 ## Image description

The image shows the diagram that explains, through an electrical circuit, the first law of the physicist Kirchhoff, that the sum of the intensity of electric currents entering a node in the circuit, is equal to the sum of the intensity of those leaving that common network node and is zero.

The common network node is located in the center, represented by an embossed circle.

The network branches between the electrical components is marked by bold lines.

The two electric currents that meet in the central common point and that enter through it are rendered by two arrows, down and right, with the tip directed towards the center.

The current starting from the right passes through a source, this being represented by a bulb or a lamp, illustrated by the specific symbol, respectively a small blank circle, crossed on its center by an x.

The two electric currents that met in the central common point and that leave it are shown by another two arrows, up and left, with the tip pointing outwards, respectively upwards and to the left.

The upward current leaves the node and passes through a resistor, continuing its path.

The resistor at the top, to the left, is a component of the electrical circuit with the role of reducing the current voltage between two points and is highlighted by a blank rectangle.

The current to the left leaves the node and enters an electric battery, continuing its route.

The electric battery is represented by the specific symbol, respectively, two parallel lines, spaced between them, the longer line being on the left and the shorter on the right, denoted by the plus and minus for its electrical charges.

## General information

Gustav Kirchhoff, a German physicist, has developed two laws related to the conservation of electrical current within the electrical circuits.

First of all, let’s summarize what an electrical circuit means: it is the path the electrical current has to travel between a power source and an electrical powered device. The circulation or travelling of electrical current between the source and the consumer is achieved using electric conductors, such as the connecting wires. In order to have a clearer image regarding the electrical circuit, we will describe a simple circuit, consisting of a battery, two connecting wires and an electric motor. We know that the battery has two terminals, meaning two ends through which it can be connected to a power source. In the case of direct current (DC), which is present in a battery, the movement of electrons, represented by the electrical current, always takes place from the positive terminal to the negative terminal. At the two ends of the motor the two connecting wires are attached. If we make the two wires from the motor touch the terminals of the battery, then the electrical circuit will be created. The current will enter through the positive end, go through the motor coil and exit through the negative end, returning to the battery. The electric current passing through the motor will determine the latter to function. At first, we could say it is an everlasting process, which is endless, unless we would disconnect the electric circuit. However, because a portion of the electrical current converts into rotation movement, therefore it does not return to where it originated, the current from the battery will be used up in time.

In order to understand Kirchhoff’s laws, we must know the main elements of an electric network. An electric network or power grid can consist of one or several power sources, as well as one or more consumers. The main elements are:

• the network node or junction, meaning the point in which at least three connecting wires are linked to each other;
• the network branch, meaning the area between two consecutive nodes;
• the network loop, meaning the closed polygonal path, formed by a succession of several sources or consumers of electrical current.

The first law by Kirchhoff refers to the conservation of electrical charge within a network node. Further on we will quote the text of the law: “the algebraic sum of the intensity of electric currents meeting in a network node equals zero” or “the sum of currents entering a junction equals the sum of currents leaving that junction” (1). This means that in any given network node the sum of the electrical currents entering that node will always be equal to the sum of electrical currents that exist in it.

To conclude, knowing the text of Kirchhoff’s laws, we can state that these laws are useful while solving equations with one or more unknowns or variables regarding the tension and intensities of the electrical current in certain points within an electrical network, based on some already known data.