76) As one penetrates a uniformly charged sphere, the electric field strength

1. increases
2. decreases
3. remains the same
4. none of the above
EXPLANATION
According to Newton’s shell theorem, which also hold for an electrostatic field caused by charges according to Coulomb’s law, the electric field inside a homogeneously charged sphere must be zero. Thus, when one penetrates the surface of the uniformly charged sphere, the electrical filed inside the sphere will go from a maximum value at the surface to zero inside the surface.

77) Two charges 4 μC and 7 mC are placed 4.5 cm apart. What is the electric potential energy of the system.

1. 5.6×105 J
2. 5.6×104 J
3. 5.6×103 J
4. 5.6×102 J
EXPLANATION
U = 1/4πε0 × Q1Q2/d = 5.6×103 J

Where, 1/4πε0=9×109

78) An electric dipole is placed in a non-uniform electric field. It experiences

1. a force but no torque
2. a force and a torque
3. a torque but no force
4. neither a force nor a torque
EXPLANATION
No answer description required for this question.

79) Two point charge are placed on x-axis. A 2 μC charge is at x=10 cm and -1 μC charge is at x=40 cm. What is the potential at x=100 cm from the origin?

1. 5 V
2. 50 V
3. 250 V
4. 5000 V
EXPLANATION
V100= 9×109[(2×10-6/(100-10)×10-2)+(-1×10-6/(100-40)×10-2)]

= 5000 V

80) Three charges each of +2 C are placed at three corners of an equilateral triangle. If the force between any two charges is F, then net force on any charge is

1. √2 F
2. √3 F
3. 2 F
4. 3F
EXPLANATION
On any charge, two forces, each of magnitude F, are acting. The angle between these equal forces is 60°. Therefore, net force on any charge= 2F cos 60°/2 = √3 F.

81) Electric lines of force about a negative point charge are

1. circular clockwise
4. circular anticlockwise
EXPLANATION
No answer description required for this question.

82) A charge of 6.75 μC in an electric field is acted upon by a force of 2.5 N. The potential gradient at that point is

1. 4.2×106 V/m
2. 3.71×105 V/m
3. 2.9×108 V/m
4. none of the above
EXPLANATION
No answer description required for this question.

83) The potential at a certain point from a point charge is 600 V and the electric field is 200 NC-1. The distance of the point of observation from the point charge is

1. 30 m
2. 0.3 m
3. 3 m
4. 300 m
EXPLANATION
No answer description required for this question.

84) If an electron experiences a force equal to its weight when placed in an electric field, then intensity of the field will be

1. 5.6×1011 N/C
2. 2.4×10-5 N/C
3. 56 N/C
4. 5.6×10-11 N/C
EXPLANATION
No answer description required for this question.

85) A hollow metallic sphere of radius 0.1 m is given a charge of 10 μC. The potential on the surface of sphere is

1. 9×103 V
2. 9×105 V
3. 4.5×106 V
4. 3×108 V
EXPLANATION
No answer description required for this question.

86) In the direction of electric field intensity, the electric potential

1. decreases
2. increases
3. remains unchanged
4. decreases up to some distance and then increases
EXPLANATION
No answer description required for this question.

87) The relative permittivity of a dielectric can be

1. 0
2. 1/3
3. -2
4. 4
EXPLANATION
The relative permittivity of all dielectric is greater than 1.

88) An electron has a charge of 1.6×10-19 C when at rest. When it moves with a speed of 0.8 C, the charge on electron will be

1. 0.8×10-19 C
2. 1.6×10-19 C
3. 0.96×10-19 C
4. none of the above
EXPLANATION
No answer description required for this question.

89) An electron is moved towards another electron. What is the effect on the electrostatic potential energy of the system.

1. There will be no effect
2. P.E decreases
3. P.E increases
EXPLANATION
No answer description required for this question.

90) A small metal ball is suspended in a uniform electric field with the help of an insulated thread. If high energy X-ray beam falls on it

1. the ball will be deflected in the direction of field
2.  the ball will be deflected opposite to direction of field
3. the ball will not deflect at all
4. the ball will fly to infinity