1. Unit.2 Vector Calculus
UNIT II VECTOR CALCULUS
Part-A
i x cy z bx y z j x cy z x y az
y z x z
k bx y z x y az
ic 1 j a 4 k b 2
1
Problem 1 Prove that div(grad ) 2
Solution:
div(grad ) .
i j k
x y z
i j k i j k
x y z x y z
2 2 2
x2 y2 z2
2 2 2
x2 y2 z2
2 .
Problem 2 Find a, b, c, if F x 2y azi bx 3y z j (4x cy 2z)k
is
irrotational.
Solution:
F
is irrotational if F 0
i j k
2 3 4 2
F
x y z
x y az bx y z x cy z
4 2 3 4 2 2
3 2 2
x y
ic 1 j a 4 k b 2
F 0 0i 0 j 0k
c 1 0, a 4 0, b 2 0
c 1, a 4, b 2.
Problem 3 If S is any closed surface enclosing a volume V and r
is the position
vector of a point, prove . 3
r n ds V
S
2. Unit.2 Vector Calculus
2
Solution:
Let r xi y j zk
By Gauss divergence theorem
F n ds FdV
S V
Here F .r
r n ds rdV
S V
.
V
i j k xi y j zk dV
x y z
111dV
3 .
r n ds V
S
Problem 4 If r a cos nt bsin nt,
where a,b, n
are constants show that
r d r na
b
dt
Solution:
Given r a cos nt bsin nt
d r na sin nt
nb cos nt
dt
r d r a cos nt bsin nt na sin nt
nb cos nt
dt
nabcos2 nt basin2 nt
nabcos2 nt absin2 nt
a b ba
n ab1 n ab
Problem 5 Prove that div curl A 0
Solution:
1 2 3
.
i j k
x y z
A A A
i 3 2 j 3 1 k 2 1
y z x z x y
3 2 3 1 2 1
x y z y x z z x y
3. Unit.2 Vector Calculus
2 2 2 2 2 2
3 2 1 3 2 1
x y x z y z y x z x z y
3
div curl A 0
Problem 6 Find the unit normal to surface xy3z2 4 at 1,1,2
Solution:
Let xy3z2 4
y3z2 i 3xy2z2 j 2xy3zk
3 2 2 2 3
1, 1,2 1 2 i 3 1 1 2 j 2 1 1 2 k
4i 12 j 4k
Unit normal to the surface is n
i j k
4 12 4
16 144
16
4 3
i j k
176
4 3 3
.
i j k i j k
16
11 11
Problem 7 Applying Green’s theorem in plane show that area enclosed by a simple
closed curve C is 1
2
xdy ydx
Solution:
Pdx Qdy Q P dx dy
P y, Q x
P 1, Q 1
y x
C R
x y
1 1 2
x dy ydx dxdy dx dy
R R
= 2 Area enclosed by C
Area enclosed by C = 1
2
xdy ydx .
and B
Problem 8 If A
are irrotational show that A B
is solenoidal
Solution:
Given A
is irrotational i.e., A 0
4. Unit.2 Vector Calculus
j 3z2 3xy 3x2 3yz
x dx x dx x dx x
2 2 2
3 3
4
B
is irrotational i.e., B 0
.A B B. A A. B
B0 A0 0
A B
is solenoidal.
Problem 9 If F grad x3 y3 z3 3xyz
find curl F
Solution:
F x3 y3 z3 3xyz
3x2 3yzl 3y2 3xz j 3z2 3xyk
i j k
3 2 3 3 2 3 3 2 3
F
x y z
x yz y xz z xy
i 3z2 3xy 3y2 3xz
y z
x z
k 3y2 3xz 3x2 3yz
x y
i3x 3x j 3y 3y k 3z 3z
i 0 j0 k 0 0
.
Problem 10 If F x2 i y2 j,
evaluate F dr
along the straight line y x from
0,0 to 1,1 .
Solution:
F.d r x2 i y2 jdxi dy
x2dx y2dy
Given y x
dy dx
1
F d r x 2 dx y 2
dy
C 0
1 1 3 1
2 2 2
0 0 0
Problem 11 What is the unit normal to the surface x, y, z C at the point x, y, z?
Solution:
5. Unit.2 Vector Calculus
5
n
.
to be solenoidal
Problem 12 State the condition for a vector F
Solution:
.F divF 0
Problem 13 If a
?
is a constant vector what is a
Solution:
Let 1 2 3 a a i a j a k
1 2 3
0
i j k
a
x y
z
a a a
Problem 14 Find grad at 2,2,2 when x2 y2 z2 2
Solution:
grad
i x2 y2x2 2 j x2 y2 z2 2 k x2 y2 z2 2
x y z
2x i 2y j 2zk
2,2,2 4i 4 j 4k
Problem 15 State Gauss Divergence Theorem
Solution:
The surface integral of the normal component of a vector function F over a closed
surface S enclosing volume V is equal to the volume integral of the divergence of F
F nds FdV
taken over V. i.e., . .
S V
Part –B
Problem 1 Find the directional derivative of x2 yz 4xz2 at the point 1,2,1 in
the direction of the vector 2i j 2k.
Solution:
x2 yz 4xz2
6. Unit.2 Vector Calculus
6
2xyz 4z2 i x2z j x2 y 8xzk
2 2 2
1, 2, 1 2 1 2 1 4 1 i 1 1 j 1 2 8 1 1 k
4 4i j 2 8k
8i j 10k
Directional derivative a
is .a
8 i j 10 k .2 i j 2 k
4 1 4
16 1
20 37 .
3 3
Problem 2 Find the maximum directional derivative of xyz2 at 1,0,3 .
Solution:
Given xyz2
yz2 i xz2 j 2xyzk
2 2
1,0,3 0 3 i 1 3 j 2 1 0 3 k 9 j
Maximum directional directive of is 9 j
Magnitude of maximum directional directive is 92 9.
Problem 3 Find the angle between the surfaces x2 y2 z2 9 and x2 y2 z 3at the
point 2,1, 2 .
Solution:
Let 2 2 2
1 x y z 9
1 2xi 2y j 2zk
1 2, 1,2 2(2)i 2 1 j 2 2 k 4i 2 j 4k
2 2
2 x y z 3
2 2xi 2y j k
22, 1,2 4i 2 j 2k
If is the angle between the surfaces then
cos .
1 2
| ||
|
1 2
4 i 2 j 4 k
.4 i 2 j 2 k
16 4 16 16 4 4
7. Unit.2 Vector Calculus
7
16 4
8
36 24
12 1
6 2 6 6
cos 1 1 .
6
Problem 4 Find the work done, when a force F x2 y2 xi 2xy y j
moves a
particle from the origin to the point 1,1 along y2 x .
Solution:
Given F
x2 y2 xi 2xy y j
F.d r x2 y2 xdx 2xy ydy
d r dxi dy j dzk
Given y2 x
2ydy dx
F.dr x2 x xdx 2y3 ydy
x2dx 2y3 ydy
1 1
Fdr x 2 dx y 3
y dy
0 0
2
C
3 1 4 2 1
x
2
y y 3
0 4 2
0
1 0 2 1 0 0
3 4 2
1 1 1
3 2 2
1
1
2
3 3
Work done . 2
F dr
3 C
Problem 5 Prove that F y2 cos x z3 i 2y sin x 4 j 3xz2 k
is irrotational and
find its scalar potential.
Solution:
Given F y2 cos x z3 i 2y sin x 4 j 3xz2 k
8. Unit.2 Vector Calculus
y2 cos x z3 i 2y sin x 4j 3xz2 k i j
8
i j k
2 cos 3 2 sin 4 3 2
F
x y z
y x z y x xz
i0 0 j 3z2 3z2 k2y cos x 2y cos
0i 0 j 0k 0
F 0
is irrotational
Hence F
F
x y yz
Equating the coefficient i, j, k
y2 cos x z3 y2 cos x z3dx
x
2 3
1 1 y sin x z x C
2y sin x 4 2y sin x 4dy
x
x y y C
2
2 2 2 sin 4
2
3xz2 3xz2dy
x
3
3
x z C
3 3
3 y2 sin x xz3 4y C
Problem 6
If F 3xyi y2 j
evaluate F.dr
when C is curve in the xy plane
y 2x2 , from 0,0 to 1,2
Solution:
F 3xyi y2 j
dr dxi dy j dzk
F.dr 3xydx y2dy
Given y 2x2
dy 4xdx
F.d r 3x(2x2 )dx 2x2 2 4x dx
6x3dx 4x4 (4x)dx
6x3dx 16x5dx
9. Unit.2 Vector Calculus
C F d r x dx aydy x b dx
a 3 b 2 a 3
a ab
9
1
Fd r x 3 x 5
dx
0
6 16
C
4 6 1
x x
6 16 7 .
4 6 6
0
6 16
4 6
Problem 7 Find .
F dr
C
when F x2 y2 i 2xy j
where the cure C is the
rectangle in the xy plane bounded by x 0, x a, y b, y 0.
Solution:
Given F x2 y2
i 2xy j
Fdr (x2 y2 )dx 2xy dy
d r dxi dy j dzk
C is the rectangle OABC and C consists of four different paths.
OA (y = 0)
AB (x = a)
BC (y = b)
CO (x = 0)
F .
d r
C OA AB BC CO
Along
OA, y 0, dy 0
AB, x a, dx 0
BC, y b, dy 0
CO, x 0, dx 0
. 2 2 2 2 0
C OA AB BC CO
0
a b
x 2 dx 2
a ydy x 2 b 2
dx
0 0
a
a b o
x 3 y 2 3
a x b 2
x
0
2
3 2 3
o a
0 2 0 0 0 2
3 2 3
2ab2.
Problem 8 If F 4xy 3x2 z2 i 2x2 j 2x3zk
F dr
check whether the integral .
C
is
independent of the pathC .
Solution:
Given
10. Unit.2 Vector Calculus
10
F 4xy 3x2 z2 i 2x2 j 2x3zk
d r dxi dy j dzk
. 4 3 2 2 2 2 2 3
C C C C
F dr xy x z dx x dy x zdz
This integral is independent
of path of integration if
F F 0
i j k
4 3 2 2 2 2 2 3
F
x y z
xy x z x x z
i 0,0 j 6x2 z 6x2 z k 4x 4x
0i 0i 0 j 0k 0.
Hence the line integral is independent of path.
Problem 9 Verify Green’s Theorem in a plane for 2 (1 ) ( 3 3 )
x y dx y x dy where
C
C is the square bounded x a, y a
Solution:
Let P x2 (1 y)
P
x2
y
Q y3 x3
Q
3x2
x
By green’s theorem in a plane
C C
Pdx Qdy Q P dxdy
x y
Now
R
Q P dx dy
x y
3 2 2
a a
a a
x x dx dy
2 2
a a
dy x dx
a a
2 3
3
a
a
a
a
y x
2 3 3
a a a a
3
11. Unit.2 Vector Calculus
Pdx Qdy x y dx x y dy
Pdx Qdy x y dx x y dy
Pdx Qdy x y dx x y dy
11
8 4 (1)
3
a
Now
Pdx Qdy
C AB BC CD DA
Along AB, y a, dy 0
X varies from a to a
a
2 1 3 3
AB
a
2 (1 ) 0
a
a
x a dx
3
1
3
a
a
a x
3 4
1 a a 3 a 3 2 a
2
a 3
3 3
Along BC
x a, dx 0
Y varies from a to a
a
2 1 3 3
BC
a
( 3 3 )
a
a
a y dy
4
3
4
a
a
a y y
4 4
a a a a a
4 4 2 4
4 4
Along CD
y a, dy 0
X varies from a to a
a
2 1 3 3
CD a
2 (1 )
a
a
x a dx
3
1
a x dx
3
a
a
a a a
3 3
1
3
12. Unit.2 Vector Calculus
Pdx Qdy x y dx x y dy
3 4 3 4
Pdx Qdy 2 a 2 a 2 a 4 2 a 2 a 2 a
4
3 3 3 3 C
Hence Green’s theorem verified.
Problem 10 Verify Green’s theorem in a plane for
3 2 8 2 4 6
C
12
2 3 2 4
3 3
a a
Along DA,
x a, dx 0
Y Varies from a to a
a
2 1 3 3
DA a
2 1 3 3
a
a
a y dx y a dy
4
3
4
a
a
y a y
4 4
a a a a a
4 4 2 4
4 4
4 4 4 4
a a
3
8 a
4 ......(2)
3
From (1) and (2)
Pdx Qdy Q P dxdy 8 a
4 .
x y
3 C R
x y dx y xy dy where C is the boundary of the region defined by
x y2 , y x2.
Solution:
Green’s theorem states that
udx vdy v u dxdy
C R
x y
Pdx Qdy Q p dxdy
x y
C R
Given 3 2 8 2 4 6
x y dx y xy dy
C
P 3x2 8y2
13. Unit.2 Vector Calculus
Pdx Qdy x x dx x x xdx
Pdx Qdy y y y dy y y dy
13
P 16y
y
Q 4y 6xy
Q
6y
x
Evaluation of
Pdx Qdy
C
(i) Along OA
y x2 dy 2xdx
3 2 8 4 4 2 6 3 2
OA OA
1
3x 2 8x 4 8x 3 12x 4
dx
0
1
20x 4 8x 3 3x 2
dx
0
5 4 3 1
0
x x 20 8 3
x
5 4 3
20 8 3
5 5 3
4 2 1 1
Along AO
y2 x2ydy dx
3 4 8 2 2 4 6 3
Ao Ao
(6 y 5 16 y 3 4 y 6 y 3 )
dy
AO
0
6y 5 22y 3
4y dy
1
6 4 2 0
y y y
1
6 22 4
6 4 2
0
11 2 5
2 2
y 6 y 4 y 2
1
1 5 3 (1)
Pdx Qdy
2 2 C OA AO
Evaluation of
R
Q P dx dy
x y
v u dxdy 6 y 16 y
dxdy
x y
R R
14. Unit.2 Vector Calculus
14
y
1 1
2
10 10
0 0
x y
x y
y
y dx dy xy dy
1
10y y y 2
dy
0
1 3
2 3
10 y y dy
0
1
5
2 4
0
y y
10 5 4
2
10 2 1
5 4
10 8 5
20
30 3
(2)
20 2
For (1) and (2)
Hence Green’s theorem is verified.
Problem 11 Verify Gauss divergence theorem for F yi x j z2 k
over the cylindrical
region bounded by x2 y2 9, Z 0 and Z 2 .
Solution:
Gauss divergence theorem is
F .
n ds divFdV
S V
div F y x z2 2z
x y z
2
3 9
2
div F dV z dzdy dx
3 9 2
0
2
x
V
x
3 9 2
2 2
3 9 2
0
2
2
x
x
z dydx
2
2
= 4 (Area of the circular region)
4 3
2 36 .................(1)
3 9
3 9
4
x
x
dydx
15. Unit.2 Vector Calculus
15
F n ds
1 2 3
.
S S S S
S is the bottom of the circular region, S is the top of the circular region and S is the
1 2 3 cylindrical region
On S , n k, ds dxdy, z 0
1 F n ds z dxdy
1
. 2 0
S
On 2 S , n k
, ds dxdy, z 2
F n ds z dx dy
2
. 2
s
4 dxdy
4 (Area of circular region)
2 4 3 36
On 3 S , x2 y2 9
n xi y j
2 2
4
x 2 y
2
xi y j
3
F n ds yi x j z k xi y j ds
3
. 2
3 S
yx
yx ds 2
xy ds
3 3 S
Let x 3cos , y 3sin
ds 3 d dy
varies from 0 to 2
z varies from 0 to 2
2 9sin cos
3
3
2 2
0 0
d dz
2 2
18 sin 2
2
0 0
d dz
2 2
cos 2
9
2
0 0
dz
2
9 1 1 0
2
dz
0
. 0 36 0 36 ...............(2)
F n ds
S
16. Unit.2 Vector Calculus
16
from (1) and (2)
F .
n ds div FdV
C V
Problem 12 Verify
Stoke’s theorem for the vector field defined by
F x2 y2 i 2xy j
in the rectangular region in the xy plane bounded by the lines
x 0, x a, y 0, y b.
Solution:
F x2 y2 i 2xy j
F dr curl F n ds
By Stoke’s theorem . .
C S
i j k
2 2 2 0
curlF
x y z
x y xy
i0 0 j 0 0 k 2y 2y 4yk
As the region is in the xy plane we can take n k and ds dxdy
curl F . n ds 4 yk .
k dx dy
S
0 0
4
b a
y dx dy
2
0
0
4
2
b
y x a
2ab2..............(1)
F .
dr
C OA AB BC CO
Along OA
y 0dy 0,
x varies from 0 to a
2 2
0
x y dx 2
xy dy
a
OA
a x
a x dx
a 3 3
2
0 0 3 3
Along AB
x adx o, y varies from 0 to b
17. Unit.2 Vector Calculus
17
2 2
0
a y .0 2
ay dy
b
AB
y 2
b a ab
2
0
2
2
Along BC
y b, dy 0
x varies from a to 0
0
x b dx
2 2 0
BC a
3 0
x b 2
x
3 a
3
2
a ab
3
Along CO
x 0, dx 0,
y varies from b to 0
0
y
0 2 0 0 0
CO b
3 3
F dr a ab a ab
. 2 2 0
3 3 c
2ab2.........(2)
For (1) and (2)
F . d r curlF .
nds
C S
Here Stoke’s theorem is verified.
Problem 13 Find .
F nds
S
if F x y2 i 2x j 2yzk
where S is the surface in
the plane 2x y 2z 6 in the first octant.
Solution:
Let 2x y 2z 6 be the given surface
Then 2i j 2k
i j k i j k
2 2 2
2
4 1 4 3
The unit outward normal n
to the surface S is 1 2 2
n i j k
3
Let R be the projection of S on the xy plane
18. Unit.2 Vector Calculus
F n x y i x j yzk i j k
x y dx x xy where C is the boundary of the
18
F . n ds F .
n dx dy
S R .
1
n k
. 1 2 2 . 2
n k i j k k
3 3
. 2 2 2 .1 2 2
3
2 2 2 4
3 3 3
x y x yz
2 2 2
3
y yz
2 2
3
y y z
2 6 2
3
y y y x
2 6 2
3
y x
4 3
3
y x
. .
F n ds F n dx dy
n k
S S .
y x dxdy
4 3
3 2 / 3
1
x dx dy
2 3
R
3 6 2
2 3
0 0
x
x dx dy
x x y dx
3 2 6 2
2 3
2
0 0
3
3
4 3 x dx
0
4 3
0
x
4 (3 )
4
81units.
Problem 14 Evaluate 2 3
C
triangle with vertices 2,0,0,0,3,0&0,0,6 using Stoke’s theorem.
Solution:
19. Unit.2 Vector Calculus
19
Stoke’s theorem is . .
F dr curlF nds
C S
where S is the surface of the triangle and n
is
the unit vector
normal to surface S .
Given F.dr x ydx 2x z dy y z dz
dr i dx jdy k dz
F x yi 2x z j y zk
i j k
2
curlF
x y z
x y x z y z
i 11 j 0 0 k 2 1
curl F 2i k
Equation of the plane ABC is
x y z
1
2 3 6
3x 2y z 6
Let 3x 2y z 6
3i 2 j k
Unit normal vector to the surface ABCor is
3 2
n i j k
14
curl F n i k i j k
. 2 . 3 2 6 1 7
14 14 14
Hence . 7
curlF nds ds
S S 14
7
14 R
dxdy
nk
where R is the projection of surface ABC on XOY plane
7 3 2 . 1 14 1 14 14
14
R
dxdy n k i j k k
7
dxdy
R
7 Area of le OAB
73 21.
20. Unit.2 Vector Calculus
Problem 15 Verify Stoke’s theorem for F y zi yz j xzk
20
where S is the
surface bounded by the planes x 0, x 1, y 0, y 1, z 0 and z 1 above the
XOY plane.
Solution:
Stoke's theorem is . .
F d r F nds
C S
F y zi yz j xzk
i j k
F
x y z
y z yz xz
yi z 1 j k
F n ds
1 2 3 4 5
.
S S S S S S
6 S is not applicable, since the given condition is above the XOY plane.
yi z j k idydz
1
1 .
S AEGD
ydy dz
AEGD
1 1 1 2 1
y dy dz y dz
1
0 0 0 0 2
1 1
2 2
z
0
yi z j k i dydz
2
1 .
S OBFC
1 1 1 2 1
y dy dz y dz
0 0 0 0
1
2 2
yi z j k jdxdz
3
1
S EBFG
1 1 1
1
0
z 1 dx dz xz x dz
0 0 0
2 1
0
1 1 1
z z
2 2 2
yi z j k j dxdz
4
1
S OADC
21. Unit.2 Vector Calculus
y z dy dz
x z dx dz
21
1 1
z 1 dx dz
0 0
1 1
1
0
xz x z 1 dz
0 0
2 1
0
1 1 1
z z
2 2 2
yi z j k kdxdy
5
1 .
S DGFC
1 1 1
1
0
1 dxdy x dy
0 0 0
1
1
0
1 dy y 1
0
S S1 S2 S3 S4 S5
1 1 1 1 1 1
2 2 2 2
L . H . S F .
dr
C OA AE EB BO
y z dx yzdy xzdz
OA OA
0 0 0, 0, 0, 0
OA
y z dx yzdy xzdz
AE AE
0 0 1, 0, 0, 0
AE
y z dx yzdy xzdz
EB EB
0
1 dx y 1, z 0,
1
0
1 x 0 1 1
y z dx yzdy xzdz
BO BO
0 0 0, 0
x z
BO
0
0 0 1 0 1
C OA AE EB BO
22. Unit.2 Vector Calculus
22
L.HS = R.HS.
Hence Stoke’s theorem is verified.
z C (0,0,1) F
D G
O y
B(0,1,0)
x A (1,0,0) E (1,1,0)