Mock Test – JEE Main – Physics- Electrostatics

Q1.

Two point charges of 3 μC and 6 μC are 2 m apart. The force between them is:

A) 9 N
B) 4.5 N
C) 27 N
D) 12 N

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Q2.

Coulomb’s law constant $k = 9 \times 10^9 \, \text{N·m²/C²}$k=9×109N\cdotpm²/C² appears in formula:

A) $F = \frac{k q_1 q_2}{r^2}$F=r2kq1​q2​​
B) $E = k q r^2$E=kqr2
C) $V = \frac{q}{4\pi \epsilon_0 r^2}$V=4πϵ0​r2q​
D) $F = k q_1 q_2 r^2$F=kq1​q2​r2

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Q3.

Electric field at the midpoint of two equal charges +Q separated by distance 2a:

A) Zero
B) Along the line joining charges
C) Perpendicular to the line joining charges
D) Cannot be determined

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Q4.

Electric dipole moment is defined as:

A) $qd$qd
B) $\frac{q}{d}$dq​
C) $\frac{d}{q}$qd​
D) $q^2 d$q2d

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Q5.

Potential energy of a system of two charges q1 and q2 separated by distance r:

A) $\frac{k q_1 q_2}{r}$rkq1​q2​​
B) $k q_1 q_2 r$kq1​q2​r
C) $\frac{k q_1 q_2}{2r}$2rkq1​q2​​
D) $k q_1 q_2 r^2$kq1​q2​r2

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Q6.

Electric field at the center of a uniformly charged ring of radius R and total charge Q:

A) $\frac{kQ}{R^2}$R2kQ​
B) Zero
C) $\frac{kQ}{2R^2}$2R2kQ​
D) Infinite

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Q7.

Force on a small charge q placed at a distance r from an infinite uniformly charged plane (charge density σ):

A) $\frac{\sigma}{2 \epsilon_0}$2ϵ0​σ​
B) $\frac{\sigma q}{2 \epsilon_0}$2ϵ0​σq​
C) $\frac{\sigma q}{\epsilon_0 r^2}$ϵ0​r2σq​
D) Zero

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Q8.

Gauss’s law relates:

A) Electric flux to charge enclosed
B) Electric field to potential
C) Potential energy to charge
D) Electric flux to energy

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Q9.

Two charges +q and –q separated by distance 2a: net electric field at midpoint is:

A) Zero
B) $\frac{kq}{a^2}$a2kq​ along dipole axis
C) $\frac{kq}{2a^2}$2a2kq​ perpendicular
D) Infinite

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Q10.

Potential at a point on the axis of a dipole at distance r from center (r >> a):

A) $\frac{k p}{r^2}$r2kp​
B) $\frac{k p}{r^3}$r3kp​
C) $\frac{k q}{r}$rkq​
D) $k p r^2$kpr2

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Q11.

Work done in moving a charge q from point A to B in an electric field depends on:

A) Only the points A and B
B) Path taken
C) Velocity of charge
D) Acceleration of charge

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Q12.

Capacitance of a parallel plate capacitor depends on:

A) Area of plates and separation
B) Charge and voltage
C) Dielectric only
D) Temperature

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Q13.

Energy stored in a capacitor:

A) $\frac{1}{2} C V^2$21​CV2
B) $C V^2$CV2
C) $\frac{1}{2} V^2/C$21​V2/C
D) $CV$CV

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Q14.

Two capacitors C1 and C2 in series. Equivalent capacitance:

A) $C_1 + C_2$C1​+C2​
B) $\frac{C_1 C_2}{C_1 + C_2}$C1​+C2​C1​C2​​
C) $C_1 C_2$C1​C2​
D) $\sqrt{C_1 C_2}$C1​C2​​

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Q15.

Force between two point charges in vacuum is 10 N. If medium with dielectric constant 5 is inserted:

A) 2 N
B) 50 N
C) 10 N
D) 5 N

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Q16.

Electric field inside a conductor in electrostatic equilibrium:

A) Maximum at surface
B) Zero
C) Same everywhere
D) Depends on shape

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Q17.

Two point charges of opposite sign are separated by distance r. Potential at midpoint:

A) Zero
B) Maximum
C) Minimum
D) Infinite

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Q18.

Electric field due to an infinite uniformly charged plane:

A) Varies with distance
B) Constant, independent of distance
C) Zero
D) Infinite

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Q19.

Dipole placed in uniform electric field experiences:

A) Net force only
B) Net torque only
C) Both force and torque
D) Neither

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Q20.

Potential at a point due to a point charge q:

A) $\frac{kq}{r^2}$r2kq​
B) $\frac{kq}{r}$rkq​
C) $kqr$kqr
D) $kq r^2$kqr2

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Q21.

Unit of electric flux in SI:

A) N·m²/C
B) N/C
C) C/N·m²
D) V·m

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Q22.

If distance between two charges doubles, force between them:

A) Doubles
B) Halves
C) Quarter
D) Quadruples

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Q23.

For a parallel plate capacitor in vacuum, inserting a dielectric:

A) Increases capacitance
B) Decreases capacitance
C) Leaves capacitance same
D) Depends on voltage

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Q24.

Electrostatic potential energy of a system of two charges q1 and q2 separated by r:

A) $\frac{k q_1 q_2}{r}$rkq1​q2​​
B) $k q_1 q_2 r$kq1​q2​r
C) $k q_1 q_2 r^2$kq1​q2​r2
D) $\frac{k q_1 q_2}{2r^2}$2r2kq1​q2​​

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Q25.

Work done in moving a charge along an equipotential surface:

A) Maximum
B) Zero
C) Depends on path
D) Depends on charge

Answer

Question No.	Answer
1	B
2	A
3	A
4	A
5	A
6	B
7	B
8	A
9	B
10	B
11	A
12	A
13	A
14	B
15	A
16	B
17	A
18	B
19	B
20	B
21	A
22	C
23	A
24	A
25	B

Solution

ELECTROSTATICS – DETAILED SOLUTIONS

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Q1. Force between two charges

$$F = k \frac{q_1 q_2}{r^2} = 9 \times 10^9 \frac{3 \times 10^{-6} \cdot 6 \times 10^{-6}}{2^2} = 4.5\,\text{N}$$F=kr2q1​q2​​=9×109223×10−6⋅6×10−6​=4.5N

Answer: B

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Q2. Coulomb’s law constant

Formula: $F = k \frac{q_1 q_2}{r^2}$F=kr2q1​q2​​

Answer: A

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Q3. Electric field at midpoint of two equal charges +Q

• Fields due to each charge point away from charge
• At midpoint, they cancel → net field = 0

Answer: A

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Q4. Electric dipole moment

$$p = q \cdot d$$p=q⋅d

Answer: A

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Q5. Potential energy of two charges

$$U = k \frac{q_1 q_2}{r}$$U=krq1​q2​​

Answer: A

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Q6. Electric field at center of a uniformly charged ring

• Symmetry → all components cancel at the center → E = 0

Answer: B

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Q7. Force on charge near infinite plane

$$E = \frac{\sigma}{2 \epsilon_0} \implies F = q E = \frac{\sigma q}{2 \epsilon_0}$$E=2ϵ0​σ​⟹F=qE=2ϵ0​σq​

Answer: B

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Q8. Gauss’s law

$$\Phi_E = \frac{Q_\text{enclosed}}{\epsilon_0}$$ΦE​=ϵ0​Qenclosed​​

Answer: A

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Q9. Electric field at midpoint of dipole (+q, –q)

• Equal magnitude but opposite direction along dipole axis → net field: $E = \frac{kq}{a^2}$E=a2kq​ along axis

Answer: B

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Q10. Potential on axis of dipole at distance r (r >> a)

$$V = \frac{k p}{r^2} \quad \text{(approximate, far from dipole)}$$V=r2kp​(approximate, far from dipole)

Answer: B

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Q11. Work done moving a charge in an electric field

• Electric field is conservative → depends only on start and end points

Answer: A

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Q12. Capacitance of parallel plate capacitor

$$C = \frac{\epsilon_0 A}{d}$$C=dϵ0​A​

Depends on area A and separation d

Answer: A

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Q13. Energy stored in a capacitor

$$U = \frac{1}{2} C V^2$$U=21​CV2

Answer: A

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Q14. Capacitors in series

$$\frac{1}{C_\text{eq}} = \frac{1}{C_1} + \frac{1}{C_2} \implies C_\text{eq} = \frac{C_1 C_2}{C_1 + C_2}$$Ceq​1​=C1​1​+C2​1​⟹Ceq​=C1​+C2​C1​C2​​

Answer: B

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Q15. Force with dielectric medium

• Force decreases by factor of dielectric constant $K$K:

$$F_\text{new} = \frac{F}{K} = \frac{10}{5} = 2\,\text{N}$$Fnew​=KF​=510​=2N

Answer: A

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Q16. Electric field inside a conductor

• At electrostatic equilibrium, E = 0

Answer: B

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Q17. Potential at midpoint between opposite charges

• Potentials of +q and –q cancel → V = 0

Answer: A

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Q18. Electric field due to infinite plane

• Constant everywhere, independent of distance:

$$E = \frac{\sigma}{2 \epsilon_0}$$E=2ϵ0​σ​

Answer: B

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Q19. Dipole in uniform electric field

• Experiences torque but no net force

Answer: B

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Q20. Potential due to point charge

$$V = k \frac{q}{r}$$V=krq​

Answer: B

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Q21. Unit of electric flux

$$\Phi_E = E \cdot A \implies \text{N·m²/C}$$ΦE​=E⋅A⟹N\cdotpm²/C

Answer: A

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Q22. Force vs distance

• Coulomb’s law: $F \propto \frac{1}{r^2}$F∝r21​
• Distance doubles → $F \to F/4$F→F/4

Answer: C

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Q23. Dielectric in parallel plate capacitor

• Capacitance increases by factor of dielectric constant

Answer: A

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Q24. Electrostatic potential energy

$$U = k \frac{q_1 q_2}{r}$$U=krq1​q2​​

Answer: A

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Q25. Work along equipotential surface

• No change in potential → work done = 0

Answer: B

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