- [Voiceover] Alright everyone.

We've gotten to one of my all-time favorite

multi-variable calculus topics, divergence.

In the next few videos I'm gonna describe

what it is mathematically and how you compute it

and all of that but here I just want to give

a very visual understanding of what it is

that it's trying to represent.

So I've got a picture in front of us, a vector field.

I've said before that a pretty neat way

to understand vector fields is to think of them

as representing a fluid flow.

What I mean by that is you can think of

every single point in space as a particle,

maybe an air particle or a water particle.

Something to that effect.

Since what a vector field does

is it associates each point in space

with some kind of vector

and remember I mean, whenever we represent vector fields,

we only show a small subset of all of those vectors,

but in principle you should be thinking of

every one of those infinitely many points in space

being associated with one of these vectors.

The fact that they're kind of smoothly changing

as you traverse across space

,means that showing this very small finite subsample

of those infinitely many vectors

still gives a pretty good feel for what's going on.

So if we have these fluid particles

and you have a vector assigned to each one,

kind of a natural thought you might have is to say

what would happen if you let things progress over time

where at any given instant the velocity of one of

these particles is given by that vector connected to it.

As it moves it will be touching a different vector.

So it's velocity might turn.

It might go in a different direction.

For each one it will kind of traverse some path

as determined by the vectors that it's touching as it goes

and when you think of all of them doing this at once,

it will feel like a certain fluid flow.

For this you don't actually have to imagine.

I went ahead and put together an animation for you.

So we'll put some water molecules or dots

to represent a small sample of the water molecules

throughout space here and then I'm just gonna let it play

where each one moves along the vector that it's closest to.

I'll just let it play forward here

where each one is flowing along the vector

that's touching the point where it is in that moment.

So for example, if we were to go back

and maybe focus our attention on just one vector

like this guy, one particle, excuse me,

he's attached to this vector so he'll be moving

in that direction but just for an instant

because after he moves a little

he'll be attached to a different vector.

So if you kind of let it play and follow that particular dot

after a little bit you'll find him elsewhere.

I think this is the one, right.

Now he's gonna be moving along this vector

or whatever one is really attached to him.

Thinking about all of the particles all at once doing this

gives a sort of global view of the vector field.

If you're studying math, you might start to ask

some natural questions about the nature of that fluid flow.

For example, you might wonder if you were to just

look in a certain region and count the number

of water molecules that are inside that region,

does that count of yours change

as you play this animation,

as you let this flow over time?

In this particular example you can look

and it doesn't look like the count changes.

Certainly not that much.

It's not increasing over time or decreasing over time

In a little bit, if I gave you the function

that determines this vector field,

you will be able to tell me why it's the case

that the number of molecules in that region

doesn't tend to change

but if you were to look at another example,

like a guy that looks like this

and if I were to say I want you to focus

on what happens around the origin,

in that little region around the origin,

you can probably predict how once I start playing it,

once I put some water molecules in there

and let them flow along the vectors that they flow along,

the density inside that region around the origin decreases.

So we put a whole bunch of vectors there

and I'll just play it for a quick instant.

Just kind of let it jump for an instant.

One thing that characterizes this field around the origin

is that decrease in density.

What you might say if you wanted to be suggestive

of the operation that I'm leading to here

is that the water molecules tend to diverge

away from the origin.

So the kind of divergence of the vector field

near that origin is positive.

You'll see what I mean mathematically by that

in the next couple videos,

but if we were to flip over these vectors, right,

if we were to flip them around,

now if I were to ask about the density

in that same region around the origin,

we can probably see how it's gonna increase.

When I play that fluid flow over just a short spurt of time,

the density in the region increases.

So these don't diverge away, they converge

towards the origin.

That fact actually has some mathematical significance

for the function representing this vector field

around that point.

Even if the vector field doesn't represent fluid flow,

if it represents a magnetic field

or an electric field or things like that,

there's a certain meaning to this idea

of diverging away from a point or converging to a point.

Another way that people sometimes think about this,

if you look at that same kind of

outward-flowing vector field

as rather than thinking of a decrease in density,

imagining that the fluid would have to constantly

be repopulated around that point.

So you're really thinking of the origin as a source of fluid

and if I had animated this better,

a whole bunch of other points should be sources of fluid

so that the density doesn't decrease everywhere,

but the idea is that points of positive divergence

where things are diverging away

would have to have a source of that fluid

in order to kind of keep things sustaining.

Conversely, if you were to look at that kind of inward flow

or what you might call negative divergence example

and you were to play it but it were to go continuously,

you'd have to think of that center point as a sink

where all the fluid kind of just sort of flows away.

That's actually a technical term.

People will say the vector field has a sink

at such and such point or the electromagnetic field

has a sink at such and such point

and that often has a certain significance.

If we go back to that original example here

where there is no change in fluid density, right,

what you might notice, this feels a lot more like

actual water than the other ones

because there is no change in density there.

If you can find a way to mathematically describe

that lack of a change in density,

it's a pretty good way to model water flow.

Again, even if it's not water flow

but it's something like the electromagnetic field,

there's often a significance to this

no changing in density idea.

So with that I think I've jabbered on enough

about the visuals of it and in the next video

I'll tell you what divergence is mathematically,

how you compute it, go through a couple examples,

things like that.

See you next video.