Question for charges, force

15. New distance betwwen S and T = square-root[2^2 + 1^2] units = square-root[5]
Electrostatic force is inversely proportional to square of distance.
Hence, F'/F = 2^2/5 where F' is the new force
F' = 4F/5

16. Let T be the string tension and E be the electric field intensity
T.cos(5) = mg (g is the acceleration due to gravity, taken as 9.81 m/s2)
T = mg/cos(5)
and qE = T.sin(5)
Hence, qE = [mg/cos(5)].sin(5) = 0.858m
E = 0.858m/q

17. Let q1 and q2 be the charges on drops A and B respectively
hence, (q1).E = (Ma)g
(q2)E = (Mb)g
hence, q1/q2 = Ma/Mb
but q1 = (Na).e, where e is the electronic charge
q2 = (Nb).e
therefore, (Na)/(Nb) = Ma/Mb

18. Work done from X to Y = E.cos(30) where E is the electric field intensity
Work done from Y to Z = 0


2012-03-30 14:11:47 補充：
It seems that the given set of options is not for this question. What are P,Q and R? They have not been mentioned in the question, and how they relate to X, Y and Z . Morever, none of the options are correct as an answer to this question.