Two metal spheres of radius R and 3R have same surface charge density . If they are brought in contact and then separated, the surface charge density on smaller and bigger sphere become 1 and 2, respectively. The ratio 1 2 is
- 13
- 19
- 9
- 3
Answer
(4) 3
Solution
Initial charges: q1 = 4 R2, q2 = 4 (3R)2 = 9q1
Total charge Q = 10q1
On contact both spheres reach the same potential:
kq1'R = kq2'3R q2' = 3q1'
q1' = Q4, q2' = 3Q4
1 2 = q1'/(4 R2)q2'/(4 9R2) = q1'q2' 9
= 13 9 = 3
(In general, at equal potential 1r, so 1 2 = 3RR = 3.)
Answer: (4)