They do not change or reverse. The term resistance is generally used when referring to DC voltages. Impedance is the general term, and is the measure of how the flow of electrons is resisted, or impeded. Voltage, Current, Resistance Relationship Currents only flow in closed loops called circuits. These circuits must be composed of conducting materials, and must have sources of voltage.
Voltage causes current to flow, while resistance and impedance oppose it.
Knowing these facts allows people to control a flow of current. The term ground can be difficult concept to understand, completely, because people use the term for many different purposes. When you use an electric appliance that has a plug with three prongs, the third prong is the ground. It gives the electrons an extra conducting path to flow to the earth, rather than through your body. Ground can also mean the reference point, or the 0 volts level, when making electrical measurements.