Why is there only one circuit in the series connection?

Current flow direction is not dependent on whether or not it's in a series or parallel circuit. Current directional flow is solely determined by the power source. For businesses and residential power, utilities use various forms of alternating current.

For example, the United States utilizes alternating current that changes directional flow 60 times per second. Some parts of Europe use alternating current that changes direction 50 times per second.

Many electronic devices use a direct current source in which the current flow is only in one direction, from negative to positive or a converter that changes the alternating current source power to direct current.