Del Operator: Gradient, Divergence, and Curl, Summaries of Mathematics

The del operator is a vector calculus symbol used for compact notation. It is defined as the gradient operator (โˆ‡) which is the sum of partial derivatives with respect to x, y, and z axes. The gradient of a function results in a vector, while the divergence of a vector yields a scalar. The curl of a vector is expressed as the cross product of the gradient operator and the vector, resulting in a vector.

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Del Operator
The symbolic operator โ€delโ€ is widely used for compact notation. It is a vector defined as
โˆ‡=โˆ‚
โˆ‚x i+โˆ‚
โˆ‚y j+โˆ‚
โˆ‚z k= ( โˆ‚
โˆ‚x ,โˆ‚
โˆ‚y ,โˆ‚
โˆ‚z ) (1)
where i,j,kcorrespond to the unit vector in the x,y,zaxis.
Recall that the product of โˆ‚
โˆ‚x and a function f(x, y, z ) is โˆ‚ f
โˆ‚x . Then we have the gradient of a
function fexpressed as
grad f=โˆ‡f= (โˆ‚f
โˆ‚x ,โˆ‚f
โˆ‚y ,โˆ‚f
โˆ‚z ) (2)
The gradient of a function fis a vector.
Moreover, for a vector F= (u, v, w) where u=u(x, y, z), v=v(x, y , z), w=w(x, y, z) are all
functions of x,y,z, the divergence of this vector Fcan be seen as the dot product of โˆ‡and vector
F, expressed as
div f=โˆ‡ ยท F=โˆ‚u
โˆ‚x +โˆ‚v
โˆ‚y +โˆ‚w
โˆ‚z (3)
The divergence of a vector results in a scalar.
On the other hand, the curl of the vector Fcan be consideres as the cross product of โˆ‡and
vector F, expressed as
curl f=โˆ‡ ร— F=
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
i j k
โˆ‚
โˆ‚x
โˆ‚
โˆ‚y
โˆ‚
โˆ‚z
u v w
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
=i(โˆ‚w
โˆ‚y โˆ’โˆ‚v
โˆ‚z ) + j(โˆ‚u
โˆ‚z โˆ’โˆ‚w
โˆ‚x ) + k(โˆ‚v
โˆ‚x โˆ’โˆ‚u
โˆ‚y ) (4)
Note that the curl of a vector results in a vector. Equation (4) can also be expressed as
curl f=โˆ‡ ร— F=
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
i j k
โˆ‚
โˆ‚x
โˆ‚
โˆ‚y
โˆ‚
โˆ‚z
u v w
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
๎˜Œ
= [(โˆ‚w
โˆ‚y โˆ’โˆ‚v
โˆ‚z ),(โˆ‚u
โˆ‚z โˆ’โˆ‚w
โˆ‚x ),(โˆ‚v
โˆ‚x โˆ’โˆ‚u
โˆ‚y )] (5)
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Del Operator

The symbolic operator โ€delโ€ is widely used for compact notation. It is a vector defined as

โˆ‚x

i + โˆ‚ โˆ‚y

j + โˆ‚ โˆ‚z

k = ( โˆ‚ โˆ‚x

โˆ‚y

โˆ‚z

where i, j, k correspond to the unit vector in the x, y, z axis.

Recall that the product of (^) โˆ‚xโˆ‚ and a function f (x, y, z) is โˆ‚fโˆ‚x. Then we have the gradient of a

function f expressed as

grad f = โˆ‡f = ( โˆ‚f โˆ‚x

, โˆ‚f โˆ‚y

, โˆ‚f โˆ‚z

The gradient of a function f is a vector.

Moreover, for a vector F = (u, v, w) where u = u(x, y, z), v = v(x, y, z), w = w(x, y, z) are all

functions of x, y, z, the divergence of this vector F can be seen as the dot product of โˆ‡ and vector

F, expressed as

div f = โˆ‡ ยท F = โˆ‚uโˆ‚x + (^) โˆ‚yโˆ‚v + โˆ‚wโˆ‚z (3)

The divergence of a vector results in a scalar.

On the other hand, the curl of the vector F can be consideres as the cross product of โˆ‡ and

vector F, expressed as

curl f = โˆ‡ ร— F =

i j k

โˆ‚ โˆ‚x

โˆ‚ โˆ‚y

โˆ‚ โˆ‚z

u v w

= i( โˆ‚w โˆ‚y

โˆ’ โˆ‚v โˆ‚z

) + j( โˆ‚u โˆ‚z

โˆ’ โˆ‚w โˆ‚x

) + k( โˆ‚v โˆ‚x

โˆ’ โˆ‚u โˆ‚y

Note that the curl of a vector results in a vector. Equation (4) can also be expressed as

curl f = โˆ‡ ร— F =

i j k

โˆ‚ โˆ‚x

โˆ‚ โˆ‚y

โˆ‚ โˆ‚z

u v w

= [( โˆ‚w โˆ‚y

โˆ’ โˆ‚v โˆ‚z

), ( โˆ‚u โˆ‚z

โˆ’ โˆ‚w โˆ‚x

), ( โˆ‚v โˆ‚x

โˆ’ โˆ‚u โˆ‚y

)] (5)