The attractive electrostatic force between the point charges +8.44 ✕ 10-6 and q has a magnitude of 0.961 n when the separation between the charges is 0.67 m. find the sign and magnitude of the charge q.

The electrostatic force between two charges Q1 and q is given by

where 
ke is the Coulomb's constant
Q1 is the first charge
q is the second charge
r is the distance between the two charges

Re-arranging the formula, we have

and since we know the value of the force F, of the charge Q1 and the distance r between the two charges, we can calculate the value of q:


And since the force is attractive, the two charges must have opposite sign, so the charge q must have negative sign.