Electric circuits and components

An electrical device that provides a path for electrical current to flow is called an electric circuit.

Atoms

A basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons is the atom. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons. The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. Anion is an atom with extra electrons or with a deficiency of electrons, resulting in its being electrically charged. An ion with extra electrons is negatively charged and is called an anion, an ion deficient electrons is positively charged and is called a cation.

Conductors, insulators, resistors, semiconductors

In some materials, electrons move easily from atom to atom. In others, the electrons move with difficulty. And in some materials, it is almost impossible to get them to move.  An electrical conductor is a substance in which the electrons are mobile. Electrons in a conductor do not move in a steady stream. Instead, they are passed from one atom to another right next to it. This happens to countless atoms all the time. As a result, literally trillions of electrons pass a given point each second in a typical electrical circuit.

Materials that does not conduct electricity is called an insulator. It has zero conductivity and infinite resistivity.

Resistors are electrical circuit elements with a fixed resistivity. They allow for the control of current flow. The better a resistor conduct, the lower its resistence; the worse it conducts, the higher the resistance. Electrical resistance is measured in units called ohms. The higher the value in ohms, the greater the resistance, and the more difficult it becomes for current to flow.

In a semiconductor, electrons flow, but not as well as they do in a conductor.

Circuit components

Electric energy is transmitted through conductors from the source of electromotive source to a consuming device, usually called a load (zátěž), where it is converted into other forms, such as heat, light, mechanical or magnetic. Thus, in its simplest form, the circuit consists of a source, a load, and connecting wires.

Other electrical elements may be included, among them being devices that perform control, regulating and measuring functions. Electrical circuit then could interconnect batteries, which supply low voltages, generators, lamps, resistors, coils (cívky) or transformers, capacitors, various types of diodes and transistors, and fuses (pojistky). A switch, which lets the current pass through, is usually included as well.

Electric charge

Charge is a basic electrical quantity. Ordinary matter is made up of atoms which have positively charged nuclei and negatively charged electrons surrounding them, The unit of electrical charge in the International Systems of Units is the coulomb (C), where 1C is equal to approximately 6.24 x 10¹⁸ elementary charges.

In an ordinary atom, the number of protons equals the number of electrons, so the atom normally has no net electric charge. An atom becomes negatively charged if it gains extra electrons, and it becomes positively charged if it loses electrons ; atoms with net charge are called ions. Every charged particle is surrounded by an electric field, the area in which the charge exerts a force. Particles with nonzero electric charge interact with each other by exchanging photonos, the carries of the electromagnetic force. The strength and direction of the force charged particles exert on each other depends on the product of their charges : they attract each other if the  product of their charges is negative and repel each other if the product is positive. Thus two electrons, each with charge -1, will repeal each other, since -1 x -1 = +1, a positive number. Static electricity  consists of charged particles at rest, while electric current consists of moving charged particles, especially electrons or ions.

Electric current

Electric current is a flow of electricity  through a conductor. It is a movement of electrically charged particles, atoms, or ions, through solids, liquids, gases, or free space. The current is measured as the rate of movement of charge per unit time, and is counted in units of amperes. The direction of  movement of electric current is considered as the same as the direction of movement of positive charge,  or in a direction opposite to the movement of negative charge. Electric current move constantly in a single direction, called direct current (DC), or may move alternately in one direction and then the opposite direction, called alternating current (AC).

Kirchhoff’s  Circuit Laws

Kirchhoff’s Circuit Laws, firstly described in 1845, are a pair of rules that deal with the principle of conservation of charge which states that : the total electric charge of a closed system remains constant over time, or in other words that the electric charge can neither be created nor destroyed.

1)      Kirchhoff’s Current Law implies that “the sum of all current entering a node (uzel obvodu) is equal to the sum of all currents leaving the node.”

                          i₁ + i₄ = i₂ + i₃

2)      Kirchhoff’s Voltage Law states that “the directed sum of all the voltages around a loop(uzavřený obvod) must be zero.”    V₁+v₂+v₃+v₄=0