Joule effect (Joule’s law)

Joule’s law states that a resistance conductor \(R\), crossed by a direct current \(I\), transforms the following amount of energy \(\Delta E\) into heat in a time interval \(\Delta t\):

\[\Delta E = RI^2\Delta t\]

In terms of power, it is observed that the electric power dissipated in heat by a conductor is:

\[P=\dfrac{\Delta E}{\Delta t}=\dfrac{RI^2\Delta t}{\Delta t}=RI^2\]

Applying Ohm’s law is obtained by another equivalent expression:

\[P=\dfrac{\Delta V^2}{R}\]

Technical problems of the Joule effect

The Joule effect is particularly important for the purposes of the construction and operation of electrical equipment. The heating phenomenon of the conductors to the passage of electric current is almost always harmful unless it is specifically sought, as in the case of the electric heater.

When an electrical conductor is crossed by excessive electric current, overcurrent in technical terms, it may be subject to irreversible damage due to excessive heating. It is proven that the main cause of fires in civilian homes is due to the effect of overcurrents in electrical systems.

Joule heating

Joule heating, also known as resistive, resistance, or Ohmic heating, is the process by which the passage of electric current through a conductor produces heat. A conductor crossed by electric current heats up due to the friction caused by the impacts of the flow of electric charges against the atoms constituting the conductor itself. In other words, it takes place in a transformation of electrical energy into heat.

What has been said can be explained taking into account the electrical resistance concept: the conductive material within which occurs the passage of electric current opposes the circulation of the current, requiring an expenditure of energy because such circulation is ensured. The electrons passing from a lower electric potential to a higher potential, give off electric potential energy; this is dissipated in heat through the increase in molecular agitation due to the collisions between the electrons and the atoms of the crystal lattice.

This process of transforming electrical energy into thermal energy was studied by James Prescott Joule (1818-1889) and is known as the Joule effect.