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NEET Physics: Electric flux and Gauss's law Practice Questions

14 free multiple-choice questions on Electric flux and Gauss's law (Electrostatics) for NEET Physics, each with the correct answer and a step-by-step explanation. Attempt a question, then reveal the solution to check your reasoning.

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  1. Q1. Gauss's Law states that the total electric flux through any closed surface is proportional to the net electric charge enclosed within that surface. If a closed surface encloses a net charge q, what is the total electric flux passing through that surface?

    • AC = q*ε₀²
    • BB = q*ε₀
    • CD = q/A
    • DA = q/ε₀
    Show answer & explanation

    Correct answer: D — A = q/ε₀

    Gauss's Law is mathematically expressed as Φ_E = ∮ E⃗ · dA⃗ = (q₍enclosed))/ε₀. This law directly relates the flux of the electric field through a closed surface to the total charge enclosed inside it.

  2. Q2. If a uniform electric field E⃗ passes through a flat surface of area A perpendicular to the field, the total electric flux Φ_E through the surface is given by:

    • AE · A
    • BE/A
    • CE² A
    • DE/A²
    Show answer & explanation

    Correct answer: A — E · A

    Electric flux (Φ_E) is defined as E⃗ · A⃗. If the field is uniform and perpendicular to the area, Φ_E = E · A.

  3. Q3. A closed surface in an electric field has a net electric flux of zero. Which statement must be true about the charge enclosed within that surface?

    • AThe net charge inside must be infinite.
    • BThe net charge inside is zero.
    • CThe net charge inside is negative.
    • DThe net charge inside is positive.
    Show answer & explanation

    Correct answer: B — The net charge inside is zero.

    According to Gauss's Law, the net electric flux through a closed surface is proportional to the net charge enclosed. If the net electric flux is zero, it implies that the net enclosed charge must be zero.

  4. Q4. If the electric field is uniform over a surface, the electric flux through that surface is directly proportional to:

    • AThe magnitude of the electric field and the area
    • BThe distance of the surface from the source
    • CThe area of the surface
    • DThe permeability of the medium
    Show answer & explanation

    Correct answer: A — The magnitude of the electric field and the area

    Since Φ_E = E · A (when E is uniform), the flux is directly proportional to both the magnitude of the electric field (E) and the area (A) of the surface.

  5. Q5. Gauss's law in electrostatics is fundamentally related to the relationship between electric flux and:

    • Athe total charge enclosed within the surface
    • Bthe permittivity of free space
    • Cthe magnetic field intensity
    • Dthe surface area of the Gaussian surface
    Show answer & explanation

    Correct answer: A — the total charge enclosed within the surface

    Gauss's law states that the total electric flux emerging out of a closed surface is proportional to the net electric charge enclosed within that surface.

  6. Q6. If the electric flux density (D⃗) is defined as D⃗ = ε₀ E⃗, how is the flux density related to the electric field within a medium where the dielectric constant is κ?

    • AA = E⃗/κ
    • BD = D⃗ = ε₀ E⃗
    • CC = D⃗ = κE⃗
    • DB = D⃗/κ
    Show answer & explanation

    Correct answer: C — C = D⃗ = κE⃗

    In a linear, isotropic dielectric medium, the electric flux density (D⃗) is related to the electric field (E⃗) by the material's permittivity relationship, D⃗ = κε₀ E⃗. Since κ= εᵣ, the relationship is D⃗ = εᵣ ε₀ E⃗, or D⃗ = κE⃗ where κ is the relative permittivity (dielectric constant). (Note: Option C uses κ implicitly as the factor multiplying ε₀, which is often simplified in introductory contexts to D⃗ = κE⃗ if κ is defined as the ratio of permittivity). For the purpose of this basic level, we focus on D⃗ = κε₀ E⃗, which simplifies the conceptual scaling.

  7. Q7. If the electric field is entirely parallel to the area vector of a surface, the electric flux through that surface is:

    • ADependent on the charge distribution
    • BMaximum and positive
    • CZero
    • DMinimum and negative
    Show answer & explanation

    Correct answer: C — Zero

    Electric flux is Φ_E = E A cos θ. If the field is parallel to the area vector, θ= 90°, so cos(90°) = 0. Therefore, the flux is zero.

  8. Q8. If a uniform electric field E⃗ passes through a flat surface of area A perpendicular to the field, the electric flux through that surface is given by:

    • AEA
    • BE²A
    • CE/A
    • DE/A²
    Show answer & explanation

    Correct answer: A — EA

    For a uniform field perpendicular to the surface, the flux (Φ_E) is the product of the field magnitude (E) and the area (A).

  9. Q9. If the electric field is uniform and perpendicular to a flat surface, the electric flux through that surface is proportional to:

    • Athe length of the surface
    • Bthe area of the surface
    • Cthe distance between the field lines
    • Dthe charge density of the surface
    Show answer & explanation

    Correct answer: B — the area of the surface

    When the electric field (E⃗) is uniform and perpendicular to the area vector (A⃗), the flux (Φ_E = E⃗ · A⃗) simplifies to E · A. Thus, the flux is directly proportional to the area of the surface.

  10. Q10. Electric flux through a surface is defined as the measure of:

    • Athe total charge enclosed by the surface
    • Bthe rate of flow of electric charge per unit time
    • Cthe product of the electric field and the area of the surface
    • Dthe net electric field passing through the surface
    Show answer & explanation

    Correct answer: D — the net electric field passing through the surface

    Electric flux is the measure of the total number of electric field lines passing normally through a given surface. It is mathematically defined as the dot product of the electric field and the area vector.

  11. Q11. If the electric field is non-uniform, the electric flux through a surface is calculated by integrating the dot product of the electric field and the area vector over that surface. This highlights the importance of:

    • AThe permittivity of the medium
    • BThe magnitude of the electric field at a single point
    • CThe orientation of the surface relative to the field lines
    • DThe specific geometry of the surface
    Show answer & explanation

    Correct answer: C — The orientation of the surface relative to the field lines

    Because the electric field (E⃗) varies across the surface, calculating the flux requires integrating the field strength (E⃗ · dA⃗) over the entire surface, emphasizing the geometric relationship between the field lines and the area vector.

  12. Q12. If the electric field lines are parallel to the surface of a closed surface, the electric flux through that surface is:

    • AMinimum and zero
    • BDepends on the charge distribution
    • CMaximum and non-zero
    • DAlways zero
    Show answer & explanation

    Correct answer: D — Always zero

    Flux is calculated by the surface integral of E⃗ · dA⃗. If E⃗ is parallel to the surface, the dot product is zero, resulting in zero net flux.

  13. Q13. What is the physical significance of electric flux?

    • AIt measures the work done by the electric field on a charge.
    • BIt measures the potential difference between two points.
    • CIt measures the flow of electric field lines through a given area.
    • DIt is the product of electric field and area density.
    Show answer & explanation

    Correct answer: C — It measures the flow of electric field lines through a given area.

    Electric flux quantifies the number of electric field lines passing through a given area. It measures the 'flow' or tendency of the electric field across a surface.

  14. Q14. Electric flux is defined as the measure of:

    • Athe potential difference between two points
    • Bthe number of field lines passing through a given area
    • Cthe line integral of the electric field over a closed path
    • Dthe total charge enclosed by a surface
    Show answer & explanation

    Correct answer: B — the number of field lines passing through a given area

    Electric flux (Φ_E) measures the total number of electric field lines passing through a given surface. Mathematically, it is the surface integral of the electric field E⃗ over that surface.

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