where Q1 represents the quantity of charge on object 1 (in Coulombs), Q2 represents the quantity of charge on object 2 (in Coulombs), and d represents the distance of separation between the two objects (in meters). 

The symbol k is a proportionality constant known as the Coulomb's law constant. The value of this constant is dependent upon the medium that the charged objects are immersed in. In the case of air, the value is approximately 9.0 x 109 N • m2 / C2. If the charged objects are present in water, the value of k can be reduced by as much as a factor of 80. It is worthwhile to point out that the units on k are such that when substituted into the equation the units on charge (Coulombs) and the units on distance (meters) will be canceled, leaving a Newton as the unit of force. 

The charge on an electron, q is -1.6 × 10-19 C. 

1)   Dos globos están cargados con una carga de -6 nC (los hemos frotado contra nuestro pelo, por ejemplo, que está muy arriba en la serie triboeléctrica). Se colocan a una distancia de 60 cm. ¿Cuánto vale la fuerza que aparece entre ellos? ¿Se atraen o se repelen?