An electric circuit works by providing a closed loop to allow current to flow through a system. Electrons must be able to flow throughout the circuit, completing a path from one pole of the power source to the other. Along the way, this flow of electrons can be used to power lights or other electrical devices. Any interruption in the pathway halts the flow of current.
One example of a simple circuit is a battery connected to a lightbulb. Current flows from one terminal of the battery through a wire until it reaches the lightbulb and flows across the filament of the bulb, which converts some of the energy to light. Then, the current flows through a second section of wire to the opposite terminal of the battery, completing the circuit. Any break in this pathway, whether it is a physical wire disconnection or a switch designed to temporarily break the circuit, shuts down the flow of current and turns off the lightbulb.
In a series circuit, the current flows through a number of lights or other devices connected in a row, and if one fails or is disconnected, it disrupts the current for the entire system. Conversely, parallel circuits split the current flow into multiple paths, so a single failure does not shut down the entire circuit.