Electric Charges and Fields — Class 12 MCQs with Answers

Electric Charges and Fields — important questions & MCQs with answers (Class 12 Physics)

14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 76 are timed and scored when you sign in.

1. Q1Easy

   The SI unit of electric charge is:

   A.Ampere

   B.Tesla

   C.Volt

   D.Coulomb✓ Correct

   Answer: D. Coulomb

   Explanation: SI charge unit traces to current: 1 coulomb = 1 ampere flowing for 1 second (C = A·s). Ampere and volt measure current and potential, not charge — easy to confuse with those base units.

2. Q2Easy

   The SI unit of electric field is:

   A.C/m

   B.C·m

   C.N/C (or V/m)✓ Correct

   D.J/C

   Answer: C. N/C (or V/m)

   Explanation: Field is force per charge, E = F/q, giving units N/C — algebraically identical to V/m since work per charge over distance gives the same combination. Don't mistake it for C/m, a charge density, not a field.

3. Q3Easy

   The SI unit of electric flux is:

   A.V/m

   B.N/C

   C.C/m²

   D.N·m²/C✓ Correct

   Answer: D. N·m²/C

   Explanation: Flux Φ = E·A combines field (N/C) with area (m²), giving N·m²/C (equivalently V·m) as the unit — distinct from N/C, which is just the field itself with no area factored in.

4. Q4Easy

   The charge on an electron is approximately:

   A.−1.6 × 10⁻¹⁹ C✓ Correct

   B.+1.6 × 10⁻¹⁹ C

   C.9.1 × 10⁻³¹ C

   D.−9.1 × 10⁻³¹ C

   Answer: A. −1.6 × 10⁻¹⁹ C

   Explanation: The electron's charge magnitude is the elementary charge e = 1.6×10⁻¹⁹ C, negative since it's the fundamental negative carrier. The 9.1×10⁻³¹ figure is the electron's mass, not its charge — a common mix-up.

5. Q5Easy

   Electric field lines:

   A.Start on +ve and end on −ve charges✓ Correct

   B.Form closed loops

   C.Can intersect each other

   D.Are always straight

   Answer: A. Start on +ve and end on −ve charges

   Explanation: By convention field lines start on positive charges and terminate on negative ones, and two field lines can never cross — a crossing would mean two different field directions at one point, impossible.

6. Q6Easy

   Inside a conductor in electrostatic equilibrium, the electric field is:

   A.Maximum

   B.Equal to surface field

   C.Zero✓ Correct

   D.Infinite

   Answer: C. Zero

   Explanation: In electrostatic equilibrium, free charges inside a conductor redistribute onto the surface until the internal field cancels exactly, leaving E = 0 everywhere inside, even though the surface field can be large.

7. Q7Medium

   Two charges +q and +q are placed r apart. If the distance is doubled, the force between them becomes:

   A.F/4✓ Correct

   B.F/2

   C.2F

   D.4F

   Answer: A. F/4

   Explanation: Coulomb's law gives F ∝ 1/r², so doubling the separation cuts the force to (1/2)² = 1/4 of its original value — a linear 'half the force' guess is the common error; the law is inverse-square, not inverse-linear.

8. Q8Medium

   Electric field due to a point charge q at distance r is:

   A.kq/r

   B.kq/r²✓ Correct

   C.kq²/r²

   D.kq·r

   Answer: B. kq/r²

   Explanation: Field of a point charge is E = kq/r² = q/(4πε₀r²), inverse-square just like the force it would exert on a unit test charge — don't drop the r² and use kq/r, which describes potential, not field.

9. Q9Medium

   Gauss's law states that electric flux through a closed surface equals:

   A.q × ε₀

   B.ε₀ · A

   C.q · A

   D.q / ε₀✓ Correct

   Answer: D. q / ε₀

   Explanation: Gauss's law: Φ = q_enclosed/ε₀ — total flux through any closed surface depends only on enclosed charge, and this is what lets symmetric charge distributions be solved for E without integrating Coulomb's law directly.

10. Q10Medium

    Force between two charges of 1 μC each separated by 1 m in vacuum is:

    A.9 × 10⁹ N

    B.9 × 10³ N

    C.9 × 10⁻³ N✓ Correct

    D.9 N

    Answer: C. 9 × 10⁻³ N

    Explanation: F = kq₁q₂/r² = (9×10⁹)(10⁻⁶)²/1² = 9×10⁹ × 10⁻¹² = 9×10⁻³ N — keep track of the (10⁻⁶)² = 10⁻¹² from squaring both microcoulomb charges, the step most often dropped.

11. Q11Medium

    Three equal +q charges at vertices of equilateral triangle. Field at centroid:

    A.Zero✓ Correct

    B.kq/r²

    C.3kq/r²

    D.kq/(3r²)

    Answer: A. Zero

    Explanation: Three equal charges at the vertices of an equilateral triangle produce three field vectors of equal magnitude at 120° to each other at the centroid, and vectors spaced 120° apart always sum to zero.

12. Q12Hard

    Four equal charges +q at corners of a square of side a. Net force on any one charge magnitude:

    A.kq²/a² (1 + 1/√2)

    B.kq²/a² × (2√2 + 1)/2✓ Correct

    C.kq²/a² (2 + 1/√2)

    D.Zero

    Answer: B. kq²/a² × (2√2 + 1)/2

    Explanation: Two side forces (kq²/a² each, perpendicular) combine to √2·kq²/a² along the diagonal, plus the diagonal force kq²/(2a²) in the same direction, giving a total of kq²/a² × (2√2+1)/2 — not zero by naive symmetry.

13. Q13Hard

    Electric field on the axis of an electric dipole at distance r (r ≫ a) is:

    A.2kp / r³✓ Correct

    B.kp / r³

    C.kp / r²

    D.kp / r

    Answer: A. 2kp / r³

    Explanation: The axial field of a short dipole comes from adding the fields of +q and −q along the axis and expanding for r ≫ a: the result is E = 2kp/r³ — twice the equatorial value at the same distance.

14. Q14Hard

    Electric field due to an infinite plane sheet of uniform surface charge density σ is:

    A.σ / ε₀

    B.Zero

    C.σ · ε₀

    D.σ / (2ε₀)✓ Correct

    Answer: D. σ / (2ε₀)

    Explanation: An infinite sheet gives E = σ/(2ε₀) from a Gauss pillbox straddling it — half the σ/ε₀ you'd get just outside a charged conductor, because a lone sheet radiates flux out both faces instead of only one.

Electric Charges and Fields — FAQs

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• 02Electrostatic Potential and Capacitance
• 03Current Electricity
• 04Moving Charges and Magnetism
• 05Magnetism and Matter
• 06Electromagnetic Induction
• 07Alternating Current

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