Parallel circuits differ from series circuits in two major ways. Parallel circuits have multiple branching pathways for electrical current whereas a simple series circuit forms a single path. The components of a parallel circuit are connected differently than they are in a series circuit; the arrangement affects the amount of current that flows through the circuit.
Basic Circuit Structure
Simple circuits consist of a power source, such as a battery, conducting wires to carry current, and devices such as a string of holiday lights for a tree. Each end of the wire is connected to one of the battery terminals. When the circuit is active, electric current flows in a continuous loop, from the positive battery terminal, through the wire and devices, and back to the negative battery terminal.
Current Flow and Resistance
The flow of current in a circuit is different for series and parallel arrangements. The current depends partly on the electrical resistance of the devices in the circuit. For a string of holiday lights, the bulbs are identical so they have the same resistance; therefore, the current flowing through any of them is the same.
In a series arrangement of holiday lights, the bulbs are connected together in a daisy-chain, one after the other. Current flows through each bulb because it has no other paths to follow. Each bulb reduces the total current from the power source; so the more bulbs you have in a series circuit, the less current flows through the circuit.
Light bulbs connected in a parallel circuit resemble the rungs of a ladder, in which each rung is connected to the ladder legs, and all rungs are parallel to one another. In this arrangement, current flowing through one bulb has no effect on the current in the others. Each additional bulb increases the total current, so a string with 20 bulbs uses 20 times the current of a single bulb.