Test: Divergence

QUESTION: 1

The divergence of a vector is a scalar. State True/False.

A.
True
B.
False

Solution:

Answer: a
Explanation: Divergence can be computed only for a vector. Since it is the measure of outward flow of flux from a small closed surface as the volume shrinks to zero, the result will be directionless (scalar).

QUESTION: 2

The divergence concept can be illustrated using Pascal’s law. State True/False.

A.
True
B.
False

Solution:

Answer: a
Explanation: Consider the illustration of Pascal’s law, wherein a ball is pricked with holes all over its body. After water is filled in it and pressure is applied on it, the water flows out the holes uniformly. This is analogous to the flux flowing outside a closed surface as the volume reduces.

QUESTION: 3

Compute the divergence of the vector xi + yj + zk.

A.
0
B.
1
C.
2
D.
3

Solution:

Answer: d
Explanation: The vector given is a position vector. The divergence of any position vector is always 3.

QUESTION: 4

Find the divergence of the vector yi + zj + xk.

A.
-1
B.
0
C.
1
D.
3

Solution:

Answer: b
Explanation: Div (yi + zj + xk) = Dx(y) + Dy(z) + Dz(x), which is zero. Here D refers to partial differentiation.

QUESTION: 5

Given D = e^-xsin y i – e^-xcos y j
Find divergence of D.

A.
3
B.
2
C.
1
D.
0

Solution:

Answer: d
Explanation: Div (D) = Dx(e^-xsin y) + Dy(-e^-xcos y ) = -e^-xsin y + e^-xsin y = 0.

QUESTION: 6

Find the divergence of the vector F= xe^-x i + y j – xz k

A.
(1 – x)(1 + e^-x)
B.
(x – 1)(1 + e^-x)
C.
(1 – x)(1 – e)
D.
(x – 1)(1 – e)

Solution:

Answer: a
Explanation: Div(F) = Dx(xe^-x) + Dy(y)+Dz(-xz) = -xe^-x + e^-x + 1 – x =
e^-x(1 – x) + (1 – x) = (1 – x)(1 + e^-x).

QUESTION: 7

Determine the divergence of F = 30 i + 2xy j + 5xz² k at (1,1,-0.2) and state the nature of the field.

A.
1, solenoidal
B.
0, solenoidal
C.
1, divergent
D.
0, divergent

Solution:

Answer: b
Explanation: Div(F) = Dx(30) + Dy(2xy) + Dz(5xz²) = 0 + 2x + 10xz = 2x + 10xz
Divergence at (1,1,-0.2) will give zero. As the divergence is zero, field is solenoidal.
Alternate/Shortcut: Without calculation, we can easily choose option b, as by theory when the divergence is zero, the vector is solenoidal. Option b is the only one which is satisfying this condition.

QUESTION: 8

Find whether the vector is solenoidal, E = yz i + xz j + xy k

A.
Yes, solenoidal
B.
No, non-solenoidal
C.
Solenoidal with negative divergence
D.
Variable divergence

Solution:

Answer: a
Explanation: Div(E) = Dx(yz) + Dy(xz) + Dz(xy) = 0. The divergence is zero, thus vector is divergentless or solenoidal.

QUESTION: 9

Find the divergence of the field, P = x²yz i + xz k

A.
xyz + 2x
B.
2xyz + x
C.
xyz + 2z
D.
2xyz + z

Solution:

Answer: b
Explanation: Div(P) = Dx(x²yz) + Dy(0) + Dz(xz) = 2xyz + x, which is option b. For different values of x,y,z the divergence of the field varies.

QUESTION: 10

Identify the nature of the field, if the divergence is zero and curl is also zero.

A.
Solenoidal, rotational
B.
Divergent, rotational
C.
Solenoidal, irrotational
D.
Divergent, irrotational

Solution:

Since the vector field does not diverge (moves in a straight path), the divergence is zero. Also, the path does not possess any curls, so the field is irrotational.