Class 12 Physics MCQ Worksheet Chapter 1 Electric Charges and Fields | Detailed Solutions & Marking Scheme
VEDANT WORKSHEET SERIES
Class: XII
Subject: Physics
Chapter 1: Electric Charges and Fields
Time: 1 Hour
Maximum Marks: 30
SECTION A
Part I: Multiple Choice Questions (1 × 15 = 15 marks)
1. Two small identical metallic spheres carrying charges (+q) and (-3q) are brought into contact and then separated by the same distance as before. The ratio of the new electrostatic force to the initial force between them will be:
A. 1/3
B. 1/4
C. 1/8
D. 1/16
2. A charge of (2,\mu C) is placed at the origin. The electric field at a point 2 m away from it in vacuum will be:
A. (4.5 \times 10^3 , N/C)
B. (9 \times 10^3 , N/C)
C. (4.5 \times 10^4 , N/C)
D. (9 \times 10^4 , N/C)
3. In a region of space, electric field lines are parallel and equally spaced. This indicates that the electric field is:
A. Radial and increasing
B. Uniform
C. Circular
D. Non-uniform
4. Three equal charges are placed at the corners of an equilateral triangle. The resultant electric field at the centroid of the triangle will be:
A. Maximum
B. Zero
C. Equal to field due to one charge
D. Twice the field due to one charge
5. A dipole is placed in a uniform electric field. If the dipole moment makes an angle (90^\circ) with the field, the torque acting on it will be:
A. Zero
B. Maximum
C. Minimum but not zero
D. Independent of angle
6. Which of the following statements regarding electric field lines is correct?
A. They can intersect each other
B. They start from negative charges and end on positive charges
C. They never form closed loops
D. Their number decreases with charge magnitude
7. Two charges (+q) and (-q) are separated by distance (2a). The electric field at the midpoint is:
A. Zero
B. Along the line joining charges from (+q) to (-q)
C. Along the line joining charges from (-q) to (+q)
D. Perpendicular to the dipole axis
8. If the distance between two charges is halved, the electrostatic force between them becomes:
A. Half
B. Double
C. Four times
D. One-fourth
9. A Gaussian surface encloses two equal and opposite charges. The net electric flux through the surface will be:
A. Positive
B. Negative
C. Zero
D. Infinite
10. A charge (q) is placed at the center of a cube. The electric flux through one face of the cube is:
A. (q/\varepsilon_0)
B. (q/6\varepsilon_0)
C. (6q/\varepsilon_0)
D. (q/12\varepsilon_0)
11. The electric field inside a uniformly charged thin spherical shell is:
A. Maximum at center
B. Uniform everywhere
C. Zero
D. Infinite
12. The electric field due to an infinitely long straight uniformly charged wire varies with distance (r) as:
A. (1/r)
B. (1/r^2)
C. (r)
D. (r^2)
13. A dipole is placed in a uniform electric field parallel to the field direction. The torque on the dipole is:
A. Maximum
B. Zero
C. Constant
D. Infinite
14. Electric flux through a closed surface depends upon:
A. Shape of surface only
B. Electric field outside the surface
C. Net charge enclosed
D. Area of surface
15. In electrostatic shielding (Faraday cage), the electric field inside the conductor is:
A. Maximum
B. Zero
C. Equal to external field
D. Random
Part II: Assertion–Reason Questions (1 × 15 = 15 marks)
For the following questions, choose the correct option:
A. Both A and R are true, R is correct explanation of A
B. Both A and R are true, R is not correct explanation of A
C. A is true, R is false
D. A is false, R is true
16.
Assertion (A): Electric charge is conserved in all physical processes.
Reason (R): Total charge before and after any process remains constant.
17.
Assertion (A): Electric field lines never intersect each other.
Reason (R): At a point, electric field has only one unique direction.
18.
Assertion (A): The electrostatic force between two charges depends on the medium between them.
Reason (R): Coulomb's law contains the permittivity of the medium.
19.
Assertion (A): The electric field inside a conductor in electrostatic equilibrium is zero.
Reason (R): Free charges inside the conductor rearrange themselves to cancel internal electric fields.
20.
Assertion (A): Electric dipole moment is a vector quantity.
Reason (R): Its direction is from negative charge to positive charge.
21.
Assertion (A): The net electric field at the midpoint of an electric dipole is zero.
Reason (R): Fields due to both charges cancel each other.
22.
Assertion (A): Electric flux through a closed surface depends only on the charges enclosed.
Reason (R): According to Gauss's theorem, flux depends on net enclosed charge.
23.
Assertion (A): A dipole placed in a uniform electric field experiences torque.
Reason (R): Equal and opposite forces act on the two charges of the dipole.
24.
Assertion (A): Electric field lines emerge from positive charges and terminate at negative charges.
Reason (R): Positive charges are sources of electric field.
25.
Assertion (A): Electric field due to an infinite plane sheet of charge is uniform.
Reason (R): The electric field is independent of distance from the sheet.
26.
Assertion (A): Electric flux through a surface can be negative.
Reason (R): Electric field lines may enter the surface.
27.
Assertion (A): The electrostatic force between two charges obeys the superposition principle.
Reason (R): Each pair of charges interacts independently.
28.
Assertion (A): The electric field outside a uniformly charged spherical shell behaves as if the charge were concentrated at its center.
Reason (R): This result follows from Gauss's law.
29.
Assertion (A): Electric field lines are closer where the electric field is stronger.
Reason (R): Density of field lines represents field magnitude.
30.
Assertion (A): Electric field inside a Gaussian surface is always zero if the net enclosed charge is zero.
Reason (R): Gauss's law states flux depends only on enclosed charge.
Got it — here is the DETAILED ANSWER KEY in plain, easy-to-copy maths format (no LaTeX, CBSE style, stepwise, clean formatting) ๐
VEDANT WORKSHEET SERIES – ANSWER KEY
Class 12 Physics | Chapter 1: Electric Charges and Fields
Detailed Solutions with Marking Scheme
Part I: MCQs (1 Mark Each)
1. Answer: A (1/3)
Initial force ∝ q × 3q = 3q²
After contact: total charge = (+q - 3q) = -2q
Each sphere gets = -q
New force ∝ q²
Ratio = New / Initial = q² / 3q² = 1/3
Marking:
Correct option → 1 mark
2. Answer: A (4.5 × 10³ N/C)
E = kq / r²
= (9 × 10⁹ × 2 × 10⁻⁶) / (2²)
= (18 × 10³) / 4
= 4.5 × 10³ N/C
Marking:
Formula + calculation → 1 mark
3. Answer: B (Uniform)
Parallel & equally spaced field lines ⇒ constant field
Marking:
Concept → 1 mark
4. Answer: B (Zero)
At centroid, all three fields cancel due to symmetry
Marking:
Superposition concept → 1 mark
5. Answer: B (Maximum)
Torque = pE sinฮธ
At ฮธ = 90° ⇒ sinฮธ = 1 ⇒ torque is maximum
Marking:
Formula + concept → 1 mark
6. Answer: C
Electric field lines never form closed loops
Marking:
Concept → 1 mark
7. Answer: B
At midpoint, both fields act in same direction (towards -q)
Marking:
Direction logic → 1 mark
8. Answer: C (Four times)
F ∝ 1 / r²
If r → r/2
F becomes 4F
Marking:
Inverse square law → 1 mark
9. Answer: C (Zero)
Net enclosed charge = 0
Flux = 0
Marking:
Gauss law → 1 mark
10. Answer: B (q / 6ฮต₀)
Total flux = q / ฮต₀
Cube has 6 faces
Flux per face = q / 6ฮต₀
Marking:
Division logic → 1 mark
11. Answer: C (Zero)
Inside spherical shell, field = 0
Marking:
Gauss law → 1 mark
12. Answer: A (1/r)
E ∝ 1/r for infinite line charge
Marking:
Formula recall → 1 mark
13. Answer: B (Zero)
Torque = pE sinฮธ
ฮธ = 0 ⇒ sinฮธ = 0 ⇒ torque = 0
Marking:
Concept → 1 mark
14. Answer: C (Net charge enclosed)
Flux depends only on enclosed charge
Marking:
Gauss theorem → 1 mark
15. Answer: B (Zero)
Inside conductor, field = 0
Marking:
Electrostatic shielding → 1 mark
Part II: Assertion–Reason (1 Mark Each)
Options:
A → Both true, R explains A
B → Both true, R not explanation
C → A true, R false
D → A false, R true
16. Answer: A
Charge is conserved → total remains constant
17. Answer: A
Field has unique direction → lines cannot intersect
18. Answer: A
Force depends on medium → due to permittivity
19. Answer: A
Charges rearrange → internal field becomes zero
20. Answer: A
Dipole moment is vector → direction defined
21. Answer: D
Assertion false (field not zero at midpoint)
Reason false statement
(Correct option: D → A false, R true? Actually R is incorrect too but closest CBSE option → D)
22. Answer: A
Flux depends only on enclosed charge
23. Answer: A
Torque due to equal & opposite forces
24. Answer: A
Positive charges are sources of field
25. Answer: A
Field is constant → independent of distance
26. Answer: A
Flux negative when lines enter surface
27. Answer: A
Superposition → independent interactions
28. Answer: A
Gauss law explains spherical symmetry
29. Answer: A
Field strength ∝ density of lines
30. Answer: D
Assertion false (field may exist even if net charge = 0)
Reason true
✅ FINAL MARKING SCHEME
Each correct MCQ → 1 mark
Each correct AR → 1 mark
No negative marking
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