68 free MCQs on Electric Charges and Fields with worked answers and explanations. Coulomb's law, electric field, potential, capacitors, Gauss's law.
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Field lines always point from positive to negative charge, are denser where the field is stronger, and never cross; an isolated positive charge has lines radiating symmetrically outward, while a dipole's lines curve from the positive to the negative charge.
Q1.
SI unit of electric charge is:
Answer: B. Coulomb
Why: The SI unit of electric charge is the Coulomb (C). 1 C = 1 A x 1 s.
Q2.
Like charges:
Answer: B. Repel
Why: Like charges (both positive or both negative) repel each other. Unlike charges attract.
Q3.
Charge on an electron is:
Answer: B. -1.6 x 10<sup>-19</sup> C
Why: An electron carries charge -e = -1.6 x 10<sup>-19</sup> C. Proton has charge +1.6 x 10<sup>-19</sup> C.
Q4.
Coulomb's law: force between charges q<sub>1</sub> and q<sub>2</sub> at distance r:
Answer: B. F = kq<sub>1</sub>q<sub>2</sub>/r<sup>2</sup>
Why: F = kq<sub>1</sub>q<sub>2</sub>/r<sup>2</sup> where k = 9 x 10<sup>9</sup> N m<sup>2</sup>/C<sup>2.</sup> Force is proportional to product of charges and inversely to r<sup>2.</sup>
Q5.
Electric field lines originate from:
Answer: B. Positive charges
Why: Electric field lines begin at positive charges and end at negative charges.
Q6.
Electric field due to a point charge Q at distance r:
Answer: B. E = kQ/r<sup>2</sup>
Why: E = kQ/r<sup>2.</sup> Direction: away from positive charge, toward negative charge.
Q7.
Inside a conductor in electrostatic equilibrium, electric field is:
Answer: C. Zero
Why: Inside a conductor in electrostatic equilibrium, free charges redistribute until E = 0 everywhere inside.
Q8.
SI unit of electric potential is:
Answer: C. Volt
Why: Electric potential is measured in Volts (V). 1 V = 1 J/C.
Q9.
Equipotential surfaces are always _____ to electric field lines:
Answer: B. Perpendicular
Why: Equipotential surfaces are always perpendicular to electric field lines. No work is done moving a charge along an equipotential.
Q10.
Electric potential due to a point charge decreases as:
Answer: B. 1/r
Why: V = kQ/r. Electric potential decreases as 1/r (not 1/r<sup>2</sup> like the field does).
Q11.
A hollow conductor placed in an external field: the interior has:
Answer: C. Zero field
Why: Faraday cage effect: the inside of a hollow conductor is shielded from external electric fields. E = 0 inside.
Q12.
Two like electric charges placed near each other:
Answer: A. repel each other
Why: Like charges (both positive or both negative) repel.
Q13.
Two unlike electric charges placed near each other:
Answer: A. attract each other
Why: A positive and a negative charge attract each other.
Q14.
The SI unit of electric charge is the:
Answer: A. the coulomb
Why: Charge is measured in coulombs (C).
Q15.
The charge carried by an electron is:
Answer: A. negative
Why: The electron carries a negative elementary charge.
Q16.
The charge carried by a proton is:
Answer: A. positive
Why: The proton carries a positive elementary charge.
Q17.
Electric charge is quantised, meaning it can only be:
Answer: A. an integer multiple of e
Why: Charge occurs in whole-number multiples of the elementary charge e.
Q18.
An object carrying equal positive and negative charge is electrically:
Answer: A. neutral
Why: Equal opposite charges cancel, leaving the object neutral.
Q19.
The region around a charge in which it exerts a force is called the:
Answer: A. electric field
Why: The electric field is the region where a charge experiences an electric force.
Q20.
Electric field lines begin on ___ charges:
Answer: A. positive
Why: Field lines start on positive charges and end on negative charges.
Q21.
Two charges of +4 C and -1 C are 3 m apart. Force between them (k = 9 x 10<sup>9</sup>):
Answer: A. 4 x 10<sup>9</sup> N attractive
Why: F = kq<sub>1</sub>q<sub>2</sub>/r<sup>2</sup> = 9 x 10<sup>9</sup> x 4 x 1 / 9 = 4 x 10<sup>9</sup> N. Negative product means attractive.
Q22.
Electric field between two parallel plates (surface charge density sigma, area A):
Answer: B. sigma / epsilon<sub>0</sub>
Why: For infinite parallel plates: E = sigma/epsilon<sub>0</sub>. Each plate gives sigma/(2epsilon<sub>0</sub>); combined = sigma/epsilon<sub>0</sub>.
Q23.
A charge +Q is inside a closed surface. Total outward flux:
Answer: A. Q/epsilon<sub>0</sub>
Why: Gauss's law: flux = Q<sub>enclosed</sub>/epsilon<sub>0</sub>, regardless of the shape or size of the closed surface.
Q24.
Electric field at surface of a charged conductor with surface charge density sigma:
Answer: A. sigma / epsilon<sub>0</sub>
Why: E = sigma/epsilon<sub>0</sub> just outside a conductor surface, perpendicular to the surface.
Q25.
Torque on an electric dipole of moment p in uniform field E:
Answer: B. pE sin(theta)
Why: Torque tau = pE sin(theta), where theta is the angle between dipole moment and field direction.
Q26.
Coulomb’s law says the force between two charges is inversely proportional to the ___ of the distance between them:
Answer: A. square
Why: F ∝ 1/r², the inverse square of the separation.
Q27.
The electric field due to a point charge q at a distance r is:
Answer: A. kq/r²
Why: For a point charge, E = kq/r².
Q28.
The SI unit of electric field strength is:
Answer: A. N/C
Why: Electric field is force per unit charge, so its unit is newton per coulomb.
Q29.
The direction of the electric field is the direction of the force on a ___ test charge:
Answer: A. positive
Why: By convention, the field direction is the force on a small positive test charge.
Q30.
Two electric field lines can never:
Answer: A. intersect
Why: Field lines cannot cross, since the field has a unique direction at each point.
Q31.
The relation between the flux through a closed surface and the charge it encloses is given by ___ law:
Answer: A. Gauss's
Why: Gauss's law relates net electric flux to the enclosed charge.
Q32.
An electric dipole consists of two equal and opposite charges separated by a small:
Answer: A. distance
Why: A dipole is a pair of equal opposite charges a short distance apart.
Q33.
The electric dipole moment points from the ___ charge to the ___ charge:
Answer: A. negative to the positive
Why: By convention the dipole moment points from the negative to the positive charge.
Q34.
The electric field inside a charged hollow conductor is:
Answer: A. zero
Why: The field inside a charged hollow conductor is zero (electrostatic shielding).
Q35.
If both charges are doubled, the Coulomb force between them becomes ___ times the original:
Answer: A. four
Why: F ∝ q₁q₂, so doubling both multiplies the force by 4.
Q36.
If the distance between two charges is doubled, the force becomes ___ of the original:
Answer: A. one quarter
Why: F ∝ 1/r², so doubling r reduces the force to one quarter.
Q37.
Electric flux is a measure of the number of field lines passing through a:
Answer: A. surface
Why: Flux counts the field lines threading a given surface.
Q38.
The SI unit of electric flux is:
Answer: A. N·m²/C
Why: Electric flux has units of N·m²/C (equivalently V·m).
Q39.
The electric field inside a conductor in electrostatic equilibrium is:
Answer: A. zero
Why: In equilibrium, charges arrange so that the field inside the conductor is zero.
Q40.
A pair of +2 μC and −2 μC charges a small distance apart forms an electric:
Answer: A. dipole
Why: Two equal opposite charges close together form a dipole.
Q41.
A uniformly charged thin shell of radius R and charge Q. Electric field at r < R:
Answer: C. Zero
Why: Shell theorem: the electric field inside a uniformly charged spherical shell is zero. All charge is on the surface.
Q42.
Work done to bring charge q from infinity to surface of charged sphere of radius R and charge Q:
Answer: A. kqQ/R
Why: W = q x V<sub>surface</sub> = q x kQ/R = kqQ/R. This is positive if charges are same sign (work done against repulsion).
Q43.
Five equal charges Q are placed at 5 corners of a regular hexagon. Electric field at center due to 5 charges:
Answer: B. kQ/a<sup>2</sup>
Why: 6 equal charges would give zero field at center. 5 charges = 6 - 1. Field = field of the missing 6th charge = kQ/a<sup>2</sup> pointing toward the empty corner.
Q44.
Charge Q is distributed uniformly on a sphere of radius R. At distance r > R, electric field:
Answer: A. kQ/r<sup>2</sup> (same as point charge)
Why: Outside a uniformly charged sphere, field = kQ/r<sup>2</sup> (same as if all charge were at center: shell theorem).
Q45.
Electric field inside a uniformly charged solid sphere at distance r from center (R = radius, Q = total charge):
Answer: B. kQr/R<sup>3</sup>
Why: Inside solid sphere: only the charge within radius r contributes. Q<sub>enclosed</sub> = Q(r/R)<sup>3.</sup> E = kQ_enclosed/r<sup>2</sup> = kQr/R<sup>3.</sup>
Q46.
The force between two 1 C charges placed 1 m apart is approximately:
Answer: A. 9 × 10⁹ N
Why: F = kq₁q₂/r² = 9 × 10⁹ × 1 × 1 / 1² = 9 × 10⁹ N.
Q47.
By Gauss’s law, the total electric flux through a closed surface enclosing a charge Q is:
Answer: A. Q/ε₀
Why: The net flux equals the enclosed charge divided by ε₀.
Q48.
The electric field due to an infinitely long straight charged wire varies with distance r as:
Answer: A. 1/r
Why: For a line charge, E = λ/2πε₀r, so E ∝ 1/r.
Q49.
The electric field due to an infinite charged plane sheet is:
Answer: A. independent of distance
Why: For an infinite sheet, E = σ/2ε₀, the same at all distances.
Q50.
The torque on an electric dipole of moment p placed in a uniform field E is:
Answer: A. pE sinθ
Why: τ = p × E = pE sinθ.
Q51.
The potential energy of a dipole in a field is minimum when the dipole is aligned ___ the field:
Answer: A. parallel to
Why: U = −pE cosθ is least (most negative) when θ = 0°, i.e. parallel alignment.
Q52.
On the axis of a short electric dipole, the field falls off with distance as:
Answer: A. 1/r³
Why: The axial field of a dipole varies as 1/r³.
Q53.
Two identical charges q are 2r apart. The electric field exactly at the midpoint is:
Answer: A. zero
Why: The two equal fields point in opposite directions at the midpoint and cancel, giving zero.
Q54.
The number of electrons that make up a charge of 1 coulomb is about:
Answer: A. 6.25 × 10¹⁸
Why: n = 1 / (1.6 × 10⁻¹⁹) ≈ 6.25 × 10¹⁸ electrons.
Q55.
A charge Q sits at the centre of a cube. The electric flux through one face is:
Answer: A. Q/6ε₀
Why: The total flux Q/ε₀ is shared equally among six faces, giving Q/6ε₀ per face.
Q56.
Electric field is a ___ quantity:
Answer: A. vector
Why: Electric field has both magnitude and direction, so it is a vector.
Q57.
Coulomb’s law has the same inverse-square form as ___ law of gravitation:
Answer: A. Newton's
Why: Both Coulomb's law and Newton's law of gravitation follow an inverse-square dependence on distance.
Q58.
When a dielectric medium is placed between two charges, the force between them:
Answer: A. decreases
Why: The dielectric reduces the force by the factor of its dielectric constant K.
Q59.
The work done in rotating a dipole from parallel to perpendicular to a field E is:
Answer: A. pE
Why: W = U<sub>perp</sub> − U<sub>parallel</sub> = 0 − (−pE) = pE.
Q60.
The net electric flux through a closed surface that encloses no net charge is:
Answer: A. zero
Why: With no enclosed charge, Gauss’s law gives a net flux of zero.
Q61.
Two charges attract with force F in vacuum. When the same charges are placed in a medium of dielectric constant 2 at the same separation, the force becomes:
Answer: A. half
Why: Force in a medium = F/K = F/2, so it becomes half.
Q62.
For a short dipole, the ratio of the electric field on its axis to the field on its equator at the same distance is:
Answer: C. 2:1
Why: Axial field = 2kp/r³ and equatorial field = kp/r³, so the ratio is 2:1.
Q63.
A point charge q is placed at the centre of a cube. The electric flux through one face of the cube is:
Answer: B. q/6ε₀
Why: Total flux q/ε₀ divides equally among 6 faces, giving q/6ε₀ per face.
Q64.
The electric field inside a uniformly charged solid conducting sphere (in electrostatic equilibrium) is:
Answer: A. zero
Why: In a conductor at equilibrium the field inside is zero; all excess charge resides on the surface.
Q65.
Two equal positive charges +q are separated by a distance d. The electric field at the midpoint of the line joining them is:
Answer: A. zero
Why: The two fields at the midpoint are equal and opposite, so they cancel and the net field is zero.
Q66.
An electric dipole in a uniform electric field experiences maximum torque when the angle between the dipole moment and the field is:
Answer: C. 90°
Why: Torque = pE sinθ is maximum at θ = 90°.
Q67.
An electron is placed in a uniform electric field of 1000 N/C. Its acceleration (e = 1.6×10⁻¹⁹ C, m = 9.1×10⁻³¹ kg) is about:
Answer: B. 1.76×10¹⁴ m/s²
Why: a = eE/m = 1.6×10⁻¹⁹·1000/9.1×10⁻³¹ ≈ 1.76×10¹⁴ m/s².
Q68.
A uniform electric field of 2×10³ N/C passes perpendicularly through a flat surface of area 0.5 m². The electric flux is:
Answer: C. 1000 N·m²/C
Why: Φ = EA cos0° = 2×10³·0.5 = 1000 N·m²/C.