Series and Parallel Circuits

Definitions and Basic Concepts

A series circuit is a circuit where components are connected end-to-end, forming a single path for electrons to flow.
A parallel circuit is a type of circuit where the components are connected across each other, forming separate paths for electrons to flow.
A component in a circuit is a generic term for any electrical element in the circuit which can include a resistor, switch, capacitor, inductor, or a source of electrical power.

In a series circuit, the current is the same across all components. However, the voltage is divided amongst the components.
The total resistance (Rt) of a series circuit is the sum of the resistances of each component. In other words, Rt = R1 + R2 + R3…
If one component fails (e.g. if a bulb breaks) in a series circuit, all components will stop working.

In a parallel circuit, the voltage is the same across all components. But, the current is divided amongst the components.
The total resistance (Rt) in a parallel circuit is found using the formula: 1/Rt = 1/R1 + 1/R2 + 1/R3 …
If one component fails in a parallel circuit, it does not affect the functioning of the other components.

Series circuits are used in Christmas tree lights, in old-style homes where one switch controlled all lights, and in some low-voltage landscape lighting.
Parallel circuits are used in wiring of most buildings, in battery cells, in straw-bridge wiring of resistors and in combination with series circuits to create combination circuits.

Always remember to convert units appropriately when using resistances in these equations.
Be careful when drawing circuit diagrams. Remember to use the correct symbols for each component and make sure the connections are clear.
The position of voltmeters and ammeters in these circuits matter - an ammeter is connected in series to measure current, while a voltmeter is connected in parallel to measure voltage.
In mixed circuits (circuits that are a combination of parallel and series), you may have to use a combination of the rules for series and parallel circuits.