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

12th Standard

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Physics

Time : 01:00:00 Hrs
Total Marks : 105
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. _______ 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

4. A force of 40 N is acting between two charges in air if the space between then is filled with glass εr = 8. Then the force between then is

(a)

20 N

(b)

10 N

(c)

5 N

(d)

the same and does not change

5. The expression for electric field in vector form is

(a)

$\frac { 1 }{ 4\pi { \varepsilon }_{ 0 }r } \frac { q }{ r } \hat { r }$

(b)

$\frac { -1 }{ 4\pi { \varepsilon }_{ 0 }r } \frac { q }{ r } \hat { r }$

(c)

$\frac { -1 }{ 4\pi { \varepsilon }_{ 0 }r } \frac { q }{ r^2 } \hat { r }$

(d)

$\frac { 1 }{ 4\pi { \varepsilon }_{ 0 }r } \frac { q }{ r^2 } \hat { r }$

6. The electric field created by a _________ is basically a non-uniform electric field.

(a)

Test charge

(b)

Positive charge

(c)

Negative charge

(d)

Point charge

7. The expression for electric potentid difference is

(a)

$\int _{ R }^{ P }{ +\overset { \rightarrow }{ E } .\overset { \rightarrow }{ dr } }$

(b)

$\\ -\int _{ \infty }^{ P }{ +\overset { \rightarrow }{ E } .\overset { \rightarrow }{ dr } }$

(c)

$\\\int _{ \infty }^{ P }{ \overset { \rightarrow }{ E } .\overset { \rightarrow }{ dr } }$

(d)

$\int _{ R }^{ P }{ -\overset { \rightarrow }{ E } .\overset { \rightarrow }{ dr } }$

8. An uniformly charged conducting shell of 2cm diameter has a surface charge density of 80μC / m2. The charge on the shell is

(a)

100.48 nC

(b)

100.48 μC

(c)

100.48C

(d)

100.48x10-12C

9. A charge QμC is placed at the center of a cube. The flux coming out from any surface will be

(a)

$\frac { Q }{ { 24\varepsilon }_{ 0 } }$

(b)

$\\ \frac { Q }{ { 8\varepsilon }_{ 0 } }$

(c)

$\frac { Q }{ { 6\varepsilon }_{ 0 } } \times { 10 }^{ -6 }$

(d)

$\frac { Q }{ { 6\varepsilon }_{ 0 } } \times { 10 }^{ -3 }$

10. Charge Q on a capacitor varies with voltage Vas shown in graph, where Q is along X-axis and V along Y-axis. The area of triangle OAB represents

(a)

capacitance

(b)

capacitive reactance

(c)

magnetic field between the plates

(d)

energy stored in the capacitor

11. In two concentric hollow spheres of radii r and R (>r), the charge Q is distributed such that their surface densities are some. Then the potential at their common centre is

(a)

$\frac { Q({ R }^{ 2 }+{ r }^{ 2 }) }{ 4\pi { \varepsilon }_{ 0 }(R+r) }$

(b)

$\frac { QR }{ R+r }$

(c)

zero

(d)

$\frac { Q({ R }+{ r }) }{ 4\pi { \varepsilon }_{ 0 }({ R }^{ 2 }+{ r }^{ 2 }) }$

12. When a point charge of 6mC is moved between two points in an electric field, the work done is 1.8 x 10-5 J. The potential difference between the two points is

(a)

1.08 V

(b)

1.08 μV

(c)

2 V

(d)

30 V

13. Electric field intensity and electric potential are related by_________.

(a)

E=-$\frac{dV}{dt}$

(b)

E=-$\frac{dV}{dx}$

(c)

E=$\frac{dV}{dt}$

(d)

E=$\frac{-dx}{dV}$

14. The intensity of the electric field that produces a force of 10-5 N on a charge of 5 μC is

(a)

5 x 10-11NC-1

(b)

50 NC-1

(c)

2 NC-1

(d)

0.5 NC-1

15. The unit of electric dipole moment is ________.

(a)

volt/metre $[\frac{V}{m}]$

(b)

coulomb / metre$[\frac{C}{m}]$

(c)

volt. metre [Vm]

(d)

Coulomb. metre (Cm)

16. 2 Marks
10 x 2 = 20
17. Write down Coulomb’s law in vector form and mention what each term represents.

18. Define ‘Electric field’.

19. Give a comparison of electrical and gravitional forces?

20. What is a Capacitor?

21. What is the electric flux through a cube of side 1cm which encloses on electric dipole?

22. 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 $\frac { Q }{ { \varepsilon }_{ 0 } }$

23. Asphere of charge +Q is fixed. Asmaller sphere of charge +q is placed near the larger sphere and released from rest. The small sphere will move away from large sphere with
a. decreasing velocity & decreasing acceleration.
b. decreasing velocity & increasing acceleration.
c. decreasing velocity &constant acceleration d. increasing velocity & decreasing acceleration
e. increasing velocity & increasing acceleration
Which of the above statement is correct? Explain.

24. Apoint charge Q is placed at point O.potential difference VA - VB is positive. Is the charge Q negative or positive?

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

26. How does the energy stored. in a capacitor change it
(i) after disconnecting the battery, the plates of a charged capacitor are moved faster.
(ii) The battery remaining connected capacitance c∝$\frac{1}{d}$, when plates of capacitor.

27. 3 Marks
10 x 3 = 30
28. Calculate the number of electrons in one coulomb of negative charge.

29. Consider the charge configuration as shown in the figure. Calculate the electric field at point A. If an electron is placed at points A, what is the acceleration experienced by this electron? (mass of the electron = 9.1 × 10-31 kg and charge of electron = −1.6 × 10-19 C)

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 the torque experienced by a dipole due to a uniform electric field.

32. Obtain Gauss law from Coulomb’s law.

33. Obtain the expression for capacitance for a parallel plate capacitor.

34. Explain in detail the effect of a dielectric placed in a parallel plate capacitor.

35. Derive the expression for resultant capacitance, when capacitors are connected in series and in parallel.

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. (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.

40. Consider an electron travelling with a speed vo and entering into a uniform electric field
$\vec { E }$ which is perpendicular to $\vec { { v }_{ 0 } }$ as shown in the Figure. Ignoring gravity, obtain the electron’s acceleration, velocity and position as functions of time.

41. 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.

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

43. A capacitor of capacity 10μF is subjected to charge by a battery of 10V. Calculate the energy stored in the capacitor.

44. The electric potential in region is represented as V = 2x +3y - z. Obtain an expression for electric field strength.

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. Three capacitors each of capacitance 9pF are connected in series
(i) What is the total capacitance of the combination?
(ii) What is the potential difference across each capacitor, if the combination is connected to a 120 V supply.