What is a circuit?

An electronic circuit is simply a path for electron flow. It must consist of a source of power, and someplace for that power to go. If there is a gap in that circuit, it’s considered an open circuit and electron flow cannot happen. We will use the positive flow model which will show electron flow from Positive(+) to Negative(-). Also keep in mind that we will generally be dealing with DC or Direct Current. This means that the positive and negative sides of a circuit are discreet and do not change (where as an Alternating Current circuit flips back and forth). This mean the polarity (whether a connection is positive or negative) is especially important while building or diagnosing circuits.

To illustrate this, take a look at the following images.

An open circuit

This image shows an open circuit. Without a path for electron flow, the light does not illuminate. The open section can be considered a switch which will close and allow flow.

A closed circuit

The light is now lit that the path is closed all the way around. This is now considered a closed circuit, electron flow can finish the path from positive to negative.