Question

Two point charges of +3 MUa C and +4 MUa C repel each other with a force of 10 N. If each is given an additional charge of - 6 MUa C, then the new force is.

Answer 1

q1 = 3μC

q2 = 4μC

K= 9 x 10^9 Nm^2C^−2

F = (K x q1 x q2) / r^2

10 = ( 9 x 10^9 x 3 x 10^-6 x 4 x 10^-6) / r^2

then

r^2 = 0.108/10 = 0.0108------(i)

now -6μC add in every charge

now q1 = -3μC

q2 = -2μC

we don't take charge, -ve or +ve charge show repletion of attraction.

and here it show repletion

F = (K x q1 x q2) / r^2

F = (9 x 10^9 x 3 x 10^-6 x 2 x 10^-6) / 0.0108 from 1st equation

F = 5 N

this is your answer