An electric circuit is a system of conducting elements designed to control the path of electric current for a particular purpose. Circuits consist of sources of electric energy, like generators and batteries; elements that transform, dissipate, or store this energy, such as resistors, capacitors and inductors; and connecting wires. Circuits often include a fuse or circuit breaker to prevent a power overload.

Devices that are connected to a circuit are connected to it in one of two ways: in series or in parallel. A series circuit forms a single pathway for the flow of current, while a parallel circuit forms separate paths or branches for the flow of current. Parallel circuits have an important advantage over series circuits. If a device connected to a series circuit malfunctions or is switched off, the circuit is broken, and other devices on the circuit cannot draw power. The separate pathways of a parallel circuit allow devices to operate independently of each other, maintaining the circuit even if one or more devices are switched off.

The first electric circuit was invented by Alessandro Volta in 1800. He discovered he could produce a steady flow of electricity using bowls of salt solution connected by metal strips. Later, he used alternating discs of copper, zinc, and cardboard that had been soaked in a salt solution to create his voltaic pile (an early battery). By attaching a wire running from the top to the bottom, he caused an electric current to flow through his circuit. The first practical use of the circuit was in electrolysis, which led to the discovery of several new chemical elements. Georg Ohm discovered some conductors had more resistance than others, which affects their efficiency in a circuit. His famous law states that the voltage across a conductor divided by the current equals the resistance, measured in ohms. Resistance causes heat in an electrical circuit, which is often not wanted.