Final answer:
To find the magnitude of the electric force on one of the masses, use the equation F = qE if the electric field and the charge are known, otherwise use Coulomb's Law, which accounts for the charges and distance between them.
Step-by-step explanation:
To determine the magnitude of the electric force on one of the masses, we would typically use Coulomb's Law, which states that the electric force (F) between two charges is proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. However, if we know the electric field (E) and the charge (q), we can use the equation F = qE to find the force on a charge in an electric field. This relation comes from the concept that the electric force is the charge multiplied by the electric field strength. Keep in mind that for two massive, positively charged particles, as they are moved farther apart, their mutual electrostatic force will change as an inverse square of the distance, just like their mutual gravitational force.