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Chapter 3. Electric Flux Density, Gauss’s Law, and DIvergence. Application of Gauss’s Law: Differential Volume Element. We are now going to apply the methods of Gauss’s law to a slightly different type of problem: a surface without symmetry.

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Application of Gauss’s Law: Differential Volume Element


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    1. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Application of Gauss’s Law: Differential Volume Element • We are now going to apply the methods of Gauss’s law to a slightly different type of problem: a surface without symmetry. • We have to choose such a very small closed surface that D is almost constant over the surface, and the small change in D may be adequately represented by using the first two terms of the Taylor’s-series expansion for D. • The result will become more nearly correct as the volume enclosed by the gaussian surface decreases. We intend eventually to allow this volume to approach zero.

    2. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Taylor’s SeriesExpansion A point near x0 Only the linear terms are used for the linearization

    3. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Application of Gauss’s Law: Differential Volume Element • Consider any point P, located by a rectangular coordinate system. • The value of D at the point P may be expressed in rectangular components: • We now choose as our closed surface, the small rectangular box, centered at P, having sides of lengths Δx, Δy, and Δz, and apply Gauss’s law:

    4. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Application of Gauss’s Law: Differential Volume Element • We will now consider the front surface in detail. • The surface element is very small, thus D is essentially constant over this surface (a portion of the entire closed surface): • The front face is at a distance of Δx/2 from P, and therefore:

    5. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Application of Gauss’s Law: Differential Volume Element • We have now, for front surface: • In the same way, the integral over the back surface can be found as:

    6. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Application of Gauss’s Law: Differential Volume Element • If we combine the two integrals over the front and back surface, we have: • Repeating the same process to the remaining surfaces, we find: • These results may be collected to yield:

    7. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Application of Gauss’s Law: Differential Volume Element • The previous equation is an approximation, which becomes better as Δv becomes smaller, and in the following section the volume Δv will be let to approach zero. • For the moment, we have applied Gauss’s law to the closed surface surrounding the volume element Δv. • The result is the approximation stating that:

    8. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Application of Gauss’s Law: Differential Volume Element • Example • Let D = y2z3ax + 2xyz3ay + 3xy2z2aznC/m2in free space. (a) Find the total electric flux passing through the surface x = 3, 0 ≤ y ≤ 2, 0 ≤ z ≤ 1 in a direction away from the origin. (b) Find • |E| at P(3,2,1). (c) Find an approximate value for the total charge contained in an incremental sphere having a radius of 2 mm centered at P(3,2,1). (a)

    9. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Application of Gauss’s Law: Differential Volume Element (b)

    10. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Application of Gauss’s Law: Differential Volume Element (c)

    11. Divergence Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence • We shall now obtain an exact relationship, by allowing the volume element Δv to shrink to zero. • The last term is the volume charge density ρv, so that:

    12. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Divergence • Let us no consider one information that can be obtained from the last equation: • This equation is valid not only for electric flux density D, but also to any vector field A to find the surface integral for a small closed surface.

    13. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Divergence • This operation received a descriptive name, divergence. The divergence of A is defined as: “The divergence of the vector flux density A is the outflow of flux from a small closed surface per unit volume as the volume shrinks to zero.” • A positive divergence of a vector quantity indicates asource of that vector quantity at that point. • Similarly, a negative divergence indicates a sink.

    14. Divergence Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Rectangular Cylindrical Spherical

    15. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Divergence • Example • If D = e–xsinyax – e–x cosyay + 2zaz, find div D at the origin and P(1,2,3). Regardless of location the divergence of D equals 2 C/m3.

    16. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Maxwell’s First Equation (Electrostatics) • We may now rewrite the expressions developed until now: Maxwell’s First EquationPoint Form of Gauss’s Law • This first of Maxwell’s four equations applies to electrostatics and steady magnetic field. • Physically it states that the electric flux per unit volume leaving a vanishingly small volume unit is exactly equal to the volume charge density there.

    17. Chapter 3 Electric Flux Density, Gauss’s Law, and DIvergence Homework 4 • D3.6. • D3.7. • D3.8. • For D3.6., Replace P(2,–1,3) with P(StID,–1,StID+3). StID is the last two digits of your Student ID Number.Example: Yasin (002201200006) will do D3.6 with P(6,–1,9). • All homework problems from Hayt and Buck, 7th Edition. • Due: Monday, 5 May 2014.