CH-1 WS-1

STD: 12 SUBJECT: PHYSICS
TOPICS: ELECTRIC CHARGES AND FIELD WORKSHEET NO: 1

1. A change 'q' placed at the centre on the line joining two charges 'Q' will be in equilibrium if
'q' will be equal to ____________.
2. How much charge should be placed on a spherical shell of radius 25 cm to have a surface charge
density of (3/π) C/m2?
3. A sphere of radius 5 cm has a charge of 31.41 μC. Calculate the surface density of charge.
4. Why can the interior of a conductor have no excess charge in the static situation?
5. Two-point charges qA = 5 µC and qB = – 5μC are located at A and B separated by 0.2 m in
vacuum.
(a) What is the electric field at the mid point O of the line joining the charges?
(b) If a negative test charge of magnitude 2μC is placed at O, what is the force experienced by the test
charge?
6. The electrostatic force between two-point charges kept at a distance ‘d’ apart, in a medium,
εr = 6, is 0.3 N. The force between them at the same separation in vacuum is ______________.
7. Two-point charges + 9e and +e is located at a distance of 16 cm from each other. Where should
a charge q be placed between them so that it is in equilibrium?
8. Two identical conducting balls A and B have charges – Q and + 3Q respectively. They are brought in
contact with each other and then separated by a distance d apart. Find the nature of the Coulomb force
between them.
9. Two identical metallic spherical shells A and B having charges +4Q and –10Q are kept a certain
distance apart. A third identical uncharged sphere C is first placed in contact with sphere A and then
with sphere B, then spheres A and B are brought in contact and then separated. Find the charge on the
spheres A and B.
10. Plot a graph showing the variation of Coulomb force (F) versus 1/r2, where r is the distance between the
two charges of each pair of charges (1μC, 2μC) and (1μC, –3μC). Interpret the graphs obtained.
11. Two concentric metallic spherical shells of radii R and 2R are given charge Q 1 and Q2 respectively. The
surface charge densities on the outer surfaces of the shells are equal Determine the ratio Q 1 : Q2.
12. The point charges having equal charges separated by 1m distance experience a force of 8N. What will
be the force experienced by them, if they are held in water at the same distance? (Given, k water = 80)
13. An infinite line charge produces a field of 9 × 104 NC–1 at a distance of 2 cm. Calculate the linear charge
density.
14. Two-point charges of + 1μC and +4μC are kept 30 cm apart. How far from the +1μC charge on the line
joining the two charges, will the net electric field be zero?
15. Plot the graph to represent the variation of electric field intensity as a function of distance from the
centre of a uniformly charged spherical shell of radius R?
16. A charge Q is placed at the centre of a cube. The electric flux through one if its face is________.
17. Two metallic spheres of radii R1 and R2 are charged. Now they are brought into contact with each other
with a conducting wire and then are separated. If the electric fields on their surfaces are E 1 and E2
respectively E1 / E2 is ..........
(a) R1/R2 (b) R2/R1 (c) (R1)2/(R2)2 (d) (R2)2/(R1)2
18. Two-point charges + 4q and + q is placed 30 cm apart. At what point on the line joining them the
electric field is zero?
19. A dipole with its charge –q and +q located at the points (0, –b, 0) and (0, +b, 0) is present in a uniform
electric field E. The equipotential surfaces of this field are planes parallel to the YZ plane.
(i) What is the direction of the electric field E?
(ii) How much torque would the dipole experience in this field?
20. S1 and S2 are two parallel concentric spheres enclosing charges Q and 2Q
respectively as shown in the figure below. What is the ratio of electric flux through S1
and S2?
21. Consider two hollow concentric spheres, S1 and S2, enclosing charges 2Q and 4Q
respectively as shown in the figure. (i) Find out the ratio of the electric flux through
them. (ii) How will the electric flux through the sphere S1 change if a medium of
dielectric constant 'εr' is introduced in the space inside S1 in place of air? Deduce the
necessary expression.

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STD: 12 SUBJECT: PHYSICS

TOPICS: ELECTRIC CHARGES AND FIELD WORKSHEET NO: 1

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