Si tienes una línea de 120V ¿Podrías iluminar varias lámparas de 6V sin quemarlas?

a and b sometimes have a positive value

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.

the moving of the box is a result of centripetal force. think of that  as a car or a truck in this case creates a turn, the force of friction performing upon the turned wheels of the car runs centripetal force obligatory for circular motion for the things inside it.