Electric current

What Is Voltage or Electrical Tension?

The voltage or electrical potential difference is a physical quantity that indicates the difference in electrical voltage between two points in an electrical circuit. It coincides with the electrical voltage and indicates the energy or work necessary to separate electrical charges of opposite sign.

What is voltage or electrical tension?

It is the difference between the potential electrical energy that a charge has at the two points due to the presence of an electric field divided by the value of the charge itself. Under stationary conditions it is equal to the work done to move a unit charge through the field from one point to another, changed sign.

If we join two points that have a potential difference through a conductive material, a flow of electrons will occur through the closed circuit that we have created. Part of the charge will move from the point of highest potential to the point of lowest potential (electric current).

The definition of electrical voltage and the unit of measurement volt is due to Alessandro Volta, who, together with the concepts of "electrical capacity" and "electrical charge", used for the first time the concept of "electrical voltage" to account for the intensive and extensive properties of electricity.

What Is the Unit of Measurement for Voltage?

Voltage is measured in volts (V) according to the international system of units. The electronic device for measuring the potential difference between two points is the voltmeter.

Voltage Concept with an Example: Hydraulic Analogy

The concept of voltage can be explained by making an analogy with a hydraulic circuit. The difference in tension can be associated with the difference in pressure that is generated in a closed pipe filled with liquid with the ends placed at different heights. The voltage between two points of the electrical circuit corresponds to the pressure difference between two points of the hydraulic circuit.

The potential difference between the poles of the electric generator can be seen as the pressure difference of the tanks of the analogous hydraulic circuit. The equivalent of the dissipation of electrical energy would be as a consequence of the friction of the liquid with the internal walls of the pipe. Finally, the intensity of the electric current flowing in the conductor can be put in analogy with the flow rate of liquid in the tube.

In this analogy, the flow of water can flow from a point of high pressure to a point of low pressure. In the same way, charges that move between two points with different potentials constitute an electric current.

Voltage in a Static Electric Field

Electric voltage across a path is defined as the amount of work per unit charge developed by the electric field to move an electric charge.

Since the field is conservative under stationary conditions, it admits potential and, therefore, the line integral of the electric field depends only on the extremes of integration. In this case, the voltage is equal to the potential difference.

Electromotive Force Induced by a Variable Magnetic Field

In a coil that encloses a surface traversed by a magnetic flux, an electromotive force is generated proportional to the rate of change of the flux over time.

A potential difference is also generated between the ends of an electrical conductor that moves perpendicular to a magnetic field.

When we subject a conductive material to a variable electromagnetic field, an electrical voltage is produced called induced voltage.

Ohm's Law

At the base of the behavior of circuits with a purely resistive load, is Ohm's law. It states the voltage applied to a resistance is proportional to the intensity and the resistance:

V = IR

The electric current that passes through a resistive component (R) generates a power dissipation whose value is given by the product of the intensity (I) and the potential difference (V):

P = VI

This phenomenon is called the Joule effect.

How Are the Different Types of Voltages Classified?

The classification of electrical voltage is different depending on the area to which it refers and the type of alternating current or direct current.

In particular, according to the provisions of the CEI-EN 50110-1 standard "Operation of electrical systems", the electrical voltage is classified as indicated in the following table:



In alternating current

In direct current

Very low voltage



≤ 50 V

≤ 120 V (in rippled direct current)

Low voltage



50-1,000 V

120-1,500 V

Medium voltage



1-30 kV

1.5-30 kV

High voltage



> 30 kV

> 30 kV

In some countries, high voltage is divided into first, second and third category or special category voltage.


Published: October 8, 2021
Last review: September 8, 2021