# Circuit

An electrical circuit is a system of conductors that form a closed path. The term conductor includes any medium that has mobile charge carriers and is therefore capable of transporting electrical charges . Everyday examples are copper cables as conductors through which electrons move. In order for electrical current to flow, energy must be supplied to the system , which is then transported in the circuit as electrical energy and finally converted back into another form of energy .

In electrical installation , each part of an electrical system that can be separated by its own fuse or an LS switch is referred to as a circuit.

On the other hand, one speaks of an open circuit if the path discussed at the beginning is interrupted at at least one point. An interruption can occur unintentionally due to a loose contact or a missing line or intentionally due to an electrical switch . A closed circuit can also be unintentional, see e.g. B. short circuit , body current or local element .

Closed and open circuit (with circuit symbols according to EN 60617)

## Elementary circuit

The two circuits shown as a circuit diagram each show

In the upper circuit, the switch (and thus the circuit) is closed; if the design is appropriate, the incandescent lamp lights up, indicating that the current is flowing.

The switch in the lower circuit is open, current is not possible. The light bulb does not light up.

## Components of a circuit

### source

In almost all versions, the electrical energy comes from a voltage source. At its output, it generates an electrical voltage that is almost independent of the respective consumer . The electrical current that it delivers at the same time depends largely on the consumer. These sources are widespread, for example accumulators and batteries supply a direct voltage , whereas the power grid and the secondary side of a transformer supply alternating voltage .

### consumer

The components in which the electrical energy is converted into another form of energy are often referred to as electrical consumers . The strength of the electrical conductor current is set on the one hand by the level of the voltage, on the other hand by a property of the consumer, which is referred to as electrical resistance . The greater the resistance, the lower the strength of the current supplied with voltage sources. Instead of specifying its resistance, the consumer is partially characterized by specifying its electrical power at nominal voltage .

A consumer can often be viewed as a linear resistance , for example an ohmic resistance . But consumers with non-linear behavior such as semiconductor components, magnetic components ( chokes , transformers) and incandescent lamps are also used.

A resistance can be as large as desired for voltage sources. In this sense, an open switch represents an infinitely high resistance. However, it must not be arbitrarily small, because then a short circuit occurs. The level of the short-circuit current only depends on the internal resistance of the voltage source. With powerful sources, the high current can melt the conductor, cause a fire or explode a battery.

The connections drawn in circuit diagrams are considered to be lossless. If a real electrical conductor does not meet the idealization of negligibly small line coverings, it must be viewed as a consumer.

The switch shown is single-pole. He can interrupt or close the circuit. For a potential-free separation of the consumer from a non-potential-free source, a two-pole switch is required that interrupts both conductors. Otherwise the consumer can still be at dangerous voltage (compared to "ground" ), even without an electrical current flowing.

## Series and parallel connection

Closed circuits consisting of a voltage source and two resistors,
above in series connection: without branching,
below in parallel connection: with branching

With further sources or consumers, depending on their arrangement, series or parallel connections are created. If all components are connected in series without a branch, then it is a series connection . The same current flows through them all. The total voltage is divided between the individual components.

If components are connected in parallel to one another, there are several routes for the current. Each of these paths is called a branch of electricity . The same voltage is applied to parallel loads. The total flow is divided between the individual branches.

Circuits that contain elements of both series and parallel circuits are called mixed circuits . Every mixed circuit can be traced back to series and parallel circuits. The regularities of these circuits are described by the Kirchhoff rules .

A number of other circuits are listed under Electrical Circuit . A more generalized treatment can be found under Network (electrical engineering) .