Two isolated, positively charged spheres are held together with an insulating string of negligible mass so that the centers of the spheres are a
distance L apart. Sphere 1 has charge Q₁ and mass M₁. Sphere 2 has charge 2Q, and mass 2M₁. The two-sphere system has internal energy U.
What is the internal energy of the system after the string is cut and the spheres have moved far away from each other?