Elements Of Electromagnetics
Elements Of Electromagnetics
7th Edition
ISBN: 9780190698614
Author: Sadiku, Matthew N. O.
Publisher: Oxford University Press
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Chapter 7, Problem 57P

(a)

To determine

Prove that the curl of the gradient function ×(V)=0 in cylindrical coordinates.

(a)

Expert Solution
Check Mark

Explanation of Solution

Calculation:

The Del operator in a cylindrical coordinates is expressed as,

=ρaρ+1ρϕaϕ+zaz

The function V is expressed as,

V=(Vρaρ+1ρVϕaϕ+Vzaz)

Therefore, the function ×(V) is,

×(V)=1ρ|aρρaϕazρϕzVρρ(1ρVϕ)Vz|=1ρ|aρρaϕazρϕzVρVϕVz|

×(V)=1ρ[(ϕ(Vz)z(Vϕ))aρ(ρ(Vz)z(Vρ))ρaϕ+(ρ(Vϕ)ϕ(Vρ))az]        (1)

The point of interchange of base values of the partial derivatives gives the same values. That is,

ϕ(Vz)=z(Vϕ)ρ(Vz)=z(Vρ)ρ(Vϕ)=ϕ(Vρ)

Therefore, the Equation (1) is re-written as follows,

×(V)=1ρ[(ϕ(Vz)ϕ(Vz))aρ(ρ(Vz)ρ(Vz))ρaϕ+(ρ(Vϕ)ρ(Vϕ))az]=1ρ[(0)aρ(0)ρaϕ+(0)az]=0

Conclusion:

Thus, the curl of the gradient function ×(V)=0 in cylindrical coordinates and it is shown.

(b)

To determine

Prove that the divergence of the curl function (×A)=0 in cylindrical coordinates.

(b)

Expert Solution
Check Mark

Explanation of Solution

Calculation:

The function ×A is expressed as,

×A=1ρ|aρρaϕazρϕzAρρAϕAz|=1ρ[(ϕ(Az)z(ρAϕ))aρ(ρ(Az)z(Aρ))ρaϕ+(ρ(ρAϕ)ϕ(Aρ))az]=[1ρ(Azϕ(ρAϕ)z)aρ1ρ(AzρAρz)ρaϕ+1ρ((ρAϕ)ρAρϕ)az]=[1ρ(Azϕ(ρAϕ)z)aρ(AzρAρz)aϕ+1ρ((ρAϕ)ρAρϕ)az]

Therefore, the function (×A) is,

(×A)=(ρaρ+1ρϕaϕ+zaz)[1ρ(Azϕ(ρAϕ)z)aρ(AzρAρz)aϕ+1ρ((ρAϕ)ρAρϕ)az]=[ρ(1ρ(Azϕ(ρAϕ)z))aρaρ0+0+0(1ρϕ(AzρAρz))aϕaϕ+0+00+(z(1ρ((ρAϕ)ρAρϕ)))azaz] {aρaϕ=0aρaz=0aϕaz=0}=[ρ(1ρ(Azϕ(ρAϕ)z))(1ρϕ(AzρAρz))+(z(1ρ((ρAϕ)ρAρϕ)))] {aρaρ=0aϕaϕ=0azaz=0}=[ρ(1ρAzϕAϕz)(1ρϕ(AzρAρz))+(z(1ρ(ρAϕ)ρ1ρAρϕ))]

Simplify the above equation.

(×A)=[ρ(1ρAzϕ)ρ(Aϕz)1ρϕ(Azρ)+1ρϕ(Aρz)+z(1ρ(ρAϕ)ρ)z(1ρAρϕ)]

(×A)=[1ρρ(Azϕ)ρ(Aϕz)1ρϕ(Azρ)+1ρϕ(Aρz)+z(Aϕρ)1ρz(Aρϕ)]        (2)

The point of interchange of base values of the partial derivatives gives the same values. That is,

ρ(Azϕ)=ϕ(Azρ)ρ(Aϕz)=z(Aϕρ)ϕ(Aρz)=z(Aρϕ)

Therefore, the Equation (2) is re-written as follows,

(×A)=[1ρρ(Azϕ)ρ(Aϕz)1ρρ(Azϕ)+1ρϕ(Aρz)+ρ(Aϕz)1ρϕ(Aρz)]=0

Conclusion:

Thus, the divergence of the curl function (×A)=0 in cylindrical coordinates and it is shown.

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