This is the case for a neutral atom. Current flows from negative material to positive material and is essentially the number of electrons per second that are carried through a conductor. If a potential difference exists, then energy can be extracted. Property of material that helps prevent the flow of electrons in it. Metals are good conductors due to low resistance.
Wood is a poor conductor due to high resistance.
Resistance, R , is measured in ohms and depends upon both the type of material and its size. R is also temperature dependent. We can control this buy putting the right resistance in the circuit for a given battery Voltage. This is a linear relation. Standard US household voltages in an outlet is 120 Volts.