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Joule effect

Joule effect

The Joule effect, also called Joule's law, is the thermal manifestation of electrical resistance. If electricity circulates in an electric conductor, a part of the kinetic energy of the electrons is transformed into heat due to the shock that the electrons experience with the molecules of the conductor through which they circulate, which increases the temperature of the conductor. It is named in honor of the English physicist James Prescott Joule.

Definition of the Joule effect:

The amount of heat energy produced by an electric current is directly proportional to the square of the intensity of the current when it is flowing through the conductor and to the resistance that this conductor opposes to the passage of the current.

This definition can be expressed mathematically as follows:

Q = I2 x R x t

where:

Q = Heat energy produced by current
I = Current intensity circulating
R = Electrical resistance of conductor
t = Time

The operation of the bulbs is based on the Joule effect: the filament is a resistance that with the passage of the current heats up to become incandescent.

In the expressed formula of the definition of the Joule effect, the magnitudes must be expressed in the same system of units. Thus, if we express the intensity in amperes (A), the resistance in ohms and the time in seconds, we obtain the heat produced in joules (J).

Many appliances are based on the Joule effect to work: electric ovens, toasters, electric heaters ... In all these cases, it is intended to generate thermal energy with electricity passing through its conductors. This heat they give off is due to the Joule effect.

In the vast majority of applications, however, it is an undesired effect and the reason why electrical and electronic devices need heatsinks, apart from one or more fans that scare away the heat generated and thus avoid excessive heating of the different components and / or devices. In these cases, heat is lost energy and therefore a decrease in efficiency.

Relation between the Joule effect and thermodynamics

The Joule effect has a special connection with the second law of thermodynamics. The second principle of thermodynamics states that: "The amount of entropy in the universe tends to increase over time."

The second principle of thermodynamics then establishes the irreversibility of physical phenomena, especially during heat exchange.

Joule's law in the more general formulation involves the transformation of electric energy into other forms of energy in which the heat energy developed is only an undesired effect and, insofar as it can be neglected, some examples of transformations of energy. energy regulated by Joule's law: mechanical energy (electric motors), light (discharge lamp, LED), electromagnetic waves (antennas, lasers), chemistry (electrochemistry) ...

In this more general formulation of Joule's law, from a principle point of view, the product of voltage for current transforms electrical energy into other forms of energy in principle reversibly, without the limitations imposed by thermodynamics.

For example, in electric motors where electrical energy is transformed into mechanical energy, an efficiency can be defined as the ratio between electric power (Joule I · V law) and mechanical power, even if currently the most efficient electric motors they do not exceed 50% efficiency due to the electrical resistance of copper, the best existing conductor, the possibility of greater efficiency has been demonstrated with motors with superconducting windings. Therefore, it is possible to conceive a reversible transformation in which all electrical energy is transformed into mechanical energy.

In the case of antennas, the efficiency of the antenna is defined as the ratio between the radiated power and the alternating average power supply and, in this case, efficiencies greater than 90% are achieved.

In terms of light, the luminous efficiency is linked to the relationship between the power dissipated by the Joule effect and the light energy useful for the perception of the human eye. In this case, while ordinary incandescent lamps have a typical efficiency of 2%, a discharge lamp can have a luminous efficiency of 29%. If we could find an efficient mechanism to transform electricity into green light (for which human perception is maximum), luminous efficiency would be 100%.

Therefore, the limitations of the second law of thermodynamics do not apply to Joule's law if it is interpreted in a non-reductive manner.

Electricity high-voltage alternating current transmission

The overhead power lines transfer electricity from electricity producers to consumers. These power lines have a resistance other than zero and, therefore, are subject to the Joule effect or Joule heating, which causes losses in the transmission.

The division of the power between the transmission losses (Joule heating in the transmission lines) and the load (useful energy delivered to the consumer) can be approximated by a voltage divider. To minimize transmission losses, the resistance of the lines should be as small as possible compared to the load (resistance of consumer appliances). The resistance of the line is minimized by the use of copper conductors, but the specifications of the resistance and the power supply of the consumer devices are fixed.

Usually, a transformer is placed between the lines and the consumption. When a high-voltage, low-current electrical current in the primary circuit (before the transformer) becomes a low-voltage, high-intensity current in the secondary circuit (after the transformer), the equivalent resistance of the secondary circuit increases and losses of transmission are reduced proportionally.

During the War of Currents, the installations of alternating current could use transformers to reduce the losses of line by the heating of Joule, at the cost of a greater voltage in the lines of transmission, in comparison with the facilities of direct current.

Heating efficiency and the Joule effect

As heating technology, Joule heating has a coefficient of performance of 1.0, which means that each joule of electric power supplied produces a joule of heat. In contrast, a heat pump can have a coefficient of more than 1.0, since it moves additional thermal energy from the environment to the heated element.

The definition of the efficiency of a heating process requires defining the limits of the system to be considered. When heating a building, the overall efficiency is different when considering the heating effect per unit of electrical power supplied on the customer's meter side, compared to the overall efficiency when also considering the losses at the power plant and the transmission of energy.

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Last review: November 28, 2018

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