Name: isothermal expansion and heat transfer
Uploaded: 2025-08-19T15:10:28.000Z
Duration: 1 min 21 s
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Isothermal Processes

Un processus thermodynamique qui se produit à température constante est appelé processus isotherme.
La conversion de l’eau bouillante en vapeur à une température constante de 100 degrés Celsius est un exemple de processus isotherme. De même, la conversion de l’eau en glace à 0 degré Celsius est également un processus isotherme.
La variation de l’énergie interne est nulle pour ces processus car la température ne change pas. Ainsi, en utilisant la première loi de la thermodynamique, le travail effectué est égal à la chaleur.
Considérons maintenant un système de gaz parfait enfermé dans une seringue à gaz scellée à une température constante.
Lorsque le piston est poussé lentement de manière contrôlée, le gaz à l’intérieur de la seringue est comprimé. Le volume réduit du gaz provoque une augmentation de la pression, mais la température reste constante.
Le relâchement contrôlé du piston entraîne une expansion du gaz à température constante.
La courbe pV de tout processus isotherme est hyperbolique, car la pression est inversement proportionnelle au volume pour un gaz parfait à température constante.

<ol>
	<li>Young, H. D, et Freedman, R.A. (2012). La physique universitaire avec la physique moderne. San Francisco, Californie : Pearson. p. 635.</li>
	<li>OpenStax. (2019). Physique universitaire Vol. 2. [Version Web]. pp 121 Extrait de <a href="https://openstax.org/books/college-physics/pages/15-2-the-first-law-of-thermodynamics-and-some-simple-processes">https://openstax.org/books/college-physics/pages/15-2-the-first-law-of-thermodynamics-and-some-simple-processes</a></li>
</ol>

les processus isothermes

Un processus thermodynamique qui se déroule à température constante est appelé un processus isotherme. La chaleur coule lentement dans le système ou hors du système afin de maintenir l'équilibre thermique. Les processus impliquant des changements de phase comme l'évaporation de l'eau en vapeur ou la congélation de l'eau en glace à une température constante sont des exemples de processus isothermes.

Un gaz idéal peut également subir une expansion ou une compression isotherme.

Par exemple, considérons 1 mole d'un gaz idéal à l'intérieur d'un cylindre isolé avec un volume initial V et une pression p. Supposons que la chaleur ajoutée au système entraîne une expansion quasi-statique du gaz à une température constante de 300 K. Si le volume du gaz change pour atteindre 2 V, quelle est la quantité de chaleur ajoutée au système?

Ici, les quantités connues sont le nombre de moles du gaz, le volume initial et final du gaz. La quantité inconnue, la chaleur, doit être calculée.

Selon la première loi de la thermodynamique,

<img alt="Equation1" src="/files/ftp_upload/13682/13682_Equation_PT_1.svg" style="text-align: center; height: 13px;" />

Comme la température est constante, le changement d'énergie interne pour ce processus est nul. Ainsi, la chaleur ajoutée au système est égale au travail effectué par le système.

<img alt="Equation2" src="/files/ftp_upload/13682/13682_Equation_PT_2.svg" style="text-align: center; height: 13px;" />

Maintenant, le travail effectué par un gaz idéal lors d'une expansion isotherme est donné par,

<img alt="Equation3" src="/files/ftp_upload/13682/13682_Equation_PT_3.svg" style="text-align: center; height: 32px;" />

En substituant le nombre de moles, la constante des gaz, la température, le volume initial et le volume final du gaz, le travail effectué est égal à 1728.85 J.

Étant donné que le travail effectué par le système est égal à la chaleur ajoutée au système, on considère que 1728.85 J de chaleur sont appliqués au système pour le faire se dilater isothermiquement jusqu'à 2 V.

Les processus isothermes

Un processus thermodynamique qui se produit à température constante est appelé processus isotherme. La chaleur s’écoule lentement dans le système ou hors du système pour ...

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isothermal expansion and heat transfer

An isothermal process is a thermodynamic process that happens at constant temperature. During the process, heat slowly flows into the system or out of the system so thermal equilibrium is maintained.

Phase changes are common examples of isothermal processes. Water can evaporate into steam at a constant temperature, and water can also freeze into ice at a constant temperature. An ideal gas can also undergo isothermal expansion or compression.

Here, an ideal gas in an isolated cylinder expands quasistatically at 300 K. The gas starts with 1 mole at volume V and pressure p. The volume then changes to 2V, and the heat added to the system must be found.

By the first law of thermodynamics, the change in internal energy is zero because the temperature stays constant. That means the heat added to the system equals the work done by the system. For an ideal gas in isothermal expansion, the work is calculated from the initial and final volume.

Substituting the number of moles, gas constant, temperature, and volumes gives a work value of 1728.85 J. Since the work done by the gas equals the heat added, 1728.85 J of heat is applied to expand the gas isothermally to 2V.

Isothermal Expansion and Heat Transfer

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isothermal processes

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Learn how isothermal processes keep temperature constant while heat equals work during ideal gas expansion, with phase change and cylinder examples.

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Isothermal Expansion and Heat Transfer

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