Describe a situation in which the net external force on a system is not zero, yet its speed remains constant.

One splits the atoms and molecules  the other combines them

Electrons have a negative charge if that's what you're asking

Current supplied to consumers from generating plants like coal power plant, nuclear power plant, geothermal and others are in form of alternating current instead of direct current because it can be transported easily in a long distance. also, the alternating current has a wide range of frequency and the amount or energy can be increased or decreased.