Voltage Sources: batteries and power supplies
Because of the “pumping” nature of voltage sources, we need to have a complete circuit before we have a current.
If we have an air gap (or rubber gap) in the circuit, no current will flow - just like if we have a solid block (like a cap) in a water circuit, no water will flow.
If the gap is small, and the voltage is high enough, the current will cross over the gap - somewhat like water, if the pressure is high enough, will break through a plug.