Main content

## Multivariable calculus

### Course: Multivariable calculus > Unit 2

Lesson 10: Curl- 2d curl intuition
- Visual curl
- 2d curl formula
- 2d curl example
- Finding curl in 2D
- 2d curl nuance
- Describing rotation in 3d with a vector
- 3d curl intuition, part 1
- 3d curl intuition, part 2
- 3d curl formula, part 1
- 3d curl formula, part 2
- 3d curl computation example
- Finding curl in 3D
- Symbols practice: The gradient

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# 3d curl intuition, part 2

Continuing the intuition for how three-dimensional curl represents rotation in three-dimensional fluid flow. Created by Grant Sanderson.

## Want to join the conversation?

- At2:00the z component is added. Why isn't it z instead of 0? I thought with z = 0 all the time you just get a vectorfield in the xy-plane.(11 votes)
- Think of the 3rd component as of z*0. If you input z=1 or 2 or 3 you will have still have to plot y^3-9y and x^3-9x in a flat plane, but since z=1,2 or 3 each plotted vector that lied in xy plane will have to start higher. By adding z*0 as 3rd component you don't change the direction or magnitude of the plotted vector, but the starting position (the beginning of a vector communicates the input, the direction and magnitude is the output- which doesn't change )(21 votes)

- when imagining the curl in 3-D I was wondering if it is possible to associate the idea of curl with the rotation of ceiling fan and the air flow. the rotation of the fan can be considered the rotation in 2D i.e. in the plane of the fan whereas the flow of air can be considered as the Z -(normal vector) pointing out of the plane of the fan.

If I curl my fingers in direction of rotation of the fan, I get the downward flow of air and thus right-hand rule holds perfectly true here. similarly, if the rotation is reversed the flow is in the upward direction(which again follows right-hand thumb rule.

I am asking the question because I was wondering if the association of this idea is correct here, it can be further extended to turbines and windmills as well. of course, the engineering design of fans, windmills etc play an important role here but I would like to ask if such an association as at all possible in general?

thank you for your immensely helpful videos.(3 votes)- The curvature of the blades of the fan is what determines the airflow direction.(4 votes)

- Thank you so much ilysm!(4 votes)
- They say that gravity is a conservative vector field, because at any point in the field, the curl is zero. So that must mean that gravity by itself would not cause a rotation. Why, then, do the planets rotate as they revolve around the Sun?

Maybe because the rotation due to gravity represents the torque, but not the angular velocity of the planet. So does this fact account for why the speed of rotation of planets is constant in the universe?(2 votes)- I believe the conservation of inertia causes this to be. If you took a system of just a star and a planet on the xy plane, where the planet rotates at some speed clockwise, the planet and star must take on some rotation to conserve inertia. You can see this on a much smaller scale if you sit on a chair that can spin and hold a spinning bike wheel by the axle, so that the axle is horizontal to the ground. If you rotate the bike wheel so that its axle is vertically aligned, you will feel a torque applied and you will begin rotating (barring other forces).(2 votes)

- Just a quick question to hopefully confirm my understanding. If you have a vector field with no dependance on z, could the curl vector field essentially be written as a scalar field so that a positive number represents the curl vector when it appears along the positive z direction, and a negative number when it appears along the negative z direction ? Thanks very much, I hope that makes sense !(2 votes)
- The curl is 3(X^2-Y^2) not 3X^2 +3Y^2 (4:30)(2 votes)
- I was 15 when I first started watching your (Grant Sanderson's) series on multivariate calculus. At first I was very sceptical about the content of your videos because it appeared as if you were assuming too much own knowledge / understanding. But now (16th birthday today!) things are finally beginning to make sense and hopefully I'll be able to review some line and surface integrals before the very hectic Year 12 starts in September !!(2 votes)
- But wouldn't the ball in the last example spin around? I mean, wouldn't the thumbs tumble?(1 vote)
- thats why he told you to use magic to stick that in place(2 votes)

## Video transcript

- [Voiceover] So where we left off, I had this two-dimensional vector field V, and I have it pictured here as
kind of a yellow vector field and I just stuck it in three dimensions in kind of an awkward way
where I put it on the XY plane and said pretend this
is in three dimensions. And then when you describe the rotation, around each point what we
were familiar with is 2D curl, that's where you get this
vector field, it's not quite a 3D vector field because
you're only assigning points on the XY plane to
three dimensional vectors, rather than every point
in space to a vector, but we're getting there. So here let's actually
extend this to a fully three dimensional vector field, and first of all let me just kind of clear up the board from the computations we did in the last part. And as I do that kind
of start thinking about how you might want to
extend the vector field that I have here that's
pretty much two dimensional into three dimensions. And one idea you might, we'll
kind of get rid of the circles and the plane, is to
take this vector field and then just kind of copy
it to different slices. So, you might get something
kind of like this. And then, I've drawn each
slice a little bit sparser than the original one, so
technically that original one if you look on the XY plane I've pictured many more vectors, but it's really the same vector field, and all I've done here is
said at every slice in space, just copy that same vector field. So if you look from above,
you can maybe see how really it's just the same vector
field kind of copied a bunch, and if you look at it each
slice, you know in the same way that in the XY plane,
you've got this vector field sitting on the slice,
every other part of space will have that. And even though there's only
what, like six or seven slices displayed here, in principal
you're thinking that every one of those infinitely
many slices of space has a copy of this vector field. And in a formula, what does that mean? Well what it means is
what it means is that we're taking not just X
and Y as input points, But we're gonna start taking Z in as well. So if I go, I'm gonna say that
Z is an input point as well. And I want to be considering these as vectors in three dimensions, so rather than just saying
that that it's got X and Y components, I'm gonna pretend
like it has a Z component that just happens to
be zero for this case. And the fact that you
have a Z in the input, but the output doesn't depend on the Z, corresponds to the fact that
all the slices are the same, as you change the Z direction the vectors won't change at all, they're just carbon copies of each other. And the fact this output
has a Z component, but it just happens to be
zero is what corresponds to the fact that it's very flat looking, you know, none of them point
up or down in the Z direction, they're all purely X and Y. So, as three dimensional vector fields go, this one is only barely a
three dimensional vector field, it's kind of phoning it in
as far as three dimensional vector fields are concerned, but it'll be quite good
for our example here, because now if we start
thinking of this as representing a three dimensional fluid
flow, so now rather than just kind of the fluid
flow like the one I have pictured over here, where
you've got water molecules moving in two dimensions
and it's very easy to understand clockwise rotation,
counter clockwise rotation, things like that, whereas
over here it's a very kind of chaotic three
dimensional fluid flow, but because it's so flat
if you view it from above it's still loosely the same just kind of counter clockwise over here on the right, and clockwise like there above, so if I were to draw like a column, you could think of this column as being, having a tornado of fluid flow, right, where it's, everything is
kind of rotating together in that same direction. So if you were to assign a
vector to each point in space to describe the kind of rotation happening around each one of those points in space, you would expect that
those inside this column, inside this sort of counter
clockwise rotating tornado, and I say counter
clockwise, but if we viewed it from below it would look clockwise, so that's the tricky part
about three dimensions and why we need to
describe it with vectors, but you expect these using
your right hand rule, where you curl the
fingers of your right hand around the direction of rotation here, you would expect vectors that
point up in the Z direction, the positive Z direction,
and if I do that, if I show what all of the
rotation vectors look like, you'll get this, and maybe
this is kind of a mess because there's a lot of things
on the screen at this point. So for the moment I'll kind of remove that original
vector field and remove the XY plane, and just
kind of focus on this new vector field that
I have pictured here. Inside that column where we
have that tornado rotation I was describing, all of the vectors point in the positive Z direction, but if we were to view it elsewhere, like over in this region, those are pointing in
the negative Z direction, and if you stick your
thumb in the direction of all of these vectors in
the negative Z direction, that tells you the direction
of, that tells you how the fluid, maybe you're
thinking of it as air kind of rushing about the room, how that fluid rotates
it in three dimensions. So what curl is gonna
do, here I'll kind of clear things up, I have
the formula from last time, that hopefully hasn't looked too in the way while I've been doing this, that described curl for a
two dimensional vector field if we imagine that's
not just taking X and Y as its inputs because it's a
three dimensional vector field, but if we imagine it taking X, Y and Z, so it's a proper three
dimensional vector field, the output is gonna tell
you at every point in space what the rotation that
corresponds to that point is. And in the next video I'm
gonna give you the formula and tell you how you
actually compute this curl given an arbitrary function, but for right now we're just
getting the visual intuition, we're just trying to understand what it is that curl
is going to represent. And in this vector field,
this one that was just kind of copies of a 2D put above,
it's almost contrived because all of the rotation
happens in these perfect tornado-like patters that
doesn't really change as you move up and down
in the XY direction, but more generally you might
have a more complicated looking vector field, so I'll go ahead and finally erase this since it's
been a little bit in the way for a while, and erase this guy too, and if you think about just arbitrary three dimensional vector
fields, like let's say this one that I have here,
so I don't know about you, but for me it's really hard
to think about the fluid flow associated with this, I have
a vague notion in my mind that okay, like fluid is
flowing out from this corner and kind of flowing in here, but it's very hard to
think about it all at once, and certainly if you start
talking about rotation, it's hard to look at
a given point and say, oh yeah there's gonna be
a general fluid rotation in some certain way and I can
give you the vector for that. But as a more loose and
vague idea, I can say, okay, given that there's
some kind of crazy air current fluid flow
happening around here, I can maybe understand
that at a specific point, you're gonna have some kind of rotation, and here I'll picture it
as if there's like a ball or a globe sitting there
in space, and maybe you're imagining your new vector field and saying what kind of rotation is it kind of induce in that ball that's just
floating there in space? Maybe you're imagining this
as like a tennis ball that you're sort of holding in
place in space using magnets or magic or something like that,
and you're letting the wind sort of freely rotate
it, and you're wondering what direction it tends to rotate, and then when it does and
when you have this rotation, you can describe that 3D rotation
with some kind of vector, and in this case it would
be a vector that points out in that direction because we're
kind of curling our fingers, curling our right hand fingers
over in that direction, and if you don't understand
how we describe 3D rotation with a vector, I have a
video on that, maybe go back and check out that video,
but the idea here is that when you have some sort
of crazy fluid flow that's induced by some
sort of vector field, and you do this at every point and say, hey what's the rotation
at every single point, that's gonna give you the
curl, that is what the curl of a three dimensional vector
field is trying to represent. And if this feels confusing,
if this feels like something that's hard to wrap your
mind around, don't worry we've been there, 3D curl is
one of the most complicated things in multivariable calculus
that we have to describe. But I think the key to
understanding it is to just kind of patiently think through and
take the time to think about what 2D curl is instead of
thinking about how you extend that to three dimensions
and slowly say, okay, okay, I kind of get it, tornados of rotation, that sort of makes sense,
and if you understand how to represent three
dimensional rotation around a single point with a
vector, then understanding three dimensional curl
comes down to thinking about doing that at every single point in space according to whatever rotation the wind flow around
that point would induce. Like I said, it is complicated,
and it's okay if it doesn't sink the first
time, it certainly took me awhile to really wrap my head
around this 3D curl idea, and with that I'll see you next video.