Note I.

31 5 October

• Definition: The curl of a vector field F = F1 i + F2 j + F3 k is the vector field

curl : vector fields →

7 vector fields
F → curl F (1)

with      
∂F3 ∂F2 ∂F1 ∂F3 ∂F2 ∂F1
curl F = − i+ − j+ − k (2)
∂y ∂z ∂z ∂x ∂x ∂y
and, in the symbolic notation this is

curl F = ∇ × F (3)

This is the easiest way to remember the formula, using the determinant formula for
the cross product
i j k
∂ ∂ ∂
curl = ∂x ∂y ∂z (4)
F1 F2 F3

• Example: If F = xyi + y 2 j + zk then applying the formula above gives

curl F = −xk (5)

Again, it is not easy at first to get a picture of what the curl does. One rough idea is
that it measures the rotation at a point of the vectors in a vector field. Certainly, this is
what happens when you take the curl of the rotational field. Consider the velocity field

u=w×r (6)

where r = (x, y, z) is the position vector and w = (w! , w2 , w3 ) is some constant vector.
Now, u is perpendicular to both r and w and the length of u is constant on circles around
w, hence the velocity field corresponds to rotation around w. We will take its curl, first

i j k
u = w1 w2 w3
x y z
= (w2 z − w3 x)i + (w3 x − w1 z)j + (w1 y − w2 x)k (7)

Now, substituting this into the curl formula we get

∇ × u = 2w (8)

• Definition: A vector field is called irrotational if it has zero curl.

1
Conor Houghton, houghton@maths.tcd.ie, see also http://www.maths.tcd.ie/~houghton/231

1
 The gradient of a scalar field is irrotational:

curl grad φ = 0 (9)

or, using the symbolic notation ∇ × ∇φ = 0. This is proved by calculation, using the
subscript to denote component, so v = (v1 , v2 , v3) for any vector, we have

(∇ × ∇φ)1 = ∂y (∇φ)3 − ∂z (∇φ)2

= ∂ y ∂z φ − ∂ z ∂y φ (10)

and the other components follow in the same way. We have used the useful notation where
∂
∂x = (11)
∂x
and so on.

Vector identities
There are a number of usefull identities involving grad, div and curl. These are usually
proved by direct calculation, expand out the various terms.
Let φ and ψ be scalar fields and F and G be vector fields, then

1.
∇(φψ) = φ∇ψ + ψ∇φ (12)
This is a direct consequence of the product rule.

2.
∇(φF) = ∇φ · F + φ∇ · F (13)
and again this follows by just expanding it out

∇(φF) = ∂x (φF1 ) + ∂y (φF2 ) + ∂z (φF3 )

= F1 ∂x φ + F2 ∂y φ + F3 ∂z φ + φ(∂x F1 + ∂y F2 + ∂z F3 )
= ∇φ · F + φ∇ · F (14)

3.
∇ × (φF) = ∇φ × F + φ∇ × F (15)
This can easily be proved too, just check, say, the x-component by direct calculation.

4.
∇ · (F × G) = (∇ × F) · G − F · ∇ × G (16)
The proof of this is handwritten as (Picture I.3.1)

2
5.
∇ × (F × G) = (∇ · G)F + (G · ∇)F − (∇ · F)G − (F · ∇)G (17)
This is one of the harder ones to prove since it involves the unusual operator

(G · ∇) = G1 ∂x + G2 ∂y + G3 ∂z (18)

and the proof is given as an exercise on the problem sheets.

6.
∇(F · G) = F × (∇ × G) + G × (∇ × F) + (F · ∇)G) + (G · ∇)F) (19)
This is also given as an exercise.

7.
∇ · (∇ × F) = 0 (20)
or, the curl of a vector field is solenoidal. This is one of the important vector identities
which hints at some of the beautiful constructions in differential geometry. It is easy
enough to prove by direct calculation.

8.
∇ × ∇φ = 0 (21)
was proved above.

9.
∇ × (∇ × F) = ∇(∇ · F) − 4F (22)
where
4φ = ∇ · ∇φ = (∂x2 + ∂y2 + ∂z2 )φ (23)
is the Laplacian, an operator which occurs frequently in physically significant equa-
tions. Obviously
4F = (4F3 , 4F2 , 4F3 ) (24)