electromagnetic field and wave theory assignment backup

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The field of electrical engineering can be broadly classified into two namely the electric circuit theory and electromagnetic fields and wave theory. While circuit theory deals with electrical analysis, synthesis and design of a wired networks, electromagnetic theory focuses on the propagation of electrical signals through a wireless medium. This assignment takes an in depth look at some concepts in electromagnetic theory as well as its applications. In the field of electromagnetic theory, one name stands very tall, the man whom many have described as the wizard of electromagnetic theory. He is James Clerk Maxwell. In 1863, James Clerk Maxwell before the Royal Academy of science proved that electrical signals can be propagated through a wireless medium in form of electric and magnetic fields. This he proved possible through the introduction of a displacement current concept. Years latter, this theory of Maxwell was practically verified and proved correct by the likes of Guglielmo Marconi and Hertz. James Clerk Maxwell developed four basic equations that characterized both electric and magnetic fields, these equations till date form the basis for the study and analysis of electromagnetic fields and wave theory.


THE CONCEPT OF ELECTRODYNAMICS Electrodynamics deals with electric and magnetic fields, its inter-

relationships under various charge arrangements. Electrodynamics can be classified into two areas viz: Electrostatics and Magneto statics ELECTROSTATICS: This deals with theory that describes physical phenomena related to the interaction between stationary electric charges or charge distributions in space. Recall from coulombs law, the force on a charge q located at a distance r due to the existence of another charge relation located at a distance is given by the

The interpretation of the above equation(1) is that the electrical force between two charges separated by distance R is directly proportional to the product of the magnitude of the charges and inversely proportional to the distances separating the charge quantities. The distance R is obtained by the vectorial differences of the position coordinates of the two charge quantities (ie components of the equation (ie 4 space. Hence the electrostatic field produced by the charge as force per unit charge. (2) on the test charge q is given R= r ). The other ) is a constant associated with the geometry of

In equation (2) above, the limit taken as q tends to zero implies that the test charge q is so small to the extent that it has no effect on the field produced by the charge under study ( ) Substituting equ (1) into equ (2)

(3) Here equation (3) is similar to equation (1) above except that the electrostatic field is produced by the charge under study due to the earlier assumption that the quantity of the test charge tends to zero and hence the electric field produced by the test charge is negligible. Suppose that the magnitude of the discrete electric charges are so minute and the number of charges so massive, then the concept of electric charge density introduced. Hence, will be

..........................(4) The above equation(4) is exactly the same as equation(3) but while equation (3) considers a point charge, equation (4) considers charge density difficult to study point charges). For the purpose of analysis and study of the property of the electrostatic field E, the Curl and divergence of the field is taken as below. Prior to this analysis, lets look at what divergence ( and Curl ( means. within a volume (It is worthy of note here that in reality, equation (4) is what is obtainable as it is

First of all, divergence and curl are both operators used to study vector and scalar fields. Now, the divergence of a vector is the outflow of flux( a flux is nothing other than lines of electric and magnetic fields) from a small closed surface per unit volume as the volume shrinks to zero. Divergence ordinarily tells us how much flux is leaving a small area on a per unit volume basis without any direction. Similarly, the Curl of a vector is the measure of the strength of rotation or vorticity of the vector field around the corresponding coordinate direction. In order to obtain the characteristics of a vector or scalar field, divergence and curl operators are applied to such quantities. Recall, from vector field identities, the curl is equivalent to zero[2] i.e . Applying this in the electrostatic field E. of any well behaved scalar field

(5) The physical interpretation of equ (5) is that the electrostatic field is irrotational. This is so because the electrostatic field is a scalar quantity(ie it has magnitude without direction). In all of the equations used here, the subscripts indicate the position coordinate of the vector quantity (for instance, implies the charge

density at the position coordinate of (

). Similarly the integral sign(

in any

of the equations implies the summation of the total charge density within the enclosed volume. Similarly, taking the divergence of the electrostatic field E, we have.

(6) The above equ(6) is a differential form of Gausss law. That is to say that equation (6) conforms to the earlier statement of Gauss that the total amount of fields radiating from a closed surface is equal to the total number of charges within the same closed volume.



FIELD OF N-POINT CHARGES Since coulomb forces are linear, the electric field intensity due to two point charges Q1 and Q2 is the sum of the forces on Q1 caused by Q1 and Q2 acting alone. This also implies that electric fields produced by different point charges are also linear. Mathematically this can be expressed thus:





+ - - - +


FIELD DUE TO A CONTINOUS VOLUME CHARGE DISTRIBUTION The small amount of charges = in a small volume is given by relation . (8)

Hence the total charge within some finite volume is given thus: Q = E=


FIELD OF A LINE CHARGE ( ) In the determination of electric field E at any point resulting from a uniform line charge density , we use symmetry to determine two factors: (a) With which coordinate does the field not vary and (b) which components of the field are not present. The line field varies only with r coordinate. No element of charge produces a Hence = component . Each element does produce an Er and EZ component, but the

contributions of charge due to + EZ and - EZ which are equal distances above and below the point at which the field is being determined will cancel out leaving only the Er component, which varies only with r. Hence, OR

= .(11) Equation (11) looks at the differential electric field within the length of the sheet, when integrated within the entire length of the wire, equation (12) is obtained. Replacing charge. by and summing the contribution from every element of




Another basic charge configuration is the infinite sheet of charge having a uniform density of such a charge distribution may be used to approximate that found on the conductors of a strip transmission here that static charges reside on the conductor surfaces and not in their interiors The line charge density or charge per unit length is Where This is the position vector of the charge density(that is the vectorial distance) ..... (13)





The first line of the above equation looks at the differential field dE in the x direction while the second line takes the integral of differential field so as to obtain the total field of the charge distribution within the sheet.

1.3 GAUSSS LAW This states that the electric Flux passing through any closed surface is equal to the total charge enclosed by that surface. The total flux ( ) passing through the closed surface is obtained by adding the differential contributions crossing each surface element s That is .. (15)

Expressing Gausss statement mathematically, we have (16) Equation (16) implies that total the amount of electric or magnetic flux emanating from an enclosed surface filled with charges is equal to the total number of charges( ) within that volume. This is true because each charge quantity radiates its own field so the total fields from a volume is the same as the sum of the fields from each of the charges within the volume. Where = Electric Flux Density One the applications of Gausss Law that it is used to convert surface integral to volume integral.



The objective here is to use Gauss law to determine (ie the electric flux density) if the charge distribution is known. To be able to do this a closed surface which satisfies two conditions has to be chosen. The conditions are (a) where is everywhere either normal or tangential to the closed surface so that is not zero, becomes or zero respectively. (b) on that portion of the closed surface for which = constant. This allows the replacement of the dot product with the products of the scalars and and then to bring outside the integral sign(ie the above two conditions if met, makes the integration of the resultant integral possible). The remaining integral then becomes over that portion of the closed surface which crosses normally and this is simply the area of this secti


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