If an object with charge -3 nC moves from a location that has a potential of -20 V to a location with a potential of -10 V, what has happened to the potential energy of the system?

The potential energy decreases.

Explanation:

Given-

Charge of the object, q = -3nC

Initial potential, Vi = -20V

Final potential, Vf = -10V

Potential energy, U = ?

We know,

Potential, V = U/q

initial potential energy = Viq

                                                        = -20V X -3nC

                                                        = 60 nJ

Final potential energy = Vfq

                                     = -10V X -3nC

                                     = 30nJ

We can see that the potential energy decreases as the object moves from initial to final position which means that the work is done against the electric field.