CHAUBEY SIR CLASSES

Shop no 138 sector 31 faridabad

CHAPTER 1 NCERT
Class 12 - Physics
Time Allowed: 1 hour Maximum Marks: 70

1. A point charge causes an electric flux of −1.0 × 10 3 2

Nm /C to pass through a spherical Gaussian surface of [1]
10.0 cm radius centred on the charge. If the radius of the Gaussian surface were doubled, how much flux would
pass through the surface?

a) −10 3
Nm /C
2
b) 4
10 Nm /C
2

c) 10 3
Nm /C
2
d) 2
−10 Nm /C
2

2. When air is replaced by a dielectric medium of dielectric constant K, the maximum force of attraction between [1]
two charges separated by a distance:

a) decreases K2 times b) decreases K times

c) increases K times d) remains unchanged

3. Two parallel large thin metal sheets have equal surface charge densities of 2.56 × 10-11 Cm-3 of opposite signs. [1]
The electric field between these sheets is

a) 3 NC-1 b) 3 × 10-10 NC-1

c) 1.5 NC-1 d) 1.5 × 10-10 NC-1

4. A half ring of radius R has a charge per unit length equal to λ . The field at the center is [1]

a) zero b) 2λ

4πε0 R

c) d) None of these
λ

4πε0 R

5. Two small charged spheres A and B have charges 10 μ C and 40 μ C respectively and are held at a separation of [1]
90 cm from each other. At what distance from A, the electric intensity would be zero?

a) 30 cm b) 22.5 cm

c) 18 cm d) 36 cm
6. Two insulated charged copper spheres A and B have their centres separated by a distance of 50 cm. What is the [1]
mutual force of electrostatic repulsion if the charge on each is 6. 5 × 10 −7
C ? The radii of A and B are
negligible compared to the distance of separation.

a) 3. 5 × 10 −2
N b) 4. 5 × 10
−2
N

c) 1. 5 × 10 −2
N d) 2. 5 × 10
−2
N

7. Careful measurement of the electric field at the surface of a black box indicates that the net outward flux through [1]
the surface of the box is 8.0 × 10 3 2
Nm /C . What is the net charge inside the box?

a) 0.04 μC b) 0.06 μC

c) 0.05 μC d) 0.07 μC

1/4
Phone no 8920776645
 8. A long solid non-conducting cylinder of radius R is charged such that the volume charge density is proportional [1]
to r, where r is the distance from the axis. The electric field E at a distance r(r < R) will depend on r as:

a) E ∝ ( 1

r
2
) b) E ∝ ( ) 1

c) E ∝ r2 d) E ∝ r

9. A wire is bent into a ring of radius R is given a charge q. The magnitude of the electrical field at the centre of the [1]
ring is:

a) 1

2
b) Zero

c) Two d) Infinite
10. If a positive charge is displaced against the electric field in which it was situated, then [1]

a) the intensity of the electric field decreases b) work will be done by the electric field on
the charge

c) energy of the system will decrease d) energy will be provided by external source
displacing the charge
11. Point out whether the following statement is right or wrong: [1]
Statement - The mutual forces between two charges do not get affected by the presence of other charges.
12. Two point charges q1 and q2 are placed at a distance d apart as shown in the figure. The electric field intensity is [1]

zero at the point P on the line joining them as shown. Write two conclusions that you can draw from this.

13. Point charge (+q) is kept in the vicinity of an uncharged conducting plate. Sketch electric field lines between the [1]
charge and the plate.
14. What do you mean by additivity of electric charge? [1]
15. What do you mean by conservation of electric charge? [1]
16. The figure shows five charged lumps of plastic and an electrically neutral coin. The cross-section of a Gaussian [1]
surface S is indicated. What is the net electric flux through the surface if q1 = q4 = +3.1 nC, q2 = q5 = -5.9 nC
and q3 = -3.1 nC?

17. What is the relation between electric intensity and flux? [1]
18. Why do the electric field lines never cross each other? [1]
19. A positive point charge (+q) is kept in the vicinity of an uncharged conducting plate. Sketch electric field line [1]
originated from the point on to the surface of the plate.

20. The distance between the electron and proton in a hydrogen atom is 5.3 × 10-11 m. Determine the magnitude of [1]

the ratio of the electrostatic and gravitational force between them. Given me = 9.1 × 10-31 kg, mp = 1.67 × 10-
27 kg, e = 1.6 × 10-19 C and G = 6.67 × 10-11 Nm2 kg-2.
21. Find the magnitude of the resultant force on a charge of 1μC held at P due to two charges of +2 × 10 −8
C and [2]
-10-8C at A and B respectively. Given AP = 10 cm, BP = 5 cm, ∠AP B = 90
∘

2/4
Phone no 8920776645
22. Consider a coin of Example 1.20. It is electrically neutral and contains equal amounts of positive and negative [2]
charge of magnitude 34.8 kC. Suppose that these equal charges were concentrated in two point charges seperated
by
i. 1 cm ( ∼ 1

2
× diagonal of the one paisa coin)
ii. 100 m ( ∼ length of a long building), and

iii. 106 m (radius of the earth).

Find the force on each such point charge in each of the three cases. What do you conclude from these results?
23. A conducting sphere of radius 10 cm has an unknown charge. If the electric field, 20 cm from the centre of the [2]
sphere is 1.5 × 10 3
N /C and points radially inward, what is the net charge on the sphere?
24. a. An electrostatic field line is a continuous curve. That is, a field cannot have sudden breaks. Why not? [2]
b. Explain why two field lines never cross each other at any point?
25. State the superposition principle for electrostatic force on a charge due to a number of charges. [2]
26. How does the speed of an electrically charged particle affect its mass and charge? [2]
27. Three equally charged small objects are placed as shown in Fig. Object A exerts an electric force on object B [2]
equal to 3.0 × 10-6 N.

i. What electric force does C exert on B?

ii. What is the net electric force on B?
28. A point charge causes an electric flux of −1.0 × 10 3 2
N m /C to pass through a spherical Gaussian surface of [2]
10.0 cm radius centered on the charge.
a. If the radius of the Gaussian surface were doubled, how much flux would pass through the surface?
b. What is the value of the point charge?

29. A charged particle having a charge of -2.0 × 10-6 C is placed close to a non-conducting plate having a surface [2]

charge density of 4.0 × 10-6 Cm-2. Find the force of attraction between the particle and the plate.
30. An electric dipole of length 1cm when placed with its axis making an angle of 60° with a uniform electric field, [2]
–
experiences a torque of 6√3N − m . Calculate the potential energy of the dipole, if it has charge ± 2nC.
31. A point charge +10μC is a distance 5 cm directly above the center of a square of side 10 cm, as shown in [3]
figure. What is the magnitude of the electric flux through the square? (Hint: Think of the square as one face of a
cube with edge 10 cm)

32. Four particles, each having a charge q, are placed on the vertices of a regular pentagon. The distance of each [3]
corner from the centre is a. Determine the electric field at the centre of the pentagon.
33. A charged particle, of charge 2 μ C and mass 10 milligram, moving with a velocity of 1000 m/s enters a uniform [3]

electric field of strength 103 NC-1 directed perpendicular to its direction of motion. Find the velocity and
displacement, of the particle after 10 s.
34. Two equal balls having equal positive charge q coulombs are suspended by two insulating strings of equal [3]

3/4
Phone no 8920776645
 length. What would be the effect on the force when a plastic sheet is inserted between the two charges and angle
between them?
35. Consider a uniform electric field E = 3 × 10
3^
i N /C . [3]
a. What is the flux of this field through a square of 10 cm on a side whose plane is parallel to the yz plane?
b. What is the flux through the same square if the normal to its plane makes a 60° angle with the x-axis?
36. Two-point charges +q and -q are placed distance d apart. What are the points at which the resultant electric field [5]
is parallel to the line joining the two charges?
37. a. Using Gauss's law, derive an expression for the electric field intensity at any point outside a uniformly [5]
charged thin spherical shell of radius R and charge density σ C /m . Draw the field lines when the charge
2

density of the sphere is (i) positive, (ii) negative.

b. A uniformly charged conducting sphere of 2.5 m in diameter has a surface charge density of 100μC /m . 2

Calculate
(i) Charge on the sphere (ii) Total electric flux passing through the sphere.

38. i. Derive an expression for the electric field at any point on the equatorial line of an electric dipole. [5]
ii. Two identical point charges, q each, are kept 2m apart in the air. A third point charge Q of unknown
magnitude and sign is placed on the line joining the charges such that the system remains in equilibrium.
Find the position and nature of Q.

4/4
Phone no 8920776645