Electrostatics Part 1

According to Coulomb's law, the force of attraction or repulsion is directly proportional to the product of distance between them and inversely proportional to the square of the distance between them.
F=K‌

q1q2

r2

Here we have, q1 is the charge of the first mass, q2 is the charge of the second mass, r is the distance and K is the proportionally constant.
Given: Charge of first sphere q1=2.1‌nc
and Charge of second sphere q2=−0.1‌NC
when they connect with each other the charge becomes, q=‌

q2−q1

2

⇒q=‌

2.1−0.1

2

⇒q=1‌nC
σ

According to Coulomb's law, we have;
F=‌

1

4πEo

‌

q1q2

r2

Now, on putting the given values we have;
F=9×109×‌

1×10−9×1×10−9

0.52

F=9×109×‌

10−18

0.25

⇒F=36×10−9N
‌ Hence, ‌F=36×10−9N