Solve the following problems:
Problem I: What is the total charge of the neptunium nucleus? (The neutral neptunium atom has 93 electrons.)
Problem II: What is the magnitude of its electric field at a distance of 2.16×10-10 m from the nucleus?
Problem III: What is the magnitude of the force on an electron at that distance?
Problem IV: What would the magnitude of the force be if the distance of the electron from the nucleus were quadrupled?
Write your solutions in appropriate format and provide references.