Guess Paper – 2012

Class – XII

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 change?

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 change?

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 0.8x10‑15m.

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 zero.

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 them.

Question 12. 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.