Two identical conducting spheres with negligible volume have 2.1 nC and –0.1 nC charges, respectively. They are brought into contact and then separated by a distance of 0.5 m. The electrostatic force acting between the spheres is__________× 10^–9 N. [Given : 4πε0 = 1/9×10^9 SI unit] respectively. They are brought into contact and then separated [...]