BITSAT Physics Electric Charges and Fields MCQs

Question: A disk of radius a/4 having a uniformly distributed charge 6 C is placed in the x - y plane with its centre at (–a/2, 0, 0). A rod of length a carrying a uniformly distributed charge 8 C is placed on the x-axis from x = a /4 to x = 5a/4. Two point charges – 7 C and 3 C are placed at (a/4, – a/4, 0) and (– 3a/4, 3a/4, 0), respectively. Consider a cubical surface formed by six surfaces x = ± a/2, y = ± a/2, z = ± a/2. The electric flux through this cubical surface is

• a)

  

• b)

  

• c)

  

• d)

  

Answer:

Question: Two concentric conducting thin spherical shells A, and B having radii r_A and r_B ((r_B > r_A) are charged to Q_A and –Q_B (|Q_B| > |Q_A|). The electric field along a line passing through the centre is

• a)

  

• b)

  

• c)

  

• d)

  

Answer:

Question: Two identical charged spheres suspended from a common point by two massless strings of lengths l, are initially at a distance d (d << l) apart because of their mutual repulsion. The charges begin to leak from both the spheres at a constant rate. As a result, the spheres approach each other with a velocity v. Then v varies as a function of the distance x between the spheres, as :

• a)

  

• b)

  

• c)

  

• d)

  

Answer:

Question: The surface charge density of a thin charged disc of radius R is σ. The value of the electric field at the centre of the disc is  With respect to the field at the centre, the electric field along the axis at a distance R from the centre of the disc

• a) Reduces by 70.7%
• b) Reduces by 29.3%
• c) Reduces by 9.7%
• d) Reduces by 14.6%

Answer: Reduces by 70.7%

Question: At the corners of an equilateral triangle of side a (1 metre), three point charges are placed (each of 0.1 C). If this system is supplied energy at the rate of 1 kw, then calculate the time required to move one of the mid-point of the line joining the other two.

• a) 50 h
• b) 60 h
• c) 48 h
• d) 54 h

Answer: 50 h

Question: A solid sphere of radius R has a charge Q distributed in its volume with a charge density r = kr^a, where k and a are constants and r is the distance from its centre. If the electric field at r =

times that at r = R, the value of a is

• a) 3
• b) 5
• c) 2
• d) 7

Answer: 2

Question: Two point dipoles  are located at (0, 0, 0) and (1m, 0, 2m) respectively. The resultant electric field due to the two dipoles at the point (1m, 0, 0) is

• a)

  

• b)

  

• c)

  

• d) None of these

Answer:

Question: Three equal charges (q) are placed at corners of an equilateral triangle. The force on any charge is

• a) zero
• b)

  

• c)

  

• d)

  

Answer:

Question: In a region of space having a uniform electric field E, a hemispherical bowl of radius r is placed. The electric flux Φthrough the bowl is

• a) 2πRE
• b) 4πR²E
• c) 2πR²E
• d) πR²E

Answer: 4πR²E

Question: The electric field intensity just sufficient to balance the earth’s gravitational attraction on an electron will be: (given mass and charge of an electron respectively are 9.1 × 10^–31 kg and 1.6 × 10^–19 C.)

• a) –5.6 × 10^–11 N/C
• b) –4.8 × 10^–15 N / C
• c) –1.6 × 10^–19 N/C
• d) –3.2 × 10^–19 N / C

Answer: –5.6 × 10^–11 N/C

Question: In an uniformly charged sphere of total charge Q and radius R, the electric field E is plotted as function of distance from the centre. The graph which would correspond to the above will be

• a)

  

• b)

  

• c)

  

• d)

  

Answer:

Question: A charge Q₁ exerts some force on a second charge Q₂. If a 3rd charge Q₃ is brought near, then the force of Q₁ exerted on Q₂ –

• a) Will increase
• b) Will decrease
• c) Will remain unchanged
• d) will increase if Q₃ is of the same sign as Q₁ and will decrease if Q₃ is of opposite sign

Answer: Will remain unchanged

Question: A charge +q is at a distance L/2 above a square of side L. Then what is the flux linked with the surface?

• a)

  

• b)

  

• c)

  

• d)

  

Answer:

Question: Two metallic spheres of radii 1 cm and 3 cm are given charges of –1 × 10^–2 C and 5×10^–2 C, respectively. If these are connected by a conducting wire, the final charge on the bigger sphere is

• a) 2 × 10^–2 C
• b) 3 × 10^–2 C
• c) 4 × 10^–2 C
• d) 1 × 10^–2 C

Answer: 3 × 10^–2 C

Question: If an electron has an initial velocity in a direction different from that of an electric field, the path of the electron is

• a) A straight line
• b) A circle
• c) An ellipse
• d) A parabola

Answer: A parabola

Question: The ratio of magnitudes of electric field due to an electric dipole on the axis and on the equatorial line at equal distances will be

• a) 4 : 1
• b) 1 : 2
• c) 2 : 1
• d) 1 : 1

Answer: 2 : 1

Question: A square surface of side L meter in the plane of the paper is placed in a uniform electric field E (volt/m) acting along the same plane at an angle θ with the horizontal side of the square as shown in Figure. The electric flux linked to the surface, in units of volt m, is

• a) EL²
• b) EL² cos θ
• c)  EL² sin θ 
• d) zero

Answer: zero

Question: Charge Q is distributed to two different metallic spheres having radii R and 2R such that both spheres have equal surface charge density, then charge on large sphere is

• a)

  

• b)

  

• c)

  

• d)

  

Answer:

Question: A charge q is placed at the centre of the open end of a cylindrical vessel. The flux of the electric field through the surface of the vessel is

• a) zero
• b) (q/ε0)
• c) (q/2ε₀)
• d) (2q/ε₀)

Answer: (q/2ε₀)

Question: A solid spherical conductor of radius R has a spherical cavity of radius a (a < R) at its centre. A charge + Q is kept at the centre. The cahrge at the inner surface, outer and at a position r (a < r < R) are respectively

• a) + Q, – Q, 0
• b) – Q, + Q, 0
• c) 0, – Q, 0
• d) + Q , 0, 0

Answer: 0, – Q, 0