Two large, parallel, metal plates carry opposite charges of equal magnitude. They are separated by 45.0 mm, and the potential difference between them is 360 V. There is a particle with charge +2.40 nC.

Find the change of potential energy of the particle computed from the electric potential.

Answer:

The charge is moving from a higher potential energy to a lower potential energy, with the change in potential given by U = qV = (2.40 109 C)(360 V) = 8.64 107 J, which is equal to the negative of the work done by the electric force.

My question: How does one know if the charge is moving from a higher potential energy to a lower potential energy (making 360 V negative)?