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Del operator

∇ = (∂/∂x,  ∂/∂y, ∂/∂z)

Gradient is partial derivative of a function with respect to x or y or z

∂p/∂x is pressure gradient in x direction

∂p/∂y is pressure gradient in y direction

∂p/∂z is pressure gradient in z direction

∂v/∂x,  ∂v/∂y, and ∂v/∂z are velocity gradients.

∂V/∂x,  ∂V/∂y and ∂V/∂z are voltage gradients.

Laplacian operator

The divergence of a gradient of a function  is called Laplacian or Laplace operator. It is usually denoted by the symbols ∇·∇ or ∇2, or Δ. It is the second derivative of a function.

Laplacian operator in different common coordinate systems

Sr.No. Co-ordinate


 Laplace operator , ∇2t
1. Cartesian co-ordinates, x,y,z 2t/∂x2 +∂2t/∂y2 +∂2t/∂z2
2. Cylindrical co-ordinates, r,θ,z (1/r) ∂/∂r(r ∂t/∂r) +(1/r2) ∂2t/∂θ2 +∂2t/∂z2
3. Spherical co-ordinates, r,θ,φ (1/r2) ∂/∂r(r2 ∂t/∂r) +(1/r2sin2φ) ∂2t/∂θ2 +(1/r2sinφ) ∂/∂φ(sinφ ∂t/∂φ)



Divergence is a scalar function but it is an operator of a vector function. It is defined for a function t (which is a function of x and y) and it can be differentiated with respect to x and y in a unit vector direction at some predefined point. Directional derivative is found at a point with the use of partial derivatives as explained below. Example to determine the directional derivative of function f(x, y) = x 3 y 2 + 8 x4 y at point (1, −2) in the direction of a unit vector in z direction at (√3/2, 1/2).

The partial derivative of f(x, y) with respect to x at (1,-2) will be

= 3x2y2 + 32 x3 y =3 12(-2)2+32(1)3(-2) = 12- 64= -52

The partial derivative of f(x, y) with respect to y at (1,-2) will be

=2x3y + 8 x4 = 2 13(-2)+ 8(1)4= –4+8=4

Then Divergence D at (x, y) and in the unit vector at (√3/2, 1/2) will be = -52(√3/2) + 4(1/2) = 2 – 26√3