#### 12th Standard Physics English Medium Electrostatics Reduced Syllabus Important Questions with Answer key 2021

12th Standard

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Physics

Time : 01:00:00 Hrs
Total Marks : 100
Multiple Choice Questions
15 x 1 = 15
1. Which charge configuration produces a uniform electric field?

(a)

point Charge

(b)

infinite uniform line charge

(c)

uniformly charged infinite plane

(d)

uniformly charged spherical shell

2. The total electric fl ux for the following closed surface which is kept inside water

(a)

$\frac { 80q }{ { \varepsilon }_{ 0 } }$

(b)

$\frac { q }{ { 40\varepsilon }_{ 0 } }$

(c)

$\frac { q }{ { 80\varepsilon }_{ 0 } }$

(d)

$\frac { q }{ { 160\varepsilon }_{ 0 } }$

3. An electric field $\vec { E } =10x\hat { i }$ exists in a certain region of space. Then the potential difference V = Vo – VA, where Vo is the potential at the origin and VA is the potential at x = 2 m is:

(a)

10 J

(b)

-20 J

(c)

+20 J

(d)

-10 J

4. In electrostatics if the charges are in motion, another force named __________ comes into play in addition to coulomb force

(a)

Lorentz force

(b)

Repulsive force

(c)

Attractive force

(d)

electromagnetic force

5. _______ and Coulomb's law form fundamental principles of electrostatics

(a)

Newton's law of gravitation

(b)

superposition principle

(c)

ohm's law

(d)

Kepler's law

6. An isolated metal sphere of radius 'r' is given a charge' q'. The potential energy of the sphere is

(a)

$\frac { { q }^{ 2 } }{ 4\pi { \varepsilon }_{ 0 }r }$

(b)

$\frac { { q }^{ } }{ 4\pi { \varepsilon }_{ 0 }r }$

(c)

$\frac { { q }^{ } }{ 8\pi { \varepsilon }_{ 0 }r }$

(d)

$\frac { { q }^{ 2 } }{ 8\pi { \varepsilon }_{ 0 }r }$

7. Which one of these is a vector quantity?

(a)

Electric charge

(b)

Electric field

(c)

Electric flux

(d)

Electric potential

8. The electric potential V as a function of distance x (metres) is given by V = ( 5x2 + 10x -9) volt. The value of electric field at a point x = 1m is

(a)

20 Vm-1

(b)

6 Vm-1

(c)

11 Vm-1

(d)

-23 Vm-1

9. A non-conducting material which has no free electrons is called

(a)

capacitor

(b)

Dielectric

(c)

conductor

(d)

Inductor

10. An electric dipole placed at an angle in a nonuniform electric field experiences

(a)

neither a force nor a torque

(b)

torque

(c)

both force and torque

(d)

force only

11. Value of k in Coulomb's law depends upon

(a)

magnitude of charges

(b)

distance between charges

(c)

both (a) and (b)

(d)

medium between two charges

12. One Joule per Coulomb is called

(a)

Gauss

(b)

ampere

(c)

(d)

volt

13. The concentric spheres of radii R and r have similar charges with equal surface densities (σ). What is the electric potential at their common centre?

(a)

$\frac { \sigma }{ { \varepsilon }_{ 0 } } (R-r)$

(b)

$\frac { \sigma }{ { { \varepsilon }_{ 0 } } } (R+r)$

(c)

$R\frac { \sigma }{ { \varepsilon }_{ 0 } }$

(d)

$\frac { \sigma }{ { \varepsilon }_{ 0 } }$

14. Gauss law is another form of ________.

(a)

Newton's law

(b)

Kepler's law

(c)

Ohm's law

(d)

Coulomb's law

15. The repulsive force between two like charges of 1 coulomb each separated by a distance of 1 m in vacuum is equal to :

(a)

9 x 109 N

(b)

109 N

(c)

9 X 10-9 N

(d)

9 N

16. 2 Marks
10 x 2 = 20
17. Define ‘Electric field’.

18. Define ‘electrostatic potential”.

19. Define ‘electrostatic potential energy’.

20. Define ‘electric flux’.

21. What is corona discharge?

22. What is dielectric breakdown.

23. A charge Q μc is placed at the centre of a cube what would be the
(i) flux through one face?
(ii) flux passing through two opposite faces of the cube?
Electric flux through whole cube $\frac { Q }{ { \varepsilon }_{ 0 } }$

24. Represent the variation of electric field due to point charge Q with
a) magnitude of charge Q
b) r and
c) $\frac{1}{r^2}$ where r is the distance of the observation point from the charge. graphically.

25. Aparallel plate capacitor is charged by a battery after some time, the battery is disconnected and a dielectric slab with its thickness equal to the plate so reparation is insected between the plates. How will
(i) the capacitance of the capacitor
(ii) potential difference between the plates &
(iii) the energy stored in the capacitor the affected?
Qo - charge; Vo - potential difference,
Co - capacitance, Eo- electric field.
U0 - energy spred, before the dielectric slab is inserted.
${ Q }_{ 0 }={ C }_{ 0 }{ V }_{ 0 };\frac { { B }_{ 0 } }{ d } ;{ U }_{ 0 }=\frac { 1 }{ 2 } { C }_{ 0 }{ v }_{ 0 }^{ 2 }$

26. Gauss law is true for any closed surface, no matter what its shape or size is. Justify.

27. 3 Marks
10 x 3 = 30
28. Two small-sized identical equally charged spheres, each having mass 1 mg are hanging in equilibrium as shown in the figure. The length of each string is 10 cm and the angle θ is 7° with the vertical. Calculate the magnitude of the charge in each sphere.(Take g = 10 ms−2)

29. Calculate the electric field at points P, Q for the following two cases, as shown in the figure.
(a) A positive point charge +1 μC is placed at the origin.
(b) A negative point charge -2 μC is placed at the origin.

30. A block of mass m and positive charge q is placed on an insulated frictionless inclined plane as shown in the figure. A uniform electric field E is applied parallel to the inclined surface such that the block is at rest. Calculate the magnitude of the electric field E.

31. Derive an expression for electrostatic potential due to a point charge.

32. Obtain an expression for potential energy due to a collection of three point charges which are separated by finite distances.

33. Obtain the expression for energy stored in the parallel plate capacitor.

34. Derive the expressions for the potential energy of a system of point charges.

35. How is electric flux is related to electric field.

36. Write the special features of Gauss law.

37. Define and derive an expression for the energy density in parallel plate capacitor.

38. 5 Marks
8 x 5 = 40
39. Consider a point charge +q placed at the origin and another point charge -2q placed at a distance of 9 m from the charge +q. Determine the point between the two charges at which electric potential is zero.

40. (i) In figure (a), calculate the electric flux through the closed areas A1 and A2.
(ii) In figure (b), calculate the electric flux through the cube.

41. When two objects are rubbed with each other, approximately a charge of 50 nC can be produced in each object. Calculate the number of electrons that must be transferred to produce this charge.

42. Explain in detail the Electrostatic Potential difference between the charges.

43. A thin metallic spherical shell of radius R carries a charge Q on its surface. A point charge $\frac{Q}{2}$ is placed at the centre C and another is placed at the centre C and another a distance x from the centre as shown in the figure
(i) Find the electric flux through the shell.

(ii) Find the force on the charges at C and A.

44. How many electrons are there in one coulomb of negative charge?

45. An electric dipole of length 4cm, when placed with its axis making an angle of 60° with a uniform electric field, experiences a torque of 4√3 Nm. Calculate the potential energy of the dipole, if it has charge ±8nC.

46. Two insulated charged copper sphere A and B have their centres separated by a distance of 50cm.
(i) What is the force of electrostatic repulsion, if the charge on each is 6.5 X 10-7C and the radii of A and B are negligible compared to the distance of separation?
(ii) What is the force of repulsion, if each sphere is charged double the above amount and the distance between them is halved?