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Max. marks:10
PHY-204
Time Allowed:15 minutes
Quiz 1A: Electricity and Magnetism
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1. Two solid spheres, both of radius 5 cm, carry identical total charges of 2 µC. Sphere A
is a good conductor. Sphere B is an insulator, and its charge is distributed uniformly
throughout its volume.
(i) How do the magnitudes of the electric fields they separately create at a radial
distance of 6 cm compare?
(a) EA > EB = 0
(b) EA > EB > 0
(c) EA = EB > 0
(d) EA = EB = 0
(e) 0 < EA < EB
(f) 0 = EA < EB .
(ii) How do the magnitudes of the electric fields they separately create at radius 4 cm
compare? Choose from the same possibilities. [3 marks]
Answer
Date: 16 September, 2013
1
Max. marks:10
Time Allowed:15 minutes
2. Twelve equal charges, q, are situated at the corners of a regular 12-sided polygon. In
equilibrium, a test charge placed at the center of polygon faces no electric force on it.
Suppose one of the 12 q’s is removed. What is the resulting force on the test charge
at the center? [2 marks]
(a)
(c)
1 11q 2
4πε0 r2
1 12q 2
4πε0 11r2
(b)
1 q2
4πε0 r2
(d) 0.
q
q
q
q
q
q
q
q
q
q
q
q
q
FIG:1
Answer
Date: 16 September, 2013
2
Max. marks:10
Time Allowed:15 minutes
3. Two identical beads each have a mass m and charge q. When placed in a hemispherical bowl of radius R with frictionless, nonconducting walls, the beads move, and at
equilibrium they are a distance R apart (Figure 2). Determine the charge on each
bead. Hint: Consider a force that is normal to the hemispherical bowl. [5 marks]
FIG. 2:
Two beads on a Hemispherical bowl
Answer
Date: 16 September, 2013
3
Max. marks:10
PHY-204
Time Allowed:15 minutes
Quiz 1B: Electricity and Magnetism
Name:
Roll no:
Attempt all questions.
1. A solid insulating sphere of radius 5cm carries electric charge uniformly distributed
throughout its volume. Concentric with the sphere is a conducting spherical shell with
no net charge as shown in Figure. The inner radius of the shell is 10cm, and the outer
radius is 15cm. No other charges are nearby.
(i) Rank the magnitude of the electric field at points A (at radius 4cm), B (radius
8cm), C (radius 12cm), and D (radius 16cm) from largest to smallest.
(a) A > B > D > C
(b) A > D > C > B = 0
(c) B = D > A > C
(d) A > B > C > D
(e) A = D > B > C.
FIG. 1
(ii) Similarly rank the electric flux through concentric spherical surfaces through
points A, B, C, and D. Choose from the same possibilities. [4 marks]
Answer
Date: 16 September, 2013
1
Max. marks:10
Time Allowed:15 minutes
2. Eight equal charges, q, are situated at the corners of a simple cube as shown in Figure.
In equilibrium, a test charge placed at the center of cube faces no electric force on it.
Suppose one of the 8 q’s is removed. What is the resulting force on the test charge at
the center? [2 marks]
q
FIG: 2
Date: 16 September, 2013
2
Max. marks:10
(a)
(c)
1 7q 2
4πε0 r2
1 8q 2
4πε0 7r2
Time Allowed:15 minutes
(b)
1 q2
4πε0 r2
(d) 0.
Answer
3. Two small beads having positive charges 3q and q are fixed at the opposite ends of
a horizontal insulating rod, extending from the origin to the point x = d. As shown
in Figure, a third small charged bead is free to slide on the rod. At what position
is the third bead in equilibrium? Explain for what charge on the third ball, stable
equilibrium will create. [4 marks]
FIG: 3
Answer
Date: 16 September, 2013
3