Name: gas temperature changes in adiabatic processes
Uploaded: 2023-04-30T12:32:28.000Z
Duration: 1 min 18 s
Channel: JoVE

K12 Standards

new-york-state-science-learning-standards

New York State Science Learning Standards

High School

Physical Science

HS. Energy

HS-PS3-3.

Adiabatic Processes for an Ideal Gas

Un processo termodinamico senza trasferimento di calore all'interno o all'esterno del sistema è definito adiabatico.
Quindi, secondo il primo principio della termodinamica, la variazione dell'energia interna è il negativo del lavoro svolto in un processo adiabatico.
Ad esempio, un pistone antincendio costituito da un tubo isolato termicamente ha un'estremità chiusa e l'altra estremità ha uno stantuffo mobile. 
Se un piccolo batuffolo di cotone viene posizionato all'interno del tubo e lo stantuffo viene lasciato spingere rapidamente verso il basso a temperatura ambiente, il batuffolo di cotone prende fuoco nonostante non venga applicato calore. Ciò accade a causa della compressione adiabatica.
Qui, al diminuire del volume, il lavoro svolto diventa negativo; quindi, la variazione di energia interna è positiva, con un corrispondente aumento della temperatura del gas.
Al contrario, quando si apre una bottiglia di bevanda gassata raffreddata, il gas intrappolato nella bottiglia subisce un'espansione adiabatica, con conseguente positività del lavoro svolto.
Di conseguenza, l'energia interna diminuisce con un corrispondente calo di temperatura, condensando il vapore evaporato in una nuvola visibile.

<ol>
	<li>Young, H.D. e Freedman, R.A. (2012). Fisica universitaria con fisica moderna. San Francisco, CA: Pearson. p. 640.</li>
	<li>OpenStax. (2019). Fisica universitaria Vol. 2. [Versione web]. Estratto da <a href="https://openstax.org/books/university-physics-volume-2/pages/3-6-adiabatic-processes-for-an-ideal-gas">https://openstax.org/books/university-physics-volume-2/pages/3-6-adiabatic-processes-for-an-ideal-gas</a></li>
</ol>

processi adiabatici per un gas ideale

Quando un gas ideale viene compresso adiabaticamente, cioè senza aggiunta di calore, viene svolto lavoro su di esso e la sua temperatura aumenta. In un'espansione adiabatica, il gas svolge un lavoro e la sua temperatura diminuisce. Le compressioni adiabatiche avvengono, per esempio, nei cilindri di un'automobile, dove le compressioni della miscela gas-aria avvengono così rapidamente che non c'è tempo per la miscela di scambiare il calore con il suo ambiente. Tuttavia, siccome durante la compressione viene svolto lavoro sulla miscela, la sua temperatura aumenta significativamente. Infatti, l'aumento di temperatura può essere così elevato che la miscela può esplodere senza l'aggiunta di una scintilla. Tali esplosioni, poiché non sono temporizzate, fanno funzionare male un'automobile - di solito "battendo". Poiché la temperatura di accensione aumenta con l'ottano della benzina, l'uso di benzina ad ottani più alti è un modo per superare questo problema.

Un altro interessante processo adiabatico è l'espansione libera di un gas. Immagina un gas confinato da una membrana su un lato di un contenitore a due compartimenti, isolato termicamente. Quando la membrana viene forata, il gas si riversa nel lato vuoto del contenitore, espandendosi liberamente. Visto che il gas si espande "contro il vuoto", la pressione diminuisce (p = 0, non svolge lavoro e, poiché il recipiente è termicamente isolato, l'espansione è adiabatica. Con Q = 0 e W = 0 nella prima legge, dU = 0, quindi l'energia interna nello stato di equilibrio iniziale e finale è la stessa per l'espansione libera. Se il gas è ideale, l'energia interna dipende solo dalla temperatura. Pertanto, quando un gas ideale si espande liberamente, la sua temperatura non cambia.

Processi adiabatici per un gas ideale

Quando un gas ideale viene compresso adiabaticamente, cioè senza aggiungere calore, viene eseguito un lavoro su di esso e la sua temperatura aumenta. In un'espansione adiabatica, ...

Education

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Physics

Fisica

Unit 1

Unità 1

The First Law of Thermodynamics

Il Primo Principio della Termodinamica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

gas temperature changes in adiabatic processes

Adiabatic processes change the temperature of an ideal gas without heat transfer. In an adiabatic compression, no heat is added, but work is done on the gas. That work raises the gas temperature. In an adiabatic expansion, the gas does work and its temperature drops.

A common example is the compression of a gas-air mixture inside a car engine cylinder. The compression happens so quickly that there is no time for heat to move between the mixture and its surroundings. Even so, the work done on the mixture can raise its temperature enough for it to explode without a spark. When that explosion happens at the wrong time, the engine knocks and runs poorly. Higher-octane gasoline can help because it has a higher ignition temperature.

The transcript also describes free expansion of a gas. In this case, a gas starts on one side of a thermally insulated two-compartment container. When the membrane is punctured, the gas rushes into the empty side and expands into a vacuum. Because the pressure outside is zero, the gas does no work. Since the container is thermally insulated, the process is adiabatic. With Q = 0 and W = 0, the first law gives dU = 0, so the internal energy stays the same before and after the expansion. For an ideal gas, internal energy depends only on temperature, so the temperature does not change during free expansion.

Gas Temperature Changes in Adiabatic Processes

gas-temperature-changes-in-adiabatic-processes

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adiabatic processes for an ideal gas

Adiabatic processes change the temperature of an ideal gas without heat transfer. In an adiabatic compression, no heat is added, but work is done on the gas. That work raises the gas temperature. In an adiabatic expansion, the gas does work and its temperature drops. A common example is the compression of a gas-air mixture inside a car engine cylinder. The compression happens so quickly that there is no time for heat to move between the mixture and its surroundings. Even so, the work done on...

pressure and volume in an adiabatic process

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Learn how adiabatic compression and free expansion affect an ideal gas. See why temperature rises in compression, drops in expansion, and stays the same in free expansion.

Gas Temperature Changes in Adiabatic Processes | JoVE K12

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Gas Temperature Changes in Adiabatic Processes

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