An open circuit is a type of electrical circuit that has some type of opening or gap along the circuit that makes current no longer flow properly. This is in contrast to a closed circuit that has been designed and implemented properly to allow the electrical current to flow. It is, however, opposite to a short circuit in which part of a circuit is established that allows current to bypass resistors and therefore increase the flow of current through the circuit. This is because an open circuit has infinite resistance at the gap or break, since it allows no current to flow through the circuit.
In general, a circuit is any type of system designed to allow electrical current to flow through it. Whether the circuit utilizes direct current (DC) or alternating current (AC) does not matter, though for illustrative purposes DC electricity is often easier to reference. DC electricity runs in one direction through a circuit, traveling with the flow of electrons from a negative charge toward a positive. A simple circuit could consist of a battery connected to the circuit at both its positive and negative ends, and a resistor that can receive the current.
The electrical current flows from the negative end toward the positive and is utilized by the resistor, typically to provide power to a part of the device. This simple circuit can be made into an open circuit simply by creating a gap or opening in the circuit at any point. It does not matter if the gap is created before the resistor or after the resistor; if the gap is created at any point, it becomes an open circuit and current no longer flows properly. This is because the open circuit disrupts the connection at some point and so the current can no longer flow from negative to positive.
For mathematical purposes, the gap or opening on an open circuit is considered to have infinite resistance, since it completely stops the flow of electrical current. This makes an open circuit the opposite of a short circuit, which establishes an alternative route for current to flow in a circuit that avoids resistors and thereby reduces resistance in a circuit. For illustrative purposes, an open circuit is often seen as the opposite of a closed circuit, which simply refers to any complete circuit in which current can flow. A circuit will often be designed to be either open or closed depending on the position of a switch in the circuit, allowing a user to change the position of the switch and turn a device powered by the circuit on or off.