Gauss' Law and Divergence Theorem Quiz
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Questions and Answers

What is the unit of measurement for electric flux?

  • Newtons per square meter
  • Coulombs per square meter
  • Volts per meter
  • Coulombs (correct)
  • What describes the direction of electric flux density D at a point?

  • The direction of the surface area vector
  • The direction of the flux lines at that point (correct)
  • The direction of the charge Q
  • The direction of the electric field E
  • How is the magnitude of electric flux density D calculated?

  • By counting the total number of charge carriers crossing the surface
  • By dividing the electric flux by the surface area (correct)
  • By dividing the total charge by the surface area
  • By multiplying the electric field by the surface area
  • Which equation correctly shows the relationship between electric flux density D and electric field intensity E in free space?

    <p>D = ε0 E (B)</p> Signup and view all the answers

    What does Gauss' Law state about electric flux through a closed surface?

    <p>It is proportional to the total charge enclosed by the surface. (D)</p> Signup and view all the answers

    What special conditions must a closed surface satisfy for Gauss' Law integration?

    <p>D must be either normal or tangential to the surface. (C)</p> Signup and view all the answers

    Which of the following describes the differential flux crossing a surface area dS?

    <p>dψ = D ∙ dS (A), dψ = D dS cos θ (C)</p> Signup and view all the answers

    At a point located at distance r from a point charge Q, what is the formula for electric flux density D?

    <p>D = Q / (4πε0r^2) a (A)</p> Signup and view all the answers

    What is the expression for the electric displacement field D created by an infinite plane of sheet with surface charge density 𝜌𝑠?

    <p>𝐃 = rac{𝜌𝑠}{2𝜖𝑜} 𝐚𝑛 (C)</p> Signup and view all the answers

    In the region between two infinite parallel-plate capacitors where the plates are charged, what is the electric field E?

    <p> rac{𝜌𝑠}{2𝜖𝑜} 𝐚𝑦 (D)</p> Signup and view all the answers

    What happens to the electric field E outside the region of two charged parallel plates?

    <p>It decreases to zero. (B)</p> Signup and view all the answers

    When applying Gauss' law to a small volume with a non-uniform distribution of D, what shape is typically chosen for the Gaussian surface?

    <p>A small cuboid (A)</p> Signup and view all the answers

    In the context of the infinite plane of sheet problem, how does the charge density affect the displacement field?

    <p>It increases the magnitude of D. (B)</p> Signup and view all the answers

    What is the expression for the total charge of a surface charge distribution?

    <p>$Q = \int \rho_S , dS$ (B)</p> Signup and view all the answers

    In the context of Gauss' Law for a point charge, what represents the relation between the electric displacement field $D$ and the total charge $Q$?

    <p>$D = \frac{Q}{4\pi r^2} \hat{r}$ (A)</p> Signup and view all the answers

    Which component of the electric displacement field D crosses normally through the faces of a Gaussian surface in a differential volume?

    <p>Only one component crosses normally. (A)</p> Signup and view all the answers

    When using Gauss' law, what is the relationship between the total electric flux through a closed surface and the enclosed charge?

    <p>Flux equals the enclosed charge divided by ε₀. (C)</p> Signup and view all the answers

    For an infinite line charge along the z-axis, how is the electric displacement field $D$ expressed?

    <p>$D = D_\rho \hat{\rho}$ (A)</p> Signup and view all the answers

    Which of the following integrals represents the total charge contained in a cylindrical volume with a line charge?

    <p>$Q = \rho_L L$ (B)</p> Signup and view all the answers

    For a Gaussian surface enclosing a charge, what is the mathematical expression used to calculate the electric flux?

    <p>∮ 𝐷 ∙ 𝑑𝐒 (D)</p> Signup and view all the answers

    In spherical coordinates, how do you express the electric field $E$ in terms of the electric displacement field $D$ for a point charge?

    <p>$E = \frac{D}{4\pi\epsilon_o r^2}$ (B)</p> Signup and view all the answers

    What is the main characteristic of the Gaussian surface used for an infinite line charge?

    <p>It is cylindrical in shape. (C)</p> Signup and view all the answers

    How is the integration of $D$ performed over the surfaces of the cylindrical Gaussian surface for an infinite line charge?

    <p>Only on the curved surface. (B)</p> Signup and view all the answers

    What does the expression $D = \frac{\rho_L}{2\pi\rho} \hat{\rho}$ represent in the context of an infinite line charge?

    <p>Electric displacement field due to line charge (C)</p> Signup and view all the answers

    What is indicated by a positive divergence of the vector flux density 𝐃?

    <p>A source exists at that point. (C)</p> Signup and view all the answers

    Which mathematical expression represents the divergence of 𝐃 based on the given content?

    <p>div 𝐃 = lim (∮ 𝐃 ∙ 𝑑𝐒) / Δ𝑣 (B)</p> Signup and view all the answers

    What does a negative divergence of the vector flux density 𝐃 indicate?

    <p>The existence of a sink. (B)</p> Signup and view all the answers

    In evaluating the divergence of 𝐃, what occurs as the volume element Δ𝑣 approaches zero?

    <p>The outflow of flux is measured per unit volume. (A)</p> Signup and view all the answers

    Which relation corresponds to the contribution from the front face of the surface integral for 𝐃?

    <p>∫front = [D_x + Δ𝑥] Δ𝑦 Δ𝑧 (D)</p> Signup and view all the answers

    Which of the following expressions correctly represents the total contribution from all faces of the closed surface?

    <p>∮ 𝐃 ∙ 𝑑𝐒 = (∂D_x/∂x + ∂D_y/∂y + ∂D_z/∂z) Δ𝑥 Δ𝑦 Δ𝑧 (B)</p> Signup and view all the answers

    What happens to the divergence of 𝐃 when there is no source or sink present?

    <p>It equals zero. (C)</p> Signup and view all the answers

    What is the expression for divergence in rectangular coordinates?

    <p>$\frac{\partial D_x}{\partial x} + \frac{\partial D_y}{\partial y} + \frac{\partial D_z}{\partial z}$ (C)</p> Signup and view all the answers

    In cylindrical coordinates, what forms the differential volume element?

    <p>$\rho d\rho d\phi dz$ (C)</p> Signup and view all the answers

    Which coordinate system requires the term $r^2 \sin(\theta)$ in its volume differential for the divergence calculation?

    <p>Spherical (A)</p> Signup and view all the answers

    What does the divergence theorem state about total flux crossing a closed surface?

    <p>It equals the total charge within the surface. (C)</p> Signup and view all the answers

    What is the correct point form of Maxwell's first equation?

    <p>$\nabla \cdot D = \rho_v$ (A)</p> Signup and view all the answers

    Which expression represents the divergence in spherical coordinates?

    <p>$\frac{1}{r^2} \frac{\partial(r^2 D_r)}{\partial r} + \frac{1}{r \sin(\theta)} \frac{\partial(sin(\theta) D_{\theta})}{\partial \theta} + \frac{1}{r \sin(\theta)} \frac{\partial D_{\phi}}{\partial \phi}$ (A)</p> Signup and view all the answers

    How is the Del operator defined in Cartesian coordinates?

    <p>$\nabla = \frac{\partial}{\partial x} \hat{a_x} + \frac{\partial}{\partial y} \hat{a_y} + \frac{\partial}{\partial z} \hat{a_z}$ (C)</p> Signup and view all the answers

    What relationship does Gauss’ law illustrate regarding electric displacement and charge?

    <p>The outward flux is proportional to the total charge enclosed. (B)</p> Signup and view all the answers

    Flashcards

    What is Gauss's Law?

    The total amount of electric flux passing through a closed surface.

    What is a Gaussian Surface?

    A closed surface used for applying Gauss's Law.

    What is Electric Flux?

    A scalar quantity representing the number of electric flux lines associated with a charge.

    What is Electric Flux Density (D)?

    A vector quantity representing the density of electric flux lines at a point.

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    How is the direction of Electric Flux Density related to the electric field?

    The direction of the electric flux density at a point is the same as the direction of the electric field line at that point.

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    What is the relation between D and E in free space?

    The relationship between Electric Flux Density (D) and Electric Field Intensity (E) in free space.

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    What is the mathematical expression for Gauss's Law?

    The total electric flux passing through a closed surface is equal to the total charge enclosed by that surface.

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    What are the conditions for choosing a Gaussian surface?

    A Gaussian surface is chosen to simplify the integration in Gauss's Law by aligning its surface to be either normal or tangential to the electric flux density.

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    Constant D over Surface

    The electric flux density (D) is constant over the portion of the surface where D dot dS is non-zero, meaning the electric flux lines are perpendicular to the surface.

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    Gauss's Law

    The total charge enclosed within a closed surface is equal to the integral of the electric flux density (D) over the entire surface.

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    Electric Field of a Point Charge

    The electric field intensity (E) due to a point charge Q at a distance r from the charge is given by E = Q / (4πε₀r²), where ε₀ is the permittivity of free space.

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    Electric Field of an Infinite Line Charge

    The electric field intensity (E) due to an infinite line charge with uniform charge density ρL at a radial distance ρ from the line charge is given by E = ρL / (2πε₀ρ), where ε₀ is the permittivity of free space.

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    Gaussian Surface

    The electric flux density (D) is constant over a closed surface, allowing for easier calculation of the total charge enclosed.

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    Charge Enclosed and Flux Density

    The integral of the electric flux density (D) over a closed surface is equal to the total charge enclosed within the surface. This is known as Gauss's Law.

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    D Normal to Surface

    The electric field lines are perpendicular to the surface, and the electric field strength is constant over the surface. This simplifies the calculation of the electric flux.

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    Radial Electric Field

    The electric field lines are radial and decrease in strength as the distance from the line charge increases.

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    Electric Field of Infinite Plane

    The electric field intensity (E) due to an infinite plane of charge with a constant surface charge density (ρs) is directly proportional to the charge density and inversely proportional to the permittivity of free space (εo).

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    Divergence of Electric Field

    Divergence of a vector field, in this context, represents the outward flux density per unit volume at a point. It measures how much the field 'diverges' or 'spreads out' at that point.

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    Divergence Theorem

    The divergence theorem is a mathematical theorem that relates the divergence of a vector field over a volume to the flux of the field across the surface enclosing the volume. It's particularly useful in electromagnetism to relate the electric field to the enclosed charge.

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    Calculating Divergence of Electric Field

    To calculate the divergence of the electric field at a point, we apply Gauss's Law to a small cuboid enclosing that point. The electric flux through each face of the cuboid is related to the component of the electric field perpendicular to that face, and the divergence is calculated from the sum of these fluxes.

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    Gauss's Law in integral form

    The integral of the dot product of the electric flux density (D) and the differential surface area (dS) over a closed surface. It represents the total electric flux passing through the surface.

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    Divergence of D

    The divergence of the electric flux density (D) is defined as the outflow of flux from a small closed surface per unit volume, as the volume approaches zero.

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    ρv (Volume Charge Density)

    The volume charge density is the amount of charge per unit volume. It represents how densely charge is distributed in space.

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    Positive Divergence

    A positive divergence indicates a source of electric flux at that point. This implies that more electric flux is flowing out of the point than into it.

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    Negative Divergence

    A negative divergence indicates a sink of electric flux at that point. This means that more electric flux is flowing into the point than out of it.

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    Zero Divergence

    A zero divergence indicates a point where there is no net source or sink of electric flux. This implies that the electric flux entering and leaving the point is equal.

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    Integral of D ⋅ dS over back face

    The integral of D ⋅ dS over the back face of a small volume element. It can be approximated by the product of the electric flux density (D) on the back face, the area of the back face (ΔyΔz), and the negative change in the x-direction (−Δx).

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    Divergence of a vector field

    The divergence of a vector field, 𝐃, represents the outward flux per unit volume at a point in space. It quantifies how much the vector field is expanding or contracting at that point.

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    Divergence in Rectangular Coordinates

    The divergence of a vector field 𝐃 in rectangular coordinates is calculated as the sum of partial derivatives of each component of 𝐃 with respect to its corresponding coordinate.

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    Divergence in Cylindrical Coordinates

    The divergence of a vector field 𝐃 in cylindrical coordinates is calculated using partial derivatives of 𝐃 with respect to radial distance (𝜌), azimuthal angle (𝜙), and the z-axis.

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    Divergence in Spherical Coordinates

    The divergence of a vector field 𝐃 in spherical coordinates is calculated using partial derivatives of 𝐃 with respect to radial distance (r), polar angle (𝜃), and azimuthal angle (𝜙).

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    Del Operator (𝛁)

    The Del operator (𝛁) is a vector operator used in vector calculus, particularly in electromagnetic theory, for operations like gradient, divergence, and curl.

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    Divergence using Del Operator

    The divergence of a vector field 𝐃 can be expressed using the dot product of the Del operator (𝛁) and 𝐃.

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    Study Notes

    Gauss' Law and Divergence Theorem

    • Electric Flux Lines (ψ): A scalar field where Q coulombs of charges produce ψ (=Q) lines of electric flux. Direct proportionality exists between electric flux and charge (ψ = Q). Unit is coulombs.
    • Electric Flux Density (D): A vector field. Direction of D at a point is the direction of flux lines at that point. Magnitude is the number of flux lines crossing a surface normal to the lines, divided by the surface area. Measured in coulombs per square meter (C/m²) or lines per square meter.
    • Differential Flux: Differential flux (dψ) crossing a differential area (dS) normal to its direction is given by dψ = D * dS * cos θ, where θ is the angle between D and the normal.
    • Gauss' Law: The total electric flux passing through any closed surface equals the total charge enclosed by that surface. Mathematically represented as ∫ D ⋅ dS = Qenc.
    • Special Gaussian Surfaces: To simplify integration, choose a closed surface where D is either normal or tangential to the surface, and D is constant over the relevant portion.
    • Enclosed Charge: This is based on charge distribution. Point charge: Q; Multiple point charges: ∑Qn; Line charge: ∫ρL dL; Surface charge: ∫ρS dS; and Volume charge: ∫ρv dv, where ρL, ρS, ρv are linear, surface, and volume charge densities respectively.
    • Relation Between D and E: A point charge (Q) produces flux lines directed outward. Electric flux density (D) at a point (r) for the flux passing symmetrically through an imaginary spherical surface (area 4πr²) is given by D = Q/(4πr²). In free space, D = ε0E.

    Application of Gauss' Law

    • Point Charge: Gaussian surface is a sphere centered at the charge. D is normal and constant across the surface, allowing D to be factored out of the integral, which yields D = Q/(4πr²).
    • Infinite Line Charge: Gaussian surface is a cylinder, where D is only in the radial direction. This permits the integration, concluding with D = ρL/(2πr), where ⍴L is line charge density and r is the variable radius.
    • Infinite Plane of Sheet: Gaussian surface is a closed cylinder, normal to the plane, with flux only through the top and bottom surfaces. This leads to D = ρs / (2ε0), where ρs is surface charge density and ε0 is permittivity of free space.

    Divergence

    • Differential Volume: The divergence of D at a point is calculated by finding the total flux leaving a small, closed surface around that point per unit volume (as the surface shrinks to zero). The resulting formula is div D = (∂Dx/∂x) + (∂Dy/∂y) + (∂Dz/∂z)(or , div D = ∇·D).
    • Divergence Theorem: The total flux leaving a closed surface is equal to the volume integral of the divergence of D throughout the enclosed volume (∫S D⋅dS = ∫v(∇ ⋅ D)dV).

    Maxwell's First Equation

    • Integral Form:S D ⋅ dS = Qenc, where D is electric flux density, dS is an infinitesimal surface area element, Qenc is the enclosed charge.
    • Point Form: ∇ ⋅ D = ρv, where ρv is the volume charge density.

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    Test your understanding of Gauss' Law and the Divergence Theorem with this comprehensive quiz. Explore concepts like electric flux, flux density, and the mathematical principles governing these laws. Perfect for students in advanced physics courses.

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