CBSE - NEW DELHI
Guess Paper – 2012
Class – XII
Subject – Physics
Electrostatics Charges, Forces And Fields
Question 1. Force between two points electric charges kept at a distance d apart
in air is F. If these charges are kept at the same distance in water, how does
the force between them
Question 2. Two point charges 10µC and 20µC are separated by a distance r in air.
If an additional charge of -8µC is given to each, by what factor does the force
between the charges
Question 3. Two extremely small charged copper spheres have their centres
separated by a distance of 50cm in vacuum. (a) What is the mutual force of
electrostatic repulsion if the charge on each is 6.5x10‑7 C? (b) What will the
force of repulsion if (I) the charge on each sphere is doubled and their
separation is halved (ii) Two spheres are placed in water (dielectric constant
of water is 80)?
Question 4. Calculate the Coulomb force between a proton and an electron
separated by a distance of
Question 5. Two point charges Q are kept at a distance r from each other. A third
charge q is placeon the line joining the above two charges such that all the
three charges are in equilibrium, What is the magnitude, sign and the position
of the charge q?
Question 6. A charge q is placed at the centre of the line joining two equal
charges Q and Q. Calculate the value of charge q such that all the three charges
are in equilibrium. Also mention the nature of this charge.
Question 7. Two point charges of charge values Q and q are placed at a distance
of x and x/2 respectively from a third charge of charge value 4q, all charges
being in the same straight line. Calculate the magnitude and nature of charge Q
such that the net force experienced by the q charge is
Question 8. Two point electric charges of values q and 2q are kept at a distance
d apart from each other in air. A third charge Q is to be kept along the same
line in such a way that the net force on q and 2q is zero. Calculate the
position of the charge Q in terms of q and d.
Question 9. Force of attraction between two point charges placed at a distance
‘d’ apart in a medium is ‘F’. What should be the distance in the same medium so
that the force of attraction between them becomes 9F?
Question 10. Two similarly and equally charged identical metal spheres A and B repel
each other with a force of 2x10-5 N. A third identical uncharged sphere C is
touched with A and then placed at the mid point between A and B. Calculate the
net electric force on C.
Question 11. Two identical metallic spheres, having unequal, opposite charges are
placed at a distance of 0.90 m in air. After bringing them in contact with each
other, they were again placed at the same distance apart . Now the force of
repulsion between them is 0.025 N. Calculate the final charge on each of
Four point charges of qa = 2 mC, qb = - 5 mC, qc = 2 mC and qd = -5 mC are
located at the corners of a square abcd of side 10 cm. Find the force on a
charge 1 mC placed at the center O of the square.