51
and as i said right one gram is one mil
00:04:55
but at the maximum density and so it's
00:04:57
exactly one
00:04:59
at about 3.9 degrees centigrade
00:05:05
and then as you heat up water it becomes
00:05:09
less and less dense
00:05:14
so we saw that at the lake yesterday
00:05:16
where there's a little bit warmer water
00:05:18
sitting on top of the cooler water
00:05:20
because it was it's heating the surface
00:05:22
is being heated by the light
00:05:24
and it's warmer and so it'll just stay
00:05:27
sitting on the top
00:05:29
now there wasn't a very big temperature
00:05:30
difference so if a big wind came it
00:05:31
would have
00:05:32
knocked that apart but we will talk
00:05:34
about that more in a little bit
00:05:37
so there's this break in this graph
00:05:39
right here
00:05:41
right and all of a sudden we're jumping
00:05:42
a whole bunch of units
00:05:44
and then we're right down here much less
00:05:47
dense
00:05:48
than water is at any temperature even
00:05:51
almost all the way up to boil all the
00:05:52
way up to boiling
00:05:54
so what what is that
00:05:57
what's happening at zero degrees that's
00:05:59
leading to water to become
00:06:01
much less dense
00:06:05
they're freezing exactly it's ice right
00:06:08
so ice
00:06:09
freezes and it floats it sits on top
00:06:13
and that's an absolutely essential
00:06:15
property a very unusual property that
00:06:17
that the solid form is actually
00:06:20
less dense than liquid form but it's
00:06:23
really important because if it was not
00:06:25
true
00:06:25
most of the water would be locked up uh
00:06:28
in the oceans because
00:06:29
once it froze it'd go down to the bottom
00:06:31
and just sit there frozen
00:06:33
right so it's it's a really important
00:06:34
aspect
00:06:36
of the way water works
00:06:40
the other thing we can see that this
00:06:42
curve is not straight right here that
00:06:43
it's actually falling off more and more
00:06:45
quickly as we go
00:06:46
up here right and so this next graph is
00:06:50
a percent density
00:06:51
per degree c increase right and we see
00:06:54
that what's happening is that density
00:06:56
decreases happening faster and faster
00:06:59
and this will be important when we talk
00:07:00
about stratification of lakes
00:07:03
in the tropics in particular so there's
00:07:07
the maximum density of water
00:07:09
and we can see these two rates that what
00:07:12
this graph is showing is that this rate
00:07:14
right here
00:07:15
is not as high as this rate is right
00:07:17
here
00:07:22
and here's another version of this of
00:07:25
the same graph
00:07:26
where now we're going all the way up to
00:07:28
80 degrees c i mentioned that ice
00:07:30
and no break right so ice is still less
00:07:33
dense than water when you're almost
00:07:35
all the way up to boiling right and then
00:07:37
we're zoomed in a little bit without
00:07:39
that break
00:07:40
here and we're zoomed way in right here
00:07:43
so we're going from
00:07:44
zero to ten and this is another point to
00:07:47
be made
00:07:48
is that you know the maximum density is
00:07:50
right here at about three point nine
00:07:53
or we could say four it would be fine if
00:07:54
you said that um it's standard
00:07:56
temperature
00:07:58
but what happens here is kind of curious
00:08:00
because it's as you get
00:08:01
colder it's starting to get less dense
00:08:06
right so water that's cooler than four
00:08:09
degrees can float on top of
00:08:10
four degree water and water that's
00:08:12
warmer than four degrees can float on
00:08:14
top
00:08:14
of four degree water what's happening
00:08:18
here is those hydrogen bonds
00:08:20
are basically there's weak hydrogen
00:08:22
bonding
00:08:23
between the water until you get up to
00:08:25
you know boiling and then eventually
00:08:26
it's all flying out in the atmosphere
00:08:28
and it's not there very much anymore
00:08:31
as you get cooler it gets more and more
00:08:34
dense right
00:08:35
but then if you get below 4 degrees c
00:08:38
you start getting those hydrogen bonds
00:08:40
forming the the structure that ice will
00:08:44
eventually have and it starts opening up
00:08:46
because it's starting to form that
00:08:48
crystalline structure albeit transiently
00:08:51
so it's starting to actually become less
00:08:52
dense and those hydrogen bonds are
00:08:54
starting to order things
00:08:55
more and more and and cause the density
00:08:57
to decrease
00:09:02
in addition to temperature change in the
00:09:05
density of water
00:09:06
the amount of salts also have a big
00:09:08
effect
00:09:10
and so we have right here seawater is
00:09:12
this point
00:09:13
is is this point right here
00:09:16
and we see that the density of seawater
00:09:19
is 1.00
00:09:23
or so like that right
00:09:26
so if you go back if we go back to this
00:09:28
graph so think about
00:09:30
0.01 difference in density
00:09:35
these dense differences up to 40 degrees
00:09:37
or points
00:09:38
in the point zero zero one so ten fold
00:09:41
left
00:09:42
so if you have saline water you can
00:09:44
basically stratify a system
00:09:46
and go against these temperature things
00:09:50
so that'll become important too when we
00:09:52
talk about saline lengths
00:09:59
and anybody tell me what this is
00:10:04
good abigail water strider and why is it
00:10:07
there
00:10:09
because it holds water tension allows it
00:10:11
to move across the surface of the
00:10:12
water right so it's one of a number of
00:10:15
organisms that can use water tension to
00:10:17
stay on the surface of the water and
00:10:18
it's
00:10:19
specifically adapted to using it you can
00:10:21
see it's pushing down in there but that
00:10:23
what that the hydrogen bonding is
00:10:25
holding that
00:10:26
that surface tension and sort of
00:10:27
depressing a little bit
00:10:30
so this is a spectacular picture
00:10:35
okay so i've got a question of the day
00:10:39
for you all
00:10:40
and i think you all have your names on
00:10:41
your on your um
00:10:43
screen so just go ahead and chat
00:10:47
throw your answer in the chat and i'll
00:10:50
that will record your name and i'll be
00:10:51
able to use it for
00:10:53
your attendance question of the day
00:10:58
point
00:11:06
all right so now everybody's had a
00:11:08
chance to answer
00:11:10
in the chat the first one maximum
00:11:12
density of
00:11:13
water is at about four degrees c that's
00:11:15
true
00:11:16
ice is more dense than cold water no
00:11:18
it's less dense
00:11:19
it floats density decreases an ever
00:11:22
greater rate as temperature increases
00:11:24
from four degrees c that's also correct
00:11:26
and that was the graph where i had the
00:11:27
two
00:11:28
lines as it went up the units of gram
00:11:31
are based on water at its minimum
00:11:33
density no
00:11:34
its weights based on water at its
00:11:36
maximum density
00:11:38
so that d is incorrect
00:11:49
so now we're going to get into some
00:11:51
stuff about reynolds number and this is
00:11:53
stuff
00:11:53
that perhaps you've not had in other
00:11:55
places but it's really important as far
00:11:57
as the way
00:11:58
organisms have evolved to operate
00:12:01
in their natural environment and it
00:12:04
tells us a lot about the way systems
00:12:06
work
00:12:07
and there's the reason for this is that
00:12:10
hydrogen bonding
00:12:11
tends to become more important at
00:12:13
smaller scales
00:12:14
and this alters both viscosity and
00:12:16
inertia
00:12:18
so if you think of the hydrogen bonding
00:12:20
being these little this little net thing
00:12:22
that's holding things
00:12:24
and you think of something like a fly
00:12:26
pushing through a spider web right
00:12:29
you use that as an analogy fly can't
00:12:32
push through a spider web it gets stuck
00:12:34
we just walk through a spider web right
00:12:37
it's because of the difference in scale
00:12:40
the same thing is going to happen with
00:12:42
we have a bacterium trying to swim
00:12:43
through water
00:12:45
or release them through water right it's
00:12:47
going to be more resistant and feel more
00:12:49
viscous
00:12:51
viscosity is a resistance to change in
00:12:53
form it's
00:12:54
a form of internal friction
00:12:58
and inertia is a resistance of a body to
00:13:00
a change in its state of motion
00:13:02
so you probably all know about inertia
00:13:05
reynolds number
00:13:06
incorporates both foods so reynolds
00:13:09
number is a
00:13:12
unitless number that describes
00:13:14
fundamental properties of a fluid that
00:13:15
are based on
00:13:16
scale and how the the
00:13:20
object that's moving through the fluid
00:13:23
and what the sk what um what the density
00:13:26
is
00:13:29
so here are some equations don't panic
00:13:31
um
00:13:32
because you can go look these up again
00:13:34
easily
00:13:36
the first viscosity is
00:13:39
a and this this is a function of
00:13:42
the dynamic viscosity so dynamic
00:13:45
viscosity
00:13:46
is sort of the intrinsic viscosity of a
00:13:48
fluid regardless of scale
00:13:51
and we can think of some fluids like um
00:13:53
we use corn syrup as an
00:13:55
as an analog for a very viscous
00:13:58
situation it happens at microscopic
00:14:01
microscopic scales but oil
00:14:04
oils are more viscous than water a lot
00:14:07
of heavy oils are
00:14:08
light gasoline is a little bit less
00:14:10
viscous than water
00:14:13
so the viscosity becomes higher as the
00:14:15
intrinsic viscosity is greater
00:14:18
as the surface area is greater as the
00:14:21
water velocity is greater and less
00:14:23
with a longer object so we'll think
00:14:26
about these in practical terms
00:14:28
the first is viscosity as a function of
00:14:32
surface area
00:14:34
so when you're in a car and you're a kid
00:14:37
and you're going 50 miles per hour down
00:14:39
the highway and the windows open what do
00:14:43
you do with your hand
00:14:50
stick it up window you stick it out the
00:14:52
window right
00:14:54
you stick it out the window and then you
00:14:56
do
00:14:57
one thing right you go like you stick it
00:14:59
out the window and then you turn it like
00:15:00
this
00:15:01
and two things happen one you get you
00:15:03
put more surface area
00:15:05
and the second thing is is bernoulli's
00:15:08
force
00:15:08
the air is going over faster over the
00:15:10
top and the bottom and it pushes it up
00:15:11
right
00:15:12
and then you do this and it pushes it
00:15:13
down right
00:15:16
also think about if you're waiting in a
00:15:17
stream if you're waiting upstream
00:15:20
into the water velocity right
00:15:23
you you you can
00:15:27
you have more viscosity than if you're
00:15:29
waiting downstream
00:15:31
if you're trying to pull something
00:15:32
upstream with a big surface area
00:15:35
it'll feel more viscous than if you're
00:15:37
trying to pull it down
00:15:39
okay so there's the water velocity
00:15:42
higher velocity
00:15:43
feels more viscous higher surface area
00:15:45
it feels more viscous
00:15:47
what did you call that principle where
00:15:49
it's like the wing and
00:15:50
the air goes oh that's the bernoulli
00:15:53
effect right so if you have a wing where
00:15:55
you have
00:15:55
if you have fluid flow that's faster
00:15:57
over one side of an object than the
00:15:59
other
00:15:59
it forces it towards that that's like
00:16:02
bird
00:16:03
wings birdslings airplane wings yeah
00:16:06
okay
00:16:10
and then characteristic length um
00:16:13
that what happens is if you have a long
00:16:15
object it
00:16:16
it influence it's less influenced by the
00:16:19
viscosity
00:16:21
than if you have a short object so this
00:16:24
is why we have
00:16:25
um javelins right javelins can go a long
00:16:28
ways
00:16:29
they want to minimize the viscosity of
00:16:32
the air they're going through
00:16:34
and that to do that they make themselves
00:16:36
long and skinny
00:16:37
so they have minimal surface area in the
00:16:39
front and a lot of length
00:16:41
and here's where if we're in a classroom
00:16:43
i throw the pointer sideways or
00:16:45
the other way and everyone laughs but
00:16:47
that's not going to work online so um
00:16:50
sorry about that uh the next
00:16:54
so so that gives you sort of a physical
00:16:56
description of how viscosity
00:16:58
works this the second one
00:17:02
is inertia and you can think of inertia
00:17:06
as being a function of the density
00:17:09
of the object the surface area of the
00:17:11
object and the velocity
00:17:15
so let's start with
00:17:18
a softball and a shot put assuming that
00:17:21
they are the same
00:17:22
size and i'm going to throw a softball
00:17:26
at you
00:17:26
or a shot put at you at the exact same
00:17:29
velocity
00:17:30
which one would you prefer me to throw
00:17:33
at you
00:17:35
depending how far away you are probably
00:17:38
the softball
00:17:39
right next to you at the same velocity
00:17:41
it will strike you at the same velocity
00:17:44
softball softball right why because it's
00:17:47
less dense so it's going to have
00:17:48
less inertia right that's why
00:17:52
you know you want to play with nerf
00:17:53
balls with kids not hard balls
00:17:55
hard baseballs right same thing
00:17:59
okay so the den that's how the density
00:18:01
works in that
00:18:02
surface area um you have a bigger
00:18:05
surface area
00:18:06
and you're going to have more inertia so
00:18:09
if
00:18:10
you are um in and
00:18:13
swimming in the ocean and a big wave
00:18:15
comes at you right
00:18:18
you want to dive under it dive through
00:18:21
it right
00:18:21
if you stand there it'll slap you hit
00:18:23
you with a full surface there and you'll
00:18:25
get that
00:18:25
full that that full
00:18:29
force of the wave or if you have
00:18:32
a little water balloon or a big water
00:18:34
balloon a giant water balloon hits you
00:18:36
it'll knock you over a little water
00:18:37
balloon
00:18:38
right and then velocity
00:18:41
it's a fact of
00:18:45
related to the square of the velocity
00:18:49
this is why it's so much worse to get an
00:18:51
automobile accident on the highway than
00:18:53
it is
00:18:54
you know it's not it's not a linear
00:18:57
relationship between 30 miles per hour
00:19:00
and 60 miles per hour
00:19:01
right it's squared worse it's not just
00:19:04
twice as bad because you're going twice
00:19:06
as fast
00:19:08
so you put these two things together and
00:19:11
you get the reynolds number as a
00:19:13
function
00:19:13
of the density the velocity
00:19:17
the characteristic length divided by the
00:19:20
dynamic viscosity
00:19:22
so let's talk about this in practical
00:19:23
terms about aquatic organisms
00:19:29
um so the first one is is how viscosity
00:19:32
varies
00:19:33
as a function of temperature of water
00:19:37
and so we see at 25 degrees and 30
00:19:39
degrees which is where we were
00:19:41
at the lake the willow lake yesterday
00:19:44
that the viscosity relative to zero
00:19:47
right is about 0.75
00:19:51
as opposed to 1.5 so it's almost
00:19:55
you know i'm sorry 1.5 so it's almost
00:19:57
twice as high
00:19:59
when the water is cold than when it's
00:20:01
warm
00:20:02
that means when you are a
00:20:06
fish you are much less you need to
00:20:09
extend much more
00:20:10
energy to move through the fluid
00:20:13
when it's cold than when it's warm
00:20:17
so what do most fish do around here
00:20:21
in the winter we will not talk about the
00:20:22
trout that they stock in there just
00:20:24
in the winter but we'll talk about some
00:20:26
of the native fishes what what's the
00:20:28
sort of the main
00:20:30
thing that fishes do in the winter as
00:20:32
opposed to the summer
00:20:36
lower the metabolism cassie said good
00:20:40
they don't stay in one spot
00:20:43
yeah they hang out right they don't do
00:20:45
much right why is it because there's
00:20:48
this balance
00:20:49
between stay close to the bottom right
00:20:50
they don't move much they they hide
00:20:53
they don't hunt much they don't move
00:20:55
around a lot because it takes a lot of
00:20:57
energy to move number one
00:20:58
their metabolism is lower so they don't
00:21:00
need as much energy
00:21:02
right so it'll take more energy to find
00:21:04
food and they don't need as much so all
00:21:05
they do is they
00:21:06
slow it down and that's because in part
00:21:10
because of the viscosity
00:21:12
now the trouts are are evolved to lower
00:21:15
temperatures and so
00:21:17
they specialize on that and that's why
00:21:19
that's why they
00:21:20
they do better with cool water than warm
00:21:22
water they still have the same effect
00:21:24
though
00:21:24
technically they can swim faster in this
00:21:26
warmer water and they'd swim as fast as
00:21:28
they could to get
00:21:28
colder water if you put them in
00:21:34
so we have these um these
00:21:37
various organisms here
00:21:40
that are on the reynolds number axis so
00:21:44
we're gonna
00:21:44
we're gonna talk about what the reynolds
00:21:47
number axis
00:21:47
the organisms that are found in
00:21:49
freshwater are operating on
00:21:52
so bacterium are about a micron in size
00:21:56
they are um they can go about 10 to the
00:22:00
minus fifth
00:22:01
meters per second
00:22:04
and they are
00:22:08
um really low reynolds numbers about 10
00:22:11
to minus fifth
00:22:12
on the other extreme salmon
00:22:16
right can are pretty big they're like a
00:22:18
meter bigger than a meter
00:22:19
uh can be and they can swim pretty fast
00:22:23
they can they can swim maybe 10 meters
00:22:26
per second right maybe not quite that
00:22:27
but
00:22:28
certainly more than a meter per second
00:22:30
and so their reynolds numbers are
00:22:32
running around
00:22:32
10 to the 7th
00:22:37
this leads to a bunch of different
00:22:39
characteristics of the organisms
00:22:46
what would one of the things that is
00:22:48
really obvious
00:22:49
is that the salmon has a different shape
00:22:53
than these organisms and the
00:22:57
bacterium can be almost you know almost
00:23:00
any shape
00:23:00
we'll talk about a little bit
00:23:04
so why does this cause the decrease
00:23:08
with increased water temperature
00:23:14
is there a relationship with density
00:23:18
it's related to that why does density
00:23:21
decrease with increased temperature
00:23:23
because the particles are farther apart
00:23:26
there's less to get through
00:23:28
they're farther apart yeah there's less
00:23:29
to get through and there's less
00:23:31
one other thing
00:23:36
hydrogen bonding yes exactly
00:23:39
there's less hydrogen bonding right so
00:23:40
that hydrogen bonding becomes less of
00:23:42
that
00:23:42
resistance to moving through the works
00:23:45
okay so now we're going to talk about
00:23:47
small organism and what happens
00:23:49
with varied by scale we have
00:23:52
reynolds number small organisms have low
00:23:55
grails numbers and large organisms have
00:23:57
high
00:23:57
reynolds numbers viscosity is high for a
00:24:00
bacterium
00:24:01
and it's low for a salmon inertia is low
00:24:05
for a bacterium and it's high for a
00:24:07
salmon or
00:24:08
us so for what if inertia is really low
00:24:11
you can't coast if a bacteria stops
00:24:13
swimming
00:24:14
it will coast an angstrom like
00:24:18
the diameter of a hydrogen item that's
00:24:20
how viscous
00:24:21
and low inertia things are for them
00:24:24
if the salmon stops swimming it will
00:24:27
coast for
00:24:29
i don't know 20 meters or something like
00:24:31
that right i mean that's how a lot of
00:24:32
fish get away they burst and then
00:24:33
you just you watch them they just glide
00:24:35
off so that's inertia
00:24:38
we'll talk about water movement flow is
00:24:40
going to be laminar
00:24:42
or no flow and turbulent for large
00:24:44
organisms
00:24:46
large organisms will streamline small
00:24:47
organisms don't have to we'll talk about
00:24:49
why that happens
00:24:51
we'll also talk about molecular
00:24:52
diffusion versus transport or eddy
00:24:54
diffusion
00:24:55
so this is related to turbulent flow
00:24:57
versus nod
00:24:59
small particles sink more slowly than
00:25:03
uh large particles because they're in a
00:25:05
more viscous solution
00:25:07
and the relative energy requirement for
00:25:09
motility because of this high volatility
00:25:11
is
00:25:12
very high for bacterium relative to us
00:25:15
or a salmon
00:25:23
so we talk about movement of water now
00:25:26
at the finest scales brownian motion
00:25:29
takes place
00:25:29
and brownian motion is just the jiggling
00:25:32
around
00:25:33
of particles related to the fact
00:25:37
that when you get to the very smallest
00:25:39
scale the probability
00:25:41
that a particle is being hit on one side
00:25:44
or another
00:25:47
by an uneven number of molecules
00:25:51
is higher so basically you look at under
00:25:54
the microscope of the bacterium and
00:25:55
that's doing this and that's because
00:25:57
slightly
00:25:57
more water molecules hit it at one
00:25:59
millisecond on one side than the other
00:26:01
and
00:26:01
push it that way and then around the
00:26:03
other way
00:26:04
we don't experience that at our scales
00:26:08
we're being bombarded by air molecules
00:26:11
all at the same time
00:26:13
from all directions if for some reason i
00:26:17
had all the air molecules on one side of
00:26:19
me hit me at once it's possible
00:26:21
it's highly highly improbable would
00:26:23
never happen
00:26:24
and none of them hit me on the other
00:26:26
side it would just throw me across the
00:26:27
room and splat them against the wall
00:26:29
and i'd be done right that just doesn't
00:26:32
happen with our scales because of the
00:26:34
numbers
00:26:37
um so that means that that motion
00:26:40
motion water's moving and and water
00:26:43
molecules are moving very quickly
00:26:45
but they're not going much of anywhere
00:26:47
they're just they're bouncing around
00:26:48
like crazy but they're not going much of
00:26:50
anywhere
00:26:51
that can lead to diffusion eventually
00:26:53
but we'll talk about that
00:26:56
as we get up the larger scale we get
00:26:58
laminar flow
00:26:59
where there's not mixing but there's
00:27:01
movement
00:27:02
all the way all the fluid is moving
00:27:04
together
00:27:06
and then as we get to higher scales and
00:27:08
higher velocities we get turbulent flow
00:27:10
so the flow vectors are not just in one
00:27:13
direction like in laminar flow
00:27:15
you've got this and the net movement is
00:27:17
in one direction but there's all this
00:27:18
mixing
00:27:20
and the important concepts here to talk
00:27:22
about are flow boundary layer
00:27:24
and streamlining
00:27:31
so we'll talk about the flow boundary
00:27:33
layer real quickly
00:27:35
and if we move towards a solid surface
00:27:37
the flow decreases
00:27:40
in the open channel we have turbulent
00:27:42
flow as we move into
00:27:44
this layer the flow becomes laminar
00:27:49
and it decreases so the closer you are
00:27:51
to a solid surface the lower the flow is
00:27:54
at some point you hit the no slip zone
00:27:56
where there's zero flow
00:27:59
right where the water the hydrogen bond
00:28:02
bonding is
00:28:03
interacting with the surface and the
00:28:04
water doesn't move at all
00:28:08
this is an exponential function
00:28:13
and when we get to 99 of the open
00:28:15
channel velocity then we can say we're
00:28:16
out of flow boundary layer
00:28:19
what did you call that surface water
00:28:22
thing they're saying the top of the
00:28:23
water is not moving
00:28:25
no no slip zone right at the bottom
00:28:28
so if you were walking on a wet floor
00:28:31
right if there was not
00:28:32
no slips on you you'd your feet would go
00:28:35
out from under you but
00:28:36
because the water there's so water
00:28:38
between you and the floor
00:28:39
but the hydrogen bonding is kind of
00:28:41
keeping it from from moving
00:28:43
you can walk in a wet floor and not
00:28:45
slide
00:28:47
okay um
