Electric current

Basic electricity laws and theorems

Basic electricity laws and theorems

Electricity is the movement of electrical charges flowing through an electrical conductor. This movement is carried out according to specific physical properties collected in some laws and theorems.

The most significant laws and theorems related to basic electrical engineering are:

1. Coulomb's law

Coulomb's law states that the electric force of two charged objects is inversely proportional to the square of the distance between them. Coulomb's law also says that this force is directly proportional to the product of the charges.

This law was first introduced in 1785 by the physicist Charles-Augustin de Coulomb. 

2. Ampère's Law

Ampère's law was developed by the Frenchman André-Marie Ampère in 1831. Ampère's law relates a static magnetic field to the cause that causes it. Later, James Clerk Maxwell corrected it, becoming part of Maxwell's equations.

This law indicates that the circulation of the intensity of the magnetic field in a closed loop is proportional to the electric current that flows in that contour.

3. Ohm's law

Ohm's law states that the electrical power’s current flowing from a conductor connecting two points is directly proportional to the voltage between the two points. It is also inversely proportional to the electrical resistance.

Ohm's law manages to describe the behavior of almost all electrically conductive materials with great precision. However, some conductive materials do not follow this law - non-ohmic conductive materials.

The law owes its name to the German physicist George Ohm. In 1827, George Ohm described the currents and voltages in simple electrical circuits. In his honor, the resistance is expressed in Ohms (ω).

4. Faraday's Law

Faraday's law of electromagnetic induction is a fundamental law of electromagnetism, with:

  • a transformer

  • an inductance element

  • a plurality of generator operation closely.

The law states that:

The magnitude of the induced electromotive force in any closed circuit is equal to the magnetic flux rate of change through the circuit.

Fleming's left-hand rule and right-hand rule are two techniques to know each vector's direction in Faraday’s formula.

Michael Faraday discovered this law in 1831. Joseph Henry found this law before Faraday in an independent study in 1830, but he did not publish this discovery. Therefore, this law is called Faraday's law.

Traditionally, there are two ways to change the magnetic flux through the circuit. First, regarding the induced electromotive force, it is what changes is your own electric field, like changing the field's current (like a transformer.) Concerning the electromotive driving force, what changes is the movement of all or part of the circuit in the magnetic field as in a generator of the same polarity.

5. Kirchhoff's current law (Kirchhoff's first law)

Kirchhoff's current law applies to a current that passes through a node of a closed electrical circuit at a steady state.

According to this Kirchhoff's law, the algebraic sum of the currents entering any node in an electrical circuit (with a different sign if they enter or leave) is zero.

6. Kirchhoff's voltage law (Kirchhoff's second law)

In general, Kirchhoff's voltage law states that the algebraic sum of the voltage drops acting between the pairs of points in space that form any closed (oriented) sequence is equal to zero.

In the most straightforward formulation, the law says that the algebraic sum of the electric potential along a closed line (with the appropriate sign depending on the direction of displacement of the mesh) is equal to zero.

7. Thévenin's theorem

Thévenin's theorem refers to any linear circuit with only voltage and current sources and resistors. The theorem states that if points A and B are available, it is equivalent to a single voltage source V and a single resistor R in series.

Bernard Thévenet's theorem states that any source can be replaced equivalently by an ideal voltage source connected in series and internal resistance.

This theorem is a dual statement of Norton's theorem about the equivalent replacement of an arbitrary circuit with an ideal current source and a resistor connected in parallel.

In other words, the current in any resistor Zn connected to any circuit is equal to the current in the same resistor Zn connected to an ideal voltage source with a voltage equal to the open-circuit voltage of the circuit.

Furthermore, it has an internal resistance Zi equal to the total resistance of the "closed part" of the circuit. The Zn terminal side determines this resistance provided that all the sources within the circuit are replaced by impedances equal to the internal impedances of these sources.

8. Norton's theorem

In the field of electrical circuits, Norton's theorem states that any linear circuit, no matter how complex, seen from two nodes A and B, is equivalent to a real current generator consisting of an ideal current generator in parallel with an endurance. The equivalence is limited to the voltage and current at nodes A and B.

Norton's theorem is an extension of Thévenin's theorem and was obtained in 1926 by two different people at the same time:

  • Hans Ferdinand Mayer (1895-1980), a Hause-Siemens researcher

  • Edward Lawry Norton (1898-1983), a Bell laboratory engineer.

Only Mayer published his work, but Norton released his work through an internal Bell Laboratories technical report.

9. Superposition theorem

The superposition theorem states that in a linear circuit with more than one independent source, the effect of all sources on an impedance is the sum of the impact of each source independently, replacing other voltage sources with a short circuit and replacing all other current sources with an open circuit.

Publication Date: September 12, 2021
Last Revision: September 12, 2021