Electric Charges and Fields Quiz
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Questions and Answers

What is the correct expression for the torque on a dipole in a uniform electric field?

  • $p × E$ (correct)
  • $2 q a E sinθ$ (correct)
  • $E × p cosθ$
  • $qE × 2 a$
  • What happens to the torque when the dipole moment is aligned with the electric field?

  • The torque increases significantly.
  • The torque remains constant.
  • The direction of torque reverses.
  • The torque becomes zero. (correct)
  • How does a non-uniform electric field affect a dipole's motion?

  • It decreases the torque on the dipole.
  • It aligns the dipole with the electric field.
  • It creates a net force on the dipole. (correct)
  • It has no effect on the dipole.
  • In which scenario is the torque on a dipole non-zero?

    <p>When the dipole is in a non-uniform electric field.</p> Signup and view all the answers

    What is the impact of increasing the strength of the electric field on the net force acting on the dipole in a uniform field?

    <p>It has no effect on the net force.</p> Signup and view all the answers

    What does the torque's direction indicate when a dipole is placed in an electric field?

    <p>It is normal to the plane formed by the dipole and the electric field.</p> Signup and view all the answers

    What role does the angle θ play in determining the torque on a dipole?

    <p>It determines how much torque is exerted on the dipole.</p> Signup and view all the answers

    If the charges of a dipole are separated under an external electric field, what is the net force on the dipole?

    <p>The net force is zero.</p> Signup and view all the answers

    What is meant by the macroscopic density of a liquid?

    <p>It considers the continuous fluid properties of the liquid.</p> Signup and view all the answers

    In the context of a continuous charge distribution, how is the charge in a small volume element DV defined?

    <p>As the charge density multiplied by the volume element size.</p> Signup and view all the answers

    What does the superposition principle allow us to determine in electric fields?

    <p>It permits the addition of electric fields from different volume elements.</p> Signup and view all the answers

    What does Gauss's law state about the electric flux through a closed surface with no enclosed charge?

    <p>The electric flux is zero.</p> Signup and view all the answers

    What is the relationship between the volume elements in continuous charge distributions and electric fields?

    <p>Every charge element contributes to the electric field at point P.</p> Signup and view all the answers

    In Gauss's law, what is being assessed through a surface enclosing a charge?

    <p>The total electric flux through the surface surrounding the charge.</p> Signup and view all the answers

    In the context of Gauss's law, what happens to the electric flux through the curved part of a closed cylindrical surface in a uniform electric field?

    <p>It is zero.</p> Signup and view all the answers

    What are the effects on the electric flux if the charge enclosed by a closed surface is positive?

    <p>The electric flux will be positive.</p> Signup and view all the answers

    What mathematical operation must be conducted to accurately describe the electric field as the volume elements approach zero?

    <p>Integration over the charge distribution.</p> Signup and view all the answers

    Which of the following statements accurately reflects Gauss's law?

    <p>It applies to any closed surface, regardless of shape.</p> Signup and view all the answers

    What does the variable r represent in the equation for the electric field due to a charge element?

    <p>The variable distance between the charge element and point P.</p> Signup and view all the answers

    What do f1 and f2 represent in the context of the cylindrical surface and Gauss's law?

    <p>Flux through the ends of the cylinder.</p> Signup and view all the answers

    How does the charge density ρ vary in a continuous charge distribution?

    <p>It changes from point to point.</p> Signup and view all the answers

    When the electric field E is directed outwards, how do the flux values f1 and f2 behave in a closed surface?

    <p>f1 is negative and f2 is positive.</p> Signup and view all the answers

    How does one conclude that the total charge contained in a closed surface is zero?

    <p>When the electric flux through the surface is zero.</p> Signup and view all the answers

    What does the term q represent in Gauss's law?

    <p>The total charge enclosed by the closed surface.</p> Signup and view all the answers

    What is the net charge within the cylinder calculated using Gauss's law?

    <p>$2.78 \times 10^{-11}$ C</p> Signup and view all the answers

    How is the electric field characterized around an infinitely long uniformly charged wire?

    <p>It is radial and does not depend on position along the wire.</p> Signup and view all the answers

    What does the total electric field at any point P due to an infinite wire depend on?

    <p>The radial distance r from the wire</p> Signup and view all the answers

    Which factor is NOT considered when determining the direction of the electric field around a uniformly charged wire?

    <p>The temperature of the surrounding medium</p> Signup and view all the answers

    What does the electric field at points equidistant to an infinitely long wire result from?

    <p>The additive effect of all charge elements on the wire</p> Signup and view all the answers

    What is the expression used to find the net charge within a cylinder using Gauss's law?

    <p>$q = e_0 \cdot f$</p> Signup and view all the answers

    What is the total outward flux calculated through the cylinder?

    <p>3.14 N m² C⁻¹</p> Signup and view all the answers

    Which statement best describes the symmetry in the electric field around an infinitely long charged wire?

    <p>It has rotational symmetry about the wire axis.</p> Signup and view all the answers

    What is the flux through the end caps of the cylindrical Gaussian surface?

    <p>Zero</p> Signup and view all the answers

    Which of the following variables affects the magnitude of the electric field around the wire?

    <p>Distance from the wire</p> Signup and view all the answers

    How does the electric field E change if the linear charge density λ becomes negative?

    <p>E reverses its direction</p> Signup and view all the answers

    What is the relationship between the scalar A and the unit vector n̂ when A is negative?

    <p>The direction of A is opposite to n̂</p> Signup and view all the answers

    Which of the following statements about the Gaussian surface is correct?

    <p>The charge enclosed is proportional to the length of the cylinder.</p> Signup and view all the answers

    Why is the assumption of an infinitely long wire crucial in this context?

    <p>It rationalizes the use of cylindrical symmetry.</p> Signup and view all the answers

    What does the term $E \times 2πrl$ represent in the context of the cylindrical Gaussian surface?

    <p>The flux through the curved part of the surface</p> Signup and view all the answers

    Study Notes

    Electric Charges and Fields

    • The electric field ( E ) generated by a dipole can be expressed as: [ E = \frac{p}{4 \pi \epsilon_0 r^3} \text{ (for ( r/a >> 1 ))} ]
    • Substituting the known values yields: [ E = \frac{5 \times 10^{-8} \text{ C m}}{4 \pi (8.854 \times 10^{-12} \text{ C}^2 \text{ N}^{-1} \text{ m}^{-2}) (15 \times 10^{-6} \text{ m})^3} \approx 1.33 \times 10^5 \text{ N C}^{-1} ]
    • The direction of the electric field from a dipole is opposite to the dipole moment vector.

    Dipole in a Uniform External Field

    • A permanent dipole possesses a dipole moment ( p ) independent of an external electric field ( E ).
    • Despite the absence of a net force on the dipole in a uniform field, a torque arises due to the separation of charges.
    • Torque magnitude can be calculated as: [ t = p \times E ]
    • When ( p ) aligns with ( E ), the torque is zero. In a non-uniform field, a net force on the dipole occurs.

    Continuous Charge Distribution

    • Discrete charges create a discontinuous charge distribution; for continuous distributions, charge density ( \rho ) varies, allowing for a different approach.
    • Electric field due to a small volume element with charge density ( \rho ) at point ( P ) can be expressed through differential forms of Coulomb's law.
    • Superposition gives the total electric field ( E ) from the charge distribution: [ E \approx \sum \frac{\rho \Delta V}{4 \pi \epsilon_0 r'^2} \hat{r}' ]

    Gauss's Law

    • Gauss's Law states: [ \text{Electric flux through a closed surface S} = \frac{q}{\epsilon_0} ]
    • Total electric flux is zero if no charge is enclosed.
    • For a cylindrical surface in a uniform electric field, the flux is zero across the curved surface due to perpendicular normals.

    Applications of Gauss's Law

    • The electric field for symmetric charge configurations can often be easily calculated using Gauss's Law.
    • For an infinitely long straight wire with uniform linear charge density ( \lambda ):
      • The electric field magnitude at distance ( r ) from the wire is: [ E = \frac{\lambda}{2 \pi \epsilon_0 r} ]
      • The electric field direction is radial: outward if ( \lambda > 0 ) and inward if ( \lambda < 0 ).
    • A cylindrical Gaussian surface can be used to derive the electric field produced by the charged wire.

    Key Points

    • Superposition and integrals can be used to find electric fields due to various charge distributions.
    • Gauss’s law provides a crucial theoretical foundation and practical tool for analyzing electric fields in symmetric situations.

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    Description

    Test your understanding of electric charges and fields with this quiz. Topics include calculations related to electric field intensity and examples illustrating the principles of electromagnetism. Perfect for students delving into physics concepts related to electric forces.

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