#### Electrostatics Important Question Paper

12th Standard EM

Reg.No. :
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Physics

Time : 01:00:00 Hrs
Total Marks : 50
5 x 1 = 5
1. Two identical point charges of magnitude –q are fi xed as shown in the fi gure below. A third charge +q is placed midway between the two charges at the point P. Suppose this charge +q is displaced a small distance from the point P in the directions indicated by the arrows, in which direction(s) will +q be stable with respect to the displacement

(a)

A1 and A2

(b)

B1 and B2

(c)

both directions

(d)

No stable

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. The electrostatic force is always greater in magnitude than gravitational force for_________ object.

(a)

bigger size

(b)

smaller size

(c)

medium size

(d)

all the above

4. The relative permittivity of water is _______.

(a)

εr= 70

(b)

εr=75

(c)

εr=80

(d)

εr=85

5. 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)

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

6. 5 x 2 = 10
7. Define ‘electrostatic potential”.

8. What is corona discharge?

9. When does a dielectric said to be polarized?

10. (i) Electric field lines donot have sudden breaks why is it so?
(ii) Explain why two field lines never cross each other at any point.

11. What is the work zone by the field in moving a small positive charge from A to P? Given reason.

12. 5 x 3 = 15
13. 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)

14. 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.

15. Derive an expression for the torque experienced by a dipole due to a uniform electric field.

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

17. What is dielectrics or insulators.

18. 4 x 5 = 20
19. Four charges are arranged at the corners of the square PQRS of side a as shown in the figure.(a) Find the work required to assemble these charges in the given configuration. (b) Suppose a charge q′ is brought to the center of the square, by keeping the four charges fixed at the corners, how much extra work is required for this?

20. Suppose a charge +q on Earth’s surface and another +q charge is placed on the surface of the Moon. (a) Calculate the value of q required to balance the gravitational attraction between Earth and Moon (b) Suppose the distance between the Moon and Earth is halved, would the charge q change?
(Take mE = 5.9 × 1024 kg, mM = 7.9 × 1022 kg)

21. What charge would be required to electrify a sphere of radius 25cm. So as to get a surface charge density of $\frac{3}{\pi}$ cm-1?

22. The dielectric constant of water is 80. What is its permittivity?