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Calculate the Laplacian of the following functions:

(a)

(b)

(c) .

(d)

Short Answer

Expert verified
  1. The value ofis 2.

  2. The value ofis.

  1. The value ofis 0.

  2. The value ofis.

Step by step solution

01

Define the laplacian

The divergence of gradient is called as Laplacian of a vector.The vector is defined as. theoperator is defined as. The value of Laplacianis obtained as

Here,is the second ordered partial derivative of function.

02

Compute

(a)

To compute an expression substitute the vectors and other required expression and then simplify

The vectoris defined as. The Laplacian of the vector ois defined as.

Substitutefor into.

Thus the value of is 2.

03

Compute

(b)

To compute an expression substitute the vectors and other required expression and then simplify.

The vectoris defined as. The Laplacian of the vectoris defined as.

Substitutefor into .

Solve further as,

Thus, the value ofis.

04

Compute

(c)

To compute an expression substitute the vectors and other required expression and then simplify.

The vectoris defined as. The Laplacian of the vectoris defined as.

Substitutefor into.

Solve further as,

Thus, the value ofis 0.

05

Compute

(d)

To compute an expression substitute the vectors and other required expression and then simplify.

The vectoris defined as. The Laplacian of the vectoris defined as.

Substitutefor into .

Solve further as,

Thus the value ofis.

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Most popular questions from this chapter

Compute the gradient and Laplacian of the functionT=r(cosฮธ+sinฮธcosฯ•). Check the Laplacian by converting Tto Cartesian coordinates and using Eq. 1.42. Test the gradient theorem for this function, using the path shown in Fig. 1.41, from (0, 0, 0) to (0, 0, 2).

(a) If A and B are two vector functions, what does the expression Aยทโˆ‡B mean?(That is, what are its x, y, and z components, in terms of the Cartesian componentsof A, B, and V?)

(b) Compute r^ยทโˆ‡r^, where r^ is the unit vector defined in Eq. 1.21.

(c) For the functions in Prob. 1.15, evaluate vaยทโˆ‡vb.

(a) Show that xddx(ฮดx)=-ฮด(x)

[Hint:Use integration by parts.]

(b) Let ฮธ(x)be the step function:

ฮธ(x)={1ifx>00,ifxโ‰ค0

Show that dฮธdx=ฮด(x)

Suppose that f is a function of two variables (y and z) only. Show that the gradient โˆ‡f=(โˆ‚f/โˆ‚y)y^(โˆ‚f/โˆ‚z)z^transforms as a vector under rotations, Eq 1.29. [Hint: (โˆ‚f/โˆ‚yยฏ)=(โˆ‚f/โˆ‚y)(โˆ‚f/โˆ‚yยฏ)+(โˆ‚f/โˆ‚z)(โˆ‚z/โˆ‚yยฏ),and the analogous formula for โˆ‚f/โˆ‚zยฏ. We know that localid="1654595255202" yยฏ=ycosฯ•+zsinฯ•and z=-ycosโˆ…+zcosโˆ…;โ€solveโ€ these equations for y and z (as functions of localid="1654325243865" yยฏand z(as functions of yand z), and compute the needed derivatives โˆ‚f/โˆ‚y,โˆ‚z/โˆ‚y, etc]

Although the gradient, divergence, and curl theorems are the fundamental integral theorems of vector calculus, it is possible to derive a number of corollaries from them. Show that:

(a)โˆซvโˆ‡Tdฯ„=โˆฎsTโ€‰da. [Hint:Let v = cT, where c is a constant, in the divergence theorem; use the product rules.]

(b)โˆซvโˆ‡ร—vdฯ„=โˆฎsvร—โ€‰da. [Hint:Replace v by (v x c) in the divergence

theorem.]

(c)โˆซvTโˆ‡2U+โˆ‡Tโ‹…โˆ‡Udฯ„=โˆฎsTโˆ‡Uโ€‰โ‹…da . [Hint:Let in the

divergence theorem.]

(d)โˆซvUโˆ‡2T+โˆ‡Uโ‹…โˆ‡Vdฯ„=โˆฎsUโˆ‡Tโ€‰โ‹…da. [Comment:This is sometimes

called Green's second identity; it follows from (c), which is known as

Green's identity.]

(e) โˆซSโˆ‡Tร—da=โˆฎPTโ€‰โ‹…dl[Hint:Let v = cT in Stokes' theorem.]

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