Series and Parallel Circuits 1: The Basics

This is Part 1 of a 5 Part Series

  1. Series and Parallel Circuits 1: The Basics (this article)
  2. Series and Parallel Circuits 2: Resistors
  3. Series and Parallel Circuits 3: Capacitors
  4. AC Series and Parallel Circuits 4: Inductors and Capacitors
  5. Series and Parallel Circuits 5: More About Circuits

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Electrical circuits come in two basic configurations, series and parallel.  Examples of each are shown below.[1]  Each circuit has an input and an outlet to the power source (batteries in this case).   

[1] Adapted from:

In each case, electrical current (I) flows from the positive side of the power source to the negative side of the power source.  However, since electrons are negatively charged, electron flow (i.e., what is really happening in almost all circuits) is in opposite direction compared to current, so electrons flow from negative to positive (thanks Ben Franklin![2]).

It is important to note that many parallel circuits have portions that are series circuits such as shown below, since they have one output (+ terminal of the battery in this case) and one return (- terminal of the battery in this case).  Additionally, nodes denote where each circuit element begins and ends.  Typically, each of the terminals of a device (series or parallel) is a node as shown below.  In circuit diagrams it is assumed that all wires are ideal, having zero resistance.  Thus, a node may consist of the entire section of wire between elements, not just a single point.


Generally, there is a voltage change (e.g., a voltage increase, in the case of a battery, or a decrease in the case of a lamp or resistor, flowing current, etc.) across the two nodes of a component operating within a circuit.  Since each leg of a parallel circuit terminates at the same node, the voltage change across each parallel leg of a circuit that connects to a common set of nodes must be the same as shown below.  In these circuits, the overall voltage drop of the circuit is the same as that of the power source (e.g., the battery).

A series circuit is โ€œopenโ€ (switched off) if any portion of the circuit is open, while opening a leg of a parallel circuit disconnects only that leg of a parallel circuit as shown below.

Further, in a series circuit, the current is the same throughout the circuit, while the currents in each leg of a parallel circuit add up to the total current as shown below.

An understanding of these key basics helps to enable a designer to configure myriad circuits.  Next, we will discuss how different components (resistors, inductors, capacitors, diodes, batteries and certain combinations) act in series and in parallel circuits, as well as a few classic circuits and their functions.


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