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#### Electrostatics Book Back Questions

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. An electric dipole is placed at an alignment angle of 300 with an electric fi eld of 2 × 105 NC-1. It experiences a torque equal to 8 N m. Th e charge on the dipole if the dipole length is 1 cm is

(a)

4 mC

(b)

8 mC

(c)

5 mC

(d)

7 mC

3. 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)

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

4. Two identical conducting balls having positive charges q1 and q2 are separated by a center to center distance r. If they are made to touch each other and then separated to the same distance, the force between them will be

(a)

less than before

(b)

same as before

(c)

more than before

(d)

zero

5. Rank the electrostatic potential energies for the given system of charges in increasing order.

(a)

1 = 4 < 2 < 3

(b)

2 = 4 < 3 < 1

(c)

2 = 3 < 1 < 4

(d)

3 < 1 < 2 < 4

6. 5 x 2 = 10
7. Write a short note on ‘electrostatic shielding’.

8. What is Polarisation?

9. What is dielectric strength?

10. Define ‘capacitance’. Give its unit.

11. What is corona discharge?

12. 5 x 3 = 15
13. Obtain the expression for energy stored in the parallel plate capacitor.

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

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

16. Explain in detail how charges are distributed in a conductor, and the principle behind the lightning conductor.

17. Explain in detail the construction and working of a Van de Graaff generator.

18. 4 x 5 = 20
19. An electron and a proton are allowed to fall through the separation between the plates of a parallel plate capacitor of voltage 5 V and separation distance h = 1 mm as shown in the figure.

(a) Calculate the time of flight for both electron and proton
(b) Suppose if a neutron is allowed to fall, what is the time of flight?
(c) Among the three, which one will reach the bottom first? (Take mp = 1.6 x 10-27 kg, me = 9.1 x 10-31 kg and g = 10 m s-2)

20. During a thunder storm, the movement of water molecules within the clouds creates friction, partially causing the bottom part of the clouds to become negatively charged. This implies that the bottom of the cloud and the ground act as a parallel plate capacitor. If the electric field between the cloud and ground exceeds the dielectric breakdown of the air (3 × 106 Vm-1 ), lightning will occur.

(a) If the bottom part of the cloud is 1000 m above the ground, determine the electric potential difference that exists between the cloud and ground.
(b) In a typical lightning phenomenon, around 25C of electrons are transferred from cloud to ground. How much electrostatic potential energy is transferred to the ground?

21. For the given capacitor configuration
(a) Find the charges on each capacitor
(b) potential difference across them
(c) energy stored in each capacitor

22. Capacitors P and Q have identical cross sectional areas A and separation d. The space between the capacitors is filled with a dielectric of dielectric constant εr as shown in the figure. Calculate the capacitance of capacitors P and Q.