Question

Two small identical spheres having charges +10 mC and –90 mC attract each other with a force of F newton. If they are kept in contact and then separated by the same distance, the new force between them is –

• A. F/6
• B. 16F
• C. 16 F/9
• D. 9F

Answer 1

Answer: (C)

16 F/9

Answer 2

F = ....(1)

After contact, change on each,

=$q _ { 2 } ^ { \prime }$ = $\frac { 10 + ( - 90 ) } { 2 }$ = – 40 mC

F ' = $\frac { \mathrm { k } ( 40 \times 40 ) } { \mathrm { r } ^ { 2 } }$ \ = $\frac { 40 \times 40 } { 10 \times 90 }$ =$\frac { 16 } { 9 }$ \ F¢ = $\frac { 16 } { 9 }$F