Electric current

What is electric pressure? Definition of potential difference

What is electric pressure? Definition of potential difference

The electric pressure - also known as voltage or electrical potential difference - is a physical scalar quantity that indicates the difference in electrical voltage between two points in an electrical circuit. It coincides with the electrical voltage and shows the energy or work necessary to separate electrical charges of opposite signs.

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, a changed sign.

If we join two points in a circuit with a potential energy difference through a conductive material, it will generate a flow through the closed circuit. This is because 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 voltmeter is an electronic device measuring the potential difference between two points.

Electric pressure concept with an example: hydraulic analogy

The concept of voltage in a DC circuit can be explained by analogy with a hydraulic circuit. The difference in tension can be associated with the pressure difference 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 equivalent hydraulic circuit. Furthermore, the equivalent of the dissipation of electrical energy would be a consequence of the friction of the liquid with the internal walls of the pipe, reducing its kinetic energy. Finally, the intensity of the electric current flowing in the conductor can be compared to the liquid flow rate 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.

What is the direction of the current in an electrical circuit?

The direction of current in an electrical circuit is defined as the direction of the flow of positive charge carriers. Historically, it was believed that electric current flows from positive to negative terminals, and this convention is still used in some cases. In reality, electrons drive the current in most materials, and electrons are negatively charged. Therefore, the actual direction of electron flow is opposite to conventional flow.

To avoid confusion, the direction of current flow is usually assumed to be from the positive terminal to the negative terminal of a battery or power supply, even though electrons flow in the opposite direction. In a circuit diagram, the direction of the arrow used to represent the current flow is also assumed to be the direction of the positive charge flow. 

Voltage in a static electric field

The 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 electric 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

Ohm's law is at the base of the behavior of circuits with a purely resistive load. It defines that the electric potential applied to resistance is proportional to the amperage and the resistance:

V = I·R

The resistance provokes a voltage drop 

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 = V·I

This phenomenon is called the Joule effect.

What does a coulomb of charge mean?

A coulomb (C) of charge is a unit of electric charge in the International System of Units (SI), named after the French physicist Charles-Augustin de Coulomb. One coulomb of charge is equivalent to the charge transferred by a steady current of one ampere in one second.

It is defined as the amount of charge that passes through a cross-section of a conductor in one second when a current of one ampere flows through it. One coulomb of charge is also equivalent to the charge of approximately 6.241 x 10^18 electrons.

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 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

High voltage is divided into first, second, and third categories or particular category voltage in some countries.

Data de publicació: October 8, 2021
Última revisió: September 8, 2021